MOSFET damage

By the sounds of your explanation there is nothing wrong or unexpected with your results or the MOSFET. I assume you are using a digital multimeter with a 10 Megohm input resistance on the voltage scale you are using.

If that is true, then take a look at the datasheet's drain-source leakage specification. It claims a maximum leakage current of 20uA at 16V drain-source with 25 deg. C junction temp. Ohms law says the 4.5V you measured across your meter's internal 10 Mohm resistance suggests a current of 450nA. Your meter was likely functioning as a pull up resistor, albeit

It doesn't really make much sense to be measuring voltages on high impedance nodes anyway. When the MOSFET is off the drain is close to floating (assuming no load is attached to the drain), though not totally (pulled to source potential with a small amount of leakage). When you attached your meter to 12V and the drain you made a high impedance voltage divider. When you attached your meter from drain to gnd, you effectively put the 10Mohm meter resistance across the leakage "resistor", so you no longer had a voltage divider, just two resistors pulling to ground. Hence the zero volt reading.

I read in sci.electronics.design that Nathan Hunsperger wrote (in ) about 'MOSFET damage', on Sat, 29 Jan 2005:

No, everything is very probably OK. Your voltmeter is simply responding to a **very** small leakage current through the FET. If you connect a few LEDs to the connector you will find everything works. Almost certainly.

The die is bonded to the heatsink. The die substrate is usually the drain ( or collector for bipolars ).

As for your other question, I suggest you look at some currents as well as voltage. A voltage reading can be meaningless ( leakage currents etc ).

Graham

I read in sci.electronics.design that Terry Given wrote (in ) about 'MOSFET damage', on Sun, 30 Jan 2005:

I'm not sure what you mean by that, but silicon isn't glass. Nor is silica (silicon dioxide); almost all glass is metallic silicate and/or borate.

I read in sci.electronics.design that Terry Given wrote (in ) about 'MOSFET damage', on Sun, 30 Jan 2005:

This probably explains why two successive staple guns I bought failed after about 50 staples. I had a full refund, otherwise I would have changed the triac and, of course, bolted the new one to the heat sink.

I agree that spring clamps are even safer than bolts, BUT you need very strong springs to ensure good thermal contact.

Mechanically, it's pretty glass-like.

Hi Nathan,

Others have covered the MOSFET readings, and Graham mentioned the die attachment. But your thought process *is* correct - connecting the wobbly bit to a large lump of copper is less than desirable from an emi standpoint, and is often inconvenient electrically too. Some v*cor power supply modules use single-ended or push-pull converters with FETs to the positive, rather than negative, DC rail. Although the gate drive needs to be level shifted, it allows the FET to be directly mounted on a DCB substrate (ie heatsink) without injecting a huge amount of noise through the drain-substrate capacitance, as would occur with the conventional implementation. I gutted one once, and it was a marvel of production engineering. The Faraday shields around the cores were almost works of art.

But wait, there's more. Copper and Silicon have different CTEs, and so change size at different rates as the temperature changes, just like a bimetallic strip. This then stresses the die-tab interface, which is usually a solder joint [I assume this is true for TO220]. These stresses tend to make the solder joint deteriorate - and the greater the temperature change, the worse the degradation. Luckily the degaraded connection increases the temperature rise, thereby exacerbating the problem, eventually leading to thermal-fatigue induced runaway failure - this is a leading cause of failure among high-power IGBT modules, and I just read an interesting paper on its occurrence in TO-247 packages too.

As if thats not bad enough, the copper tab does a great job of transmitting forces to the silicon (ie glass). Over-tightening of a bolt can deform the copper tab, placing a direct mechanical strain on the die, degrading lifetime. Another interesting thing that can happen with tight bolts is the copper tab bends at the bolt interface, actually lifting the die off the heatsink. Pop-riveting applies a fairly uncontrolled force with a very rapid risetime, and can easily break the die. motorola AN1040 covers this subject in detail, is on-line and a must to read.

Cheers Terry

Which was entirely the point - thump it and it will break.

Cheers Terry