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[Moved] Voltage drop on load in flyback SMPS

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Newbie level 1
Oct 10, 2014
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I have designed a fixed frequency flyback SMPS (5V, 1A) using STM's design studio using Viper17 series. I do not have much knowledge about SMPS practical issues.
The supply is working fine ( with some voltage variation) on no-load but the voltage drops to half value on a load of as small as 10mA.

I am unable to understand what is causing extreme drop in voltage. I do not have scope with me to check output at the viper.I have attached the circuit i am using. Any direction on this will be helpful. SMPs_ckt.JPG


i hope the schematic is according a datasheet or an application directely from the IC manufacturer. I didn´t check this.

You say the output voltage drops even with a small load.
The whole circuit is a regualtion loop. The output voltage is monitored and via the optocoupler feed back to FB pin of the IC.
So the first thing is to check if the FB pin voltage is in it´s valid range. If yes, then the problem is in the circuit from output voltage to FB pin.

If the FB pin voltage is out of range, then the IC tries to push more energy to the output but this fails.
To check on this, first check the DC input voltage to the primary of the transformer. if this is not in a valid, stable range, then inspect the power input circuitry, where there is a too high votage drop.

If DC supply is OK, then the problem is either in power switching (MOSFET), the transformer or it´s connections. or in the voltage catch at the secondary side.
My hint is to check the transformer circuit according correct wiring, especially polarity.

Please try to do the trst and tell us what you find out. All the test should be possible with a DVM.

Be careful, there is dangerous voltage! Take care of the remaining voltage in capacitors, even when power is switched off.

Good luck


Ropt should be connected to d1 cathode really.
Rc is too big value at 1.6megohms

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These are often due to tripping some protection mechanism in the controller.
Eg UVLO, output overload, overvoltage on bias coil etc

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