EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: cryptoprojects on November 12, 2022, 05:36:43 pm
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Hi all,
we are all familiar with the standard transistor avalanche based pulse generator:
https://www.analog.com/media/en/technical-documentation/application-notes/an72f.pdf (https://www.analog.com/media/en/technical-documentation/application-notes/an72f.pdf)
However, the 2n2369 transistor is not designed for avalanche operation, they are very hard to get at the moment, and I just killed the last 2N2369 I had in my parts bin.
MOSFET transistors are designed for avalanche operation and usually have avalanche ratings published in the data sheet.
Therefore I tried building a pulse generator using a 60V MOSFET, the STP55NF06L. Despite a 150-180V DC power supply voltage, I cannot get the transistor to break down. I tried open circuit as well as resistances from 2.2k to 50k (swept using pot) from gate to ground.
Are MOSFETs fundamentally different in what it takes to trigger avalanche breakdown? Any suggestions?
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I don't know if anyone has had any success with avalanching MOSFETs in this application, you could maybe try biasing the gate a bit higher but I don't know.
People have had success avalanching other bipolar transistors, eg the (avalanche rated) FMMT413, but also the ZTX415, BSX20, and others, even the BC547...
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/msg127582/#msg127582 (https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/msg127582/#msg127582)
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This circuit seems to rely on collector avalanche current entering the base layer, forward biasing the BE junction and effectively turning the BJT on for a moment to quickly conduct large current from the collector to the emitter at low voltage drop. I suspect that using the BC junction alone (or any other random diode) in the transistor's place wouldn't be nearly as effective.
While MOSFETs also contain a parasitic NPN/PNP structure, the base and emitter (body and source) are shorted internally. Accidental triggering of the parasitic bipolar device can occur during avalanche or fast turn-off, resulting in prolonged conduction of current after turn-off, large power dissipation and destruction of the device. Every effort is made to disable this parasitic transistor, even under abnormal conditions.
International Rectifier published full application note about avalanche ruggedness. It's the opposite of what you want to achieve.
https://www.infineon.com/dgdl/an-1005.pdf?fileId=5546d462533600a401535590ab660f3a (https://www.infineon.com/dgdl/an-1005.pdf?fileId=5546d462533600a401535590ab660f3a)
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Magic said what I was going to.
Avalanche in a MOSFET happens with the parasitic collector-base junction which forms the body diode, and since the parasitic base-emitter junction is deliberately shorted, the amplification is not enough.
Try other transistors, including RF transistors. Zetex even made a transistor specifically for avalanche operation, so try some of those "super e-line" transistors.
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Thanks David for the very clear explanation and thanks Magic for the link, that's exactly what I was looking for. Too bad!