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Registered Member #4537
Joined: Sun Apr 22 2012, 06:59AM
Location:
Posts: 9
Hi - i need some help on this issue with this chip as well -
I am using the LT3750 capacitor charger with the coilcraft DA2034 (N=10, 10uH, 10A Ipk) and a IPB530N15N3 MOSFET D2pak (150vDS, 23miliohm, 9nC, 21A, 84A puls). Current sense resistor is 8 miliohm, like the LT3750 datasheet suggests for 10A current limiting, all other components similar to the datasheet.
my output capacitor is 500uF 900v. i charge it up to 800v. this is 160 joules, it takes about 8 seconds. the thing is, that the MOSFET get very hot after a few repetitive charging cycles. after about 2-3 minutes of repetitive charging (charge to 800v, discharge capacitor with large resistor, in 100ms, and re-charge immediately) it gets so hot that it starts melting its own solder on the PCB ! :evil:
my rough instinct tells me that charging 160 joules in 8 seconds is 20 joules per second which means 20 watts. if we take a very bad case scenario, of 50% efficiency, the MOSFET will waste another 20watts of power, or almost. can this case lead to such high temperature that it melts its own solder ? or i might have some other problem that causes efficiency to be way below 50% ? is there some formula or thumb rule to predict the heating or power loss in such a setup ?
according to bottom of page 9 in the LT3750 datasheet, the Iavg of the mosfet is 4.34A at 12v which is about 50W. (N is 10, VOUTpk is 800, Vtrans is 12v) can it be that it wastes so much power as heat ? it is relatively low Rdson and very low gate charge (9nC) on the last page (16) of the datasheets, it shows a graph charging 110Joules in just 5 seconds at 12v, with 90% efficiency. that means that out of the 50W power passing in the mosfet, less than 5W SHOULD BE wasted in heat. this is surely not my case...
I know that i might need to move to a TO220+heatsink setup for higher currents, but that issue make me stop and check, because it might be a bad sign...
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
IPB530N15N3 has a junction-to-ambient thermal resistance of 62 C per W when using a minimal PWB footprint. With minimal copper pour for the drain, you will rapidly approach 150 C with only 2 W MOSFET dissipation. Once you reach high junction temperatures, the MOSFET Rds_on will also increase, further accelerating the MOSFET temperature.
At high peak currents, I would use additional copper pour for the MOSFET drain to pull more heat away from the part. Another option would be to reduce your peak charging current, thus reducing your MOSFET dissipation.
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LT3750 Capacitor Charger
