TOP245G problem
Hi, we are using TOP242G and TOP245G through 9 years in a buck topology application. From a DC in of 325 to 375 Vdc and to obtain 15Vdc non isolated. We use a open core inductor and the output current is about 200 mA average maximum. This circuit feeds all the control electronics for a bigger power supply.
The heatsink is a double sided pcb copper plane 25x25 mm. In the past we had to move from 242 to 245 to avoid overheating protection from de IC.
Now the problem is we have had 4 IC failures among 100 pieces and we don't see any overload or temperature problem in the application. We have measured 60 degrees max at the copper plane and you may see in the attached captures that current is between limits. We don't see current limiting nor overtemp protection in any case. The fault is solved just changing the TOP245G and everything start to function again. The current inductor is 470uH and is so low to avoid audio noise. Attached are some captures of the inductor current with a 3.3mH one.
We see that with the 470uH inductor the IC is operating at a freq lower than 123kHz and this indicates how light is the operating point so it has to lower the freq to maintain the regulation.
So, we will be very glad to know if there is some wrong in our application to explain the failures and also if there is any trick to solve the acoustic annoying noise.
Thanks in advance
Note1: The schematic is as your app note DI-1
Note2: In the captures the zero reference is the mid screen line. THe trace is referenced lower to compensate the offset in the current probe
Note3: With load the inductor current appears to be negative. This is probably due to noise picked up into the current probe
Note4: the images are:
470uH (standby): http://www.powerint.com/en/470ufidlejpg
470uH (loaded): http://www.powerint.com/en/470ufloadjpg
3.3mH (standby): http://www.powerint.com/en/3300ufidlejpg
3.3mH (standby, zoom): http://www.powerint.com/en/3300ufidlezoomjpg
3.3mH (loaded): http://www.powerint.com/en/3300ufloadjpg
Comments
Thanks for your support. You can see our schematic application here:
http://www.powerint.com/en/schematic-3
and your DI-1 PDF document is this:
http://www.powerint.com/sites/default/files/di1.pdf
The maximum voltage Vds we have is around 450V, far of 700V maximum absolute rating.
Thanks in advance.
We have analized all parameter and all are inside specifications. We use this design from 9 years ago, and no problem. Recently we have changed the inductor, from 1mH to 470mH, because now we are working with a higher input DC level (before 325V and now 400V) and this 470mH inductor avoid the audible noise. We have a very low comsumption, but we have not more space in the PCB and we have o use TOP246 to avoid temperature protection. So, the TOP is working at minimum dutty cycle, and minimum frequency, low temperature. We are thinking to change the TOP245 and use a TOP246, to test if there are some change. We think we don´t need it, but we dont now we can do. You think the change of the inductor can be the problem? Any idea? Thx in advance
Hi, The document DI-1 you are referring to is not available on our website. Would you please post the schematic again? I understand that you've checked the temperature and current. However, I think your current waveform is not stable - it oscillates sometimes. Would you please also check the peak Vds to make sure it is less than 675V at maximum input and maximum load?