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Guidelines Relating to Operation of Switching Elements Under Heavy Loading

2021.10.27

Points of this article

・Under heavy loading, when t_rr 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 t_rr is longer.

・In a PSFB circuit, it is important that MOSFETs with small t_rr 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.

When, under heavy loading, the recovery time t_rr of the body diode of a MOSFET is long and current remains, there is the possibility that a parasitic bipolar transistor may erroneously turn on while the MOSFET in a leading leg is turned off, causing failure of the MOSFET. This occurs because the parasitic bipolar transistor spontaneously turns on (erroneous turn-on) due to a charging current flowing to the drain-source capacitance C_DS that occurs at turn-off, so that a large current flows momentarily.

In inverters and other circuits, in a state in which a forward current is flowing through the body diode of a MOSFET, by applying a high-voltage reverse bias, the charge in the body diode (Q_rr) can be forcibly discharged rapidly. The time required for this discharge is t_rr, and so as a result, t_rr is shortened.

On the other hand, in a PSFB circuit, because the bias applied to a body diode during recovery is substantially 0 V, discharging of charge is slow, and consequently t_rr is longer. The diagram below shows in summary the V_DS, I_D, and recovery current of a leading-leg MOSFET.

When t_rr becomes longer, the current occurring due to recovery shifts as indicated by the red dashed line in the diagram. That is, charge remains in the MOSFET when turned off, current flows more readily, and erroneous turn-on of the parasitic bipolar transistor tends to occur.

t0 to t1：	Discharge from MOSFET output capacitor, forward current begins to flow through body diode
t1 to t3：	Interval during which body diode is conducting
t1 to t5：	MOSFET turns on, on state interval
t3 to t4：	Interval during which body diode recovery current flows; When t_rr is long, this interval is longer
t5 to t6：	MOSFET turns off. At this time, if t_rr is long, erroneous turn-on of the parasitic bipolar transistor tends to occur, and the MOSFET fails

For reasons such as this, MOSFETs with a short t_rr must be used in a PSFB circuit. Quite simply, the smaller the value of t_rr, the more effective. SJ MOSFETs are devices on the market with a short t_rr and which feature fast recovery, but parameters relating to t_rr differ depending on the manufacturer and the series, and so thorough study is required to select an appropriate device.

Even in lagging-leg MOSFETs, there is the possibility of erroneous turn-on of the parasitic bipolar transistor occurring at turn-off. However, as explained when discussing PSFB circuit operation, because the interval during which I_D is positive is long compared with leading legs, the effect of t_rr is not so great, and MOSFET failure due to erroneous turn-on of the parasitic bipolar transistor occurs less readily in lagging legs. A summary diagram of the recovery current similar to that for a leading leg is shown below.

t0 to t1：	Discharge from MOSFET output capacitor, forward current begins to flow through body diode
t0 to t2：	Interval during which body diode is conducting
t1 to t4：	MOSFET turns on, on state interval
t2 to t3：	Interval during which body diode recovery current flows; When t_rr is long, this interval is longer
t4 to t5：	MOSFET turns off. Compared with leading legs, there is little influence from recovery, so that erroneous turn-on of the parasitic bipolar transistor does not readily occur.

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