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Flyback Driver, Half-bridge

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NovoRei

Tue Feb 23 2010, 12:31AM

Registered Member #2468 Joined: Mon Nov 16 2009, 02:09AM
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Posts: 2

Hi.

Im having problems with this topology.

The driver is TL494 directly connected to mosfet gate.

Then B+ is between mosfets and B- have 2 caps like this:
Halfbridge

Running at 12v. Opposing Zeners protect gate.

I do not know the inductance of primary but the matching cap is at 2uf-0.1uf with a resonance at ~6khz.

Problem: Very small spark, Low current too ~100ma.

Increasing voltage or winding a new primary would fix?

Below is gate and b- at ~1khz.

radiotech

Tue Feb 23 2010, 06:29AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

I was curious why you chose the full wave voltage doubler after the variac.
This places the 0V AC line at +200 volts DC if the midpoint of the capacitors
gets grounded to the case. A half wave doubler would make -400 the same
potential as 0VAC.

hboy007

Tue Feb 23 2010, 03:39PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374

Hi there,

I have some experience driving flyback transformers. First of all, many are intended to be driven with a primary voltage of 70-150V.
The internat diodes permit operation at output voltages of +21 ..+48kV depending on the model. Normally, flyback chokes (actually, they should not be called transformers)
are driven by a single switch in a flyback topology, symmetrical push-pull drivers excert additional stress to the rectifier diodes because the negative half-wave swing of a flyback
transformer under load is dampened. There is no full-wave rectification so either you're interpreting the pull phase of the half bridge as a demagnetizing pulse (check the timing and
pulse duration on the scope and perhaps limit it to a few Âµs with a monoflop, use high voltage MOSFETs and TVS diodes / RCD snubbers in parallel) or you run the risk of reducing
the internal diode lifetime drastically.
For example, to obtain 30kV output in push-pull topology with a bit of safety overhead of 30% the diodes will have to withstand 26kV in reverse each (assuming you use 3 diodes).
The diodes normally used can take 16 or 20kV.

Coming to speak of your circuits, really high voltage output is only obtained when the choke resonates, either with its parasitic intrinsic winding capacity or with a small external cap. The
oscillation frequency is high compared to the driving signal that takes some time to ramp up the primary current (expect 50-150kHz oscillations). If you're not getting high voltage, try increasing
the primary voltage to ~100V.

Antonio

Tue Feb 23 2010, 04:31PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

"The driver is TL494 directly connected to mosfet gate. "
This does not make sense in the picture. Try to post the exact structure, otherwise all is guesswork.

ragnar

Tue Feb 23 2010, 07:13PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

NovoRei wrote ...

The driver is TL494 directly connected to mosfet gate.

Yes, a bit vague. Aren't TL494 currents open-collector? You'd generally put a MOSFET driver between the TL494 and the gate... or do you already have that? A photograph of your circuit or a sketch of your exact schematic would help greatly with the remote diagnosis.

NovoRei

Wed Feb 24 2010, 02:08AM

Registered Member #2468 Joined: Mon Nov 16 2009, 02:09AM
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Posts: 2

Thanks for the help.

Im using pull-down to ensure off state. Not the best solution but should work.

I think the problem is at the bridge, the upper gate dont work properly. In fact, Vgs excursion from 0v to ~-1.2v .

#D3 is connected to D1 anode.

ragnar

Wed Feb 24 2010, 03:25AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

NovoRei wrote ...

Im using pull-down to ensure off state. Not the best solution but should work.

OK... at low frequencies... kinda...

NovoRei wrote ...

I think the problem is at the bridge, the upper gate dont work properly. In fact, Vgs excursion from 0v to ~-12v.

Of course sir -- if you wish to connect two MOSFETs to a TL494 like that, you would need a gate driver and transformer or a level shifter!

You could consider a push-pull topology (MOSFET sources at 0V, drains to respective ends of a V+-center-tapped primary), although you run the risk of something cooking at DC if other parts of the circuit aren't working.

hboy007

Wed Feb 24 2010, 03:42AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Blackplasma,

I tried the hi-side driver the day before yesterday (see

My conclusion: you're better off with an IR2110, the circuit I refer to happens to be in the datasheet of said device.

The important thing is to get the base signal relative to the substrate which happens to be connected to the source pin. Otherwise the input in NovoReis spice schematic would have to be pulled to some volts above the positive supply rail which is not an option.

Many guys I know like to use gate driver transformers with two secondary windings to drive both mosfets at a time.

hboy007

Wed Feb 24 2010, 04:06AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Here is what I mean. If the frequency gets too low, the high side shuts off as the capacitor is drained.

ps. the internal transistors of the TL494 are floating, you can tie them to whatever potential you like within 0 .. V_supply, but they are not push-pull-outputs.

Steve Conner

Wed Feb 24 2010, 10:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

NovoRei wrote ...

Thanks for the help.

Im using pull-down to ensure off state. Not the best solution but should work.

I think the problem is at the bridge, the upper gate dont work properly. In fact, Vgs excursion from 0v to ~-1.2v .

#D3 is connected to D1 cathode.

If you're trying to copy my old high-side thing, you've got it wrong. There's nothing to pull up the top gate.

I didn't design it as a flyback driver, but some people decided to try it.

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