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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I want to use some hyperfast diodes to block reverse conduction in a mosfet/igbt. They will go from the power supply to the source. Can I combine the same diodes in parallel for twice the current capacity?
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
If by "hyperfast diodes" you mean Silicon Carbide Shottkey Barrier diodes, you can parallel them without problem to double current capacity (SiC negative tempco).
The one thing to remember is SiC diode Vfwd is 1 volt or higher.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I may switch to MOSFETs. They need blocking, right? I was thinking of something like RHRG5060 or RHRG75120 for blocking the mosfet.
Steven, when you say "don't need blocking" do you mean the diode before the SOURCE? I was blowing igbts until I put a blocking diode from the supply to the source, and a reverse current diode from the emitter to the source.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To answer your original question: The diode datasheet should tell you if it's OK to combine sections in parallel. Generally the answer is yes.
Now, IGBT's don't have a source. Their terminals are called gate, collector and emitter.
IGBTs don't have a body-drain diode like MOSFETs. If you take the collector negative with respect to the emitter, it breaks down like a zener diode at 7-15V. You're not supposed to do this, and more than a few amps of current will destroy the device.
IGBTs come either with a co-packaged diode or without. The co-packaged diode is an ultrafast diode on a separate die, bonded between collector and emitter. These are the kind you need for virtually all hobby projects I know of, and they don't need any extra blocking diodes, etc.
If you accidentally bought some IGBTs without diodes, then you need to add them. But unlike MOSFETs, they don't need the series Schottky to block the body-drain diode that they don't have.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
Steve McConner wrote ...
If you accidentally bought some IGBTs without diodes, then you need to add them. But unlike MOSFETs, they don't need the series Schottky to block the body-drain diode that they don't have.
I see that the ultrafast diodes divert the reverse current around the device for both the IGBT and MOSFET. I would think that both devices need a blocking diode to ensure that no current also runs parallel through the IGBT. Does the IGBT inherently not allow reverse current like you alluded to up to a voltage of 15v? Again, if this is the case, why not put a blocking diode that can withstand much higher voltages?
What is so important about the need to block the body-drain diode, but no need to do this for the IGBT?
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
IamSmooth wrote ...
What is so important about the need to block the body-drain diode, but no need to do this for the IGBT?
In MOSFETs, the antiparallel diode is made in the doping process or somesuch. Any IGBTs with a diode actually have a separate die for the IGBT and diode. The mosfet's diode is not very good, but they normally put rather good diodes in the IGBT.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
You can find FREDFETS which are MOSFETS with enhanced body diodes, usually Platinum-doped, to reduce recovery time in the 200ns realm. To use faster diodes requires an anti-parallel diode circuit.
IGBT's can come with a monolithic fast-recovery diode built onto the same substrate as the IGBT or as a co-packaged die, or with no diode at all. The really high performance IGBT modules will have SiC SB diodes. There is essentially no reverse recovery time for these diodes.
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hyperfast diodes
