Isolated 10V power supply using viper222LSTR IC

[Pemendra]

Hi ST team,

I have design the 10V isolated power supply design using viper222LSTR IC having universal input AC supply.

 Schematic of the circuit is attached here.

When i am increasing the input voltage, voltage across the capacitor C7 maintain constant of 10V, while voltage across capacitor C12 is in increases, winding of the transformer, W3= W2= 30 turns, are same and W1= 180 turns. 

So why?

1.  My voltage across the C12 is incases with increasing the input voltage.

2. With increasing the dummy load (i.e. R8= 1K) then viper222 get on & off frequently.

I have required constant 10V across C12 in isolated manner while increasing the input voltage ( I am extracted from C12 because of need of isolated power supply and and i want to avoid use of optocoupler in the circuit)       

How i can fixed it, please help use.

Thanks and Regards,

Pemendra Pardhi

[Peter BENSCH · Accepted solution]

To your question 1:

The behaviour is most likely related to two things:

1. the construction of your transformer
2. the connected load

Regarding point 1: A flyback controller requires information about the current voltage at the output for precise control. A TL431 or similar is typically used for this purpose, which measures the voltage and reports its value back to pin EA-OUT via an optocoupler, as can be seen in the data sheet, fig 12, for example. Now you do not want to use an optocoupler and have set up a flyback according to data sheet, fig 11. However, you must also take into account that the VIPer222 only sees the voltage of the supply winding and not the output voltage. If you wind the two secondary windings of the transformer in such a way that both are closely intermeshed (e.g. by winding both wires at the same time), you will achieve the maximum possible coupling so that the output voltage can follow the supply voltage in a reasonably acceptable manner.

Regarding point 2: However, all of this only considers the case with a generally constant high load. As soon as the load varies, your output voltage will also fluctuate because the coupling between the first and second secondary winding is not infinitely high.

To your question 2:

In your design, R8 has the value 100k, which at an assumed voltage of 10V allows a current of 100µA to flow, which corresponds to a load of 1mW. This load is far too small for the controller to be able to regulate it sensibly. Now two effects occur:

• Burst mode (see data sheet, section 6.4)
• Pulse skipping (see data sheet, section 6.3)

What you are observing is most likely the pulse skipping mode, because the transformer is not sufficiently discharged during the off-time. You would therefore have to connect a sensible base load to R8 (e.g. 470...560 ohms) in order to avoid the pulse skipping that is usually even audible.

The disadvantage of a primary side flyback converter is precisely this lack of control accuracy due to insufficient coupling, which is why it is used:

• for applications without high demands on the value of the generated voltage, or
• in applications where the flyback is followed by a linear regulator

Hope this helps?

Regards
/Peter

[Pemendra]

Hi, Peter

Thanks for providing the suggestion and given the solution,

There saw the transformer winding polarity was a problem, and i also have to change the compensator capacitor value from 330pF to 100pF.

Is there any documentation available to select the compensator? 

 Now circuit is working properly at dummy load of 1k at output, instant of 100k.

Thanks and regards 

Pemendra Kumar Pardhi

[Peter BENSCH]

To calculate the compensation network, you can either read the relevant literature (e.g. AN5642, section A.1) or have the design calculated and simulated precisely for your boundary parameters using eDesignSuite.

Good luck!
/Peter