using capacitor across mosfet VGS ?!!

[Electro nS]

hello

i have seen a circuit for H bridge that uses a resistor 20ohm on power mosfet gate and ir2110 driver and a wierd (first time i see this) capacitor across Vgs (usaully zeners are used)

can any one explain the role of this component for me ( its value is around 1 to 10nF judging from size) because i cannot disassemble from PCB (very small Smd components)

thanks in advance , any idea ???

 

[FvM]

Very unusual. Looks like someone tried to artifcially decrease the MOSFET switchings speed. Normally, we would do this by increasing the gate resistance.

 

[mtwieg]

Are you actually sure it's a capacitor? Sometimes cheap diodes or resistors can look just like capacitor packages.

If it's a capacitor it might be there to improve the miller ratio to prevent dv/dt induced instability, but it's poor practice to do such a thing.

 

[Electro nS]

Are you actually sure it's a capacitor? Sometimes cheap diodes or resistors can look just like capacitor packages.

i will try to remove the components and take a picture of it and measure by multimeter the resistance and capacitance and will post to confirm

 

[KonstanU]

I assume that someone does so to prevent overvoltage on the gate-source terminals during switching process.
Drain-gate capacitance creates positive feedback. As a consequence switching process leads to increasing of gate voltage.
Could you tell what your negative voltage on the gate terminal is (off state)?

 

[Electro nS]

I assume that someone does so to prevent overvoltage on the gate-source terminals during switching process.
Drain-gate capacitance creates positive feedback. As a consequence switching process leads to increasing of gate voltage.
Could you tell what your negative voltage on the gate terminal is (off state)?

i think it is 0v since the circuit is using 2 IR2110 in full bridge configuration at 36v

and do you think it is a good thing to use this capacitor to prevent overvoltage at gate or zener and tvs is a better option ???

 

[mtwieg]

i think it is 0v since the circuit is using 2 IR2110 in full bridge configuration at 36v

and do you think it is a good thing to use this capacitor to prevent overvoltage at gate or zener and tvs is a better option ???

No, it's usually a poor solution (and can easily do more harm than good). Proper component selection and circuit layout is the best solution. If you still need protection against overvoltage, then a TVS diode is the better option.

 

[Electro nS]

No, it's usually a poor solution (and can easily do more harm than good). Proper component selection and circuit layout is the best solution. If you still need protection against overvoltage, then a TVS diode is the better option.

thanks , So TVS is the way to go , better than zener i assume ( faster clamping speed but for shorter time ?!)

 

[FvM]

I don't see a general need for voltage limiting means like TVS diodes in IR2110 or similar gate driver circuits. If you fear overvoltages in the driver supply, the voltage clamp should be placed there.

 

[KonstanU]

I have looked on the famous book and have found very interesting thing.
It says that input gate-emitter capacitance can help to reduce switch-on losses.

 

[FvM]

Interesting consideration, but not actually the switch-on losses are reduced by a capacitor. More exactly the additional losses created by the reverse recovery of the diode from which the current is commutated are reduced by lowering dI/dt while dU/dt is increased at the same time by using a smaller gate resistor.

I didn't yet find gate-source (respectively IGBT gate-emitter) capacitors in real designs and I'm not sure if the advantage shows under typical operation conditions. The technique would be only interesting if the current is commutated from a diode.

E.g. in H-bridges, you'll avoid diode conduction by strict synchronous switching and adjusting the bridge dead time respectively. It can be expected that a capacitor isn't helpful in this situation. Another problem is that the capacitor also affects switch-off behaviour, which isn't discussed in the book. I fear that the idea is a bit "too simple".

P.S.: Which book are you quoting?

 

[KonstanU]

E.g. in H-bridges, you'll avoid diode conduction by strict synchronous switching and adjusting the bridge dead time respectively. It can be expected that a capacitor isn't helpful in this situation. Another problem is that the capacitor also affects switch-off behaviour, which isn't discussed in the book. I fear that the idea is a bit "too simple".

I wonder but during the dead time, how a current flows. I assume, that diodes will be in forward state and conduct current.
I am sure that Cge will effect on switch-off losses (increase). I didn´t see also implementaion of this technics.

P.S.: Which book are you quoting?

IGBT Modules: Technologies, Driver and Applicaton. Infineon

 

[FvM]

I wonder but during the dead time, how a current flows. I assume, that diodes will be in forward state and conduct current.

I forgot to mention an important point.

In IGBT bridges, the current will always commutate from diode to IGBT in "passive" quadrants II and IV (energy flow from load to bridge) because IGBTs can't conduct reverse currents. In so far, my synchronous switching comment is only valid for MOSFETs. Here it's possible to adjust the deadtime to a low value so that the current is directly switched from high-side to low-side transistor and vice versa, without building up considerable diode current in passive quadrants.

 

[FvM]

As an update, I found that Cge is generally used with high voltage IGBT (e.g. 3300V Vce,max), but it's not considered advantageous for lower voltage classes.

 

[kalaianand]

IR2110 works on the principle of Boost strap mechanism. Continuous power available for Low side mosfets drive. Where as for HIGH side drive, this capacitor will be act as source for the small duration, based on PWM ON time duration. But for that too, capacitor will not be placed in the VGS, on the other Pin (VHH). This is used only in the continuous operation with H-bridge configuration.