Mosfets in H bridge blowing off when load is connected

Thread Starter

Seema01

Joined Jan 11, 2020
9
Hello,

Im trying to make an H bridge with mosfets....works fine on no load. But moment i connect it to an out put pulse transformer it blows off?? is it due to wrong way of soldering??
 

danadak

Joined Mar 10, 2018
3,889
No clock dead time present in drive circuit, no inductive clamping,
lots of possibilities for MOSFET destruction.

1579874060209.png

Post schematic.


Regards, Dana.
 

Janis59

Joined Aug 21, 2017
1,057
99% probability the cause is those timing gap inapropriatness. The rest a 1% is for the less disputed problem namely wire inductance between Power Source capacitor and Bridge Vcc input. Each centimeter have 10 nH of parasythics, but for amperage of 100A have deadly even 10 nH, 10 A may mean the allowance for 10 cm of length, but 1 Amp allows just the meter (Im contrasting just to show the principle). Therefore most of middle level power systems use a sandwich-line wiring in very close distance with plane sheet wires. But very high power units must use a two larger sheets where one cap leg going through and another not. Then to this cap battery in 90 deg angle in few mm distance grows out the transistor brick, those 600 A and 1200 V or anything. For large power there are no other way to survive.
 

Chris65536

Joined Nov 11, 2019
164
Im trying to make an H bridge with mosfets....works fine on no load. But moment i connect it to an out put pulse transformer it blows off?? is it due to wrong way of soldering??
I can only guess, but I'd bet you're putting DC into a low ohm load. What is the DC resistance of the pulse transformer?
 

Sensacell

Joined Jun 19, 2012
2,564
Your gate drive circuit has 2 NPN transistors - one with it's collector grounded?

This configuration is incorrect, It can never drive the gate LOW.
 

danadak

Joined Mar 10, 2018
3,889
Actually you can drive the NPN inverted, in fact for some types you wind up with lower
Vcesat when you do. But forced beta much worse in this configuration.

I am in agreement with Sensacell, there are better ways of doing this.


Regards, Dana.
 

atferrari

Joined Jan 6, 2004
3,773
Actually you can drive the NPN inverted, in fact for some types you wind up with lower
Vcesat when you do. But forced beta much worse in this configuration.

I am in agreement with Sensacell, there are better ways of doing this.


Regards, Dana.
Dana,
Could you elaborate briefly on that with a simple schematic?
Curious / interested.
Thanks
 

AlbertHall

Joined Jun 4, 2014
9,276
Actually you can drive the NPN inverted, in fact for some types you wind up with lower
Vcesat when you do. But forced beta much worse in this configuration.

I am in agreement with Sensacell, there are better ways of doing this.


Regards, Dana.
I did a simulation of this, using 2N3904, but the emitter of the lower transistor doesn't get above 43mV despite 84mA flowing in the 51Ω resistor. I think this must be that the model doesn't understand 2N3904 with reverse polarity?
 

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danadak

Joined Mar 10, 2018
3,889
I did a simulation of this, using 2N3904, but the emitter of the lower transistor doesn't get above 43mV despite 84mA flowing in the 51Ω resistor. I think this must be that the model doesn't understand 2N3904 with reverse polarity?
Sim should handle reverse polarity, its just passive constants and V and I sources.
I think.......

If emitter 43 mV off ground, is it saturated ?


Regards, Dana.
 

AlbertHall

Joined Jun 4, 2014
9,276
I changed the sim to use a 2N3906 (PNP) for the bottom transistor and it works much better.
I don't like the fact that the top MOSFET has no DC path for the gate. It works OK in the sim but would probably not in a real circuit.
 
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