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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, that's up to you as the designer, you can design for whatever level of ripple current you want, by choosing the switching frequency and the inductance. I imagine you can run a powdered iron core at a very high AC flux density, because of the low duty cycle. A QCW spark lasts maybe 10ms, but the rep rate is limited to a few per second because of heating in the IGBTs.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi again,
I just have one more question for goodchild -
what is the reason for using the apparent deadtime-generator circuits (74hc14 gates) between tl494 and the gate drivers? Are those really necesary with the controller that can generate deadtime?
I was thinking of simply using SG3525 with some drivers on the outputs and a gdt, would that work after all?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hello again and again, because there's one more important thing I'm wondering about now... What would be the proper way of sensing the output current of such a converter? I wondered about whether it is possible to implement a CT but that doesn't seem likely because the inductor current has a continuous DC component. Is a hall sensor an only option for overcurrent protection in this case?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Richie once told me about a circuit where two CTs are used, one in series with each power switch. The outputs of them are ORd with HV fast diodes in the wanted direction, and they're allowed to generate a huge spike in the other direction to reset the cores quickly. The result is a ghetto DC CT that works from maybe 10 to 90% duty cycle, but will "fail deadly" outside of that.
In this case I'd just put up the $$$ for a Hall current sensor to save hassle.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Figures 8 and 9 on page 3-279:
Works well from about 2% to 98% duty ratio if you're trying to implement average current mode control.
There are also tricks you can play with the current sensing CTs if you have two interleaved converters (buck or boost.) You can pass the current-carrying conductors from each switch through the same CT in opposite directions. This makes the CT see an alternating magnetisation with no net DC, and you full-wave rectify the output from the CT to reconstruct the switch current.
A cheap compromise solution for low power buck converters is to just include a non-inductive current sense resistor in the ground return path of the load. You then get a positive voltage proportional to load current measured across it relative to the switching side ground, or alternatively a negative voltage proportional to current measured relative to the load ground.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hm, thanks steve. I was wondering what was the best type of hall sensor to use, an enclosed one that is like a CT (first one), or a monolithic IC like the second one? I suspect the IC might be more accurate but also more prone to external magnetic noise. I'm not sure if it has a ferrite core at all inside it.
The third is a board from india that is very cheap for some reason, and it seems to have a sensor with core on it installed, not sure about it's performance.
All sensors seem to have 50-80A range, although I think I could use either parallel inductors or a multifilar wound inductor and measure the current of just one 'file' to be able to measure up to hundreds of amps.
PS. The BELL sensor has a superior bandwidth and it costs only a dollar more than the IC, and I think I'd most rather go for it.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sorry it took me so long to get back.
About the dead time circuit: I use a CM600DU-24NFH in my sync buck. This brick handles a ton of current but is also very slow. I had to add dead time to the gate drive to prevent shoot-threw condition on my buck. I believe I used 800n to 1u dead time for this brick.
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Pulsed power buck converters for DRSSTC drive
