Problem with PWM voltage at MOSFET gate in three-phase inverter

ian field

Joined Oct 27, 2012
6,539
Hello.

I have designed and made my own three-phase inverter and I am currently testing it with a regulated DC supply, but recently one of the MOSFETs failed.


note: My MOSFETs have internal body diodes.

I replaced the MOSFET and checked the gate signals with a scope.
When I checked the signal on the bottom MOSFET gates, I see a constant PWM signal of 12V, independent of the DC supply, which is good.


When I look at the signals for the top MOSFET gates, the peak voltage PWM keeps rising when I rise the DC supply!!
I got almost to 20V peak with 10V DC supply.



20V is the maximum gate to source voltage so that's why the MOSFET died the first time.
So, my question is, what could be the reason the gate voltage keeps rising?

Thanks in advance.
Your symbols show IGBTs - they're much slower than MOSFETs, but with a 20mS horizontal scale; that shouldn't be causing any problems.

Unless you're using logic level devices; you need at least 8V to switch the device fully on and avoid VA dissipation.

Some devices can take as much as 30V on the gate, but 15 - 18V is more likely - its not fast switching, so I'd try Zener clamps and current limiting resistors.
 

Thread Starter

Arne Spiessens

Joined Nov 8, 2016
17
It was just to illustrate the inverter, I am using MOSFETs.

What exactly do zener diodes do? Do they limit the voltage at 12V?
What zener should I get?


Thanks!
 

Dyslexicbloke

Joined Sep 4, 2010
559
You need to post what you actually have, n channel / p channel

Fets are switched by VGS and assuming you have n's below and p's on top with common drains you should be pulling the top gates low and liftting the bottom gates.
Of course if you are using all n's which will likly have lower RDFS ON than comparable p's then the gates on the top will need to exceed your supply rail by VGS.

If you are using dedicated FET driver chips that bootstrap the gate to exceed the supply giving G at supply + the required gate voltage then what you are looking at is correct, assuming you are measuring it relative to ground.

How much time do you have between hi off and low on, or vice versa, If you are trying to switch then simultainiously you will get shoot through when both devices are conduction, essentally creating a dead short, all be it only briefly.

Al
 

ian field

Joined Oct 27, 2012
6,539
It was just to illustrate the inverter, I am using MOSFETs.

What exactly do zener diodes do? Do they limit the voltage at 12V?
What zener should I get?


Thanks!
Zeners are specified by clamp voltage and power rating - 12V should work OK.

You'll have to work out the power rating yourself - voltage & the current as limited by the resistors. 1.3W is probably a practical starting point.
 
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