purpose of a resistor between gate and source of a switch in inverters

[thelukuntla]

hi
i have some basic doubt why a resistor is needed between gate and source of a switch in inverters.

Thanks in advance

 

[FvM]

The problem has been previously discussed, e.g. here https://www.edaboard.com/threads/220850/

 

[Tahmid]

This link has been given in the above link, however, I'd like to post it here again since it's also been discussed here.

https://www.edaboard.com/threads/197011/

Hope this helps.
Tahmid.

 

[thelukuntla]

many thanks.
Another clarification needed.
Are there any advantages in using gate drive transformers in the making of gate drive circuits for inverters.

 

[Tahmid]

It depends on the circuit using gate driver transformer and also the one using solid-state drivers. For one, you easily get isolation. For isolating a solid-state design, you'd need to have an isolated power supply.

Hope this helps.
Tahmid.

 

[thelukuntla]

thank u.
Don't be afraid becoz i am putting lot of doubts.
For making inverters either single phase or three phase by using Power MOSFETS, whether one can use MOSFET body diode as feedback diode or additional anti-parallel fast switching diodes are required, if required under what conditions like based on power rating etc.
Is it holds same for IGBT, if we purchase IGBT with internal diode.

 

[alexan_e]

I was reading **broken link removed** in page 17 and it says



so in addition to the floating gate protection this indicates an additional reason to use the gate-source resistor

Alex

 

[Tahmid]

What do you mean by feedback diode? Diode used to rectify AC to DC for feedback? For that, you should rectify the output voltage with external diodes.

 

[FvM]

so in addition to the floating gate protection this indicates an additional reason to use the gate-source resistor

I adressed power-up behaviour as a possible purpose of gate pull-down resistors in the previous discussion:

https://www.edaboard.com/threads/220850/#post940513

As far as I see, nobody has checked yet, if popular gate driver ICs like IR2110 actually need an additional pull-down resistor.

 

[Tahmid]

I've checked. The MOSFETs blow up without the additional pull-down resistors. Cost me more than 20 IR2110's and a lot more MOSFETs before I figured that out.

 

[FvM]

I've checked. The MOSFETs blow up without the additional pull-down resistors. Cost me more than 20 IR2110's and a lot more MOSFETs before I figured that out.

What's the gate threshold voltage of the MOSFETs?

 

[Tahmid]

I've used these mostly:

IRF840: 2 to 4V
IRFP450: 2 to 4V
STH15NA50: 2.25 to 3.75V

 

[FvM]

Thanks for the information. In other words, these are standard MOSFETs. By typical data, they don't sink much drain current with Vgs below e.g. 3 V. That's a point to consider when looking at gate driver IC's power up behaviour.

 

[thelukuntla]

What do you mean by feedback diode? Diode used to rectify AC to DC for feedback? For that, you should rectify the output voltage with external diodes.

In my sense feedback diodes mean it is the body diode that is antiparallel to Power MOSFET and freewheels or provides path for current when switch is OFF for inductive type of loads.My doubt is, under what conditions (like power rating or switching times) additional/external diode is required for Power MOSFET in making inverter.
Hope you got my doubt.

 

[Tahmid]

You should add anti-parallel diodes when you are operating at very high frequencies, where the reverse recovery time of the body diode would be too high. You could also decide to use anti-parallel diodes to reduce power dissipation within the MOSFET as the forward voltage of the body diode is rather high.

Hope this helps.
Tahmid.

 

[thelukuntla]

thanks for the info.
how to select the anti-parallel diode, mean parameters like reverse breakdown voltage, trr , qrr .

 

[Tahmid]

I would say that it's not very critical, just choose a diode with a fast reverse recovery time and low forward voltage, with a high enough voltage and current rating.

 

[FvM]

Only schottky diodes would help against most of the reverse current flowing in the internal diode.

I doubt, that external diodes will be used with MOSFET output stages, except for special cases. The regular method is to implement synchronous switching. In this case, the MOSFET carries the reverse current.

 

[George97]

I've checked. The MOSFETs blow up without the additional pull-down resistors. Cost me more than 20 IR2110's and a lot more MOSFETs before I figured that out.

Yes,Tahmid, I also faced same kind of problem. A pull down resistor between gate and source of a mosfet is required in most cases whatever is the reason. I find it very essential.

 

[vimalkhanna]

This is right as the charge stored in the Cap Cgs is forcing the mosfet to stay ON when the top Mosfet is switched ON and there is a delay in switch OFF of the Mosfet to ground .Both Mosfets and driver get shorted immediately .The resistance between gate and source discharges this capacitance .