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Two switch forward topology WEIRD waveform and efficiency issue

arvinfx

Member
Hi ,

I am repairing a double ended forward SMPS and I have 2 problems first see the waveform of transformer:

Core waveform.png

As you can see when mosfets go off, there is a huge jumping . normally it has to be like blue line is , but in my case it goes up and gradually comes down as I mention by read arrow.
First I found out the SMPS consumes to much power when it is on and without any load. after that I investigate more and end up to that bouncing I mention above.

So any idea would help me to know what is the main problem.

Thanks
 
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Diver300

Well-Known Member
Most Helpful Member
I think that it's just the normal ringing after the diode has turned off.

When the switch is on, the voltage is high.

After the switch turns off the voltage drops rapidly and the diode turns on, keeping the voltage from going much below 0V.

Once the inductor current has dropped to zero, the diode stops conducting. After the diode turns off, and before the next switching cycle, there are no low impedance sources connected. There is some capacitance due to the diode, switch and winding inductance, and that resonates with the winding inductance to give a damped sine wave.

In this case, there is about one cycle of oscillation before the next switching cycle.
 

arvinfx

Member
Do you have a schematic that you can share?

Looks like one of the MOSFETs is not turning on.
Don't have schematic , But generally I checked it is a normal two-switch forward or double-ended forward with FAN4800 controller. It is something like this:
ML4800-64984-14.jpg


If one mosfet doesn't turn on , SMPS won't turn on.
 
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Diver300

Well-Known Member
Most Helpful Member
Also, on that circuit, when Q1 and Q2 are off, and neither D3 nor D4 is conducting, the voltage on either end of the primary winding can be just about anything that is above ground and less than Vin.

A similar thing happens with linear power supplies, made from a mains frequency transformer feeding a bridge rectifier. When the voltage is away from the peaks, and no diodes are conducting, the voltage from a winding terminal to the output isn't well defined. The shape of the waveform, as seen on an oscilloscope, can vary a lot when the diodes aren't conducting, depending on stray capacitance etc.
 

arvinfx

Member
This wave is the PWM outpout from FAN4800 :
PWM-OUTPUT.png

And this is PWM on gate-source of low-side mosfet :

PWM-GATE-SOURCE.png

Is this two waveforms normal?
 
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arvinfx

Member
gentleman

I have found exactly same same SMPS.
Just now I have checked all wave-forms above on new SMPS ,wave-forms are same as repaired one that we were talking about.

So the only issue now is the high consuming current on repaired one. i don't know were to search for it ?! it is near double! even when SMPS is off
 

Diver300

Well-Known Member
Most Helpful Member
gentleman

I have found exactly same same SMPS.
Just now I have checked all wave-forms above on new SMPS ,wave-forms are same as repaired one that we were talking about.

So the only issue now is the high consuming current on repaired one. i don't know were to search for it ?! it is near double! even when SMPS is off
How much is double? Amperes is a common unit for current.
 

Diver300

Well-Known Member
Most Helpful Member
Can you measure the power factor?

If the power factor correction is working, then the difference of 300 mA represents around 70 W, and that means that something is getting hot. Whatever it is should be easy to find.
 

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