resistor and diode for mosfet gate 2

[yamster]

Hi All,

Im studying a circuit diagram one of my more senior workmates made when he designed a switched mode power supply.

To drive a H-bridge configuration of 4x n-channel power MOSFETS, he used a HIP4080 full bridge driver.

The thing was, he placed a resistor (13R) between the MOSFET gates and the driving pins (ALO, AHO, etc.) of the bridge driver. Parallel to the resistor is a 'soft-recovery' plastic diode.

I was wondering what was the point of putting the resistor and the diode between the driver and the mosfet? Im guessing its some sort of spike protection? how would this work?

 

[sreid]

The gate drive circuit (resistor/diode) is often used to control the turn on / turn off times of the FETs by how fast the gate capacitance is charged. Slower rise/fall times create less EMI.

 

[ScottI2R]

Hi all!
A lesson I just learned recently: Slower switching also causes HEAT!!! And the HEAT was destroying FETS in my situation. Remember my post "IRF 460 Failures" sreid? Thanks for helping with that BTW.

Just trying to pass along what I've learned.

Scott

In a hundred years, it isn't going to matter anyway.

 

[IRstuff]

I can imagine that the resistor/diode configuration could be used to tailor the turn-on and turn-off independently. The resistor could increase the rise time on the gate, thereby slowing down the turn-on. The diode could be used to bypass the resistor during turn-off.

If both pullup and and pulldown are configured similarly, this would avoid current spikes caused by having pullup and pulldown on at the same time.

TTFN

 

[sreid]

What I've primarilly used this gate drive for is in flyback power supplies. At FET turn on there is no current so the resistor allows causes slow FET turn on and reduces noise from fast edges. The diode turn the FET off fast (because the FET is pullig current) but the rise time is controlled by a RDC snubber circuit.

The diode resistor circuit was (is) used with bipolar transistors. The resistor set the base drive current so the transistor was not drven into too hard a saturation. The diode provided a high current path to quickly sweep the base charge to minimize heating from the transistor slowly comming out of saturation.

 

[yamster]

That makes more sense now! Thanks guys!