Is PI have some solution to this problem?

The bias winding should have 50% to 60% of the turns of the secondary winding (12 to 14 Volts, based on a 24 Volt output). If the voltage is higher than expected, the problem usually because the bias rectifier is peak detecting the leakage spike. Try using a standard recovery time diode for the bias diode(1N4003 for example). Also place a 22 Ohm resistor in series with this diode. If the voltage increases too much at full load, you may need to add a load resistor to the bias circuit.

Z. Cochrane

Hi Cochrane,
Thanks for ur quick response,
I agree with u. But my problem is that, due to the high bias voltage, i have asked for Xmer manufacturer to drop down the bias voltage. So they reduce the no of turns of the bias winding. I have tested this Xmer but it didnt work. The power supply gets oscillate.
Tell me where do i made changes so it works properly.
I m waiting for u reply

Thanks & Regards
SachinD

The oscillation problem may be due to the bias supply is going too low to support the control pin current. You have to be able to supply 8.5 mA into the control pin. The voltage on the pin is 6.2 Volts. For reliable operation, the bias voltage should not go below 10 Volts. During the oscillations, if the voltages drops below this level, then you need to increase the turns on the bias winding.
Z. Cochrane

Hi Cochrane,
I have measured the bias voltage of the Xmer.
It Seems to be 20V.
The output is 24V for few milliseconds and then vanish again.
This action continues till the supply is ON.
And discharge very slowly when we switch OFF the suply.

Regards
SD

It sounds like the power supply is in auto-restart mode. Verify that the control pin is getting a minimum of 2 mA of current. If the current does not get to this level before it turns off, then you need to check the feedback circuitry to determine why.

If you still have problems, then I will need to see your schematic (PDF format) and will need to see the drain current, drain voltage, output Voltage, and bias Voltage waveforms.

Best regards,
Z. Cochrane

Thanks,
Yes you are right, the supply is in auto restart mode. The voltage i m getting on C pin is 5.34V.
So to increase this voltage i have increase the series resistor connected to auto restart capacitor, but no effect. Increase up to 80 ohms.
I have also try to increase the serial resistor connected to optocoupler LED.
But still the same.
Is there any other option left?

Regards
SachinD

Thanks,

Yes, the supply is in auto restart mode. The voltage i m getting on C pin is 5.34V.

So to increase this voltage i have increased the series resistor up to 80 ohm, connected to the auto restart capacitor, but observed no effect. .

I have also tried to increase the serial resistor connected to optocoupler LED.
But still the same result.

Is there any other option left?

I have attached schematic for your reference.

Regards
SachinD

Changing the value of R9 will not solve the problem. The control pin needs >2 mA of feedback current for it not to go into auto-restart. You will need to probe the circuit with a scope to determine why you are not getting the proper feedback current. During start up, does the bias voltage get >10 V before it goes into auto-restart? If it does, then you need to check the to see if the opto is working properly.

You may want to temporally replace the TL431 with a 24 V zener to reduce the possibility that the feedback network is not stable. Do this only to help troubleshoot. Also, I noticed that the opto diode's series resistor should be 1k Ohm.

Z. Cochrane

The TL431 gets damage every time when i swtich ON the supply.

The output voltage go up 40V, because of this the TL431 gets damage.

Therefore next time when i switch ON the supply, the voltage on the c pin was below 5.8, so it goes in to the auto restart mode.

I have also compare the secondary inductance of the Xmer with the old one it seems to be OK.

What are the possibility that the output gets so high?.

Regards
Sachin D

There seems to be something wrong with the feedback circuit. The TL431 should regulate the output to 24 Volts. You should measure the bias voltage and the output voltage with a scope (use two channels to view at the same time). The bias voltage should increase proportionately with the output (via turns ratio of bias to output windings). When the output reaches 24 Volts, the bias should be about 12 Volts. At this point the control pin voltage should be 5.7 to 6.2 Volts and opto's transistor should supply at least 2 mA into the control pin.

If the TL431 gets dammaged, replace it with a 24V zener diode until you determine what is wrong with the circiut. Also increase the value to the resistor in series with the opto's diode to 1k Ohm. This is a better value for both TL431 and zener circuit.

Z. Cochrane

Hi Z. Cochrane,
My problem has been solved.
Thank for your great support.
The problem is in the Xmer bias winding tapping.

Thanks for ur support.

Thanking you.

Regards
Sachind