Low line, full load, auto restart problem
Hi,
I'm trying to make a power supply according to PI Expert. It's working OK except when trying to deliver full load power at low input voltage. The PI device goes into auto-restart mode and the output voltage fails to get to 24V. Please see the attached PDF for the schematic. There are no major deviations from the schematic.
I've investigated the soft-finish circuit and it seems to work OK. The capacitor closes the loop and allows feedback current to prolong the startup time. The problem is that even when the device is trying it's hardest the output voltage rolls off and stabilizes at 19V.
I'm inclined to say that the output voltage is being limited by the device current limit but I think that it's actually something to with the device going into CCM because the auto-restarting begins literally on the boundary between DCM and CCM.
Please can anyone give me some suggestions to solve this problem? I don't have a current probe and I don't have a loop analyzer!
Many thanks
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I love the TOPswitch but I'm struggling to figure out why this isn't working.
The only differences are: R5 is 4.7ohm, C2 is 3x100uF in parallel, R11 is 2K2/5%. The transformer differs in design from the suggested one to keep costs down as this is meant to be a prototype. The transformer is a pair of EC52/3C90 cores with a 2mm centre air gap, the layer stackup is as follows:
| outer |
| 19T of 3x Ø0,4mm ECW |
| 5T of 2x Ø0,4mm ECW |
| 9T of 4x Ø0,5mm ECW |
| 19T of 3x Ø0,4mm ECW |
| inner |
The suggested transformer construction is shown on the attached PDF. The AL of my transformer is 130nH/T2 so it gives approximately the correct primary inductance. The turn ratios have been scaled to approximately match the PI design.
The bias supply seems to be working fine so I believe the turns ratio is satisfactory for that winding.
Best regards
Also, there is 1Vpp low frequency (100-200Hz) output ripple when the output has a small load connected. I know this is a control loop issue because the ripple disappears when the load is increased.
The ripple is also massively reduced when the LED bias resistor is reduced greatly but I know that this isn't the right thing to do.
I'm trying to follow your guide to feedback analysis by current injection and I've noticed that when feedback current with frequency between 500-1000Hz is injected the output ripple significantly reduces.
Hello Coates,
Can you try with a bigger input capacitor like 400uF to see if the problem is there or not. I am wondering the low frequecy ripple comes from the input. when running at full load condition, the input bulk capacitor will have a big ripple which will also show at the output. the PIxls suggested 330uF but you only use 300uF.
Best Regards
Hi,
I've tried this but it doesn't help. I think it's a control issue. When on full load the current draw from the bus caps will be greater so the INPUT ripple voltage will be greater, and when on light load the current draw will be less so the INPUT voltage ripple will be smaller. However what I've described here is the opposite! The OUTPUT voltage ripple is small on full load (high current) and large on light load (low current).
Also, the frequency of the voltage ripple changes which suggests that the effect isn't coming from the line ripple voltage.
Best regards
Hello coates,
i am a little bit lost here. what's the issue you have. In the first forum, you said you have power delivery issue at full load low line. And this time, you mentioned you don'y have problem at full load but have big ripple at no load. can you give more detail about your issue, like some waveform if possible for us to dig in.
Best Regards
Hello coates,
Thank you very much for using PI products.
Before we go to next step, i want to learn more about the problem. Compared to PI expert, what kind of small change( no major deviations ) you did? For the transformer, did you use the same transformer PI expert suggested?
Best Regards