Heating of the boost's mosfet

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eem2am

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hello,

Boost converter
12V in,
28V out,
Power out = 10W

freq = 500KHz,

Mosfet NOT heatsinked

No room for heatsink...on either mosfet or boost diode.

TOTAL PCB area = 65mm by 15mm each side (for whole circuit)
. (its a double sided PCB)
The area of copper available for being the heatsink for the mosfet is just 5mm by 5mm!.....the same for the boost diode!.

I have done calculations and have found that this is ABSOLUTELY NOWHERE near enough copper area for cooling.

Please does any reader have any "got out of jail" storys with overheating a mosfet and getting away with it?
 

You may try to add via holes beneath them, since your PCB is double sided.
 

modbro thanks, btu my PCB is only 65mm by 15mm and i need the bottom side for ground plane too.

their just doesnt seem enough room for the copper area i need for heatsinking
 

Hi
One solution for using transistors without heat-sink is to use a higher power transistor. your power is 10W and output voltage is 28 so your power is less then half amperes
you can use Mosfet with higher currents.
 

Check if the raise and fall times of gate voltage are fast enough. If some of these times are relatively slow, try to boost transistor with more gate current. For example try to lower gate resistor value or add current booster for gate drive (1 transistor + 1 diode).

With schematic it would be easier to see problem.

If you drive mos with voltage slowly changing levels, your mos will be one nice heater because mos power looses are mainly at fall and raise times. Cure is more current to the gate capacitance.
 

In that case, you need to increase the eff. Lower Rds on of the moefet and Lower Vf of the boost diode.
 

Your solution is to enclose you project in a metal box. Bolt the Mosfets to the metal box, which then act as a heat sink.
 

for the given size of the pcb, 6x15 mm you can redesign the converter to work at lower sw. freq., let's say 100KHz, you will have enough space to fit all the parts, including the inductor. decreasing the freq to 100KHz will also substantially decrease the losses, improove efficiency and lower the dissiated power. if you don't want to redesign the circuit, just check if is possible just to decrease freq. you may need to change the inductor and increase output filtering. if you post schematic, or a photo with the pcb we may help you solve-it.
 

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