some question about gate drivers ICs

[kappa_82]

hi,
it may seem naive, but I have some basic question about IGBT/MOSFET dirver ICs.
what they do exactly? when I have isolated sources can not we drive the switches using a optocoupler directly?

thank you

 

[chuckey]

Can your optocoupler handle the highly capacitive input of a power MOSFET at your required switching frequency?
Frank

 

[kappa_82]

I think optocoupler with phototriac output can handle capacitive currents.

 

[dick_freebird]

Optocouplers are weak and many are one-sided (a single NPN,
bring your own pullup). They can also be undesirably slow if
they are based on a saturating BJT output switch.

Avago makes opto FET gate drivers. You might find -some-
optothingamabob that does the job for your application.
It's just not something that generalizes well.

 

[thunderdantheman]

In terms of what MOSFET drivers do.... they act as a buffer between the SMPS controller IC outputs (usually low current source/sink) and the MOSFET gate which is a dirty big capacitor! and if you need to switch it fast (like in a SMPS or motor controller) the current required to charge the gate cap to VgsON starts becoming significant... i.e. I = C * dv / dt if we want to charge a 10nF gate to 10V in 100ns you need 1nF * 10V / 100ns = 1A. This is just a primer. generally you wouldn't calculate your gate drive like this as the gate capacitance is dynamic and the gate charge Qgx parameters are used.. you can look that up..

So its very uncommon to drive MOSFET directly with an optocoupler for reasons stated and posted above.. A common way to maintain isolation is to use gate isolation transformers.. these are available off the shelf in most cases.