As a student, I could only afford a cheap one.

It is best to always check LOW ESR.

 

Did you check the polarity of the capacitor on the board?

Yes, I did. Without a load, it operates under room temperature as it should. Once the output current increases, it starts to get hot. When I last checked it, the output voltage was 150V , I decreased the output voltage to 150V for a load test, and the output current was 0.7A. The capacitor was about to burn.

 

I added 1 ohm resistor under the input capacitor and checked on oscilloscope.

 

Those pictures look like you have ≈8A of ripple current through the capacitor, and it's likely higher than that without the resistor, so that's probably the cause of the heating.
To minimize the heating you could use several smaller capacitors in parallel so they share the ripple current.

Remember that capacitor has to supply the peak current of the transformer primary, which is at least ten times the secondary current.

 

To minimize the heating you could use several smaller capacitors in parallel so they share the ripple current.

I'd suggest a film capacitor of several microfarads to deal with the high frequency stuff in parallel with the electrolytic. Electrolytics get quite inductive at high frequencies.

Is it possible to observe the back EMF on oscilloscope? If so, how can I do that?

It's the voltage on the drain of the MOSFET.

Totally right. As a student, I could only afford a cheap one.

Spend more on the capacitor and less on the opto-isolator! A PC817 will do!

 

Why is the waveform on SDS00002.png so irregular? Is the feedback stable? What does the 12V supply look like on the scope?

 

It is better to redesign the input filter stage to an LC filter network. With LC filter configuration, you could reduce the input capacitance. If possible, use multi layer ceramic capacitor which has low ESR

 

I'm not sure a series L will help. Firstly, the battery's ESR was originally in parallel with the capacitor ESR which reduced the hf ripple current into and out of the capacitor. Secondly, with reduced parallel capacitance, and increased series impedance the voltage ripple on the 12V rail will be increased, not reduced.
I'd bet that the high ripple current was caused by transformer saturation, due to the uneven waveform; and that if the circuit were working properly, there would be no problem with the original capacitor.

 

Why is the waveform on SDS00002.png so irregular? Is the feedback stable? What does the 12V supply look like on the scope?

This how it looks like.

 

This how it looks like.

That can't be the 12V supply, can it?

 

That can't be the 12V supply, can it?

Well, that should be 12v supply line I have this bad load tester which I have to avoid using it.
This is how it looks like on scope with 20kiloohm resistor instead of this bad load tester.

 

Sorry, I'm lost. Where is the 20k resistor? Is it all still supplied from the battery? Where does the load tester go?

 

Maybe I missed it, but HOW HOT IS IT GETTING? Capacitors rated at 105 degrees might feel terrible hot, but within specifictioins.

 

Sorry, I'm lost. Where is the 20k resistor? Is it all still supplied from the battery? Where does the load tester go?

Yes, it's supplied from battery. I used load tester first but recently I switched over to 20k resistor to see if there is any difference between the load tester and normal resistor.

 

Not funny anymore

 

Maybe I missed it, but HOW HOT IS IT GETTING? Capacitors rated at 105 degrees might feel terrible hot, but within specifictioins.

Update: Firework.

 

Hello,

Was the polarity of the capacitor the right way?
Having the capacitor in reverse polarity will heat up the capacitor with the possibility of blowing up the capacitor.

Bertus

 

Hope it didn't smell too bad. (My colleague put a 63V capacitor on 108V last week, and after it expired there was a foul smell in the lab for the rest of the day, still noticeable the following morning)
Does it work without the capacitor? Shouldn't be a problem on a battery with short leads.

 

Hello,

Was the polarity of the capacitor the right way?
Having the capacitor in reverse polarity will heat up the capacitor with the possibility of blowing up the capacitor.

Bertus

Polarity was correct, but having a heating problem, probably due to ripple current :S

 

Hope it didn't smell too bad. (My colleague put a 63V capacitor on 108V last week, and after it expired there was a foul smell in the lab for the rest of the day, still noticeable the following morning)
Does it work without the capacitor? Shouldn't be a problem on a battery with short leads.

I knew it would happen. I have been wearing my workshop glasses in case of blowing up. But couldn't protect my face from the hot liquid. It's kind of greasy and smells bad as you said. Btw, I didn't try without a cap, replaced it with a new one