PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
1434
Applications of Resonant and Soft Switching Converters
V. Sladecek, P. Palacky, T. Pavelek, and P. Hudecek
Department of Electronics, Faculty of Electrical Engineering and Computer Science
VŠB-Technical University of Ostrava, 17, listopadu 15, Ostrava-Poruba 70833, Czech Republic
Abstract— This paper covers the circuit modification of the power part of the inverter with
auxiliary resonant poles utilising configuration of switches realised with routinely produced IGBT
modules. Covered is also the control optimisation which goal is the minimisation of switching of
the auxiliary resonant pole. Presented results were gained on a prototype of an inverter laboratory
sample.
1. INTRODUCTION
High frequency operations of PWM converters allow reduction of the size and weight of their
magnetic components. However, at high switching frequency, switching losses and EMI emissions
become significant and must be reduced. Traditional high frequency switch — mode supplies, which
rely on generating an AC waveform have used power transistors to “hard-switch” the unregulated
input voltage at this rate. This means that a transistor turning on will have the whole raw input
voltage, across it as it changes state. During the actual switching interval (less than 50 microseconds) there is a finite period as the transistor begins to conduct where the voltage begins to fall
at the same time as current begins to flow. This simultaneous presence of voltage across the transistor and current through it means that, during this period, power is being dissipated within the
device. A similar event occurs as the transistor turns off, with the full current flowing through it.
More recently, new power conversion topologies have been developed which dramatically reduce the
power dissipated by the main power transistors during the switching interval. The most common
technique employed has been a constant frequency resonant switching scheme, which ensures that
the actual energy being dissipated by the active device is reduced to nearly zero. This method,
commonly called is “Zero Voltage Switching” (ZVS), “Zero Current Switching” or “Soft
Switching”. These converters use the LC resonance circuit. When using resonant circuits to reduce switching losses, by resonant inductance connected or disconnected from the resonant circuit
at zero current passing through this inductance, or that connects or disconnects the resonance capacity at zero voltage. Leader in this area are quasiresonant converters, which use the properties of
resonant LC circuit, only in moments of commutation switches. Reduction in the steepness of the
starting and trailing edge of the output voltage when using resonant inverter also has a favorable
influence on the electromagnetic interference.
2. POWER PART OF AN INVERTER WITH RESONANT POLE
In Fig. 1, there is shown a block scheme of the power part of used three-phase converter with soft
switching. The converter with resonant poles consists of a conventional voltage source inverter
with zero (reverse) diodes and resonant poles. Each branch of the inverter bridge contains switches
VT11 and VT12 (VT21 , VT22 and VT31 , VT32 ). The switches activate circuits Lr1 , Cr1 , respective
Lr2 , Cr2 and Lr3 , Cr3 (see Fig. 1). These circuits are initialized in the instants, when the current
of load is too low for fast overcharging, or wrong polarity for overcharging of resonant capacitors.
If the current of load is high enough and right polarity it is possible to overcharge without resonant
circuit utilization. The main switches of the converter use zero voltage switching and the auxiliary
switches use zero current switching.
3. DESIGN OF METHODS FOR OPTIMIZATION OF RESONANT CIRCUIT CONTROL
WITH RESPECT TO LOAD CURRENT
For design of optimalization of resonant circuit control it is used the scheme for one phase of
resonant circuit (Fig. 2). It is clear that switching of IGBT transistors has to be realised in such
way to no IGBTs use hard switching. It is possible to perform optimization of switching algorithm
with respect to the value of load current in the following way — to minimal number of resonant
circuit activation occurs and to short-term currents of resonant circuit have a necessary value only.
The optimization can be divided into two basic problems.
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1435
XR 1
VT11
XR 2
C R1A
VT 1
C R1B
VT 2
VD 1
XR 3
VT 3
VD 3
VT 5
VD 5
L R1
Ud
VD 2
VT 12
VD 4
VT 4
VD 6
VT 3
V
U
M
W
Figure 1: Inverter with resonant poles — power part.
VT11
C R1A
VT 1
VD 1
uV1
L R1
Ud
VT 12
IL
VT 2
iLR
uV2
VD 2
C R1B
Figure 2: One phase of quasi-resonant inverter.
• Optimization of switching algorithm of the switches in the bridge with respect to the value of
IL .
• Optimization of current impulse size in the resonant circuit with respect to the value of IL .
If the load current achieves sufficient value and right direction it is possible to overcharge the
resonant capacitors by the current without resonant circuit activation. In the second case the
situation is following, load current has right direction, but it is not high enough for fast overcharging
the resonant capacitors. In this case the load current overcharges the resonant capacitors with the
help of the resonant circuit without its activation. For right understanding it is presented detailed
explanation. Each situation follows from waveforms of the load current. The first assumption — we
take into account an ideal load current, we substitute it by a sinusoidal waveform. The waveform
is divided into a few sectors according to size and direction of load current (see Fig. 3). Current
directions, positive and negative, correspond to labelling in Fig. 2. The load current is also divided
into sectors I, II, III with respect to its absolute value. Switching algorithm for control of one phase
of quasi-resonant converter depends on the direction and sector, where load current IL lies. From
the fact follows that it exists five switching sequences of each IGBT in the quasi-resonant bridge.
4. BLOCK SCHEME OF THE CONTROL PART FOR ONE PHASE OF THE
CONVERTER USING SOFT-SWITCHING
In Fig. 4, it is shown a block scheme of the control part for one phase of the converter using softswitching. The input is a PWM signal connected to the analog part, where it is compared the load
current and resonant current, it is evaluated position of load current with respect to the position
in the given sector (see Fig. 4) and it is chosen a switching sequence for logical part of the control
circuits.
PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
1436
5. EXPERIMENTAL RESULTS
In Figs. 5 to 9 there are depicted the time responses of output voltage and current together with
the current of resonant coil during small and large values of the output current. On the pictures
are also included the details of switching in various regimes.
IL
Sector III
Sector II
Sector I
SENSOR UVT1=0
0
Sector I
SENSOR UVT2=0
t
PWM
SENSOR ILr
Sector II
SENSOR I L
ANALOG
PART
Sector III
Figure 3: Waveform of load current and its division into particular sectors.
DIGITAL
PART
POWER
PART
Figure 4: Block scheme of the control part for one phase
of the converter using soft-switching.
Figure 5: Inverter with resonant circuit.
Figure 6: Voltage UVT2 (blue) and resonant current
iLR (red) for small IL .
Figure 7: Voltage UVT2 (blue) and resonant current
iLR (red) for one switching cycle VT2 and small IL .
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1437
Figure 8: Output voltage, current and resonant current for f = 50 Hz and small output current.
Figure 9: Output voltage, current and resonant current for f = 50 Hz and large output current.
ACKNOWLEDGMENT
The paper was created with the support of the projects MSM: 618910007, ENET: CZ.1.05/2.1.00/03.0069.
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