Design flyback


Joined Aug 12, 2011
A flyback converter normally has an energizing MOSFET on the primary and a rectifier diode (or MOSFET) on the secondary. I assume you mean the diode on the secondary.

The diode is necessary to prevent reverse secondary currents since primary voltages pulse from zero to output at high frequency. A SEPIC converter (variation of flyback) pulses below zero.

An asynchronous DC-DC converter uses a diode rectifier. A synchronous converter uses a MOSFET rectifier. Synchronous converters can be as much as 10% more efficienct since much less power is lost in a MOSFET than a diode. However, they require a synchronous controller that provides a gate signal for the MOSFET.

Control systems are difficult to design for DC-DC converters since they need to vary a PWM signal at fairly high frequencies. A good pwm controller is usually in the 500KHz area. Normally, you would use a PWM controller chip to provide the PWM signal and voltage control. The TI TPS43000 is a good example.

There are two modes of control, voltage mode and current mode. Voltage mode is harder to design, but more efficient. All converters require some kind of feedback compensation since ouput capacitance and inductance create a low pass filter with unstable characteristics. There are some specialized PWM converter chips that simplify or remove external feedback compensation through various approaches. The MAX1709 is one example, but there are lots of others.
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Joined Jul 9, 2011
Which diode are you referring to? Can you provide a schematic?

In a flyback converter you may not absolutely need a diode on the transformer primary. You can however use them as part of a RCD voltage clamp to protect the switching element for example. When the switch turns of you don't want the energy stored in the transformer to be dissipated in a diode, you want it at the output.

Read this:

For the feedback you will usually take one of the output voltages and feed it back to your PWM controller, one way is to use a voltage divider, TL431 and optocoupler... But there are other ways too.

First you have to decide on a controller and then you decide how to do the feedback. Google "pdf flyback transformer feedback". Tons of application notes will appear.

Thread Starter


Joined Jun 23, 2008
diode in primary side of transformer which help dissipate energy from leakage inducatance of the primary should be fast reverse recovery my question is what if slow what will happen
and second question what is the different between force revese recovery and reverse recovery in FET OR IGBT