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UCC28910D - Power supply won't start up..

Andrew Frazer
Expert 2305 points
Other Parts Discussed in Thread: UCC28910

I have an issue with a Power Supply design.   It doesn't seem to want to start up, and i'm not sure why;

the schematic is here. Sorry, i can't for some reason add a file today, it seems the forum has been upgraded and this broke!

http://ibin.co/1rMBztX2tG6G

The voltage at the Anode of C1 is around 320VDC, with respect to the Neutral.        There was no output on the secondary side.

At VDD i was only measuring about 400mV,  not nearly enough to turn reach the required VDD to turn on the device.

I did make one small typo in this design, R6 is show as 10R, but in fact it is 1R.

does anyone see anything fundamentally wrong with my design?  I'm stumped.

  • 0
    TI__Mastermind 34465 points
    Please redraw the schematic and show the pin numbers you used on the UCC28910.
    It looks like you only connected up one of the ground pins.
    Pins 1,2 and 3 should be connected to your Neutral node.
  • 0 in reply to John Griffin1
    Expert 2305 points
    Pins 1,2,3 are all connected to GND
    Pin 4, is connected to IPX
    Pin 5, is connected to VS
    Pin 6, is connected to VDD
    Pin 8, is connected to DRAIN

    Appreciate that the schematic might be a tad lacking in details.

    I have doubled checked the schematics and the pcb and they are as per the data sheet.

    If its a really nessarry i will redraw tomorrow when i'm at a work. Im bought a blank pcb and the not going one
    home with me tonight ( i'm not going to plug it in, because i don't have a 'safe' workplace to do it, ) but took the time
    to check though everything with the multimeter and might have found an issue Pin 4 of my Transformer. its supposed to be connected to the N on the board, but it seems it hasn't been soldered correctly.. theres a big chuck of copper and i suspect it has not got hot enough.

    Im looking forward to mark2 of this, already, where the th transformer is going to swapped with a SMD version.

    Assuming that the pins are correct on the schmetic, is there anything that is obviously wrong.
  • 0 in reply to Andrew Frazer
    Expert 2305 points

    Please see my schematic symbol with Pin numbers and the PCB Pad layout for the component.

    http://ibin.co/1rUCJtyA8cIK

    http://ibin.co/1rUCeCCxmcSO

    I've checked and rechecked the pins are as they are supposed to be.   

    I've done a complete rebuild today ( new pcb + parts ) and i get the same result; 

    No output,  i get about 0.5VDC at pin 6.   

  • 0 in reply to Andrew Frazer
    Expert 2305 points
    I'm still at a loss to know why this won't work; :-(

    At power up, i can see that the voltage on Pin 8, rises to ~320VDC ( my input is normally 230VAC ) and sits there. Voltage ad VDD never rises above 0.5V at VDD ( Pin 6 ).. And there there is never any switching going on the transformer.

    I am right in assuming that the thing should start up with no load ( other than the 2k resistor across the output? )

    I'm getting a bit desperate now!
  • 0 in reply to Andrew Frazer
    Expert 2305 points
    Ok, feel a bit stupid now.. Went through things item by item, and found that somehow the diode from the AUX winding to VDD ( the bas20HT1g ) had been created incorrectly in the librarys.. The silk screen was 180 degrees out! and so i'd put my diode on backwards. I only picked this up when I deeply investigated things.

    Upshot of having a fault is that often it forces you into really understanding what is going on.