Want to slow down the FET turn-on speed without slowing down the turn-off speed? A design expert has come up with a clever way...
As a circuit designer, you may encounter a situation where you need to use a series gate resistor (as
described in
Why put a resistor in series with the gate of a FET
) to slow down the speed at which a FET turns on, but you don't want to slow down the speed at which it turns off. How would you do this?
Take the following circuit diagram as an example, where a 5V gate drive (U1) is used to turn on/off a low-side N-channel FET that is used to switch a mystery load.
R1 is used to slow down the turn-on speed of the FET. The gate capacitance of the FET is filled with current (its peak value is limited to 500mA (5V/10Ω) by the 10Ω resistor), and the gate voltage will rise with the time constant set by R1 and the gate capacitance of Q1 (approximate value). The time required to rise to the FET threshold voltage is the turn-on time, but what about the turn-off time?
Without any other intervention, the turn-off time is roughly the same as the turn-on time, since the gate capacitance of the FET must also discharge through R1. However, adding diode D1 with indicated polarity can shorten the turn-off time, but not the intentionally extended turn-on time.
Note that the gate voltage now includes an additional diode drop in the off state, but as long as that voltage is well below the FET threshold voltage for full turn-off, there is nothing to worry about. Also note that if the opposite effect is desired (i.e. fast turn-on, slow turn-off), simply reverse the polarity of the diode on the same concept (similarly, the diode drop for the turn-on voltage must still be well above the FET threshold voltage for full turn-on).
One application where this approach might be needed is a bridge drive circuit with high-side and low-side FETs: one FET must be completely turned off before turning on the other FET to avoid shoot-through (power is temporarily shorted to GND through both FETs when both are briefly turned on at the same time).
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