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Voltage stability problem

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Amirahp

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Hi , I`ve faced many converters having one issue . They properly work for some seconds and then the output voltage suddenly increases and if i didn`t turn them off the output DC link would be blown. Take a push-pull DC/DC converter as an example , more specifically I have been working on a MGE UPS.It has a push-pull for stepping up battery voltage to +400 and -400 volts.After starting the UPS, everything was working properly but the voltage remained higher than 800 like 890 and the control section prevented the inverter to turn on. At last, I realized there was something wrong with the control loop of push-pull and I changed one of control loop op-amps (LM324).Afterwards, the problem changed and when I started the system , the voltage went up and it was increasing to 950 and even more.If I didn`t turn it off , DC BUS would be blown. have anyone seen this problem so far?
 

If you add the schematic will be better.
Having this info, can be anything.
Control loop on open load ?
Reference voltage that follows the output voltage ?
Optocouplers not correctly polarized ( i.e too low current on the LED diode ) ?

It is very difficult to give the proper answer .

Mandi
 

Actually , drawing the whole schematic can be rigorous but I`ve drawn some part of it and the needed has attached .Thanks. 22.jpg
 

Hello Amirahp,
I would check the circuitry around the opto-coupler. The LED side of it.
It seems the feedback for voltage regulation is not working correctly.
Mainly check resistors for them going high, especially the 1% types.
Even a slight increase in their resistance can cause problems.
Regards,
Relayer
 
Hi , the problem has changed and the voltage is stabled now in rage of 800 to 850 but I think it should be below 780 and the protection circuit and Uc diagnose this as a major problem and the Uc doesn`t start the inverter and UPS starts buzzing continuously.
 

feedback resistors could be going open ckt or changing value due to high watts?
 
Dear Easy-peasy I`ve check every resistor on that PCB and nothing was wrong.For the most part, I traced the control loop and found that one Opamp held in the control board would be the problem and changing it didn`t do anything. I doubt that I even find the problem correctly , maybe somewhe3.jpg2.jpg3.jpgre else has problem.
 

Hello Amirahp,
Please refer to your modified picture below:

22.jpg

The two red arrows are indicating your Vcc points. You need to place
your multi-meter on either one and observe if there are any changes
to the voltage. That means clipping the Vcc and ground points with parrot
clips or something else you you can monitor the voltage during the start
up sequence and perhaps 5 minutes after the UPS has stabilized.

The two green arrows means that you need to trace back to the circitry
involved with where CN7A connects to. You need to pay close attention
to any zener diode/s that may be within that feedback circuitry.
You may need to lift one side of the diode/s to measure any reverse
leakage. If its an SMD type, you'll have to remove it from circuit to get
an accurate measurement.
If you haven't checked any resistors in this section, then check them.
You'll find that quite a few of them will be 1% types. You'll need to take
the close tolerances into the equation and ensure none have gone beyond
the 1% difference.
I hope the above helps.
Please keep us updated on how you get on.
Regards,
Relayer
 
Yes , I`ve changed and NOTHING has happened.
 

Hello Amirahp,
Please refer to your modified picture below:

View attachment 142615

The two red arrows are indicating your Vcc points. You need to place
your multi-meter on either one and observe if there are any changes
to the voltage. That means clipping the Vcc and ground points with parrot
clips or something else you you can monitor the voltage during the start
up sequence and perhaps 5 minutes after the UPS has stabilized.

The two green arrows means that you need to trace back to the circitry
involved with where CN7A connects to. You need to pay close attention
to any zener diode/s that may be within that feedback circuitry.
You may need to lift one side of the diode/s to measure any reverse
leakage. If its an SMD type, you'll have to remove it from circuit to get
an accurate measurement.
If you haven't checked any resistors in this section, then check them.
You'll find that quite a few of them will be 1% types. You'll need to take
the close tolerances into the equation and ensure none have gone beyond
the 1% difference.
I hope the above helps.
Please keep us updated on how you get on.
Regards,
Relayer

Thanks and I`ll precisely do what u`ve said and I will tell u what will change.
Regards,
Amirahp.
 

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