Electronic – If MOSFET is a voltage-controlled device, then why do we need to supply it with high current when using in an H-bridge


I might be confusing myself, but from what I am reading so far, a BJT is a current-controlled device and a MOSFET is a voltage-controlled device, which to me implies that a MOSFET requires very low input current.

If that is correct, than why, in say an H-bridge, do we need to use a high side driver to supply 2A? Is it because of the Miller capacitance that we need to charge the gate capacitance for a split second? Please explain.

Best Answer

Exactly what you suspect. The effect of the gate capacitance is to slow down the switching. (The Miller effect multiplies the 'effective' gate capacitance.)

If we want the H-bridge to switch only occasionally (let's say at 1 Hz) a low gate current is (in most cases) fine, because the thermal effects of the switching are spread over 1 second, which is a relatively long period.

But if PWM is used, for instance at 300 Hz, there is only ~ 3ms to spread the heat dissipated in a switch (on+off), so this heat must be minimized.