TL494 flyback driver

Published on: Jun 14, 2013. Updated on: Nov 28, 2017.


I wanted to design a versatile driver circuit that could drive a half- or full-bridge of MOSFETs or IGBTs through a gate drive transformer (GDT). This should make a driver that is able to run flyback transformers found in CRT TV sets and computer monitors.

The TL494 IC is designed for maintaining all the functions needed in a switching mode power supply using pulse width modulation (PWM). The output transistors can be run in either single ended mode or push-pull. The pulse width is normally controlled through a feedback signal in the power supply, but for this project we want to control it manually, this is done differently in almost all schematics found.



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Flyback transformers from a CRT TV are typically driven at 15 kHz and flyback transformers from computer monitors are typically driven between 30 to 150 kHz.

The TL494 IC uses a 5% dead time to insure proper switching and at frequencies over 150 kHz this minimum dead time is higher.

The design goals for this project will be a driver with a variable duty cycle from 0% to 45% and a variable frequency from 50 kHz to 150 kHz.

This should make for a efficient driver and one that works out of the audible spectrum. In order to design with components at hand, the frequency span is not going so low as 15 kHz.



Voltage supply IRFP250N: 0 VAC to 120 VAC
Frequency span 38 kHz to 150 kHz.
Duty cycle span 0% to 43%




25th May 2009

The breadboard prototype is ready to be tested, the tape is to hold the timing capacitor in place since the legs on it was too short.

In the first oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 43% duty cycle.

In the second oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 5-7% duty cycle.

In the third oscilloscope shot we see the output waveform without pull up resistors, it is about 150 kHz at 43% duty cycle.


27th May 2009

PCBs was made for both the driver and half-bridge section. The full bridge rectifier used here in the pictures is only rated for a mere 4 A. This is not enough for running a flyback with low input voltage and high duty cycle. A 25 A bridge with heat sink should be used to ensure some overhead.



29th May 2009

In the oscilloscope shot we see the waveform of the primary side of the GDT driving a MOSFET half-bridge. To test the circuit I first used a old half-bridge I had from an earlier project.

The sturdiness of this new driver shines through when I killed a flyback transformer due to over-voltage on the secondary side. Corona glow can be seen in the center towards the ferrite core.



This universal inverter makes it possible to adjust the output voltage and current exactly to ones needs. It makes a great and much more sturdy flyback driver than many simple drivers with just a single transistor, which is of course no surprise as it implements its own control IC, MOSFET driver ICs and a half-bridge of MOSFETs.

For a final constant voltage or current power supply it will not work, as there is no feedback adjusting the pulse width to a certain load.