HV low current output

Posted by: henryspl on 06/29/2012

I am trying to use TOP261YN and a transformer 720uH primary and 1780uH secondary. I want to have variable output voltage, 180 or 530V, load varies between 100uA to 80mA. My first attempt is to get it working at 180V out. I thought I could use a smaller part but it seems to current limit. The TOP261 works but if I short the output the part blows up with a 10K resistor on the X pin. I am trying a 11K resistor now. Pulsed current on D pin seems to be quite high, like 6A and the shape of the current ramp seems unusual. The transformer does not look to be saturating. Snubber behaves normally, drawing current right after D turns off. Very small current through secondary diode. I have a simulation that tells me pulsed current through D should be less than 500mA. Do I have a problem with the transformer spec?

Scope Plot shows Red trace D current 2A/div. Green trace D volts 100V/div, time scale 250ns/div.

Regulation is poor with 100uA load, output floating up higher than 200V. Is there a way to do pulse skipping or something at low load?

Thanks,

John H.

Files

SCHEMATIC1 _ PAGE2.pdf (15 KB)

Comments

I hope your current probe is de-gaussed when you made the measurement. The shape looks non-linear (dual inductance).

Try connecting R108 (X pin resisitor) to -VR directly and please check the behaviour. Did you use PI expert to design your transformer?

rgds

I tried using PI expert but it won't let me input a secondary voltage higher than 100. The transformer was actually designed several years ago and operating parameters may have been different than they are now. The schematic is not quite right, R108 is already connected to -VR. The current probe is working ok measuring other things.

I also have a query into the transformer manufacturer, waiting for a response.

Thanks for helping.

I can't find a spec for minimum duty cycle for the TOP parts. I see the operating frequency will drop to half with small load. Does anyone know what is min duty cycle?

Can anyone explain what might cause this strange current waveform?

I figured out why the posted current waveform looked strange. I had some secondary windings not connected. I wired them all in parellel and now the current waveform looks good, only 1A peak and nice and straight. Now I just need to get better regulation at low load (200uA). The pulses are in bunches and output is over 400V instead of 180V. With 4mA load, regulation is ok, measuring 180V out but pulses are still in bunches. I could add a resitive load to get regulation with the external 200uA load but when I switch to 530V output, it will waste a lot of power. I will work on compensation to get rid of the bunches.