www.ietdl.org Published in IET Power Electronics Received on 30th April 2013 Revised on 16th September 2013 Accepted on 23rd October 2013 doi: 10.1049/iet-pel.2013.0321 ISSN 1755-4535 Survey of modelling techniques used in optimisation of power electronic components Mehran Mirjafari1, Robert S. Balog2 1 Department of Electrical Engineering, Texas A&M University, 30C4 Zachry bldg., College Station 77843, TX, USA Department of Electrical and Computer Engineering, Texas A&M University, 216D Zachry Engineering Bldg., College Station 77843, TX, USA E-mail: sardis@gmail.com 2 Abstract: Design optimisation techniques for power electronic converters have been the subject of numerous research studies for the past 15 years. Accurate modelling is the most important task in the optimisation, and, because the nature of optimisation problem demands evaluating an objective function numerous times, fast and simplified modelling techniques are required. The objective of this study is to categorise and analyse the previous studies related to modelling techniques incorporated in design optimisation of power electronic converters. Common trends in modelling and optimisation of power electronic converters are analysed and suggestions for future research in this field will be presented. 1 Introduction Electricity represents 40% of the total energy consumption worldwide, and the demand for electrical power is only expected to increase [1]. This growth in demand is partly because of increase in number of digital devices, and hence, power electronic converters which must regulate power at almost any arbitrary voltage, current and frequency. Thus, power electronics can be found through the electrical system as they enable efficient conversion generation, storage and end-user applications. With technologies such as all electric aircrafts [2] and electric vehicles [3, 4] gaining more attention, it is expected that the number of power electronic converters will continue to increase in the coming years [5]. Increasing the number of digital devices will also increase the demand for power electronic converters. Cisco visual networking index forecast predicts that by year 2015 the number of digital devices connected to the network is twice the global population [6]. At the same time, all electronics are expected to become ‘better, faster and cheaper’ which require highly optimised designs. Therefore effective design techniques are required to achieve higher-performance indices for power electronic converters, which include size, weight, efficiency, cost and reliability. Design optimisation of power electronic converters is certainly not a new subject and dates back to the early days of power electronics. The evolution of digital computers made it possible to use advanced numerical and stochastic techniques for optimisation of complex power electronic circuits. Advances in modelling techniques also resulted in more accurate models and therefore a more accurate optimisation. However, despite advances in modelling power electronic converters, only a limited class of models can be used for optimisation since objective functions are evaluated numerous times in the optimisation process, which makes the optimisation problem computation-intensive. In other words, modern optimisation techniques, whether deterministic or stochastic, search for the optimal point by calculating an objective function repeatedly, moving from one candidate design point to the other, which makes it nearly impossible to use highly accurate detailed models such as those obtained from finite element analysis or similar techniques. Therefore in power electronic design optimisation, modelling usually comes down to one or a set of algebraic (or, in rare cases, differential or integral) equations which are much easier to evaluate in a computation-intensive optimisation problem. It should be mentioned that in some areas such as electrical machines, it is now a common practice to use complex finite element models [7, 8] or complex MATLAB models [9–12] for optimisation; however, use of these complex models are very limited in power electronics. With a wide variety of simplified models for different components, designers of power electronic converters face a challenge for choosing the right models for optimisation of their designs. The criteria for choosing models may be based on the nature of the objective function, the design variables of degrees of freedom for the design, or accuracy of the models. This paper presents a study of the modelling approaches used in the design optimisation of power electronic converters published in IEEE-affiliated journals and conferences after 1995. From 398 papers which deal with one form of optimisation, 110 ‘design’ optimisation papers were selected and analysed. This study will be helpful for the designers of conventional power electronic converters, as well as researchers and R&D engineers who want to pick the right components and operating conditions 1192 IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 This is an open access article published by the IET under the Creative Commons Attributiondoi: 10.1049/iet-pel.2013.0321 NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org (e.g. switching frequency) for a newly proposed converter topology. The remainder of this paper is organised as follows: in Section 2, a definition of the optimisation problem is given and different modelling approaches will be introduced, in Section 3, loss modelling of different components will be discussed, in Section 4, size modelling of converters and in Section 5 cost modelling will be presented. Sections 6 and 7 contain recommendations for future work and conclusion, respectively. 2 Problem definition The ‘optimisation’ problem can be defined mathematically and in a general form as [13] minimise: subject to: f (x), x [ V gi (x) ≤ 0, i = 1 . . . n (1) where f (x) is the ‘objective function’ defined in the space Ω and gi (x) i = 1 … n are a number of ‘constraints’. This definition establishes the scope of the optimisation considered in this survey, which explicitly excludes three other categories of optimisation found in the literature: † incremental improvement to a specific performance index and do not fall into a rigorous optimisation problem stated in (1). † optimisation of the controller and algorithm in which the performance indices are limited to the controller itself. Examples for this category are [14], in which parameters such as steady-state error, settling time, overshoot etc. are subject to optimisation, and [15] in which parameters such as steady-state error, settling time etc. are combined to form an objective function. † optimisation of the performance indices of the whole converter (efficiency, cost etc.) through changing controller parameters (i.e. controller tuning). For example, in [16] efficiency is optimised via tuning the switching frequency and dead time, whereas in [17] harmonic content is optimised via controlling the pulse width modulation (PWM) sequence. Papers included in this study pertain to optimising the whole or a part of the power stage of the power electronic circuit. Objective function f(x) creates a mapping between design variables and design objective. This mapping is created using models of the components with regard to operation of the converter. For example, if energy efficiency of a converter is being considered as the objective function, loss models of individual components with regard to effective voltages and currents across the converter create a mapping between design variables (e.g. component values) and efficiency. Component models may also be used in defining constraints. For example, in order to check if a certain LC filter complies with IEEE 519 or any other harmonic standard, a detailed model of the inductor which takes AC resistance of the inductor winding for each harmonic frequency is required. The optimisation problem is then solved using either a deterministic (e.g. Newton’s method, gradient descent method or sequential quadratic programming etc. [13]) or stochastic (simulated annealing, genetic algorithm, particle swarm optimisation etc. [18–20]) method. These methods ‘search’ for an optimal point by means of an iterative approach, involving calculation of the objective function one or more times for each iteration. Similar to any other form of modelling, in power electronics there is usually a tradeoff between the accuracy of the model and the number of variables considered. Dynamics of the system are usually simplified (or entirely ignored) in order to develop a reduced-order, and hence faster way of modelling. A model derived from finite element analysis, for example, takes into account a fine mesh inside of each different material in the component as well as accurate material properties. Such highly accurate models may not be useful for the purpose of the optimisation since they are computation-intensive, and therefore time consuming. Therefore optimisation problems usually use simpler techniques which involve mostly algebraic or simple differential or integral equations. Finding and categorising such models is the subject of this work and can be of a great help to anyone willing to work in this area. A wide variety of techniques might be used in order to find a reduced-order model for the optimisation. For instance, the Steinmetz equation is derived from curve fitting to a data series given by test measurements; Dowell equation is derived from solving diffusion equation in one-dimensional (1D) (and ignoring the 3D nature of the problem) and considering a fixed forward voltage drop for a power diode simply ignores the slight variation in the voltage drop which can be calculated by using device physics equations in [21]. In this paper, models have been categorised based on the objective function, that is, loss, size etc. In the next sections, the following aspects of models are discussed: Section 3, loss mechanisms – Different phenomenas that contribute to losses in power electronic circuits and approaches to model those phenomena; Section 4, size modelling – calculating weight and/or volume of the converter from a set of design variables; and Section 5, cost modelling – different factors which are considered to affect the cost of the converter. 3 3.1 Loss mechanisms Core losses in inductors and transformers Inductors and transformers are very common in power electronic circuits and contribute to the power losses especially in high-frequency converters. There are two mechanisms for the core losses: eddy current losses and hysteresis losses. Although these losses are well studied under single-frequency sinusoidal magnetic fields [22], these models and results are not generally applicable to switching power supplies. Owing to the non-linear nature of the hysteresis losses, it is not possible to apply superposition principle in the case of a non-sinusoidal field. Therefore methods other than those applicable to sinusoidal excitation are usually used to calculate core losses in magnetic components. 3.1.1 Steinmetz equation: The Steinmetz equation is used to calculate core losses when the magnetic field is sinusoidal. The basic form of Steinmetz equation is Pcore = K f a Bb (2) where Pcore is the volumetric core loss, f is the frequency of sinusoidal excitation waveform, B is the peak flux density of the core and α, β and K are coefficients specific to each IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 1193 doi: 10.1049/iet-pel.2013.0321 This is an open access article published by the IET under the Creative Commons AttributionNonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org core-geometry and usually determined by curve fitting to the curve provided by the manufacturer. Once the Steinmetz parameters have been identified, either given explicitly by the manufacturer or found from curve fitting the manufacturer’s data plots, it is straightforward to calculate core losses. A main limitation is that this equation is only good for sinusoidal excitation voltage and can be used whenever applicable. In [23–26], the basic form of the Steinmetz equation is used to calculate core losses in a resonant module. In [27], this equation is used to calculate core losses in the AC source side of a boost power factor correction (PFC) converter. In [28], this equation is based on an approximation. In [29], this equation is used to calculate core losses in a DC–DC converter and it is claimed that it is given in the datasheet, which is no longer available in the provided web address, and finally in [30], core losses in a high-frequency transformer is calculated using this equation; however, magnetic field waveform of this transformer is not specified. In [31], Steinmetz equation is used to calculate core losses in a transformer. The flux density is calculated from core geometry and excitation in [32] and used in the equation. 3.1.2 Modifications to Steinmetz equation: In most typical power electronic circuits, the magnetic cores are exposed to non-sinusoidal magnetic fields. Thus, the basic Steinmetz equation does not accurately predict core losses under non-sinusoidal excitation. In [33–39], a modified form of Steinmetz equation, which is developed in [40] is used. In this modified form, instead of using frequency, the time-rate of change of flux density (dB/dt) is used to calculate the core loss a dB Pcore = K1 (DB)b−a (3) dt where ΔB is the peak-to-peak flux density and K1 is chosen to be consistent with the basic Steinmetz (2) for sinusoidal waveforms. In [41–44], another form of modified Steinmetz equation, which is introduced by Albach et al. [45] was used. An equivalent frequency is derived for the arbitrary waveform to be used with the basic equation and the effects of temperature are also considered. In [46–48], a generalisation of Steinmetz equation based on [49], which seeks to calculate core losses in discontinuous (DCM) and continuous (CCM) conduction modes for a buck converter, is used. In these calculations, core losses in DCM depend only on frequency and maximum flux density while they depend on frequency, maximum flux density and flux density changes in CCM. Jieli et al. [50] also used a generalisation of Steinmetz equation based on their own work [51]. In this generalisation, time average of core losses over a period is calculated from equation below Pcore = 1 T a dB b−a K1 B(t) dt dt 0 T (4) where K1 is calculated such that (4) is consistent with the basic Steinmetz equation (2) under sinusoidal excitation. In [52], a modified Steinmetz equation, which is introduced in [53], is used. In this modified equation, an equivalent frequency is derived for non-sinusoidal waveforms, which is basically an integral form of piecewise linear equation which was presented in [45]. Table 1 Major modifications to Steinmetz equation Designations Additional considerations modified Steinmetz equation [45, 53] dB/dt generalised Steinmetz equation [51] B(t) and dB(t)/dt Formulas feq = 2 DB 2 p2 a−1 b P = k feq B̂ fr a dB P(t) = K1 |B(t)|b−a dt P= 1 T T P(t) dt 0 natural Steinmetz equation [40, 55] dB(t)/dt waveform coefficient Steinmetz equation [58] a correction factor (flux waveform coefficient) T 2 dB dt dt 0 Pcore = DB 2 b−a KN T T a dB dt dt 0 P = FWC K f a B b , for example, FWCsquare = p 4 A quadratic term was also added to the basic Steinmetz equation because of the geometry effects. In [54] an equation is used, which is introduced in [55] and called natural Steinmetz equation and uses the assumption that core losses depend on flux density and its rate of change instead of a single frequency Pcore = DB b−a KN T dBa dt T 0 dt 2 (5) KN is calculated in a way that (5) is consistent with the basic Steinmetz equation (2) for sinusoidal excitation. Another modified version of Steinmetz equation was used in [56, 57] Pcore = a k (b f )m DBn (6) Although this equation appears identical to the basic Steinmetz (2), it is claimed that coefficients α and β compensate for the non-sinusoidal excitation [56, 57]. Waveform coefficient Steinmetz equation is introduced in [58], and, as the name implies, losses are calculated by multiplying a factor to the basic Steinmetz equation. The factor is calculated from the flux waveform. Table 1 summarises modifications to the Steinmetz equation. Owing to the empirical nature of Steinmetz equation, all of the modifications are also empirical. According to [59], generalised Steinmetz equation is substantially less accurate than modified Steinmetz equation and natural Steinmetz equation. Natural Steinmetz equation is also more accurate than modified Steinmetz equation since it does not use an arbitrary averaging method. There is no direct comparison between waveform coefficient Steinmetz equation and the three aforementioned methods; however, since waveform coefficient method only takes peak flux density into account, it is expected that it is less accurate than the other three. 3.2 Copper losses in windings Copper losses are a considerable portion of the total losses of the inductor. Generally, power losses in a conductor can be 1194 IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 This is an open access article published by the IET under the Creative Commons Attributiondoi: 10.1049/iet-pel.2013.0321 NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org Table 2 Summary of copper losses Reference(s) Descriptions Formulas [25, 26, 30, 44, 54, 60] calculated from Dowell equation see (7) and (8) [23, 33–37, 61–63] equivalent resistance calculated from current wave shape and Dowell equation Reff C = 1 + A4 3 Rdc 2 dIrms /dt vIrms 5Nl2 − 1 , see (7) and (8) for Nl and A 15 definitions C= [64–66] equivalent frequency derived based on current wave shape, resistance then calculated from Dowell equation veff = [27–29, 41, 50, 67–76] Dc resistance used-high-frequency effects neglected Rac = Rdc [77] single-frequency resistance used-higher-frequency effects neglected Rac = Rac, f0 [46–48, 78] comprehensive analysis done for a flyback transformer see [78] for comprehensive analysis [31, 32] comprehensive analysis for a low-profile transformer see [32] for comprehensive analysis [39, 42, 43, 79, 80] losses calculated from 2D solution of diffusion equation P = FI 2 + GH 2 rms(d/dt)I(t) Irms n sinh n + sin n 4 cosh n − cos n sinh n − sin n G = w 2 Rdc n cosh n + cos n d n= dv F = Rdc w : conductor width I : peak current H : peak field [56, 57, 81] DC and eddy current losses calculated separately, superposition used for eddy current losses [82, 83] an analytical technique used-claims to be faster than solving diffusion equation 2D, but with a good accuracy calculated by multiplying the resistance by the square of the current. However, conductor current and current flowing in adjacent conductors create a magnetic field which leads to additional losses in the conductor. Additional losses because of the conductor current and current flowing in adjacent conductors are called skin and proximity effects, respectively. These two losses are highly frequency-dependent, and, since high-frequency waveforms are very common in power electronics, these effects must be considered if an accurate loss prediction is needed. Owing to the linear nature of eddy current losses, superposition can generally be applied for copper loss calculation, unlike in the case of core losses. Different approaches taken in optimisation papers are reviewed below and summarised in Table 2. 3.2.1 Dowell equation: The Dowell equation is a generalised form of the solution for the diffusion equation of rectangular conductors adapted and applied to round conductors [22]. In this equation, instead of calculating losses directly, the ‘AC resistance’ of the conductor is calculated. The ratio of this resistance to the DC resistance 2 Pwinding = 3RIrms + Peddy 1 I(n) 2 2 Peddy = Peddy, R n IR n=1 see [83] for comprehensive analysis is given as Rac Rdc sinh 2A + sin 2A 2(Nl2 − 1) sinh A − sin A + =A cosh 2A − cos 2A 3 cosh A + cos A (7) FR = where A is given A= p(3/4) d 4 dv (8) and d is the conductor diameter, δω is the skin depth and Nl is the number of layers in a multilayer winding. This equation is widely used in the literature along with Fourier analysis and superposition of the current waveform [25, 26, 30, 44, 54, 60]. In the case of a non-sinusoidal current waveform, AC resistance should be calculated separate for each frequency which requires that IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 1195 doi: 10.1049/iet-pel.2013.0321 This is an open access article published by the IET under the Creative Commons AttributionNonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org the circuit be solved for each frequency separately. The total copper loss can be obtained using superposition principle, that is, summing up losses of the circuits for each individual frequency. This approach, while accurate, increases the computing time and might be burdensome for an optimisation algorithm. Another approach, which is based on Dowell equation, is to calculate an effective total resistance based on the current waveform. The approach taken in [23, 24, 33–38], which is based on an approach presented in [61], calculates an effective resistance from the root-mean-square (RMS) current and its derivative. However, this equation is only useful when the current waveform and its derivatives are known. In one other approach, which is based on 1D solution of diffusion equation, an equivalent frequency is calculated for a non-sinusoidal waveform [64–66]. Current derivatives are also required in this approach. 3.2.2 Equivalent resistance: In the Dowell equation, eddy current losses and therefore AC resistance are frequency-dependent. However, some of the papers model copper losses as a single equivalent resistance. In [50], the DC resistance of the inductor was used, neglecting high-frequency effects, reportedly because of low-ripple current. In [27, 28], only DC resistance was used, stating explicitly that the high-frequency effects are neglected. In [77], transformer current waveform is considered to be sinusoidal (and confirmed by measurement), and therefore an equivalent resistance for sinusoidal waveform is used. In [84], an equivalent resistance is used to model both core losses and copper losses, and the resistance is calculated from curve fitting. In [29, 41, 52, 67–69, 71, 74, 75, 85, 86], equivalent resistance is used; however, no reason is specified for neglecting high-frequency effects. 3.2.3 Other copper loss calculation methods: The Dowell equation and series resistance approximation are the most common approaches in the literature, other techniques are also mentioned. In this section, these are reviewed. Copper losses for a flyback transformer are calculated in [46–48] based on the detailed analysis given in [78]. Main loss mechanisms are studied in detail in [78] using an analytical approach. In [31], an equation for calculating power losses in low-profile transformers is given, but the comprehensive analysis is given in [32]. In [39], copper losses are calculated using a 2D solution of diffusion equation presented in [80]. The approach of [80] is also used in [79] which gives the detailed loss analysis for [42, 43]. In [56, 57, 81], DC losses and eddy current losses are calculated separately and superposition is applied for eddy current, and finally, in [82] an approach introduced in [83] is used which is claimed to be more accurate than 1D solution to diffusion equation while being faster than solving the equation numerically. From this study on the papers, it is evident that a 1D solution to diffusion equation (i.e. Dowell equation) remains the mostly used method to calculate winding losses in power electronic converters. Although Dowell equation is obviously more accurate than using the DC resistance, it is less accurate (and also less complex and faster) than a 2D solution to diffusion equation. Some other methods such as the method used in [56, 57, 81] is less accurate than Dowell equation since it ignores proximity losses. The method mentioned in [83] is also shown to be more accurate than Dowell equation; however, the computational speed of two methods has not been compared. 3.3 Semiconductor losses Calculating semiconductor losses should be performed based on the type of the switch and the circuit. In power electronics, metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs) are used widely as primary switches, whereas diodes are mostly used for freewheeling and providing a return path for the current for one-directional MOSFETs and IGBTs. Losses in semiconductor switches are a considerable portion of the total losses of a power electronic converter, and therefore precise loss modelling techniques are necessary for accurate loss calculation. Owing to the complex nature of the semiconductor materials and the large number of the parameters involved, modelling loss mechanisms based on geometry or other device-level characteristics of the device is challenging. Therefore most papers do not include these characteristics as design variables and instead prefer to choose a switch from a database, and hence calculate losses from information provided on the specific datasheet. In [23, 24, 29, 33–36, 38, 44, 46, 50, 52, 67–69, 71, 75, 77, 85, 87–89], at least one portion of semiconductor losses are calculated from values provided by datasheet and scaling them with current and/or voltage. 3.3.1 MOSFET losses: MOSFET conduction losses are usually calculated using a simple on state equivalent resistance [23, 24, 27, 29, 33–38, 44, 46, 50, 52, 67, 71, 75, 77, 84–86, 90–92]. In most of the papers, this resistance (Rds,ON) is obtained from the MOSFET datasheet and multiplied by RMS current square to calculate conduction losses [23, 24, 29, 33–36, 38, 44, 46, 50, 52, 67, 71, 75, 77, 84, 85, 93, 94]. Some of the other papers attempt to calculate or scale Rds,ON based on some device-level characteristics or operational characteristics of the MOSFET. In [37], Rds,ON is obtained from the datasheet, Table 3 Summary of MOSFET switching losses Reference(s) [67, 75, 98] [29, 46, 68–72, 74] [41, 44, 50] [95] [73, 90, 91] [27] [23, 33–37, 39, 62, 63] [97] Descriptions from capacitance values obtained from datasheet from rise time and fall time obtained from datasheet switching energy is scaled with voltage and current capacitance values scaled with length and width of MOSFET die capacitance values from width of MOSFET die capacitance values calculated from chip area empirical equation for soft-switched converters model for a series–parallel resonant converter Table 4 Summary of conduction losses in MOSFETs Reference(s) [37] [72] [27] [73, 90] [91] [39, 41] Descriptions Rds,ON is scaled with junction temperature Rds,ON is scaled with rated voltage Rds,ON from MOSFET chip area Rds,ON from MOSFET die width Rds,ON from MOSFET die width and applied gate voltage Rds,ON from junction temperature and current extracted from datasheet 1196 IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 This is an open access article published by the IET under the Creative Commons Attributiondoi: 10.1049/iet-pel.2013.0321 NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org but is scaled with junction temperature of the MOSFET. In [52], Rds,ON is scaled with maximum blocking voltage of MOSFET. In [27], Rds,ON is calculated from MOSFET chip area. In [86, 90], Rds,ON is calculated from the MOSFET width, and in [91] Rds,ON is calculated from MOSFET width and applied gate voltage. In [39, 41], Rds,ON is calculated from junction temperature and load current based on the values extracted from the datasheet. A summary of conduction loss calculation for MOSFETs is given in Table 4. MOSFET switching losses depend highly on the type of the converter and its control strategy. In hard switched converters, energy loss during one cycle is calculated using either equivalent capacitance [27, 67, 75, 84, 86, 90, 91, 95] or rise time and fall time of the MOSFET voltage and current [29, 46, 52, 68, 69, 71, 85]. In some other papers, turn-on and/or turn-off energy is directly used to calculate switching losses [41, 44, 50], scaling them with the voltage or current. Like Rds,ON, equivalent parasitic capacitance is taken only from the datasheet in some of the papers [67, 75, 84], whereas in some others, it is calculated or scaled from the device-level characteristics. In [95], equivalent capacitance changes with the width and length of the each MOSFET. In [86, 90, 91], capacitance is calculated from width of the MOSFETs, and in [27] it is calculated from area of the MOSFET. In soft-switched converters, however, calculating switching losses cannot be performed using the above-mentioned approaches. Therefore detailed loss analysis or empirical equations are required for these types of converters. In [23, 24, 33–39], empirical equations have been derived from measurement and curve fit to express switching losses, and in [96] a model for a series–parallel resonant converter, presented in [97], is used. A summary of methods for calculating MOSFET switching losses is presented in Table 3. Choosing a method for calculating MOSFET conduction and switching losses depends highly on what the design variables are for the optimisation. If parameters such as MOSFET die area, drift region thickness etc. are among design variables, the problem is entirely different from a case where certain internal parameters of the MOSFET are not subject to the optimisation. Therefore a direct comparison between methods mentioned in Tables 3 and 4 is not meaningful. However, it can be said that if only datasheet information is used to calculate losses, they have to be adjusted to reflect changes because of operating voltage and current. These equations are highly non-linear and a simple scaling is not accurate [21]. 3.3.2 IGBT losses: Unlike MOSFETs, IGBT forward voltage drop is not purely resistive. Different ways of modelling IGBT conduction losses in papers related to power electronic optimisation can be summarised as follows. In [87, 88], forward voltage drop of the IGBT has a constant term and a non-linear term B VF = V0 + r0 IL con (9) where IL is the IGBT current. V0 is the fixed portion of forward voltage drop, Bcon is a constant found from curve fitting and r0 called ‘dynamical resistance’ is a function of gate drive voltage. A similar equation is used in [99], which is based on the model provided in [100], but details on finding the parameters are not given. Considering Bcon = 1 the second term in forward voltage drop is only resistive, which is the base of the approach used in [54]. In this Table 5 Summary of loss calculation in IGBTs Reference(s) [87, 88, 99, 100] [54] [101–106] [54, 68, 69, 87, 88] [107, 108] [109] Descriptions forward voltage drop: a fixed part and a non-linear part non-linear part is a function of gate resistance and load current switching losses from datasheet values and parasitic component values conduction and switching losses inter-dependent dependence of fixed part of forward voltage drop to switching times resistive part of forward voltage drop is a function of maximum current a comprehensive model using device-level parameters switching losses from datasheet values switching energy from a non-linear equation based on current switching losses from linear interpolation based on gate resistance approach, fixed part of the forward voltage drop is derived, based on switching characteristics of the IGBT, using a surface fitting on available commercial samples. The resistive part is derived from the commercial samples based on maximum current of the IGBT. A complete loss model for the IGBT and the diode is used in [101–103], which is based on the model presented in [104– 106]. This model uses MATLAB/SIMULINK for modelling losses based on certain physical attributes of the IGBT such as active area and width of the IGBT die, doping level and high-level carrier lifetime. The model is good for an IGBT and a diode in a chopper cell; however, as explained in this paper, most power electronic circuits can be reduced to a chopper cell for the purpose of modelling and optimisation. This model is able to calculate both conduction and switching losses. For switching losses, the usual approach is to find the switching energy from the rise and fall time of the current and voltage [54, 68, 69, 87, 88]. However, using non-linear equations to find turn-on and turn-off energy is also common [107, 108]. Linear interpolation is used in [109] to find turn-on and turn-off energy losses considering the effect of the gate resistance. Loss calculation for IGBTs is summarised in Table 5. Similar to MOSFETs, the differences between IGBT loss calculation approaches are mostly because of the design variables chosen for the modelling. It is evident from loss calculation approaches both for MOSFETs and IGBTs that loss modelling based on semiconductor design variables (e.g. die area, doping concentration etc.) is still in its early stages and more work is needed to be done in this area. Table 6 Summary of diode losses Reference(s) [23, 27, 29, 33–37, 54, 62, 63, 70, 72, 77] [39, 41, 46, 99] [67] [101–106] Descriptions fixed voltage drop from datasheet fixed part and a resistive or non-linear term Shockley equation for diode voltage drop a comprehensive model using device-level parameters IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 1197 doi: 10.1049/iet-pel.2013.0321 This is an open access article published by the IET under the Creative Commons AttributionNonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org Table 7 Summary of gate drive losses Table 8 Summary of capacitor losses Reference(s) Reference(s) [33–35, 90] [91] Descriptions from gate capacitance, gate voltage and frequency (from datasheet) gate capacitance is calculated from geometry for a monolithic buck converter This will be explained more in Section 6. However, it can be said that forward voltage drop and switching transition times are highly non-linear [21] and any attempt that takes this non-linearity into account (either via data fitting or physical modelling) is considered more accurate. 3.3.3 Diode: Diode conduction losses are usually calculated using a fixed forward voltage drop for the diode in on state which is usually extracted from the datasheet [23, 24, 27, 29, 33–38, 52, 54, 77, 85] and RMS current 2 (loss = voltage × drop × IRMS ). In some of the papers, a resistive or non-linear term is also added to this fixed drop [39, 41, 46, 99]. Other approaches used are using Shockley equation for diode voltage drop instead of a fixed voltage [67] and a comprehensive loss analysis for IGBT and diode as explained in previous section [101–103]. A summary of the diode losses calculation techniques is presented in Table 6. Considering fixed voltage drop for diode conduction losses seems to be a good approximation; however, since diode voltage drop is a non-linear term, any attempt to take this non-linearity into consideration makes the modelling more accurate. Depending on the design variables of the optimisation problem, using Shockley equation yields the best accuracy since it relies on semiconductor physics. In addition, model used in [101–106] accurately models the diode by solving the carrier distribution equation; however, since it is used to model an IGBT-diode combination that cannot be used directly in the circuits where diodes are not used with IGBTs. 3.3.4 Gate drive losses: Even though gate drive losses usually contribute little to the total power losses, they might be significant, especially in a low-power [90, 91] or high-efficiency [33–35, 37] converter. Gate drive losses are almost exclusively calculated using the gate capacitance, gate voltage and frequency [33–35, 90, 91]. In [91], this capacitance is calculated from the geometry for a monolithic buck converter. Gate drive losses are also considered in [37], but the details are not given. A summary of gate drive loss calculation is in Table 7. Gate drive losses do not change significantly with load conditions, and therefore gate drive loss calculation from datasheet values yields a good accuracy. However, in certain converters, such as monolithic converters, where gate capacitances change significantly with design variables, and there is no datasheet value for these capacitance values, they are calculated in the optimisation process, and losses are calculated accordingly [91]. 3.4 Capacitor losses Capacitor losses are usually calculated considering an equivalent series resistance (ESR) for the capacitor [23–25, 34–38, 42, 43, 54, 71, 74, 75, 85, 107, 108]. This resistance represents the power losses in the capacitor, which is because of the resistance of conducting material [23, 25, 34–37, 42, 43, 62, 63] [54, 107, 108, 111, 112] Descriptions ESR calculated from tan δ ESR calculated from tan δ and capacitor geometry and also dielectric losses. If the former part is neglected, series resistance can be expressed as ESR = tan d Cv (10) where C is the capacitance, ω is the frequency in radians and tan δ is the loss angle which is nearly constant for each type of dielectric. In [23–25, 34–38, 42, 43, 110], ESR because of the dielectric losses is calculated from the above-mentioned equation, whereas in [71, 74, 75, 85] no detail is given on how to calculate the ESR. In [107, 108], a resistance is added to the ESR to represent the losses because of resistivity of the conducting parts, which is calculated from the capacitor geometry for a wound metalised film capacitor. A similar approach for the same type of capacitor with different sets of equations is used in [54], which is based on the analysis presented in [111, 112]. A summary of capacitor loss calculation techniques is given in Table 8. Capacitors are usually selected from a list of off-the-shelf components; therefore it is customary to calculate capacitor losses from tan δ, which is relatively fixed for a certain dielectric [113]. However, tan δ only accounts for dielectric losses, whereas capacitor losses also include conduction losses in the metallic parts of the capacitor. Therefore, in some of the references, a more detailed approach is taken to calculate capacitor losses [54, 107, 108, 111, 112]. 4 4.1 Size modelling Inductors and transformers Inductors and transformers are usually custom designed, and therefore their size (weight or volume) can be calculated from their geometry, which is a part of design variable set. This approach is taken in [23–27, 33, 36, 38, 39, 42, 43, 54, 56, 57, 60, 114–121]. In another paper, volume of the inductor is calculated from the area product of the inductor core which can be considered indirect calculation from geometry [39]. In [25, 42, 43], the volume of an integrated LC module is calculated from the core and the winding geometry. Table 9 Summary of modelling size of magnetic components Reference(s) [23, 25–27, 33, 36, 39, 42, 43, 54, 56, 57, 60, 62, 63, 114– 121] [25, 42, 43] [122, 123] [46–48, 124] [29] Descriptions from geometry from geometry for an integrated LC module from stored energy from inductance, maximum current and maximum flux density directly from component catalog 1198 IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 This is an open access article published by the IET under the Creative Commons Attributiondoi: 10.1049/iet-pel.2013.0321 NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org If geometric parameters are not a part of the variable set, the size of the magnetic components are calculated from other approaches. In [122, 123], inductor volume is calculated from the stored energy, whereas in [123] it also depends on inductance and maximum current. In [46–48, 124], the inductor volume is calculated from an empirical equation based on inductance, maximum current and maximum flux density, and in [29] inductor volume is directly extracted from a catalog of off-the-shelf components. A summary of size modelling for magnetic components is given in Table 9. Calculating size from geometry is obviously the ultimate way; however, in problems where magnetic components are not detailed-designed, finding the volume from stored energy can also yield good accuracy assuming that the magnetic core is utilised efficiently, that is, magnetic core is utilised in all four quadrants of the B–H loop, and up to its maximum flux density. 4.2 Reference(s) [46] [23, 27, 33, 36, 62, 63, 125] Descriptions estimated from thermal resistance a CSPI introduced for each heat sink, optimisation looks for higher CSPI optimisation papers. In [46], heat sink volume is estimated from the required thermal resistance and is then compared with some manufacturer data; however, the manufacturer name and sample size is not known. In [47, 48], the size of the heat sink is somehow estimated but not explained, and in [23, 24, 27, 33, 36, 38], a concept called cooling system performance index (CSPI) which is presented in [125] is used. CSPI is defined as follows CSPI = Capacitors Unlike inductors, capacitors are not usually custom designed, and therefore their volume usually should be approximated, if not extracted directly from a catalog, which is the case in [29, 33, 46, 56, 57]. Other approaches include calculating capacitance volume from its stored energy [27, 122, 123], or from an empirical formula based on its capacitance obtained from a curve fit, or given by the manufacturer [23, 24, 36, 38, 48, 117–120, 124]. In some of the papers, however, capacitors are also custom designed, and therefore their volume is calculated from their geometry. In [115, 116], an integrated LC filter is designed, and therefore total volume is calculated from the geometry of the components including the capacitor. A planar integrated passive module is designed in [42, 43], in which total volume depends on geometry of the integrated components, that is, the capacitor and the inductor, and in [54], volume of a film capacitor is calculated directly from its geometry. Capacitor modelling techniques are summarised in Table 10. Size approximations for capacitors are difficult to compare since most of the approaches are mostly empirical and application-specific. This may suggest that more research should be conducted on size approximation for capacitors, which will be explained more in Section 6. 4.3 Table 11 Summary of size modelling for heat sinks Heat sink Calculating heat sink size from analytical equations for heat dissipation is very complicated, since heat sinks come in a wide variety of shapes, which makes it difficult to tie their thermal resistance to their weight or volume. Therefore some estimation techniques are applied in power electronic 1 Rth, s−a VCS (11) where Rth,s−a is the surface-to-ambient thermal resistance and VCS is the volume of the heat sink. According to [125], a higher CSPI means a higher-power density for the converter in which the heat sink is used. Therefore the approach in [23, 24, 27, 33, 36, 38] is using a heat sink with higher CSPI. Finding the volume of such a heat sink is then reduced to find the surface-to-ambient thermal resistance required by semiconductor switches. Size modelling methods of heat sinks are listed in Table 11. 5 Cost modelling Cost of the converter is an important objective function in many of the designs. In fact, high-production costs may be a barrier for using some technologies, which are desirable for certain applications. Owing to complexity of the pricing process, in which economics is involved, most of the power electronic optimisation papers either use component cost (i.e. price) from the catalogs or use a curve fit to find the cost from certain attributes of the components. In [29, 107, 108, 126], price of the commercial off-the-shelf components are used as their cost, whereas in [127–129] cost of resistor, inductor and capacitor is approximated from their resistance, inductance and capacitance, respectively. In [130–133], cost of the components is approximated from cost of actual off-the-shelf components; however, no detail is given on which attributes have been taken into account. In [65, 66, 82], cost of stranded wires is estimated from their diameter and number, and in [39] cost of a semiconductor chip is estimated from its surface area. A summary of cost modelling methods is presented in Table 12. Table 10 Summary of modelling size of capacitors Table 12 Summary of cost modelling Reference(s) Descriptions Reference(s) [29, 33, 46, 56, 57] [27, 122, 123] [23, 36, 48, 62, 63, 117– 120, 124] [115, 116] [42, 43] [54] size extracted directly from catalog from stored energy empirical equation based on capacitance from geometry for an integrated LC module from geometry for a planar integrated passive module from geometry for a film capacitor [29, 107, 108, 126] [127–129] [65, 66, 82] [39] Descriptions from price of off-the-shelf components cost of passive components is approximated from value cost of stranded wire estimated from strand diameter and number cost of a semiconductor chip is estimated from surface area IET Power Electron., 2014, Vol. 7, Iss. 5, pp. 1192–1203 1199 doi: 10.1049/iet-pel.2013.0321 This is an open access article published by the IET under the Creative Commons AttributionNonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/3.0/) www.ietdl.org Given the difficulties of estimating economic value of a certain component or converter, all of the cost modelling approaches are only low-order approximations. Cost modelling approaches in Table 12 are application-specific and cannot be compared with each other directly. 6 Recommendations for future work Even though many papers have been published on power electronic design optimisation, there is still plenty of work which can be done to make the design optimisation techniques more effective. A list of recommendations to improve power electronic design optimisation is as follows: † Modelling of core losses: as explained in Section 3.1, calculating hysteresis losses from physical attributes of the core with non-sinusoidal flux waveforms is still based on the Steinmetz equation approximation. Introducing a simple and effective way of calculating losses because of minor loops will enable a more accurate core loss calculation, which leads to a better optimisation. † Semiconductor modelling: because of the complex nature of semiconductors, little work has been done that presents a relationship between the device-level characteristics of semiconductor devices and their circuit behaviour. For instance, even though a tradeoff between IGBT speed and forward voltage drop is known for many years, lack of an easy way to estimate IGBT losses from its device-level attributes forces researchers to limit their work to existing semiconductor devices. Modelling semiconductor devices with a deeper look into semiconductor physics will help the researchers to explore the whole design space of semiconductor devices and not only the commercial off-the-shelf components. † Capacitor modelling: capacitor losses have been calculated from a very simple equation (9) for many years, which only accounts for the dielectric losses. Conduction losses because of the resistance of the conducting parts might be significant especially in aluminium electrolytic capacitors, where high losses results in the liquid electrolyte to dry out gradually and capacitor to fail. Estimating these losses from low-level characteristics of the capacitor such as dimensions or characteristics of electrolyte will enable a better estimation of losses and perhaps lifetime of the capacitor. Moreover, (9) is only valid when capacitor is carrying a sinusoidal current and superposition cannot be applied because of the dielectric losses being non-linear by nature. A solution to this problem will also be beneficial for power electronic design optimisation. † Modelling of thermal management devices: modelling of heat sinks is very important in calculating the total size of the converter, but little attention has been paid to cost and size modelling of heat sinks. This might be because of difficulties of modelling thermal conduction in heat sinks and incorporating such analysis into a power electronic design problem. However, because of significance of heat sinks in total weight and volume of power converters, a descriptive modelling technique can be really helpful. † Modelling power electronic system behaviour: most of the modelling techniques used in the optimisation use some kind of technique to represent time-domain voltage and current information by single or multiple algebraic values such as RMS values, harmonic content etc. This may result in oversimplification of some of the models. Using the time-domain information by itself is usually challenging because of its computational burden; however, with computing power becoming less expensive every day, a comprehensive converter modelling in time domain and calculating performance indices (loss, size etc.) from these models can be very beneficial. 7 Conclusion Significance of electrical energy in global energy usage and role of power electronics in processing electric power in low and high levels requires effective methods for design of power electronic converters. In this paper, a review of modelling techniques used in design optimisation of the power electronic converters was presented. Modelling techniques for power losses, weight and volume and cost were presented and analysed. Owing to the diversity of converters and components in power electronics, analysis presented in this paper would be beneficial for the researchers in this area. 8 References 1 U.S. Energy Information Administration. 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