Should MOSFETs be open all the way?

[stevenmahoney]

I have a question, perhaps a simple one.

In the following schematics driver send a signal to the gates of three MOSFETs. I expected a square signal in the output, but it is straight DC. It looks like one or all MOSFETs is open.

I replaced the driver IC, driver transistors, all MOSFETs, but result is the same.

What am I missing?

 

[Harald Kapp]

Your gate signal is mostly High, therefore turning on the MOSFET. There are only small Low going pulses turning off the MOSFETs.
Your yellow LED should show this by being almost fully lit.
Your driver signal probably needs to be inverted?

You can test the MOSFETs by removing R7 from the circuit and short-circuiting Z3. Your MOSFETs should now be off.

 

[stevenmahoney]

I understand that and feared that the duty of the signal is too high, however, service manual listed this signal as correct one:

 

[henrypereira]

I understand that and feared that the duty of the signal is too high, however, service manual listed this signal as correct one:

have you tested the signal before zener diode?
or at the output of the driver ic?

those will be the steps to identify the culprit of the fault as i can see one of your components its losing its value causing the effect you have.

 

[Electrobrains]

I have a question, perhaps a simple one.
In the following schematics driver send a signal to the gates of three MOSFETs. I expected a square signal in the output, but it is straight DC. It looks like one or all MOSFETs is open.
I replaced the driver IC, driver transistors, all MOSFETs, but result is the same.
What am I missing?

It's not easy to say what is wrong, or even if something is wrong. You must give more information about what your circuit does and why do you think the duty cycle should be lower?
It seems you have some kind of voltage or pulse regulator loop. Is it a pre-regulator for a SMPS or some Power Factor Correction circuit?
The full ground circuit is not shown (the drains of the MOSFETs hang on the positive, rectified rail). Where does the branch from node P5.Y2 go?
Perhaps your error is at a totally different place.

 

[stevenmahoney]

Yes, signal is perfect all the way yo the gates.

 

[stevenmahoney]

Electrobrains, this is an electronic ballast for HMI light (film industry). This circuit should produce a 20kHz square wave. The power factor looks OK to me, because that is what service manual said it should be on the gates. What I do not understand why I cannot see a square signal at output.

 

[Electrobrains]

Electrobrains, this is an electronic ballast for HMI light (film industry). This circuit should produce a 20kHz square wave. The power factor looks OK to me, because that is what service manual said it should be on the gates. What I do not understand why I cannot see a square signal at output.

So in fact there is no problem? The unit works?
It seems you have a square wave on the output. It's just high Duty Cycle. That's normal if you have a regulating PWM circuit involved.

 

[stevenmahoney]

Electrobrains, I do not know if there is a problem. The attached oscilloscope clip is a square wave at the gates of MOSFETs. Still getting a straight DC at the output... Could it be because I run it without a load?

 

[Electrobrains]

Electrobrains, I do not know if there is a problem. The attached oscilloscope clip is a square wave at the gates of MOSFETs. Still getting a straight DC at the output... Could it be because I run it without a load?

1. Definitely Yes! Ballast circuits often need a load to work.
2. The "output" in your diagram would need a load, otherwise you will measure DC. Your multimeter has too high resistance. C13 will discharge through the MOSFETs and will hardly be able to get any current through the multimeter. Thus C13 has 0V over it and you measure straight onto the dc supply.

 

[stevenmahoney]

It is marked "OUTPUT" in the right bottom corner.

 

[Electrobrains]

It is marked "OUTPUT" in the right bottom corner.

Sorry, I saw it too late but edited the message.