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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've looked at the TL494 datasheet here: and it looks like it's just one pot for the frequency (maybe with the ability to change the capacitance value as well, a rotary switch and several capacitors, maybe), and one pot to control the duty cycle, from 4% to 48%, if I remember correctly, by controlling voltage between 0V and 3.3V.
I imagine what Dr DC says above regarding the series resonant topology is a valid comment, and the circuit may perform better with parallel resonant topology, but apart from that is does seem to be a 'brilliantly simple', high power circuit with only one expensive part, the tank capacitor.
Registered Member #8120
Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
There is only one pot that matters - the frequency control one. You want to put a good multi-turn one there, otherwise it's too touchy.
The dead time pot does not do anything useful - the dead time does not propagate through the GDT.
The circuit works nicely with some fixes - damping resistors and zenner clamping on the IGBT gates, and careful layout of the driver board.
It's quite touchy - waving your hand near the driver or the timer gets the frequency off.
With a 10-turn pot it's quite easy to tune the resonance. No idea how hard would it be to track it across things like curie point - i haven't ran it over 40V (~100W) yet, and at that level the steel bolt is heating up slowly, and aluminium stays metal cold.
I don't see anything bad on the IGBTs when the tank is off-resonance, so detuning is a valid power control method. It seems to be cleaner if the frequency is higher than the resonance than when it is lower, which seem to be the same as in IamSmooth's design.
Registered Member #8120
Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
Meanwhile, things are going along nicely with the build. The power section is now built solid, and i'm testing at 50V.
It draws 5A unloaded, 4A with a chunk of aluminium (which heats up eagerly now), and 2A with a steel bolt. So completely opposite to a ZVS heater, which consume nothing unloaded and a lot loaded. Is that normal?
Another peculiarity - with a big chunk of steel it looses power, but keeps the frequency. With a large chunk of aluminium it keeps the power, but the frequency goes up. Why does this happen? I sort of expected the opposite, since steel is ferromagnetic.
Then, i've done some tests for possible failures. Shorting out the work coil does not seem to unsettle the bridge - some ms after the short there are spikes about 10V above the 50V, but nothing that appear major.
Turning off the gate drive while the power is applied does not produce anything catastrophic, but for some while the gates are driven at lower voltage and then chaotically as the capacitors discharge and the GDT currents decay. This might get the IGBTs into linear region or make a few short shoot-throughs, thus doing some burst heating. So, control circuitry must be on before the power is applied, and turned off last.
Am i missing any failure modes or potential issues with modes described?
Ash Small wrote ... Of course. Maybe some type of centre-tapped matching transformer setup could work? (single ended topology)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Artlav wrote ...
Ash Small wrote ... Of course. Maybe some type of centre-tapped matching transformer setup could work? (single ended topology)
Not sure what you mean here.
I think I'm going off at a tangent a bit here, I was just thinking that you could use a TL494, and two mosfet drivers to drive two mosfets (or IGBTs), one on each end of a centre-tapped matching transformer primary, and simply turn up the power using the PWM function on the TL494.
Maybe I should sketch a diagram to illustrate this? I can't see any reason why it shouldn't work, I was even thinking a similar setup could be used to drive the primary of a TC, if it's not already been done.
I'll try and get a diagram together later.
Which circuit are you using? is it the one in the first link in your first post?
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