Problem with schottky

[mail2vasu2004]

I am working on a Flyback converter with main 230V AC input to 18V, 15A DC output. I have put MBR3045 at the secondary output for rectification. After turning on the input to 220V and no load connected to the secondary, the diode is heating up to close to 55-60C.

My switching pulses are not very clean and the mosfet in the input is turning on before it completely turns off. Could this be the reason.

Or the output in the secondary is 60V peak to peak with positive peak being 31V and negative peak being 29V approx. Could this be the reason.

Or could it be the avalanche energy is too high.( Also let me know how to know this as only ST microelectronics product have this mentioned in their datasheet)

Kindly share your experiences.

Vasu

[sawreyrw]

Post a schematic.

[djoffe]

Without seeing the schematic, I can't be sure, but I think you are exceeding the peak reverse voltage...if the input pulse swings about +/-30 V, and the DC output is 30 Volts, then the diode see 60 volts in the reverse direction.
It's only rated for 45 volts, per the data sheet.

http://www.onsemi.com/pub_link/Collateral/MBR3045-D.PDF

[mail2vasu2004]

thanks for the suggestion

will change the schottky...
just clarify on how the avalanche breakdown energy can be calculated for this particular diode as the datasheet doesnt show any ratings on the same


vasu

[djoffe]

you've put nearly 60 volts across it, I think...that's more than the rated 45 Volts. Figure 3 goes to 45 volts, and depending on temperature of the diode, at even that low voltage, it will conduct from 0.4 to 40 mA.

This diode just shouldn't be used with that much voltage across it. Remember that if the duty cycle is around 50%, the ac voltage out of the transformer swings plus and minus by the same amount, owing to volt-second balance. That means the cap charges to DC=30V, and when the transformer reverses, the diode see 30--30=60 Volts reverse across it.

So far, you are fortunate that the diode is better than its ratings...

[mail2vasu2004]

Thanks a lot. The diode did blow. So with dutycycle increase more then 50% will the swing be less negative.

your suggestions will surely be precious.

Thanks
Vasu

[kubeek]

You cannot have more than 50% duty cycle in a flyback. That would make the flux walking up into saturation and you will get a dead transistor in the process.

[metalsculptor]

Originally Posted by kubeek

You cannot have more than 50% duty cycle in a flyback. That would make the flux walking up into saturation and you will get a dead transistor in the process.

Of course you can have more than 50% duty with a flyback circuit, television is proof of that, both the horizontal and vertical deflection systems run 85 to 90% duty. Car ignitions are another example of high duty cycle flyback converters. The trick is to use enough air gap to help reset the core and soften the saturation curve, the air gap also helps to increase the stored energy.
Even with no attention to the air gap, the core will just move up the saturation curve until equilibrium is found and that equilibrium should be the design point, if not the switching device will fail like you mentioned.

[kubeek]

Thanks, didn´t know that.

[metalsculptor]

Glad to be of service.