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TPS92411 High temperature in Mosfet

Miguel Camus
Prodigy 120 points

Hi dear friends,


after solving a problem with the power factor what I saw is that the mosfet I was using (FCD4N60) that looks better than the one used in the EVB is overheating, in few minutes is reaching more than 120C at room temperature of 20C. The mosfet has a good dissipation pad and there are not other components overheating around it. I replaced it with an IPD65R190C7, that is a super mosfet with really good characteristics and the result is the same. I think the problem is not the current (measured about 100mA in RCS) as it stands 8 amps without problems. The voltage that stands is 650V and I'm powering the device at 115VAC. What can it be??

Thanks in advance.

Miguel

  • 0
    TI__Mastermind 28655 points
    Hello,

    In a linear current regulator LED driver most of the power dissipation is in the MOSFET. This means that a lower RDSon MOSFET will not dissipate less since it is in linear mode. MOSFET specifications tend to advertise the switching characteristics, such as eight amps. This is with the part fully enhanced so there is little voltage across it. In the linear circuit application the part needs a proper heatsink. At 100 mA and, say, an average of 20 volts on the MOSFET it will have to dissipate two watts. This can be worse at high line since the headroom can be higher when the current is high (due to power factor correction).

    From a thermal consideration, besides the LEDs, the MOSFET is really the only part that dissipates a lot of power unlike a switcher that has the MOSFET, transformer, diode(s) and other parts to worry about.

    Also, because the MOSFET is run in linear mode, the SOA (safe operating area) is very important also. The SOA has to be checked for both steady state operation as well as start-up when the MOSFET sees rectified AC dropping until the LED bypass capacitors are charged, this can be many 60 Hz half cycles. The easiest way to see this is watching the MOSFET drain as it powers up (make sure all the bypass capacitors are discharged before measuring).