Current mirror based smps feedback

[sam_sdpl]

hii,

In one of my flyback smps configuration working in non-isolated topology iam using current mirror based feedback instead of using optoco
upler. The circuit works fine , but under higher main voltage i.e. 40% of 230V. The trannsistor in current mirror Q2, Q3 and Q1 gets shorted which inturns destroys switcher IC. Drain and source voltage gets shorted in the switcher IC.

Please refer the feedback circuit attached . I am not very sure of the resistance values of R4 required to be kept.

And also suggest what other design factors are going to influence the performance of the circuit.

Switcher IC being used is Viper25 from ST.

Output voltage is 3.6V@400mA

Sam

 

[goldsmith]

Dear Sam
Hi
If you're talking about non isolated SMPS , why optocoupler and variable zener ? you can use two resistors simply . because your grounds are not independent . BTW : what do you mean by non isolated flyback converter ? do you mean buck converter ?
Best Wishes
Goldsmith

 

[sam_sdpl]

Simple resistor divider will not work in my configuration because The IC does not have an internal error amplifier to sense the error.

Yes definately it is in non -isolated mode i am not using an opto coupler instead of it i am using a current mirror based circuit.

 

[goldsmith]

Hi sam
Which IC you're trying to use ?

 

[sam_sdpl]

Viper25H current mode Quasi resonant converter.

With this arrangement i am also experiencing problem of cycle skipping at higher loads

 

[mtwieg]

hii,

In one of my flyback smps configuration working in non-isolated topology iam using current mirror based feedback instead of using optoco
upler. The circuit works fine , but under higher main voltage i.e. 40% of 230V.

What does 40% of 230V mean? You mean 40% above 230V? (320V?)

The trannsistor in current mirror Q2, Q3 and Q1 gets shorted which inturns destroys switcher IC. Drain and source voltage gets shorted in the switcher IC.

What kind of transistors are you using?

From what you show in your schematic, I don't see how overvoltage on the mains could directly damage the transistors. But I can see ways for it to cause damage indirectly. Perhaps under high mains voltage the drain voltage is exceeded on viper25, causing it to fail, causing a fault on the output which then kills the transistors. Or maybe at high mains voltage the supply loops is unstable, again causing overvoltage on the output enough to damage the transistors. These are issues that involve the entire converter circuit, so you should post a more complete schematic. Including the turns ratio on your transformer, any snubber networks used, etc.

 

[FvM]

The shown circuit misses a current limiting means for Q1. You should add a collector series resistor that keeps Q1 power dissipation within reasonable limits.

There may be other problems with your circuit, but the said measure should at least protect the three transistors. There's a certain chance, that the current mirror circuit is actually destroyed from the VIPER25 side, but get clarity by excluding the other explanation.