EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: CopperCone on May 29, 2018, 04:16:25 pm
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So my system topology looks like it will be a
Mains protection stuff -> isolation transformer
Then diode bridge and capacitor
The isolation transformer will act as a redundant semi safety. The primary control on the capacitor and diode bank inrush will be a beefy switched in resistor on a time delay.
This hvdc will be chopped up by a igbt h bridge feeding an impedance matching stepdown transformer into the work coil.
What options exist for protecting the signal path between the filter cap and the igbt?
I figure it would need to be an active device, all i can think of that is fast enough is some kind of crowbar.. Are there any likely components to use here?
The only thing that comes to my mind is some kind of trigatron or krytron tube that can discharge a 24000uF cap at 270.
I assume it might be one use only. I imagine most semiconductor devices blowing up?
I thought also to artificially raise the impedance of the capacitor and live with the losses
I would need to take operational measurements on a prototype to get a better idea of what could work... But what are options?
I have not thought about active protection between the h bridge and the tank coil yet, other then the control system operating right. Are there consieveable supervisor independant circuits here that can prevent explosion from bad programming?
It seems too complicated to do anything other then make sure the pll works. I would like a simpler more reliable solution that supervises a more complicated solution like a pll or mcu control.
I suppose i can just double up and make some kind of redundant logic device which requires independant measurement concurrance but i prefer diversity.
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With a voltage source inverter, it is often easiest to just turn off the IGBTs if an overcurrent situation is detected. If you are confident that the driver can only turn on one IGBT per bridge leg at a time (also in the presence of EMI and during startup and shutdown), a current transformer on the bridge output rigged to a fast hardware shutdown is good. Otherwise, desaturation detection per IGBT is a good approach, and you can buy driver chips that support this directly. Tim Williams' (T3sl4co1l) excellent induction heater web page http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat6.html (http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat6.html) has a discrete implementation of desaturation detection.
If you do want a crowbar as a second line of defense, it will be a lot easier to do so if you drastically reduce your DC bus capacitance. The only advantage of having a large amount of DC bus capacitance is a reduction in the output power ripple in the heater, which rarely matters for workshop use, and a lower peak voltage seen by the tank capacitor for a given heating power. The disadvantage is significantly reduced power factor (unless active PFC is used), more damage potential from stored energy, and increased cost and size. Induction cookers usually use a tens of microfarads or less of DC bus capacitance at several kilowatts, resulting in almost perfect power factor.
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doesnt it isolate the tank from the line better? a big capacitor I mean.
The drivers I have include De-sat protection.
I figured maybe its possible to make a crowbar that also disconnects the capacitor, with some timing.. but it seems tricky
like linear mode operation to take the edge off the peak current seen by the denergization circuit. I thought it would be unwise to open up something drawing like 20 amps @ 300V