Thermal shutdown problem(plzzzzz help)

Posted by: phd.nash on 11/27/2008

< i designed flyback type smps with LNK364 < output power=3.8 W < core EE13 TDK, L=1550uH < ambient temp=85C < rms current of the LNK364 is 44mA at 290VAC input, 85C ambient temp conditions. < i separated the transformer from the pcb. after, i placed the flyback transformer another environment and the ambient temperature of this environment is 25C. < after the flyback full loaded, ten minutes later LNK364 goes over temperature protection. < there is no transformer saturation problem because of temperature and there is no critical mosfet rms current problem. < i calculate the junction temperature like that Tj=Tambient+ ((Irms)^2*Rdson)*Qja Qja= thermal impedance= 70C/W for LNK364 Rdson=50 Irms=0.044 A Tambient=85C as a result Tj=85+(0.044^2*50)*70=91.77C why lnk364 goes the thermal shutdown???

Comments

The max mabient ambient temperature is high and your VAC input is also high. The higher voltage switching will defitnately cause higher switching losses which will add to dissipation. Due to your high ambient requirements and higher input voltage you may need to use a larger device to compensate for these conditions. Since LNK364 is the largest device for thst family you might try a larger device form the TNYSWitch-lll series.

ok. thanks but eventually, all the dissipations will affect the rms current of mosfet. rms current gives us an idea about the output power and dissipations. thermal shutdown is directly related to switching losses.for calculating switching losses two parameters are used. 1)Rdson 2)RMS current of mosfet. i think lnk364 doesn't support the datasheet values maybe i'm wrong

Hi,

There are three contributors to the total loss. Turn-ON loss, Turn-OFF loss and the conduction loss.

I suggest you review our guidelines for PCB design. You need to ensure through appropriate thermal design that the part has adequate heatsink. Generally this is provided by using adequate copper area on the circuit board that is connected to the source pins. Please see the datasheet for reference.

Please see Figure.7 of our datasheet. The layout example shown in figure 7 highlights the traces that need to have sufficient width/area in order to meet the thermal requirements.

Regards

PI-Sarek