Technical Information Site of Power Supply Design

2022.09.07 Si Power Device

Improving the Power Conversion Efficiency of Phase Shift Full Bridge Circuit - Summary -

Improving the Power Conversion Efficiency of Phase Shift Full Bridge Circuits

Here are summarized important points relating to operation and current paths in order to improve the efficiency of a PSFB circuit using SJ MOSFETs.

Under light loading

  • ・Because establishment of ZVS operation is difficult, losses tend to occur at turn-on.
  • ・In order to reduce these turn-on losses, the dead time must be adjusted to prevent shoot-through currents.
  • ・By choosing MOSFETs with a good parasitic capacitance balance, shoot-through currents can be prevented.
  • ・If gate threshold voltages are high, erroneous turn-on can be prevented.

Under heavy loading

  • ・If the trr values of the body diodes of the MOSFETs are large, erroneous turn-on of the parasitic bipolar transistors in leading-leg MOSFETs become a concern.
  • ・Hence MOSFETs with a small body diode trr must be used.

From the above, demands to be made of SJ MOSFETs used in a PSFB circuit are a fast recovery characteristic (trr), optimized parasitic capacitance balance of the MOSFETs, and a high gate threshold voltage.

In comparisons of efficiency using different fast recovery-type SJ MOSFETs, the R6020JNX (in the R60xxJNx series), a latest-generation PrestoMOS™ device, yielded the highest efficiency over all load ranges. This can be attributed to the fact that the recovery characteristic of PrestoMOS™ MOSFETs is among the best of fast-recovery SJ MOSFETs in the industry, and also to the fact that shoot-through currents do not flow readily owing to the high gate threshold voltage, while turn-off losses are reduced.

Below, links to each section in this chapter as well as key points of each are provided. Please refer to them for more detailed explanations.

Basic Configuration of a PSFB circuit

Key Points:

・Phase shift full bridge (PSFB) circuits make possible dramatic reductions in switching losses through zero-voltage switching (ZVS) operation of switching elements, and so can handle greater amounts of power.

・A PSFB switching circuit essentially consists of four switching elements (MOSFETs); the leakage inductance of a transformer is used as a resonance inductance necessary for ZVS operation.

・In order to extend the range of ZVS operation, there are cases in which an inductor is added in series. Here, a circuit with an added series inductor is assumed.

・In the basic switching operation, after the on/off states of Q1 and Q2 have been swapped, the on/off states of Q3 and Q4 are swapped after a certain phase lag.

・In general, the leg of Q1 and Q2 is called the "leading leg", and the leg of Q3 and Q4 is the "lagging leg".

Basic Operation of PSFB Circuits

Key Points:

・The operation states and current paths, represented as 14 different modes, should be understood.

・Differences in current waveforms occur due to differences in operation. MOSFET losses are different in the leading leg and the lagging leg, and different heat generation results, so that caution must be exercised in thermal design.

・From the equation for the condition for ZVS operation, it is seen that under light loading IL1 is small and so ZVS operation is not readily achieved, but as the load increases, ZVS operation is more easily realized.

Guidelines Relating to Operation of Switching Elements Under Light Loading

Key Points:

・Under light loading, currents are small, and the accumulated energy in LS is small, so that it is entirely possible that switching operation may begin without complete COSS charging/discharging, as a result of which ZVS operation is not possible, and MOSFET turn-on losses readily occur.

・Due to incomplete COSS charging/discharging, VDS may remain, and so in order to prevent shoot-through currents caused by upper/lower arm short-circuits, it is important that the dead time be set appropriately.

・Because a shoot-through current may flow depending on the capacitance ratio of CGD and CGS of a MOSFET, it is important that MOSFETs with an appropriate capacitance ratio be selected.

Guidelines Relating to Operation of Switching Elements Under Heavy Loading

Key Points:

・Under heavy loading, when trr is long, there is the possibility of erroneous turn-on of a parasitic bipolar transistor upon turn-off of a leading leg, resulting in MOSFET failure.

・In a PSFB circuit, the bias on a body diode during recovery is substantially 0 V, so that discharging of charge is slow, and consequently trr is longer.

・In a PSFB circuit, it is important that MOSFETs with small trr values be used.

・Even fast-recovery type SJ MOSFETs have different performance parameters depending on the manufacturer and the series, so careful examination is necessary when selecting a device.

Evaluation of Efficiency

Key Points:

・In efficiency comparisons, the best results were obtained for PrestoMOS™ devices.

・In order to enhance the efficiency of a PSFB circuit employing SJ MOSFETs, it is important to choose SJ MOSFETs having as small a trr value as possible as well as an excellent switching characteristic.

References [1] L.Saro, et al., “High-Voltage MOSFET Behavior I  Soft-Switching Converter: Analysis and Reliability Improvements, ”International Tel-communication Conference, San Francisco, 1998.

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