high output ripple between 80 to 126Vac input voltage

Posted by: vale on 11/23/2010

Hi,
I have an 7W power supply with a TOP252PN. I have an universal range of input power supply (85-265Vac). At 80% of full load and at 230Vac input voltage, I have an output ripple voltage near 180mV. When I go under 55% of the input voltage (126Vac), the output voltage ripple start to increase and at 50% I have more than 1.5V ripple. I tried to change the Top switch with TOP253PN and TOP254PN in order to have more power available, but the result is the same. I changed the TOP switch but I maintained the same load.
The problem could be with the TOP switch setup or also with the transformer?

Thanks

Regards

Comments

Hello vale,

Without access to your schematic, my best gues will be the gain of the feedback loop. Try to use an optocoupler with lower gain or add a 500ohm resistor on parallel to the diode section of the optocoupler.

Cheers,
PICrusher

Hi,
Thank you for the quick response. Attached there is my PSU schematic. I have 4 optoisolated secondary output. In order to have an output voltage more precise, I used for each output a linear voltage regulator, so I choose to use a primary zener for feedback because I don't need an output with high precision from the top switch. I didn't used an optocoupler for feedback because some outputs could not be connected so I was not sure that the feedback from a not connected output could be precise.

thanks

Valentino

I have the same problem but my ripple is not as big as your ripple, do you find any solution to this problem, I thing it can be result of the input bulk capacitor ESR, in my converter under the 120 V AC the ripple increases nearly 3 times and its waveform distorts and it does not sinusoidal ripple any more,

In my case the problem is due to the feedback. With 230Vac input voltge everithing is ok, when I go below 120Vac, the output ripple increase a lot. If you monitor with a scope the wave on the drain pin, you can see that at 230Vac you have the ON time and then there is a sinusoidal resonance before the OFF time. When you decrease the input voltage this resonance decrease. When this resonance disappears your output ripple will start to increase. In my case this is the reason. If you want to decrease the problem you must to put the PSU in full load and change the feedback gain in order to have this resonance.
I did this and now I have an output ripple constant between 95 to 230Vac.