[SOLVED] Power MoSFET Drain-Source Voltage

[electro13]

Hi,

Im Driving two power MOSFETs (stp36nf06) using a SG3525ANG with a gate resistor, when I measure the Gate-Source voltage using the oscilloscope I get a perfect PWM of a peak-peak of 12V, but when I tried to measure the Drain-Source voltage I got a flat DC line at 12V with small spikes at the points its suppose to be switched off! I dont know what the problem is! I know im suppose to get a PWM between the drain and the source! any idea what the problem might be?

Regards,

 

[albbg]

How the two MOSFETs are connected ?
When you say peak-to-peak=12V you mean the VGS voltage is both positive and negative ?
It could also be you are using a PWM frequency that is too high. Try to give manually 0 and 12V as VGS in order to see if the transistor alone switches correctly.

 

[Centmo]

So it sounds like your FET is not turning on. Are you measuring Vgs and Vds right at the device pins? I'll throw in the possibility that your FET is fried and cannot turn on.

 

[electro13]

Im Measuring Vgs and Vds right, Vgs is normal! Ive tried using different PWM frequencies and different MOSFET but still got the same problem

 

[kripacharya]

a schematic of your setup would be useful. What have you got connected to the Drain ?

 

[Centmo]

Yes, we need to see a circuit schematic.

 

[electro13]

I have attached part of the schematic, the two mosfets are connected to a centre tapped transformer, the gate is connected to a 3525 IC output

https://obrazki.elektroda.pl/5632654600_1364241960.png

 

[kabeer02]

simply check your voltage at positive supply and drain, if their is no voltage then your fet doesn't switch

 

[kripacharya]

I have attached part of the schematic, the two mosfets are connected to a centre tapped transformer, the gate is connected to a 3525 IC output

https://obrazki.elektroda.pl/5632654600_1364241960.png

ok good.

Not kindly post details of your PWM - frequency/ duty

Also some detail of your transformer : Inductance, L/R etc.

Lastly - Presumably the resistor connected to your Source for the 'sense' input is a very small one ? What is it ?

 

[electro13]

PWM 20 KHz, 50% duty cycle

Transformer is a EC 28 (12V/230V) I dont know the inductance or L/R because I had to take out of an old PCB board

the resister connecting the source to the shutdown pin is 1K ohms, the one connecting the source to ground is 1 ohm

 

[electro13]

simply check your voltage at positive supply and drain, if their is no voltage then your fet doesn't switch

I've tried this, I can see the problem, the fet switchs on but never switchs off, how can I fixed this?

 

[Centmo]

Wait, if Vds = 12V, then the FET never switches ON.

 

[kripacharya]

Everything looks good, and your posts state that you have already changed mosfets. So the only possibilities left that i can think of are --

1) your transformer is open circuit. Check its continuity and/ or replace it with a simple resistor for a test run.
2) you are using incorrect pins of the mosfet. check the pinout again.

 

[electro13]

I've tested the mosfet using a simple circuit, connecting the drain to 12V via a 1K resister, source to ground, and gate to PWM output of a 3525 chip via 22R, I applied a 20KHz PWM to the gate and I got VGS and VDS as shown below, I changed the mosfet and I got VDS shown in figure 3, then tried it using 100 KHz and i got the last two graphs below! any suggestions on why this is happening?

 

[Centmo]

Can you simplify the circuit even more, so it is just the MOSFET, the 1k resistor and the gate resistor? Connect gate resistor directly to +12V to see if it will turn on. Triple check the MOSFET pin-out.

 

[electro13]

it turned out, the MOSFET was burned out, I had it initially in reverse, so it was burned and when I put it the right way it wasnt working, and the transformer was saturating, so I changed the MOSFET and changed the frequency and it works now!!

Thanks all for the help!!