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PMP10215: No voltage at 36V outputs

Part Number: PMP10215
Other Parts Discussed in Thread: TL431

Hi there, as per this previous thread we have just completed building the reference design.  We are getting 3.3V and 12V outputs, but not 36V output.  I followed the last thread, but don't really understand what the final solution was for obtaining the 36V (or 18V) output in the end?  Can you please summarise for me how I achieve this?  Many thanks! 

  • Hi Mark,

    Regardless to the fact you are using the revision D or E of the schematic, the DC/DC stage is turned on, if the switch S1 is in the ON position (which means pins 5 and 6 of S1 are tied).

    If S1 is switched to OFF (therefore switched to "remote" position, so S1 connects its pins 5 and 4 together), then in order to turn on the DC/DC converter a 3.3V level should be applied to pin 3 of J4 (referred to secondary side ground).

    The pin 1 of J4 (referred to secondary side ground, pin 2 of J4), defines Vout level; therefore V(pin1) = 0 --> Vout = 18V; V(pin 1)= 3.3V --> Vout = 36V.

    Please let me know if that solve the issue.

    Regards,

    Roberto

  • Hi Roberto.  I confirmed and measured the following for you:

    • S1 is in the ON position.
    • Between TP15 & TP16 = 393 V
    • TP18 = 12.2 V
    • TP23 = 22.8 V
    • TP20 = 11.9 V
    • TP24 = 3.2 V
    • TP11 = 22.8 V

    Any ideas why I'm still not getting 36V out?

  • Sorry Roberto, I forgot to add the dummy load to the 12V DC output.  i.e. a 390-Ohm resistor.  This set TP23 to 24.8V.

    So I get 36V our of J1 now (when I route the 3.2V output to pin 1 of J4).

    I get 19.4V out of J1 otherwise.  I thought it was supposed to be 18V?

    I am getting some unwanted noise when the device is powered up.  Like a high-frequency switching/buzzing sound.  I've included a short video clip that demonstrates this.  What could be the cause?  Can it be stopped?

    Thanks for your support!

    Click here to play this video

  • Hi Mark,

    Glad to see you get the output voltages by applying the dummy load to 12V output. You know, this power supply has been specifically designed to supply a Class-D audio amplifier, which had some current consumption on both outputs (3.3V and 12V).

    BTW, also as original design, the output voltage should have been set to 18V / 36V for testing purpose, but at the end the output voltages are described in the test report. In the online test report, it is specified as 19V / 36V supply (and the text on the schematic has not been updated with the latest voltage levels, sorry!).

    If you want to calculate the exact output voltages, when applying or not the 3.3V to pin 1 of J4, it is pretty straightforward:

    I suppose you added the extra-circuit of revision E, correct?

    In this case, considering that (in both revisions D and E) the reference voltage of the TL431A is 2.495V, when you apply or not the 3.3V on pin 1 of J4, the resistor R107 is connected (or not) in parallel to R20. That means, as low feedback resistor of the TL431 we have 5 KOhm or 10 KOhm. The resistor R19 is always 68.1 KOhm, so with a simple math you get the two level of output voltage to be: 19.486V and 36.477V.

    If, you want to get exactly 18V and 36V (according to the precision of the TL431A), then you should use for R20 = 10.958 KOhm, and for R107 = 9.439 KOhm. 

    Again, sorry for not updating the text in the schematic, but the test report shows more correct values.

    Regarding the noise you hear during startup, it might be dependent on the output capacitance of your load, associated to the loop compensation during startup. The gain of this compensation, only during startup, is defined by R103 (which I added in the revision E after experiencing some noise, happening during testing the revision D, which doesn't have this resistor). If you are still using the revision D, please add this resistor in series to C20. If you are already using the revision E, please increase this resistor from 15 KOhm to 22 KOhm or even 33 KOhm.

    I hope you solve al these critical aspects here.

    Best regards,

    Roberto

  • Many thanks, Roberto.  I'm not too fussed by there being 19V so long as that is what is supposed to be there.  I was concerned I'd done something wrong and wanted to investigate what it may be.  I'll also be mindful of the other tips you have provided so I can adjust if I ever need to in the future.

    I'm using revision D, so I'll add R103 in series with C20 and see if that helps.  The noise never seemed to go away, so I'm not sure it was just a start-up thing, but hopefully this addition will help!

    Kind regards,

    Mark

  • Thanks for your feedback Mark, just let me know whenever you need more help :-)