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I designed/had built a PCB to handle higher current draw. The P Channel MOSFET (Q1) I selected is used as a ON/OFF switch for the rest of the circuitry. R2 gets grounded through a switch when flipped "ON", hence I just have it grounded in the picture. It is getting very hot (230F) at 20A current draw after about 5 minutes of being on.

enter image description here

  • Q1 = SQM120P06-07L_GE3

  • D1 = SMAZ10-13-F

  • R1 = 1/10W 0603

  • R2 = 1/4W 0603

  • The MOSFET has a Absolute Max Vgs of +/-20V

  • When measuring the voltage across the Gate and Source, I get -10V which the datasheet says should provide the minimum RDS(on).

  • The MOSFET is properly heatsinked on 3 layers of 1oz copper through vias.

My question is, why is this getting so hot? Did I incorrectly select R1, R2, or D1?

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    \$\begingroup\$ The datasheet specs R=0.0067 Ohm at V_gs = -10. That's U_drop = 0.134 Volts, and at 20A, that's 2.68 Watts. I'd say that's quite a significant amount of heat. \$\endgroup\$
    – akwky
    Feb 10 at 16:02
  • \$\begingroup\$ 6.7mOhms at 20A gives over 2.5W, that is a lot of power to get out of a FET. I am not surprised that it got hot. As mentioned, a heat sink is required. \$\endgroup\$
    – Puffafish
    Feb 10 at 16:03
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    \$\begingroup\$ 20 A is absolutely not "low-ish" current. You should probably not be using a PCB heatsink for this. \$\endgroup\$
    – Hearth
    Feb 10 at 16:50
  • \$\begingroup\$ R2 will also fry (324 mW dissipation). \$\endgroup\$
    – Andy aka
    Feb 10 at 19:00

2 Answers 2

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Heatsinking is required. Doesn't matter how low the source-drain resistance seems. With a junction-to-ambient thermal resistance of 40K/W that's still 107K rise even if you only dissipate 2.7W...at room temperature The electrical resistance increases as you the MOSFET gets hotter. Between 1.5-2x when running at it's max rated temperature.

PCB heatsinks just aren't that effective. You would have to show us your thermal layout. But after working with a real heatsink that something like a TO-247 package bolts to, doing it on a PCB leaves a lot to be desired. I only used real heatsinks before and always felt that a PCB couldn't really stand up the same way. But I heard people doing it so gave it a try and it was about as effective as I thought it was. And I was using 4-layers of 4oz-6oz planes that were more than 2 square inches in area. I'll stick to discrete heatsinks now unless size or weight is a concern.

20A isn't very low either.

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  • \$\begingroup\$ Should I just select a more "efficient" MOSFET with a lower junction-to-ambient thermal resistance? I wont go anywhere near the 120A capacity that this MOSFET can provide. Most likley just 40A. \$\endgroup\$
    – Nick_Nack
    Feb 10 at 16:05
  • \$\begingroup\$ @Nick_Nack Yours is already low especially for a difficult to source PMOS. Not like a sub1mOhm NMOS but reasonable. I do not think you will get much benefit. Can you connect a heatsink to the opposite side of the PCB or something? Or a fan? \$\endgroup\$
    – DKNguyen
    Feb 10 at 16:07
  • \$\begingroup\$ Using a fan is not possible as this is located inside a watertight box. Heat sinking the bottom of the board is also not a possibility. I will either have to move to a through-hole package and put a heatsink on that or use something like FDB9503L-F085 that has 2.6mohm RDS(on) at -10V Vgs. \$\endgroup\$
    – Nick_Nack
    Feb 10 at 16:44
  • \$\begingroup\$ A heatsink won't help in an enclosed box either. The heat needs to get out. If it's in a water tight box then you might have to do something like putting a heatspreader against the MOSFET or PCB and butting up the heatspreader against the wall of the metal box, possibly even with a heatsink on the other side of the wall. If it's plastic box and not submerged then maybe you can cut away a window and replace it with a sealed aluminum or copper plate epoxied or siliconed in. \$\endgroup\$
    – DKNguyen
    Feb 10 at 16:48
  • \$\begingroup\$ Why is it water tight? Is it submerged or are you expecting spray from all directions? Down-facing vents won't work? (you would still need a fan). Also, if it's water tight you may also get condensation issues if there is temperature cycling. \$\endgroup\$
    – DKNguyen
    Feb 10 at 16:54
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As stated in previous answer, at room temperature (25 ºC), RDS(on) will be 6.7 mOhm at VGS=-10V. When the transistor switches on, power will be 2.7 W, and temperature will rise, but also RDS (positive temp. coef.). I estimated thermal resistance of PCB based on your remark: after 5 minutes, 230F = 110ºC, making one minute iteration of RDS and power losses until 15 minutes. This is only for comparision, it results about 28 ºC/W. After 15 minutes, the junction temperature will rise to near limit, RDS = 12.5 mOhm, and power losses about 5 W.

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With a heat sink below 10 ºC/W (and through-hole FET), Tj will be about 56ºC, RDS=8 mOhm, and power loss 3.2 W. Consider to use 2 parallel MOSFET (current 2 x 10 A), with PCB arrangement and same gate circuit. With 1 squared PCB, according datasheet RthJA=40 ºC/W, and temperature will rise to 57 ºC, total losses will be 2 x 0.8 = 1.6 W. This may be better supported by a closed box.

enter image description here

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