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Using off-line flybacks in reverse...

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Blueteeth

Well-Known Member
Hi,

A couple of years ago I purchased a job lot of small flyback transformers, originally designed for off-line low power switched mode power supplys, maximum 22W (according to the spec sheet).

Primary is 65-280VAC, secondaries are 5V@3.2A or 12v@1.8A max.

Now, for a while I've been trying to design a relatively simple but versatile 300V supply for capacitor charging and xenon strobes - that is, can run off a wide supply voltage, and gives the user a selectable peak current draw . I decided on flyback topology as the output voltage has such a wide range (0-300v as the cap charges), plus its 'relatively' simple to control using either IC's specific for the job, or even a boost converter IC. After spending some timegetting to know PSPICE, I have simulated some pretty cool peak-current-flyback circuits, which I am confident will be a good start.

Now, the tricky part is the coupled inductor/transformer.
With an input voltage of 5-12V, I'm tihnking a turns ratio of 1:12- 1:15 for a max output of 300V. I'm hesitant to go over this, because my design relies on the reflected voltage on the primary to measure the peak voltage on the secondary - a higher ratio means this reflected voltage will be smaller.

My question is, would it be possible to use an off-line flyback in reverse? I realise its not as simple as using a forward transformer in reverse, since it is down to inductances rather than simply turn ratio. Has anyone done this without having to completely rewind it?

Blueteeth
 

mneary

New Member
If you have the numbers, simulate it. I think the principle would be the same, just the numbers reversed.
 

Warpspeed

Member
The problem you will face is that the flyback voltage on the primary is not just due to the turns ratio, but turns ratio plus any leakage inductance. The primary voltage spike at turn off can be much higher than you may anticipate.

If that is a problem, consider using the diagonal half bridge topology.

The flyback voltage is then positively clamped to the incoming dc supply rail, and any flyback energy is then dumped back into the dc supply bulk storage capacitor.

With this topology, you can run a flyback at full power, with a completely open circuit load, and it will be completely safe. That is perfect for any capacitor charging application.

Just set your turns ratio to suit the relative dc levels, taking into account all diode drops.

It will charge up to the expected design output voltage, and stop there, regardless of any duty cycle control. Bullet proof and reliable...........
 

Blueteeth

Well-Known Member
Hi guys, thanks for the replies.

Simulating is one thing, but real world brings up a whole lot of other issues. Interestingly, The flyback I have, has a ratio of 9.8:1, with a secondary inductance of 98uH (which I'm using as a primary. With a 5V supply I can get a pretty efficient 240V out of it. With peak current limiting I can run it almost to saturation at a whopping 18W. Works like a charm at 12V but I'm after a larger input voltage range (5v to 12v input, 280-330 out), requiring higher turns ratio.

That said, with the primary voltage boosted to 25V for 240 out, I'm still 60V short of my target, so I spent a while 'baking' them in the oven to soften the epoxy holding the ferrites together. I am gonna rewind them.

I have an inducatnce meter (the PIC16F628A version) which I built many years ago, that's very handy, but I still need to measure the saturation current of my new winding. Would this just require a squarewave oscillator with variable on-time and an oscilloscope? Once I can get the sat current up to about 2-3A by gapping/adding/removing windings the secondary sohuld be easy.

I shall post my circuit if all goes well. Its a pretty simple 555 job, used purely as a set-reset latch and a few basic components to make the flyback run at pseudo-resonance. (switches off when primarycurrent reaches max, switches back on when primary voltage drops back down to the supply voltage).
 

bountyhunter

Well-Known Member
One problem: the transformer is almost always wound with the primary nearest the core for best coupling. If you drive the secondary as a primary, it won't be coupled as well to the core. Theoretically, it will work and create voltage, not sure what the transformer specs will be when it's doing it.
 

Blueteeth

Well-Known Member
Agreed! I realised that as I was setting up the test circuit..... I did getvoltage but rather a lot of leakage inductance (ergo: huge spike on the switching transistor) whichI expected, but not as much as as this.

So hey, its not optimsed in its original state, thus the rewinding. Guess I'll have to do this for every transformer I wish to use in reverse...unless its just a quick and dirty test circuit like a zener diode tester or something.
 
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