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LM5023: Unable to get an output with 30VDC - 145VDC IN to 24V out converter.

Vijai Suraj
Prodigy 10 points
Part Number: LM5023

Tool/software: Webench Power Designer

Hi.

I recently made a 30V-145V to 24V DC/DC power converter. I used the output from Webench as reference and made small changes to the circuit for my design. I have tried both the Webench design, as well as the modified design; both don't give any output. The Webench design led to burining of some components, while the modified design gave no output. I am attaching the transformer parameters, input and output waveforms for the modified design:

Transfromer parameters:
Lp = 19.0970 uH
Ls = 16.6053 uH
Laux = 3.95309 uH
Ns to Np = 0.933
Naux to Np = 0.455

Find the Webench design here.

The following are the transformer input waveforms. (Note that the attenuation of the probes ae set 10x the actual values: so take the values as 0.1x the readings)

The following are the auxiliary winding waveforms. (The attenuation is set at the right value here)

The modified designs have the following values for the parameters:
Rcs = 499 Ohm
Rqrt = 12 kOhm
Rqrb = 1.56 kOhm
Cd = 140 pF

Thanks and Regards

  • 0
    TI__Guru* 86266 points

    Hello,

     

    Could you share the Webench design and schematic for review?

     

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hi. Thank you for responding Mike. 

    webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=82A9BD91F7C3AC77

    I am a bit sceptical about the snubber resistor values. Let me know if they're alright.

  • 0 in reply to Vijai Suraj
    TI__Guru 74115 points

    Hi,

    Your modified component values are very different from your webench design. How did you modify them and why did you modify them?

    Can you put back the webnech design values to provide the test results, waveforms?

    Rcs = 499 Ohm
    Rqrt = 12 kOhm
    Rqrb = 1.56 kOhm
    Cd = 140 pF

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Hi, 

    I did the calculations as per the datasheet, and selected the values of Rqrt, Rqrb and Cd. Rcs was chosen with similar consideration and by referring to other schematics. 

    I tested the DC DC converter with Webench values as well, but some of the compoents began smoking before I could take any values. In the brief moment I observed, it had the same issue: no DC output. It remained under 1 VDC. However, in both instances, with my values and Webench values, I did observe an AC voltage of 12 Vrms (using a multimeter). 

    Update: I did some calculation: I used the peak value of current through the primary winding from Webench and calculated the voltage across the Rsense. This will be the voltage at one point of the Rcs. Internally, there is a 6.6 kOhm resistor, between the CS pin and the internal comparator (comparing to 0.5 typ.). Since, the comparator is comparing against 0.5V, there should be a voltage greater than that internally. But for this converter, the voltage across the Rsense resistor is less than 0.4V (0.385V), much less the internal voltage. So, I dont think Rcs is the issue, but all the other values are the same as in Webench design. 

  • 0 in reply to Vijai Suraj
    TI__Guru 74115 points

    Hi,

    Which components began smoking?

  • 0 in reply to Hong Huang
    Prodigy 10 points

    Most likely Rqrb or Rqrt. 

  • 0 in reply to Vijai Suraj
    TI__Guru 74115 points

    Hi,

    These resistors are on aux, should not be smoking. It looks either aux voltage not correct (i.e., your transformer has something wrong) or you used wrong resistors. Please check these.