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Out with the new, in with the old

-July 11, 2013

Once I had to design a flyback converter, which would not be difficult except for one issue–the cores we used in our other projects were not suitable for this converter. I checked it with calculations and simulations, but the result was always the same. The cores were for power flyback converters, but I needed to design converters for low-power devices.

Flyback converters provide power in cores by pulsing it in a primary winding. This power is stored in the core and provides power in the secondary winding during the pulse off time. A flyback converter has an auxiliary winding, too. This winding is used for supplying power to the control IC of a converter.

My problem was that I had unsuitable pulses in the primary winding. These pulses had strong distortions, which caused a control MOSFET to get very warm. A special circuit usually used for reducing these distortions was ineffective with any combination of its elements and dissipation power on these elements was too much.

I sat and looked on an oscillograph and cursed cores, my destiny, the boss with his idea, and these converters. I wanted to investigate a circuit to provide supply voltage to the control IC of the converter on the auxiliary winding. Accidentally, I took off a rectifier diode from this circuit that was an SMD. I do not like to solder small SMDs, so I took an old diode with terminals and placed it in the device.

When I looked at the oscillograph I could not believe my eyes–the strong distortions decreased! The pulses on the drain of the power MOSFET were suitable for proper operation of this converter. I replaced my old diode (with low recovery time) with a modern, fast diode and strong distortions appeared again. Using a diode with low recovery time decreased the distortions.

I needed to provide some shunting using the auxiliary winding from timing of the arising pulse in the primary winding and this converter would work. Replacing a fast diode with a low-recovery-time diode was impossible because the diode dissipated high power, but I found another simple and suitable solution.

We did not have to use all expensive cores because I found a new solution for reducing parasitic oscillations in flyback converters1. Naturally it was after a complete investigation of these processes, but this idea was used a few times in different flyback converters and I always had excellent results.


Reference

1. Rentyuk, Vladimir, "An easy way to reduce parasitic oscillations in flyback converters," Electronics World, October 2007.

Vladimir Rentyuk is a development engineer at Modul-98, a developer of embedded electronic systems, robotic equipment, image recognition systems, and other products for European and world markets. He holds a master's degree in radio engineering from Zaporizhzhya Machine Construction Institute (now Zaporizhzhya National Technical University) in Ukraine.

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