Current Required to charge Mosfets On/Off calculation

[surfline]

Hi,

I know the general formula to charge / discharge Mosfet's is: Qg (Total gate charge) / t (desired switching time). My question is what should my desired switching time be?

If for example, the rise / fall times of my mosfet are 25 nS , but I have a 20 kHZ PWM input to the gate of the mosfet's, should my desired switching time be 25 nS or 1/20,000 ? Or somewhere in between? Exactly how in between and why? How fast would you want to switch in this situation.

Thanks.

 

[MikeMl]

Do some simulation.

You question is how fast is fast enough. As the FET drain switches from high to low or the other way, it passes through a region where there is drain current, and the drain voltage is near the middle of its range. That is where the device dissipates serious power. The goal is to get it to move through that range as fast as possible, and as infrequently as practical.

Look at the very simple attached sim. The light blue trace in the bottom pane is the Fet's enemy; dissipation. Note how the gate capacitance and Miller capacitance slow V(g) due to the 100Ω source resistance.

LTSpice or other similar simulator will let you explore the envelope.

 

[surfline]

I'm just still a little confused whether the time part is referring to the given transistor switching parameters (rise and fall times) or your actual PWM switching frequency?

 

[Hero999]

It's both.

The slower the rise and fall times the higher the power dissipation and the higher the frequency the higher the power dissipation. At low frequencies, the rise/fall time is less important because it's switching less often but at high frequencies the rise/fall time is very important.

 

[MrAl]

Hi there,


In addition to the power dissipation which is usually an important issue, there is also the 'catch' diode reverse recovery period issue to think about. That's the time when the catch diode has to recover from being forward biased and the power to recover comes from the MOSFET being switched 'on'.

One of the ways to handle this is to limit the turn on time of the MOSFET so that it turns on gradually rather than abruptly, but not so slowly that it causes too much power dissipation. The technique usually involves a small resistor in series with the gate such as 10 or 20 ohms.
Another way is to use a 'zero' recovery time diode.

BTW, you can estimate the power loss in the MOSFET during the switching period due to the switching itself with this formula:

P=Imax*Vmax*Tp*F/3

where
Imax is the max current, and
Vmax is the max voltage, and
Tp is the switching period, and
F is the switching frequency

For an example, say we have a MOSFET switching at 10kHz and each switch period is 10us long, and the max current is 50 amps and the max voltage is 100 volts, that would mean 167 watts would be dissipated in the transistor just because of the on/off switching itself. That would probably be too high so the design would have to be improved.
Note that in that formula the MOSFET switches twice per cycle, once 'on' and once 'off', both of which dissipate power.