Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
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My tester has two diodes (Dula diode common Cathode) to the left of the MOSFET (IRFP260).
Same sort of failures showed up in aerospace applications.
This power MOSFET spec'd to dissipate up to 280W, suitably heatsinked.
Safe Operating Area (SOA) shown for various pulse widths, but graph does not include lines for DC as data sheets used to show.
Device is constructed with trenches for extra area and behaves like many MOSFETs in parallel. Problem is thermal runaway where one area gets hotter, carries more than its share of current, gets hotter, eventually fails.
https://linearaudio.nl/sites/linearaudio.net/files/NASA_Shue_On-Spirito-20100014777_Apr-2010_PDF.pdf
EEVblog #1023 - Rigol DL3021 Electronic Load Teardown - Page 2
EEVblog #1023 - Rigol DL3021 Electronic Load Teardown - Page 2
www.eevblog.com
Linear MOSFET | Microsemi
A MOSFET specifically designed to be more robust than a standard MOSFET when operated with both high voltage and high current near DC conditions (>100msecs).
www.microsemi.com
Figure 6 of NASA paper shows additional lines sloping downward, cutting off at some Vds each of the pulsewidth lines.
I saw reports that even where modified SOA said it should be OK, thermal runaway occurred.
MOSFETs either have to be kept quite cool or be operated well within, perhaps orders of magnitude within SOA.
Sounds like these battery testers are being rated 150W or 180W with a 260W transistor (rating assumes case held at 25 degrees C)
With 150 degree C max junction temperature, 260W indicates 2.2 W/degree C.
"Linear Derating Factor 2.2 W/°C"
This simply explains that power dissipation decreases with ambient temperature, because not as much power is needed at higher ambient temperature to reach maximum junction temperature. Same analysis I always do for any electronic component. It does not address thermal runaway during linear operation.
Safe operating area - Wikipedia
en.wikipedia.org
"Some (usually expensive) MOSFETs are specified for operation in the linear region and include DC SOA diagrams, e.g. IXYS IXTK8N150L."
When you use your battery tester for higher voltage packs instead of single cells, more likely to violate the modified safe operating area.
The Vishay part identified doesn't document a safe DC operating point.
You could try repairing your failed boards with a transistor that does specify DC operation. Also allow more margin especially at higher voltage.