| Electronics > Beginners |
| I keep killing mosfets - what I do wrong? |
| << < (3/6) > >> |
| PMA:
My iron was actually in ungrounded power outlet (only extension is grounded and I didn't noticed it / thought through), and there was computer etc. in the same extension. PSU was properly grounded. Luckily this is beginners area so I don't feel that bad ;D |
| magic:
Myself, I use a grounded iron so that it doesn't accumulate charge but I don't bother grounding the circuit. Instead, I touch some grounded metal and the board's ground simultaneously to discharge it and then just solder away. And that's only when dealing with MOSFETs. So far so good, but I can hear the ESD zealots screaming in horror :-DD |
| PMA:
I agree with your method. I understand that ESD measures are taken seriously in production environment but there is no need to go overboard when prototyping with single or couple boards. Even single MOSFETs seems to be quite tough guys as I was able to assemble two with my original setup without killing them :) Difference between soldering iron and GND was probably 120volts (1/2 of supply). I found the problem when I was connecting serial cable from the board to PC. My hand touched PC and outer shell of the USB cable at the same time. I sweared twice - firstly because it hurt and secondly because I realized what I have done :) |
| Ian.M:
As I commented in the thread I linked, a 1Meg resistor that is appropriate for ESD grounding wont help with the possible leakage current from a hot ceramic insulated mains powered soldering iron, as it only needs 10uA to develop 10V across the grounding resistor. It then becomes a matter of whether the peak voltage, low-pass filtered by the RC network of the grounding resistance with the MOSFET gate capacitance exceeds the gate oxide breakdown voltage. Its even worse if you have an un-grounded SMPSU feeding a low voltage iron. The max Y capacitor leakage current set by various safety standards is 0.75mA, and even if the leakage is only a tenth of that, it will develop far more than any normal MOSFET's gate oxide breakdown voltage across a 1meg resistor. Therefore, mains powered, and low voltage irons with a SMPSU that isn't specially constructed to avoid Y capacitor leakage current problems, need their bit hard-grounded to the same ground as the device you are working on (if its grounded). Yes it increases the risk of ESD damage if you aren't working on an ESD dissipative surface, but you should be on an ESD mat anyway. If you are dealing with notably ESD sensitive devices, avoid working on a silicone heat resistant mat unless its from a reputable manufacturer and specifically was sold as ESD safe. |
| Jwillis:
According to the data sheet the maximum current to the in pin(2) is -250mA. In the diagram on page 7 it shows a resistor between the switch and pin 2. Maximum drain current on the 2n7002 is typically 115mA. Did you try putting a resistor between the drain and pin 2?Just a thought. If its shunting to ground then there must be current there. |
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