R Classroom | Precautions for Switching Components Working under Heavy Load
Key Takeaways
・Under heavy load, if t
rr
is long, the parasitic bipolar transistor may be falsely turned on when the super-leading arm is turned off, thus damaging the MOSFET.
・In the PSFB circuit, the bias voltage of the body diode is almost 0V during the reverse recovery period, so the charge release speed becomes slow, which eventually leads to a longer t rr .
・It is important to use a MOSFET with a small t rr in a PSFB circuit .
・Even with fast recovery type SJ MOSFETs, performance varies depending on the manufacturer and product lineup, so it is necessary to fully confirm when selecting.
Under heavy load, if the reverse recovery time trr of the MOSFET body diode is long and there is residual current, the parasitic bipolar transistor may be mis-turned on when the leading MOSFET is turned off, thus damaging the MOSFET. This problem occurs when the parasitic bipolar transistor is spontaneously turned on (mis-turned on) by the charging current to the drain-source capacitance CDS generated at the time of shutdown, and a large current flows instantaneously.
In an inverter circuit or other circuit, when a forward current flows through the body diode of a MOSFET, a high reverse bias voltage is applied to force the charge (Q rr ) in the body diode to be quickly discharged. The time required for this discharge is t rr , so t rr is eventually shortened.
In the PSFB circuit, the bias voltage applied to the body diode during reverse recovery is almost 0V, which slows down the charge release and eventually leads to a longer trr . The figure below is a schematic diagram of VDS, ID and reverse recovery current of the super-leader MOSFET.
When t rr becomes longer, the current generated by reverse recovery changes, as shown by the red dashed line in the figure. That is, when it is turned off, there is charge remaining in the MOSFET, which makes it easier for current to flow and makes it easier for the parasitic bipolar transistor to be mis-turned on.
(Click for larger image)
|
t0～t1： |
The output capacitance of the MOSFET is discharged and forward current starts to flow through the body diode. |
|
t1～t3： |
The period of time during which the body diode is conducting. |
|
t1～t5： |
The MOSFET is on and is in the on state. |
|
t3～t4： |
The period of time during which the body diode flows reverse recovery current. The longer trr is, the longer this period is. |
|
t5～t6: |
MOSFET is turned off. At this time, if t rr is too long, the parasitic bipolar transistor is more likely to be mis-turned on, causing damage to the MOSFET. |
Therefore, in the PSFB circuit, a MOSFET with a small t rr is required. In short, the smaller the t rr , the more effective it is. There are some fast-recovery SJ MOSFETs with low t rr on the market, but t rr and its related parameters vary from manufacturer to manufacturer and product series to series , so full confirmation is required when selecting.
Even the MOSFET of the lagging arm may cause the mis-turn-on of the parasitic bipolar transistor when it is turned off. However, as explained by the operation of the PSFB circuit, the lagging arm is not easily affected by t rr because the period of time during which ID is positive is longer than that of the leading arm, and is not easily damaged by the mis-turn-on of the parasitic bipolar transistor. As with the leading arm, a schematic diagram of the reverse recovery current is also given here.
(Click for larger image)
|
t0～t1： |
The output capacitance of the MOSFET is discharged and forward current starts to flow through the body diode. |
|
t0～t2：
|
The period of time during which the body diode is conducting.
|
|
t1～t4： |
The MOSFET is on and is in the on state. |
|
t2～t3： |
The period of time during which the body diode flows reverse recovery current. The longer trr is, the longer this period is. |
|
t4～t5： |
MOSFET is turned off. Compared with the super-leader, it is less susceptible to reverse recovery, so the parasitic bipolar transistor is less likely to be falsely turned on. |
END
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