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Alright, continuing with the debugging of my new board tonight I found another issue worse than the one before: My switching MOSFET is getting so hot that I'm getting molten solder.

I replaced the MOSFET with a short circuit just to see if the components sorounding the MOS were the cause of the problem (mostly a big Inductor I have right next to it) but the copper reaches a temperature of 110°F, not enough to melt solder.

This is my schematic:

enter image description here

As you can see, my VGS is around -11V and I'm running 3A through it using an Electronic Load. Based on the datasheet of my NTD20P06L-D this thing can do up to 15.5A.

Any ideas of what might be going on?

The only thing I could think of was that the heat on the copper could be increasing RDS(on) but at that particular low current it shouldn't be so serious.

I am using the TO-252-3, DPak (2 Leads + Tab) package.

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Gustavo Corona
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  • I separated the PAD from the board and connected it via 18 AWG wire and the melting occurred almost immediately, so it is definitely the part getting super hot. – Gustavo Corona Oct 20 '12 at 02:01
  • Have a look at figure 11 (pg 5 in that datasheet). For an 11 V Vgs, the maximum dc current is more like 5 A than 15. You're still under that, but it's not surprising you need a heat sink. Also notice notes 1 and 2 on pg 1 of the datasheet. The drain current ratings assume a 1" square pad on your pcb. If you are just using the minimum pad, you should de-rate by about 40% according to the thermal resistance values. Also, the 15 A rating is an absolute maximum, not a recommended operating condition. If you want your circuit to last, you should de-rate from this to begin with. – The Photon Oct 20 '12 at 03:38
  • Is this PWM switched or essentially low speed on/off? What switching speed. Gate is driven off by 100l pullup and will a long time to turn off. If occasional that's fine enough. If PWM then you need a gate driver. Very simple driver cct available if needed. – Russell McMahon Oct 20 '12 at 04:12
  • This is steady state. It remains always on. – Gustavo Corona Oct 20 '12 at 04:32
  • Short the collector and emitter of your transistor, and see what happens. Also (in that state) measure the gate-drain voltage and the source-drain voltage of your FET. – Wouter van Ooijen Oct 21 '12 at 19:31
  • The key measurement that seems to be missing is the voltage across the MOSFET drain - source. With this and the current, you can calculate the power dissipation and compare that to the rating in your installed configuration. You can also calculate the RDSon - if the later is substantially different from the spec, you likely have a drive problem. – Chris Stratton Oct 22 '12 at 16:32

1 Answers1

11

It sounds like you need a heatsink.

Another thing to check is your gate voltage when you have the FET switched on. If your Gate-Drain voltage is too high, the MOSFET may not be properly biased on, a situation which would generate a lot of heat.

Also, how often is this switching? Is it a steady-state thing, or is this part of a switching supply? If it's a switching supply, you will also need to look at the rate at which the system switches.

Anyways, assuming you have everything biased properly (probably a safe assumption, but measure it anyways):

RDS(on) = 130mΩ @ G-D voltage of 5V

So, with 130 mΩ in series with 3A:

$$V = 0.130 * 3$$ $$V = 0.39$$ $$Power = V * A$$ $$Power = 0.39 * 3$$ $$Power = 1.17W$$

So you're going to be dissipating 1.17W of power in the MOSFET, in the best-case situation.
That will get very toasty without a heatsink. If you're just running this as a bare TO-220 device, it getting extremely hot isn't too suprising.

So, assuming we have a TO-220 in free-air:

TO-220 junction-to-air thermal to ambient equals 62.5 degree per watt.

(From here)

Therefore:

$$Δ°C = 62.5 * 1.17$$

$$Δ°C = 73.125$$ $$Device Temperature °C = 73.125 + Ambient$$ $$Device Temperature °C = 98.125$$

So assuming ideal thermal dissipation on a bare TO-220, it's still going to easily reach ~100°C.
Any environmental factors that further reduce the device's cooling will make it worse.

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