PS based on DER-185 is not starting up

Posted by: Peter Krarup on 06/03/2009

Hi,

I have used the DER-185 to design a 12V/5V power supply, but it won't start.
Vd is 308VDC with 20V/400kHz ripple at 230VAC input. The load is a microcontroller on the 5VDC and a 1K on 12VDC.
Vfb is < 300mV and sometimes it looks like the systems tries to start
Vbp is 5.9V
The transformer is a Wurth type for DER-185
What is wrong here?

Regards
Peter

Files

Protel Schematic_1.pdf (71.69 KB)

Comments

Hi Peter:
What It think is happening is that your feedback loop is open. First check for 6 volt as at the bypass pin. (I think this voltage should be ok because you say the supply sometimes is trying to start. But please check to be 100% sure). Then the next thing to check is the phase of the transformer. Measure the DRAIN voltage with an oscilloscope at the time when the supply is trying to start. Compare this waveform with pin 10 and also with pin 3 of the transformer. The 3 nodes waveforms should be in phase. The correct Phase is necessary for the controller to keep running and achieve regulation. Let us know if this is the problem …PI_Worf

Thanks for the answer. I assume that the measurement of DRAIN and pin 10 is with respect to SOURCE.

Peter

Peter: Pin 10 voltage is with respect to OUTPUT RETURN (pin 6). The GROUND lead of the scope probe that you are using to measure DRAIN waveform is connected to SOURCE pin. The GROUND lead of the probe you are using to measure the waveform at pin 10 is going to be connected to pin 6. Thank you PI-Worf

Hi again,

Bypass voltage is 5.9V
DRAIN, transformer pin 3 and pin 10 are in phase when the controller tries to start up.

Peter

Hi again,

a scope plot of the signals in phase

Peter

Peter:
The waveforms look in phase. I think the next thing to try is to see if you have some short or very low impedance path at the output. It may happen that a very low impedance at the output is being reflected to the primary side of the transformer. This conditions would drive the controller into auto-restart. So measure with an ohmmeter the impedance at output (before the linear regulator). Is should have low impedance when the output diode is in forward polarity with respect to the ohmmeter. leads. Higher impedance when the ohmmeter leads are inverter. If there is not such conditions, it means not enough voltage is being feed to the feedback pin. Are you using the same transformer design as DER185? The transformer should sample feedback voltage during the OFF time. This voltage should be 1.86 Volts at feedback pin (after your voltage divider. Please check these issues and tell us if there is any progress. PI-Worf

Hi,
The problem is solved. The Schottky diode was mounted the wrong way.
The ohmmeter measurements revealed the error.
Thanks for the help

Regards
Peter Krarup