PIC Mosfet Drive

[audioguru]

Your R1 of 1k ohms was making a voltage divider feeding 8.7V to the gate instead of 0V to turn off the Mosfet.
Since the maximum allowed Vgs is 20V for the IRFZ44 Mosfet then R1 is not needed with your supply of only 12V.

 

[Suraj143]

All clear.After removing the 1K resistor everything worked perfectly.Now the driving hardware & output wavforms are ok.

For testing I just hooked a 12-0-12 / 230V (1A) transformer & checked the output voltage.I see only 145 VAC.And my FETs are extremely hot even without a load on AC side.Battery voltage is 12.5V.

For the test circuit I used two FPQF-55N06 FETs.

How to overcome these issues ?

Thanks.

 

[Nigel Goodwin]

How big are your heatsinks?, but it doesn't look like the kind of circuit that would work at all well.

 

[Suraj143]

How big are your heatsinks?, but it doesn't look like the kind of circuit that would work at all well.

The FETs are plastic package.I didn't use any heatsinks Also I wanted to see the transformer output voltage without any load.

 

[rjenkinsgb]

For testing I just hooked a 12-0-12 / 230V (1A) transformer & checked the output voltage.I see only 145 VAC

Oddly enough, the ratio of target to actual output voltage is 1.586

Divide 12 (input) by that, to get the primary voltage needed to get around 230V out, and you get 7.56V
That pretty much matches your UPS transformer primary!

 

[audioguru]

The datasheet for the FPQF-55N06 Mosfets show that they need a Vgs of 10V to fully turn on. If the Vgs is less than 10V then they get hot and the transformer gets less than the 12.5V.
What frequency is the PIC driving the Mosfets?

 

[Suraj143]

The datasheet for the FPQF-55N06 Mosfets show that they need a Vgs of 10V to fully turn on. If the Vgs is less than 10V then they get hot and the transformer gets less than the 12.5V.
What frequency is the PIC driving the Mosfets?

Driving Frequency is 50Hz.

 

[Suraj143]

I just put my scope before going to Final MOSFET gates.I got below waveforms.

 

[ChrisP58]

I just put my scope before going to Final MOSFET gates.I got below waveforms.

View attachment 136617

*** Major problem. ***
You have both mosfet on at the same time, creating periods of high current.

Rework your code to have periods of dead time where both mosfet drive signals are off.

 

[Suraj143]

*** Major problem. ***
You have both mosfet on at the same time, creating periods of high current.

Rework your code to have periods of dead time where both mosfet drive signals are off.

You got it.My code is generating the correct code.But due to Low side driver in post 16 the wave is inverting.I need to invert the code in PIC side.

 

[Suraj143]

Hi, I did the final testing by plugin a 12-0-12 transformer. But I only get 185VAC.What's the reason to this?
Battery voltage is 13V DC.

 

[rjenkinsgb]

But I only get 185VAC.What's the reason to this?

See post #25.
Use the 7.5-0-7.5 one...

 

[audioguru]

A 12V-0-12V transformer might be actually 16V-0-16V that drops to 12V when loaded. Then when used backwards the "220V" output is actually much less.

 

[Roff]

Why do you need the diode across R4? For that matter, why do you need R4? I would use a piece of wire.

 

[Nigel Goodwin]

Why do you need the diode across R4? For that matter, why do you need R4? I would use a piece of wire.

R4 is to limit the gate current, and the diode speeds turning OFF up.

 

[Roff]

R4 is to limit the gate current, and the diode speeds turning OFF up.

Yeah, but the current is only limited during turn ON. Also, why should we limit gate current?

 

[Nigel Goodwin]

Yeah, but the current is only limited during turn ON. Also, why should we limit gate current?

To much current in the gate, and too much taken from the drivers.