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Digital Control for Improved Efficiency and Reduced Harmonic Distortion over Wide Load Range in Boost PFC Rectifiers Fu-Zen Chen and Dragan Maksimovic Colorado Power Electronics Center ECE Department, University of Colorado, Boulder, CO 80309-0425 {fchen, maksimov}@colorado.edu Abstract— This paper addresses control techniques aimed at improving light-load efficiency and reducing harmonic distortion in digitally controlled single-phase boost power factor correction (PFC) rectifiers. Based on a discontinuous conduction mode (DCM) detection circuit, it is shown how an extension to the predictive current control law leads to improved current regulation over wide load range in both continuous conduction mode (CCM) and in DCM. Furthermore, adaptive switching and adaptive switching frequency techniques are introduced to reduce switching losses and improve efficiency at light loads. Experimental results are shown for a 300W boost PFC rectifier.1 I. (DCM) over a portion of the ac line period. At light loads, the boost PFC operates in DCM over the entire ac line period. Various current control techniques addressing DCM operation have been addressed in [7-10]. It has been shown that DCM operation and CCM/DCM mode transitions can result in increased input current distortion in digitally controlled boost PFC rectifiers [6]. The increased distortion in DCM is due to the effects of ringing between the boost inductor and parasitic switch-node capacitance. In addition, DCM operation affects the dynamics of the current control loop, which may result in poor current regulation, requiring modifications to the CCM current control law [6, 7]. In this paper, the focus on DCM and DCM/CCM operation of boost PFC rectifiers over wide load range is further motivated by the increased interest in extending highefficiency operation to intermediate and light loads. The paper is organized as follows. Predictive current control techniques, including CCM and modified CCM/DCM approaches are discussed in Section II. Adaptive switching and adaptive switching frequency techniques aimed at improving light load efficiency are introduced in Section III. Experimental results on a 300W digitally controlled boost PFC rectifier shown in Fig. 1 are given in Section IV. Section V concludes the paper. INTRODUCTION Single-phase boost power factor corrector (PFC) rectifier (Fig. 1) rated at about 100-200W and above are usually designed to operate at constant frequency in continuous conduction mode (CCM) at full load. Issues in digital control of boost PFC rectifiers have been addressed in numerous publications (e.g. [1-6]), including digital average current mode control and predictive current control algorithms for low-harmonic current shaping in CCM. At intermediate loads, the boost PFC operates in discontinuous conduction mode iac L (0.5 mH) ig + vac + vL vg _ Q avL _ Ton sDCM iL,sense Current Sensing Corrector iC Vo R (220mF) ++ DTon Current Controller ei _ + ADC Vo_d u iref PREDICTIVE CURRENT CONTROL A. CCM Predictive Current Control Predictive current control techniques for CCM operation, such as the methods described in [4, 7], enable effective, highperformance current control and relatively simple digital implementation. In order to reduce the current analog-todigital (A/D) sampling rate requirements, inductor current can be sampled once per switching period. Assuming CCM operation, if the current sample is taken in the middle of the transistor conduction interval or in the middle of the diode conduction interval, it approximately represents the average inductor current over the entire switching period. Based on the geometry of the inductor current waveform, a predictive current control law can be constructed to minimize the error between the current reference iL,ref and the current sample iL,sense, _ Vo Feed Forward II. + C Driver g DPWM Ton,ff DCM comparator D (CSD04060) + iL ADC _ iD vds (STP25NM60N) _ + iQ vg ADC vg_d vg_d Voltage ev _ Vref_d + Controller Digital Controller CCM/DCM modification Fig.1 Experimental digitally controlled 300 W boost PFC. Modifications related to the proposed CCM/DCM predictive current control methods include an auxiliary inductor winding, a DCM comparator, and current sensing corrector function in the digital controller. Comparator signal (sDCM) feeds into the DCM current sensing corrector, and into DPWM for adaptive switching approaches. Ton [ n + 1] = vg L (i L,ref - i L ,sense ) + (1 - )Ts Vo Vo where Ton is the transistor on-time in the next switching period, Ts = 1/fs is the switching period, vg is the input voltage, and Vo is the output voltage. A block diagram of the controller This work has been sponsored through the Colorado Power Electronics Center (CoPEC). 1 978-1-422-2812-0/09/$25.00 ©2009 IEEE (1) 760 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. capable of implementing the predictive current control law is shown in Fig. 1. Note that the CCM predictive current control law (1) includes a proportional term DTon based on the current error, and a feed-forward term Ton,ff based on the input-tooutput voltage ratio vg/Vo. As shown in Fig. 1, the current reference iref in the boost PFC is obtained by multiplication of the sampled input voltage vg_d, and the power control signal u at the output of the output voltage-loop controller. iL,sense _ + ) Ts CCM vg 2L u (1 - ) Ts Ts Vo (7) DCM ei b a z -1 + ++ DTon ++ Ton sDCM DPWM III. iref.Ts z -1 CONTROL TECHNIQUES FOR IMPROVED EFFICIENCY AND REDUCED DISTORTION A. Adaptive Switching CCM/DCM Current Control When a constant-frequency boost PFC rectifier operates in DCM, ringing can be observed at the time when both transistor Q and diode D are turned-off, as illustrated by the experimental waveforms of the inductor current iL, transistor control signal g and transistor drain voltage vds shown in Fig. 4. The ringing is due to the resonance between the boost inductor and the total parasitic capacitance at the switching node. The oscillation period and amplitude are dependent on the inductance and capacitance values as well as the operating point. At the time when the transistor is turned on, the values of the drain voltage and the inductor current can vary significantly. The resulting average inductor current is a Tdcm Fig.2 Waveforms illustrating operation of the DCM comparator In order to simplify the calculation of the current sensing correction factor ( ), and to facilitate the adaptive switching technique described in Section III, an auxiliary inductor winding and a voltage comparator (“DCM comparator” in Fig. 1) are used to detect zero crossings of the inductor voltage (vL) in the CCM/DCM current controller proposed in this paper. The DCM comparator output signal (sDCM) is shown in Fig. 2 together with the inductor current waveform in DCM. This signal can be used to detect DCM/CCM boundary, and to measure the discontinuous conduction interval (Tdcm) using the system clock and a counter. Based on Tdcm, the correction factor can be calculated simply as (3) The complete CCM/DCM predictive current control law, including a proportional/integral (PI) feedback action and the DCM correction is give in (4)-(7), T [ n + 1] = DT [n] + T on on on, ff Fig.4 Experimental boost converter waveforms in DCM (4) 2 978-1-422-2812-0/09/$25.00 ©2009 IEEE g Ton,ff Predicitve Current Compensator Fig.3 Block diagram of the CCM/DCM predictive current controller (Ts-Tdcm) Comparator Signal sDCM Tdcm ) Ts vg Vo A block diagram of the CCM/DCM predictive current controller is shown in Fig. 3. <iL>Ts k = (1 - (6) Input emulated resistance (Re) is equal to (1/u). is the gain of the predictive controller; is related to the location of the zero in the PI compensator. Ts is the switching period. which has been shown to reduce the current distortion at light load and in CCM/DCM mode transitions [7]. Ton ei [ n] = Ts iref - (Ts - Tdcm ) iL , sense Ton, ff = The CCM predictive current control (1) is very effective when the boost converter operates in CCM [4, 7]. However, under intermediate or light load conditions, when the boost converter operates in DCM, the current sensed in the middle of the transistor or the diode conduction is no longer representing the average inductor current. In addition, the CCM current control dynamics are different from that in DCM. This results in more current distortion in DCM. A current correction approach has been proposed in [7] to extend the predictive current control law to DCM operation. A current correction factor ( ) is introduced to correct for the difference between the sensed current (iL,sense) and the average current (<iL>Ts), 2L Vo k= ( ) (2) ReTs Vo - v g Inductor Current iL (5) (1 - B. CCM/DCM Predictive Current Control iL,sense DTon[n] = a ei [n] + b ei [n - 1] + DTon [n - 1] 761 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. highly nonlinear function of the transistor duty ratio. In DCM operation of the boost PFC rectifier using standard current control algorithms optimized for CCM operation, this oscillation results in a significant current distortion. This problem has been identified in [6], and a solution was proposed based on damping the oscillation using a dissipative snubber. The input current distortion is significantly reduced, but at the expense of slightly increased losses and reduced efficiency in both CCM and DCM operation. iL,sense[n-1] Ts Tsw Tdcm Drain Voltage vds Tosc iL,sense[n] Inductor Current iL Tosc 2 vg Gate Signal g Comparator Signal sDCM Fig.6 Experimental boost converter waveforms in DCM using adaptive switching approach (fs = 80kHz) Fig.5 Waveforms illustrating adaptive switching control technique B. Adaptive Frequency CCM/DCM Current Control In order to further increase the light-load efficiency, one approach is to reduce the switching frequency in order to reduce switching losses. In the adaptive frequency CCM/DCM current controller proposed in this paper, only the DCM switching frequency is adjusted, while keeping approximately constant CCM switching frequency, fsmax = 1/Ts,min. The DCM switching frequency variation described here is based on keeping the transistor duty cycle constant in DCM. For a particular load (120 W) in the PFC rectifier of Fig. 1, Fig. 7 shows variations of the transistor duty ratio and the switching period over one half line period. When the converter operates at constant frequency in CCM, the switch duty ratio is approximately independent of load. In DCM, a lower duty An adaptive switching control technique is proposed here to address the above mentioned problem while simultaneously reducing the switching losses. The method is based on extending the transistor turn-off time until the drain voltage (vds) rings to a minimum, as illustrated by the waveforms of Fig. 5. Similar ideas of approaching zero-voltage switching based on DCM ringing have been previously applied to flyback DC-DC converters [14, 15]. The approach described here is based on the assumption that the DCM oscillation period Tosc does not change much between two consecutive switching periods. The digital controller uses the DCM comparator signal sDCM to measure and store the oscillation period Tosc. After expiration of the nominal (CCM) switching period Ts, the controller waits for a low-to-high transition of sDCM, and extends the off time by Tosc/2 so that the next DCM oscillation cycle ends at the point when the inductor current is zero and the drain voltage is at a minimum (as shown in Fig. 5). As a result of the adaptive switching period adjustment, switching losses are reduced, and the inductor current is very close to zero each time the transistor is turned on, thus reducing input current distortion in DCM. The adaptive switching control law is the same as the CCM/DCM control law (4)-(7), except that (6) is replaced by ei [ n] = Tsw iref - (Tsw - Tdcm ) i L , sense (8) Tsw is the actual switching period. where the current error is computed based on the actual adaptively adjusted switching period (Tsw) as opposed to the fixed CCM switching period Ts. Fig. 6 shows an example of experimental waveforms demonstrating operation of the adaptive switching CCM/DCM predictive current controller over several switching periods. Fig.7 Switching period and duty ratio over line period using adaptive frequency approach (fsmax =80kHz) 3 978-1-422-2812-0/09/$25.00 ©2009 IEEE 762 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. ratio is required as the load is reduced, assuming constant switching frequency. The proposed adaptive frequency controller keeps the same duty ratio during DCM operation by allowing the switching period to vary. The switching period variation is shown in the bottom part of Fig. 7. Note that the constant DCM duty ratio approach also enables a smooth transition between CCM and DCM operation of the controller. The controller is also taking advantage of the adaptive switching CCM/DCM control described in Section III.A, in which the transistor is turned on at the lowest drain voltage. Depending on the selection of the minimum switching period Ts,min and the lowest load, the adaptive frequency CCM/DCM controller may operate under 20kHz, which may cause audio noise. A limit to the maximum allowed switching period (Ts,max) is imposed to limit the minimum allowed switching frequency. The complete set of adaptive frequency CCM/DCM controller equations is as follows: Ton [n + 1] = DTon [ n] + Ton , ff (9) Ts [n + 1] = max[Ts ,min , Ts , DCM ] (10) Ts , DCM = min[Ts ,max , T 2 s ,min (1 - v g Re ) ] Vo 2 L (12) ei [n] = Tsw iref - (Tsw - Tdcm ) iL ,sense (13) vg Vo ) Ts ,min , 2 L Ts , max ] Re Ts , min period (Tdcm), gives a value slightly shorter compared to the actual Tdcm. As a result, the CCM/DCM and the adaptive switching CCM/DCM controller run as CCM controllers through the entire line period, resulting in the same distortion of the line current as the standard CCM predictive controller. Using the adaptive frequency controller, the transistor turn-off time around zero-crossing increases, which reduces the (11) D Ton [n] = a ei [ n] + b ei [n - 1] + DTon [n - 1] Ton , ff = min[(1 - Fig.8 Boost PFC experimental setup (14) Ts,min is constant switching period selected for CCM operation; Ts,max is the maximum allowed switching period. IV. EXPERIMENTAL RESULTS A 300W boost PFC rectifier (fs 80 kHz; L = 0.5 mH; C = 220 H) has been built as shown in Fig. 1, using a field programmable gate array (FPGA) development platform to implement the digital controller. Fig. 8 shows the experimental setup. The operation is tested under four different controllers, including the standard predictive CCM current controller (Section II.A), CCM/DCM predictive current controller (Section II.B), the proposed adaptive switching CCM/DCM current controller (Section III.A) and the adaptive frequency CCM/DCM current controller (Section III.B) over wide load range (15-300 W). At high power, in CCM operation, all of the controllers operate exactly the same as the standard predictive controller in CCM, resulting in lowharmonic current waveform, as shown in Fig. 9. At a reduced load, when DCM occurs only near the zerocrossings of the ac line, the CCM/DCM transition results in some distortion, as shown in Fig. 10(a). Since the inductor current around zero crossings is very low, the duty cycle is nearly 100%. The low inductor current cannot charge the switching node capacitance up to the output voltage. The DCM comparator used to detect the discontinuous conduction Fig.9 Rectifier line voltage (vg) and line current (iac), (80kHz, 300W), adaptive switching CCM/DCM current control distortion around zero crossings, as shown in Fig. 10 (b). At an intermediate load, when DCM occurs during a part of the line period, lower distortion can be observed in the waveforms obtained using the proposed CCM/DCM controller, as shown in Figs. 11(a) and 11(b). The DCM current correction improves current control and reduces distortion. At light loads, when the converter operates in DCM at all times, input current distortion using the adaptive switching CCM/DCM controller (Fig. 13(b)) is significantly reduced compared to the CCM/DCM controller (Fig. 13(a)). The DCM distortion is reduced by switching at the lowest drain voltage, 4 978-1-422-2812-0/09/$25.00 ©2009 IEEE 763 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. thus reducing nonlinear effects of inductor current ringing on the current regulation performance. Furthermore, efficiency is improved as shown in Fig. 12. The remaining distortion in the adaptive switching CCM/DCM controller is a result of the switching period discretized by the DCM oscillation period (Tosc). Hence, some current distortion happens around the jump between n to n+1 DCM oscillation cycles. Fig. 12 shows that the adaptive frequency CCM/DCM current controller improves efficiency further at light loads by reducing the switching losses. In the experimental circuit, the maximum (CCM) switching frequency is 80 kHz; while the minimum switching frequency is 20kHz. At very light loads, using the adaptive frequency CCM/DCM controller, the converter operates at the lowest allowed frequency most of the time, which is why the current harmonic distortion increases somewhat. Experimental results for 15W (5% of the rated load) are (a) (b) Fig.11 Rectifier line voltage (vg) and line current (iac), (80kHz, 75W) (a) CCM predictive current control (b) CCM/DCM predictive current control (a) Line Voltage = 110 Vrm s 96 Efficiency (%) 95 94 93 Adaptive Frequency CCM/DCM Current Control 92 Adaptive Sw itching CCM/DCM Current Control CCM/DCM Predictive Current Control 91 CCM Predictive Current Control 90 0 50 100 150 200 250 Power (W) Fig.12 Efficiency comparison (fsmax=80kHz) (b) Fig.10 Rectifier line voltage (vg) and line current (iac), (80kHz, 100W) (a) Adaptive switching CCM/DCM current control (b) Adaptive frequency CCM/DCM current control 5 978-1-422-2812-0/09/$25.00 ©2009 IEEE 764 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. 300 shown in Figs. 14(a)-14(d). Finally, Table I summarizes the performance of the four considered controllers. V. CONCLUSIONS This paper addresses efficiency improvements and reductions in input current harmonic distortion in digitally controlled single-phase boost power factor correction (PFC) rectifiers operating over wide range of loads. Proposed predictive current control methods enable low-harmonic operation in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM). A DCM detection circuit is used to facilitate a CCM/DCM predictive current control law, and adaptive switching at the minimum of the transistor drain voltage. These techniques result in improved efficiency and reduced current distortion. Furthermore, an approach to adaptive switching frequency adjustment is proposed, leading to further efficiency improvements at light loads. Experimental results are shown for a 300 W digitally controlled boost PFC rectifier. (a) VI. [1] [2] [3] [4] [5] [6] (b) Fig.13 Rectifier line voltage (vg) and line current (iac), (80kHz, 50W) (a) CCM/DCM predictive current control (b) Adaptive switching CCM/DCM current control REFERENCES W. Zhang, G. Feng, Y. Liu, and B. Wu, ”A Digital Power Factor Correction (PFC) Control Strategy Optimized for DSP,” in IEEE Trans. on Power Electron., vol. 19, no. 6, pp. 1474–1485, Nov. 2004. W. Stefanutti, P. Mattavelli, G. Spiazzi, and P. Tenti, “Digital Control of Single-Phase Power Factor Preregulators Based on Current and Voltage Sensing at Switching Terminals,” in IEEE Trans. on Power Electron., vol. 21, no. 5, pp. 1356–1363, Sep. 2006. B. Mather, B. Ramachandran, and D. Maksimovic, “A Digital PFC Controller without Input Voltage Sensing,” in Proc. IEEE APEC 2007, pp. 198-204. J. Chen, A. P. Prodic, R. W. Erickson, and D. Maksimovic, “Predictive Digital Current Programmed Control,” in IEEE Trans. on Power Electron., vol. 18, no. 1, pp. 411–419, Jan. 2003. D. M. Van de Sype, K. De Gusseme, A. P. M. Van den Bossche, and J. A. Melkebeek, “A Sampleing Algorithem for Digitally Controlled Boost PFC Converters,” in IEEE Trans. on Power Electron., vol. 19, no. 3, pp. 649–657, May 2004. K. De Gusseme, D. M. Van de Sype, A. P. M. Van den Bossche, and J. A. Melkebeek, “Input-Current Distortion of CCM Boost PFC Converters Operated in DCM,” in IEEE Trans. on Ind. Electron., vol. 54, no. 2, pp. 858–865, Apr. 2007. TABLE I Performance comparison of experimental CCM predictive current controller, CCM/DCM predictive current control, adaptive switching CCM/DCM current control, and adaptive frequency CCM/DCM current control. Output Power Efficiency Power Factor THD CCM Predictive Current Control 300W 94.9 % 0.999 2.2 % CCM/DCM Predictive Current Control 300W 94.9 % 0.999 2.2% Adaptive Switching CCM/DCM Current Control 300W 94.9 % 0.999 2.2 % Adaptive Frequency CCM/DCM Current Control 300W 94.9 % 0.999 2.2 % CCM Predictive Current Control 50W 94.7 % 0.985 17.1 % CCM/DCM Predictive Current Control 50W 94.5 % 0.993 10.9 % 5.8 % Adaptive Switching CCM/DCM Current Control 50W 95.1 % 0.993 Adaptive Frequency CCM/DCM Current Control 50W 95.1 % 0.995 3.8 % CCM Predictive Current Control 15W 91.1 % 0.923 33.7 % CCM/DCM Predictive Current Control 15W 90.9 % 0.946 20.1 % Adaptive Switching CCM/DCM Current Control 15W 92.5 % 0.943 14.0 % Adaptive Frequency CCM/DCM Current Control 15W 93.5 % 0.945 12.9 % 6 978-1-422-2812-0/09/$25.00 ©2009 IEEE 765 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply. (c) (a) (b) Fig.14 Rectifier line voltage (vg) and line current (iac), (80kHz, 15W) (a) CCM predictive current control (b) CCM/DCM predictive current control (c) Adaptive switching CCM/DCM current control (d) Adaptive frequency CCM/DCM current control [7] K. De Gusseme, D. M. Van de Sype, A. P. M. Van den Bossche, and J. A. Melkebeek, “Digitally Controlled Boost Power-Factor-Correction Converters Operating in Both Continuous and Discontinuous Conduction Mode,” in IEEE Trans. on Ind. Electron., vol. 52, no. 1, pp. 88–97, Feb. 2005. [8] P. Athalye, D. Maksimovic, and R. Erickson, “DSP implementation of a single-cycle predictive current controller in a boost PFC rectifier,” in Proc. IEEE APEC 2005, pp. 837-842. [9] Z. Z. Ye, and M. Jovanovic, “Implementation and Performance Evaluation of DSP-Based Control for Constant-Frequency Discontinuous-Conduction-Mode Boost PFC Front End,” in IEEE Trans. on Ind. Electron., vol. 52, no. 1, pp. 98–107, Feb. 2005. [10] M. Ferdowsi, and A. 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[Method for Operating a Switching Converter and Drive Circuit for Driving a Switch in a Switching Converter.] 7 978-1-422-2812-0/09/$25.00 ©2009 IEEE 766 Authorized licensed use limited to: UNIVERSITY OF COLORADO. Downloaded on September 1, 2009 at 16:26 from IEEE Xplore. Restrictions apply.