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12V primary voltage SSTC's

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Andri

Sat May 23 2009, 01:42AM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

I'd be surprised to see something like that work.

Why this will not work:

That gate driver will not bring the GE voltage of the upper devices (in the H-bridge) to the level needed.
Alternatively to #1: The upper devices should be P-type.
For such a small coil, the resonant frequency is probably way higher than any IGBT can handle.

To explain point #1 further, the current through an IGBT is controlled by the GE voltage (GS for FETs) and what you appear to be controlling is the voltage between gate and ground. This is fine for the devices of which emitter is connected to ground but it will not work for those of which the emitter is connected to the load. The most common solution for this problem is a GDT (there is an excellent article on the wiki on GDTs).

Going on to point #2: As I said before, the GS voltage controls the device. Placing a P-type device in place of the upper devices, with source facing Vcc eliminates the problem discussed in point #1. For better understanding of this principle you should study push-pull output stages (google). Mind you, there are no P-type IGBTs available. You only get P-type FETs.

One of the benefits of a low-voltage design is that you can use P-type devices (which are otherwise hard to get, expensive, have bad current rating, etc.). So you should use that. There isn't even a reason to use IGBTs in a design like this. As you go lower and lower in voltage rating the current rating of FETs goes up and on-resistance goes down. Another benefit of using FETs is that they switch much faster and thus your coil might even work in the MHz range which eliminates the problem of point #3.
I suggest you use IRF540 (n-type) and IRF9540 (p-type). They are a complimentary pair of HEXFETs from International Rectifier.
The option of using a GDT or a level shifter still remains though.

Now, you're only making things harder by using an antenna. Just use a function generator to begin with. You can try adding an antenna later when you have sparks flying.

RichM

Sat May 23 2009, 10:06AM

Registered Member #950 Joined: Sat Aug 11 2007, 02:11PM
Location:
Posts: 8

Ok, I see what you mean. I did try with a function generator and it seemed to switch ok. I guess the current was lower than excpected due to one device in each pair not being fully turned on. I did also notice uneven heating. Now I see why!

I was using IGBT's because I read that they have a linear increase in temperature with current rather than an increase proportional to the curret squared. IT should be mostly ZVS anyway so I'll try MOSFETS.

Just curious.... Is it possible to use an IC such as a L6384E to drive each pair in a full bridge? This has a gigh and low side driver.

LithiumLord

Sat May 23 2009, 12:30PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Well, if there is no GDT or other couplers (I thought you missed those out on the drawing alone in order to simplify the blueprint) you surely need one.
As for the driver you mentioned - it's pretty nice to use as a deadtime generator, however the bootstrap feed along with such a low gate current and unipolar drive isn't a very good thing for SSTC usage. I'm sure it will not do much harm on 12v though, but the frequency will be too high for it. For the driver design you can take a look through my project thread - those are pretty good for high switching rates as are both capable of driving higher currents, 8A per gate in my setup - and even more in case you use a higher-rated driver IC and use a bipolar drive mode unlike most halfbridge/H-bridge integrated drivers do. Also they will provide you with enough headspace for higher bridge voltage.

IGBTs with no ZCS and no bipolar drive are not a way to go for sure, watch this

to see how during an older test run an open-loop unipolar driver with an H-bridge of IRG4PC50W's performed in a coil that used to work perfectly with a bridge of IRFP260's and now runs off-wall with a feedback driver and bipolar gate drive with 500v FETs without a single failure ;)
Also, do not forget about the IGBT safe range curve - without ZCS running off 12v you can easily fry a 600v IGBT - even faster then off-wall.

Andri

Sat May 23 2009, 06:06PM

Registered Member #1533 Joined: Wed Jun 11 2008, 02:13PM
Location: ReykjavÃk, Iceland
Posts: 46

Yes, you can use an L6384E or any other gate driver with sufficient current ratings and a level shifter.
A high-voltage bridge driver such as the L6384E shifts the input voltage (which is relative to ground) so that it becomes relative to the HV rails (which aren't particularly HV in your case, though).

When choosing between IGBTs and MOSFETs you also need to think about switching losses. Switching losses of MOSFETs are negligible in comparison to conduction losses but switching losses are a significant factor in the power dissipation of IGBTs.

Here's a picture I drew yesterday to compare the IRFP460 with an IGBT with "similar" ratings.

1243101311 1533 FT69883 Irfp460vsirg4bc30w

As you can see the IGBT becomes a better choice anywhere above 25A (continuous) but I doubt you will be going anywhere above 10A (continuous) with your coil. But that's not the point I wanted to get across with this picture (not at least in this post). What I wanted to show you is how the power dissipation of IGBTs and MOSFETs depends on current and frequency. This is universal for all devices.

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