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TOP266. High secondary output and primary bias voltage.

Posted by: Kurre2017 on

Hi,
PI Expert 9 design. Uin 90-365Vac, output +55V/0.4A, -55V/0,4A and 24V/0.4A which one is used for the feedback loop.

The out put voltage is pre set to be 24V. However at low and high mains voltage all voltages are about 16% higher than expected both with low and high loads. The bias voltage is just below 20 V at low and high input voltage and low and high loads.
The TL431 anode to cathode voltage is around 1,9V under the same conditions as mentioned. It means TL431 does not get enough voltage to work properly. The gain adjusting resistor is 13k
but a lower value to increase the current desn't help. It cause start up problem. I have separately checked the feedback circuitry, TL431 and optocoupler PS2501-1-K-A fed from a variable dc supply and it seem to work as it should.
I have tried transformers from two suppliers but the result are almost the same with both only a small difference probably depending the difference in leakage inductance. The winding turn on 24V wdg is 8t and bias wdg is 5t.
Please give suggestion to solve this problem and to improve design.

Thanks Kurre2017

Comments

Submitted by PI-Bombadil on 01/25/2018

Hello Kurre2017,

Thank you for considering Power Integrations in your design. Let me try to help you with this.

First, can you send me the PI Expert Design results? In your actual design, do you some deviations from this?

Thanks,
Bombadil

Submitted by Easy Peasy on 01/28/2018

It appears you haven't enough current in the opto / TL431 (see data sheet for it) as the 13k resistor is too high in value. This also prevents up to 8mA in the opto led which you need to allow for a low gain opto - which explains your lack of regulation.

Check the power dissipation in the TL431 if you lower R12 (to 3k3 say)

For an Iq of 1mA in the TL431 the res across the opto led will need to be 470 ohm to limit the voltage across it

At no load the uncontrolled outputs will tend to go high due to leakage inductance spiking.

Submitted by Easy Peasy on 01/28/2018

Also you need bigger snubber caps on the diode snubbers to reduce ringing and volt rise at no load on the uncontrolled outputs 22pF is simply way too small to have any real effect.