WEBENCH® Tools/LM5036: VCC and REF not reaching UV

Hi Julez, 

Could you please have a look when increasing the maximum limitation of input current? 

Besides, please check whether the aux fly-buck triggered the OCP, since overload will also pull down the Aux1 voltage.

Regards,

Teng 

Hello Teng,

thank you for the fast reply. I did increase the input current but without different results - like i said, input voltage didnt drop so I´m sure it´s no input current issue. I also tried removing the coupled inductance from T2 and supplied the voltage from an external Power supply at 8,6V/200mA. This didnt increace the REF output, so I think its an internal regulator issue, followed by an REF UV and no gate signals to the FET/internal AUX switch. I did build up a second hardware with the same design and will try to check for either the same or different issues.

If you have any further ideas I would be really happy to try them aswell!

best regards,

Julian

Hi Julian,

Thank you for your information.

It is really difficult to understand that REF is 4.2 V. Please test REF when there is no load at REF pin.

Regards,

Teng

Hi Julian,

I haven’t heard back from you for quite sometime, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out) – thanks.

Teng

Hello Teng,

appologies for the late answer - I had to wait for parts. Indeed I fixed my issue temporarily by building up a second hardware. With the new one i got new troubles. The Aux supply did work this time. Together with the working AUX supply the REF pin also goes up in voltage (from 5V to AUX supply level). Is this normal behavior?

Also gate pulses started with an amplitude of 5V at 2.5ms while the intended start up delay was 5ms (Soft-start capacitor at 56nF). 

I did test the hardware with a 21V power supply, current limited, with high input capacity provided by electrolytic caps on the hardware itself. I wanted to slowly check the different states, so i did externally switched on the feedback voltage at UVLO for small time steps (1-4ms length). I also did remove the aux transformer connection to the secondary side by removing the diode Dsec_aux, to prevent issues from the secondary side showing in the first runs where i wanted to only test the primary side. 

This procedure ended after some successful test of the aux supply with the REF voltage coming out at 1,8V constant, resistance at 57,1ohm to GND with everything but the IC disconnected from the REF Pin. This being below the treshhold, the aux supply is now non functional again. Now the question - is it possible to feed in the 5V externally at REF Pin for further testing? Any idea what could have caused the fault of the IC?

Thanks for your help! Really appretiate it. Sorry for the maybe unexperienced questions, but I´m doing this as a thesis work at university and I can´t explain the strange behavior of the hardware after only using components listed by the WEBENCH and some MOSFETs and Transformer I dimensioned by myself.

Have a nice day!

Greetings from Germany

Julian