Viper319 Not performing for load more than 4W

[Rajesh_J]

I have attached the Actual Circuit as image. We have gone for flybuck topoogy.

Input VBus = 335VDC. We require 15VDC, 600mA for Load Requirements. (9W Least Requirement)

As per datasheet VIPER319HDTR is capable of delivering 19W. 

We designed the PCB Board and While testing we found that IC is not giving required Vout for current more than 0.25A (4.25W).

Our Readings below

15Vout, 114mA - We get stable 15Vout. Works fine without problem.

15Vout, 209mA - We get stable 15Vout. Works fine without problem.

15Vout, 283mA - Vout fluxtuates approx. at 10V for 2min, Some sparkle buzz sound heard during that 2 min. (This unstable state exists for random time 1 to 2 mins). Then Vout comes to the stable 15V without any noise. We tried several trials. Consistently we are facing same problem.

We don't want Vout to fluctuate.
Can you help us debug the issue. Recommend favourable changes.

Note: I also connected Pin 2, 4 to GND (Pin 1) - As recommended in datasheet. 

@Patrizia BELLITTO @jsong.2 @Kshah.21 @VOsin.1 

[Peter BENSCH]

Welcome @Rajesh_J, to the community!

Firstly, please note: the 19W specified in the data sheet applies to flybacks, not to your buck converter. You can simulate such a buck with the eDesignSuite (eds.st.com), but you will find that the Viper319 already gets very hot at around 250Vdc.

You have to bear in mind that the voltage conversion in flybacks is realised via the winding ratio from primary to secondary, whereas buck can only do this via the duty cycle. With large voltage differences such as your 335V to 15V, the switch-on pulse is therefore extremely short, which results in correspondingly high current pulses for your output power. On the one hand, this can cause the coil to saturate, and on the other hand, the output power that can reasonably be achieved is significantly lower than that specified for flyback.

You could try whether the converter works more stably with a coil with a saturation current (not working current!) of 5...6A. If this is not successful, you must either reduce your output power or use a different component or a different topology.

Hope this helps
/Peter