Driving a CRT monitor flyback transformer with squarewave

Wiebe Cazemier · 2011-04-01T18:52:32+00:00

Hi,I'm working on designing a flyback transformer driver and I have a few problems. It's been a while since I've been in sci.electronics.* and I've come to ask for the expertise here again :)My driver is an Atmega 168 mictrocontroler, driving a IRF840 (500V 8A) MOSFET through two stages of BC547/BC557 transistors. It drives the flyback in pure flyback mode (so no resonant circuits with capacitors and such). My problem is dealing with the back-EMF. Right now, I have 150R 15W resistance with a UF4007 75 ns diode. With this, not only does it create an enormous amount of current in the back-EMF ballast when the secondary winding is open, it also creates high voltage spikes, often destroying my 500V 8A MOSFET. And, I feel that it is severly limiting the output of the secondary winding. My squarewave is from 0 to 50V 90% dutycycle, yet breakdown distance of the transformer's output is about 1.5 cm. When the flyback was still in the CRT, it was about 2 to 2.5 cm, and that is with a sawtooth drive signal, putting less energy in the core than a squarewave.In an old style CRT TV, the flyback driver circuit does not just generate high voltage by collapsing the magnetic field, but it creates a resonance circuit with the retrace timing capacitor (also called safety capacitor). This prevents having large back-EMF problems. My first question is: how does a multiscan CRT monitor (any not-very-old VGA and better) drive its flyback transformer? These things scan at 30-150 kHz, so it's kind of difficult to make a resonant circuit with a capacitor, because that would only work at one frequency.My second question is dependent on the answer to the first, I guess, namely: how do I handle the back-EMF efficiently? Can I do it like a CRT monitor does? I'm going to try 'transorbs' (I've ordered some). These are basically very fast bipolar zener diodes, designed to suppress transients. Perhaps if I use a 400V version to shunt the back-EMF (meaning limit it to 400 volts), there will still be...

Apr 01, 2011 82 Replies

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cbarn24050 15 years ago

low Ron

IMHO.

much

ing.

voltage

the

ximal

rate

rive,

hem

g it.

As stated the upper device is a deflection transistor ( bjt ).

You would normaly bias it from the low voltage supply via a suitabe resistor to give enough on state base current.

Yes these transistors are rated at a least 1500 volts, this is exactly what the op wants.

In the on

I dont see what your talking about here!

Vote

Fredxx 15 years ago

I have no idea, but it was in the early days of solid state TVs. I would guess they went for the most cost effective method to cope with the flyback voltage.

Vote

josephkk 15 years ago

low Ron

work, IMHO.

at much

switching.

voltage

(like the

maximal

saturate

drive,

transistors?

the

them

help

describing it.

anyone

=20

Glad to find someone was awake. The term for HV V(ce) splitting usage is totem pole. Also used (borrowed) to describe TTL outputs by similarity. Doing totem pole with MOS transistors includes some interesting design trade-offs.

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