TL494 flyback driver


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.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!


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.



21 Responses to TL494 flyback driver

  1. Alex says:

    Is that 10 turns primary on a 140V DC bus? no wonder the flyback arced over lol.

    Did you leave the core spacers in or remove them? Its just I have read conflicting information regarding the core spacers when using bridge drivers.

  2. Mads Barnkob says:

    Hey Alex

    I was running it at violently high voltages, yes, 10 turns 140VDC, poor little flyback 🙂

    I did not remove the air gap and it is my understanding that you use the air gap in chokes as you want to store energy. While in transformers you have no gap as you want to transfer energy. Flyback transformers are usually described as a choke with primary and secondary coils due to their purpose. A cheap energy transfer that requires little circuitry.

    When using a bridge to drive a transformer there should be no air gap as we do not need to store energy as we transfer energy to it for each cycle.

    Kind regards

  3. Alex says:

    Hi Mads.

    I did some experimenting and found that the gap seems to prevent the core from going into saturation, but at the cost of increased MOSFET heating. For example without the gap and a DC blocking capacitor inserted in series I was still able to saturate the core from just drawing arcs (current draw shot up).

    But with a gap it seemed to give some current limiting action but at the expense of more MOSFET heating.

    Magnetics, they are black magic lol.


  4. Pingback: TL494 flyback driver | Kaizer Power Electronics

  5. Yuri says:

    It’s not a magic – it’s a lack of snubbing. This bridge have no effective snubber (parallel diodes are not enough) and energy stored in load inductance is heating up MOSFETs. The air gap have two functions:
    -Makes core saturation harder to achieve
    -Increase energy stored in core
    First function is alway wanted, but second increase “inductive kick” heating up poorly protected MOSFETs. However, bridge designed like in attachment should have no troubles since energy disharged from load will be safely disposed (this circuit is popular among local tesla coil builders, compared to simple designs like showed here it greatly increase stability and decrease MOSFET heating while offering also much more durability ).

  6. Kit says:

    I’m slowly gathering information on how to build a CO2 laser to be used in a CNC laser cutter. The HV power supply is a critical part of the design and this looks a very promising design to use.
    I would want to put a rectifier on the output of the flyback transformer to achieve around 30kV peak DC to make the tube strike and then maintain approximately 30mA current during lasing. I have my own ideas on how to achieve a safe way to monitor output current and provide feedback to the controlling chip.

    I’d be very grateful for any comments you have on this idea, particularly regarding the use of gaps (or not) in the core and whether these cores would saturate in this application.

  7. Mads Barnkob says:

    Hi kit

    The driver itself will be able to deliver 900W as your design goal is. A smalle flyback transformer from a Tv set or computer monitor will not. Maybe you can use more transformers in parallel or as a last resort wind your own on a large ferrite core.

    Kind regards and happy new year

  8. Kit says:

    Thank you very much for the prompt reply, and a happy new year to you too.

    This is going to be a long project so I think I’ll build and test the power supply with a standard transformer core and then experiment with alternative cores once I’m somewhere near having a working laser tube.

    Thanks again


  9. hvlaser says:

    Hats off to Mads for providing such a great place to find this kind of information!

    I also want to say hi to Kit. Over the past few weeks, I’ve also been designing a power supply to drive a CO2 laser. Much like you, I’ve been researching this area heavily. I currently have a working machine, but my aim is to make a second one as much DIY as possible. If you like, feel free to reach out to me to toss around ideas and share insights.

    Thanks again guys and take care

  10. kit says:

    Hi to hvlaser!
    I’ve seen DIY lasers using Tesla coil style PSUs on YouTube, but, like you I’m sure, I want something with a more stable, measurable and controllable output. A lot less electrical interference so close to the computer and other digital circuitry performing the CNC function would be a good idea as well.



  11. Mads Barnkob says:

    Hi kit and hvlaser

    I hope you find some good solutions in cooperation and please return with a link to your work if you happen to document it on your own site or some forum.

    I am looking forward to see what you get to.

    Kind regards

  12. KhaSieu says:

    Hi Mads Barnkob,
    I was participate in my school technology competition for the previous time (I won the second prize! And there’s no first prize) and haven’t got much time to see this page, today I saw it and feel I really need to build one.
    I have some advise: why don’t you use the built-in primary? Usually it’s the two first pin on the left of the flyback, the first pin is gnd and the second one next to it is Vcc (in this case :140V DC input).
    In any case I think we need to ensure as least 3-5% dead time to be on the safe side. Then over current protection should be added as well, voltage regulation is always welcomed.
    The circuit show that currently the switch is wired in a half bridge configuration, making a full bridge will increase output power. To make it work with 220v lines then what could I do? Note that the built-in primary couldn’t withstand 220v directly. Wire two primary in series and the secondaries in series?

  13. Mads Barnkob says:

    Hi Seiu

    Congratulations on the second prize 🙂

    The built-in primary coil have too many windings for what we want to do. It is perfect for its application of having a voltage for CRT operation and where there is high coupling. Instead we use much fewer primary turns to get some proper current and energy dumped into the transformer.

    If you want to feed a high voltage transformer with 320VDC from just rectified mains, a offline driver is the name for that, you will need to do some proper calculations and make your own transformer from scratch. There is simply not enough window room on a flyback transformer to fit enough primary windings in at the power levels you want from using mains.

    The TL494 got built in dead time and you can use one of the two built in op-amps to add current limiting.

    Kind regards

  14. saattvik thourwal says:

    The driver you are using is used to drive tesla coils. No wonder the flyback died !!

  15. Mads Barnkob says:

    Hi saattvik

    I killed them all on purpose, not as much of a test of the transformers, but how sturdy the driver and bridge was in regard to driving transformers.

    Kind regards

  16. KhaSieu says:

    Hi Mads,
    I have build it, but suffer from the lack of parts, particular the driver IC. I used the C2383-A1013 pair for an alternative for the IC but that don’t seem to work out. The waveform on the scope is attached, can you give me some suggestion? I think increase the number of transistors in parallel with the output of the chip (and hence, the current capability?).
    I see you are using a MKP-type capacitor in the oscillation section of the IC, I’m using a ceramic capacitor with the same value, I wish to know if it have any negative effect here?
    Finally, what have happened to I’m trying to register, but I’ve tried dozens times but I got no email about exactly how to activate my account. And “send a email to forum at”??? How could I send such a email?? There’s no email address??

  17. Mads Barnkob says:

    Hi KhaSieu

    You should use the small transistors to drive a output stage of some TO247 MOSFETs to get a solid and sturdy GDT driver to replace the driver ICs.

    It is no problem to replace the MKP capacitor with ceramic in the oscillator, ceramic is really only inferior to MKP when it comes to higher currents.

    The email is written in “spam” protected syntax, what it means is “forum” @ “”, put together.

    Kind regards

  18. KhaSieu says:

    Hi Mads,
    I didn’t understand, so I need to use those TO-92 transistor to drive some TO247 fets, then the fets provides the driving power to the GDT and then the GDT drives the half bridge!?

  19. Mads Barnkob says:

    Hi Sieu

    Yes, looking at it simply you are just building amplifiers that are strong enough to drive the GDT.

    You should have no problems finding schematics of this via f.ex. google.

    Kind regards

  20. Marek Fiala says:

    Hey there,
    I built this TL494 oscillator just as a test on a breadboard, with the pullup resistors at the output collectors as shown in the schematic, but found one problem about it. Using the output this way, obviously, shifts the output waveform by 180°. But, therefore, it also affects the built-in deadtime, which is now inverted, and instead of making sure the two outputs don’t overlap each other when switching, it does exactly the opposite! (see attachment drawing) I haven’t got to the stage of winding the GDT yet, i just hooked it up directly to the MOSFETs just to probe the outputs and see how it behaves. If i use a GDT, the problem can be solved just by reversing the secondaries as needed, but the problem is that the “safe” stage of the driver still suffers from a brief short circuit during the “not-dead-time” caused by inverted outputs. I decided to hook up the driving stage of the MOSFETs (just a couple of BD139s/140s) in between the IC emitters and GND, is this okay to do? Also i decided to remove the 1k resistor from the divider where you regulate PWM, that should not cause any problem either, or does it? My understanding is that at 0 Volts the duty cycle should be 50%.
    Thank you.

  21. Thank you very much for sharing this to the world. Greatly appreciated! Keep up the great teaching, professor!!!!!!!

Leave a Reply

Your email address will not be published. Required fields are marked *