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More thoughts on this IRFZ24N destruction.
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1.? ?It occurred with just 3 or 4 dits / dahs with "drive" set to 68% on 80 meters -- but with the intervening tuner in an indeterminate state into the ancient SB200 amplifier.? ?
2.? ?There was no indication of a malfunction of the SB200 amplifier (a plate arc would easily explain damage to the exciter)
3.? ?The IRFZ24N is rated at up to 17A current -- which the little switching supply that I used, cannot produce.? ? I think it is rated at 12 Amps or so at best, maybe less.? ?The first indication was a total shutdown -- so the supply went into current overlimit shutdown.? ??
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4.? The IRFZ24N is rated for 55V drain-to-source.? ? That particular device had huge 5W zeners from gate to ground, with up to 1000 pf of capacitance (so I didn't get much power at 10 meters, (laugh) ).? ??
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5.? The best explanation appears to be over voltage on the drain due to some oddity of the un-tuned intermediate auto-tuner state.? ?Over current seems less likely since the supply can't really generate enough.
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6.? ?Based on this failure, Ashhar's move back to 100V-rated parts seems quite reasonable from a manufacturer point of view.? However, for MY particular interests,? I note that there are available IRF520 parts? (available on Mouser)? ?with similar input capacitance to the IRFZ24N, and I could certainly replace the 5W zeners and their 1000pf capacitance (!)? with the 3pf TVS diodes that just arrived from Mouser.? [There are other IRF520's that have more input capacitance and questionable V dss ratings.....not so good....]? ? ? ?I might then have significantly more "headroom" for power? on 80/40/30 than you can get with the IRF510, and be comfortable around 30 W power output with good linearity for digital operations.? ?I like being well within "max" power for linearity.? ? ?And maybe less risk of suddenly blowing a MOSFET due to a poor termination impedance.
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Gordon KX4Z
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