Driving a CRT monitor flyback transformer with squarewave

Well, if the voltage is equal to what it was in the CRT monitor, I'm happy. I'm not trying to do anything beyond what the flyback was meant to do.

I think I have that kind of voltage now, but not very efficiently, since my MOSFET driving the thing does get very hot. Perhaps it's over-saturation. I will give the scope reading Grant suggested a go, see if I can detect the saturation point.

One thing I have noticed, is that when I have an arc, the arc gets a bigger orange glow when I reduce the duty cycle. At 95% for instance, the arc is a purple line. At 80, it's almost a flame. It also feels hotter (the air above it)

You need to saturate the Mosfet to reduce heat. This also goes to say that you need to select a mosfet with the lowest turn on R you can find.

You really should be current limiting the primary side. There are a couple of ways I can think of to do this. One way not to do it is biasing the transistor, this will generate heat in the Mosfet but can be done if you so desire.

Years ago I had to make a 20kv dielectric tester to generate 1 ms transient pulse across mica capacitors. I was able to achieve this using an automobile coil. I used 2 555's. one was a current regulated switching supply and the other was for gating that end point supply to the primary side of the coil. Mosfets were used on the forefront here since it made perfect sense. I elected to use a low voltage design since it was easier to find Fets that had much lower Ron and a car coil was a perfect candidate for the job.

Jamie

Same time you don't want to saturate the transformer, unless you're making a self-oscillating LV switcher that uses that magnetic saturation point to turn off the gate drive. Then you could regulate the output like a CCFL backlight inverter circuit, there's lots of those on the 'net too. Though they regulate to constant current, you can see the topology. Some here would name it, I forgot the name.

Yup. TV EHT tripler has the xformer, tripler built in, but I've not looked at the driving circuit for years. There's space to wind one's own 12V primary around them. I collected a couple more monitors to dismantle, see if I find a more recent one.

Grant.

The (bipolar in that case) transistor I removed from my own colour TV recently had a rating in excess of 1000V.

Well, you'll only get a high voltage in the secondary while the transistor is off (and then only while there is energy remaining in the field).

At 95% I wouldn't be surprised if the transistor turns on while there is still energy in the field, which would also reduce the amount of energy that can be added, and increase the likelihood of driving the transformer into saturation.

Sylvia.

So Dimbulb IS an engineer!

I used to file them on the neck anywhere near the socket. Used a = jewelers file so it was slow, but no implosions ever. Not that i claim that it = was safe.

I've determined the inductance value of the primary coil of my flyback. I charged it up to 20V through 150R (with a squarewave). It took 30uS for the voltage across the coil to reach about zero. With T=L/R and assuming it takes 5 time constants to charge it, inductance is 900 uH.

By calculating the back-EMF ballast resistor with the same formula and the

Now > You need to saturate the Mosfet to reduce heat. This also goes to say

The IRF840 has Ron of 0.85 Ohm. At 5 A, it would dissipate 21W. I have a feeling mine does more.

I drive it with a two stage of bc547 and bc557. Those have a maximal current of 100 mA (200 peak). Perhaps that's not enough to saturate the MOSFET. But I don't understand this transconductance thing of MOSFETs... BJT's are easier...

That's fairly ordinary. But than you don't get high voltage and low Ron so easy. Just had a look at IRF840, that's too small for this work, IMHO. IR are very optimistic on their claimed specs, I'd be looking at much higher V and I ratings for a MOSFET doing 5A high voltage switching.

If you're using the common pnp + npn emitter follower for gate drive, allow for amp or more drive current, try a pair of 1A transistors?

Grant.

The problem is, that the high current ones (like 40A) have lower voltage rating, like 200V. And there is a problem with the IR product selector. I can't find any >300V MOSFETs with it, even though they exist (like the IRF840).

I'll see about adding some extra drive transistors.

Ron

O.

Why not cascade a bjt transitor with your fet? Maybe one of those tv deflection types might work.

Then simply use a IGBT Transisor. Jamie

If the OP tries cascading, then he needs to put resistors across the drain and source of each transistor, to balance the voltage across them when they're turned off.

Also need to watch the drain-gate voltages.

Sylvia.

Look up the switching transistor from a monitor circuit, 1500V peak, 25A peak (800V open base, 10A continuous, BU2520). What OP is using is nowhere near that rating. Mind you, with a peak base turn off current at max 6A, peak base drive 9A, continuous base current 6A, these puppies wont be the easiest to drive at speed either ;) But they'll do it for years.

MOSFETs not necessarily the best fit.

Grant.

Ron

O.

age

You need to lookup how a cascade circuit works.

If you're thinking about cascaded amplifiers, that's not going to help the OP in the slightest when it comes to managing the high voltage.

If you have something else in mind, post a link to something describing it.

Of course, it's always easy to post vague comments implying ignorance on the part of some other person, while not saying enough to allow anyone to contradict you.

Sylvia.

I have seen two transistors cascaded in a Sony Trinitron, where the voltage experienced between them was split using a capacitor across each which made up the resonant circuit. Each was driven by a pulse transformer.

Did they have problems with high cost or low beta of 1500V transistors?

w Ron

MHO.

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Cascode is what I meant to say, hope that clears it up.

Well, it clarifies your intent.

I don't see that it addresses the problem, though. Referring to the diagram in

and paying due attention to the comment about biasing...

In the transition from the on state to the off state, the voltage on the drain (Vout) that's connected to the transformer primary will rise from near zero to whatever high voltage is produced by the primary. In the on state, the gate of that transistor will also be not far from zero. In the absence of some special handling of the gate voltage in the transition to the off state, the drain-gate voltage will rise beyond the tolerance of the transistor, which will fail.

Sylvia.