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Re: Parallel resonant DRSSTC

Original poster: Sparktron01@xxxxxxxxxxx

Hi Sue

What you are mentioning is basically what is termed an 'LCL' inverter (Induction Heating Applications). Using an LCL impedance matching circuit to "boost " primary resonant current (and isolate he**ish peak currents for switching devices) is a worthy goal. The difficulty with such a circuit is if there are load "discontinuities" there could be significant voltage kickback at the switching devices. This is a significant design issue with high power SS induction heating drivers (100kVA - 1MVA +). Here are several references concerning this topology...


Modular, high power SS driver capable that can be scaled as large as your pocket book is deep... ;^D


Design Optimiization Paper for Voltage fed LCL Converter


Richie Burnett's (EXCELLENT!!!!) site with detailed explanation of a LCL Induction Heating Converter. The impedance matching aspect of this design (IMHO) should be carefully considered. Having <100A versus 1kA + going through your power devices might improve reliability just a slight bit... ;^D.

I am doing some proprietary IH work at this time (for work ;^D ), and I can provide numerous references that indicate:
1. The fewer power devices in the power path, the more reliable the converter.

2. A half bridge converter will oustensibly be nearly 2X more reliable (at least half as likely to have a destructive DC shoot through event) as a full bridge, and are commerically built for IH converters up to 25kVA+. I don't think (at last check) anyone in the WORLD has a SSTC / SS Converter that big on the "amateur" level...

3. Placing a DC buck regulator in series with the half bridge as a high speed current limiting/controlling means will also "dramatically" improve converter robustness and tolerance to "crazy" loads. That's why LEPEL, Ameritherm, and other major IH manufacturers use this topology for IH applications for 100-400kHz, and IGBT converters up to 25 (maybe 50? ) kVA. The only way you will damage an LCL converter of a half-bridge topology is if the load "shorts" or you short out the primary (which again IMO is highly unlikely).

4. There are several converter topologies (Neutral Point Clamped [NPC] converter), as a half-bridge, may allow operation to "very high" input voltages (>1kV). If you want high power, I truly believe that is the direction that MUST be taken.

I hope this information is useful, and Happy New Year to the Tesla List!

Best Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

> Original poster: Sue Gaeta <sgsparky@xxxxxxxxxxx>
> Hi all,
> I was curious if anyone has ever tried designing a DRSSTC using a capacitor
> in parallel with the primary rather than in series with it. If something
> like that could work it would have a big advantage that the extreamly high
> primary current would be between the capacitor and primary, and there would
> be almost no current throgh the switching devices.
> Has anyone ever thought of something like this, or played with the idea
> with simulation software?
> What would be the disadvantage of something like that?
> Sue