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TPS23754: fails to start

John Ford
Intellectual 530 points
Part Number: TPS23754
Other Parts Discussed in Thread: TPS23753, TPS23753A, TLV431A

Hi all.

I am trying to bring up a board based on the '420 evem, using the optimized components in SLVA475, the efficiency optimized synchronous flyback converter.  I have a couple of questions.

First, the board was sort of working, i,e, it started up and would ramp up the output to about 2.5 volts before stopping, and this repeated forever.  This was with no-load, and also with a 3A load.  The frequcncy of the start/stop was about 75 Hz. 

Vb was OK, and stable, and both gates were pulsing in what seemed to be correct sequence, i.e. alternating on-off with each other at a low duty cycle.

I was suspecting that something in the feedback was wrong, so I was probing the optocoupler pins, and I I accidentally shorted the anode and cathode of the optocoupler together with my probe, which I would think might send the system into full-power output.  Alas, this seems to have killed the system.   Vc never goes above ~1V on startup, and stays there forever.  No buzzing on and off.

Here are some questions to help me get back to operation.

1) If Vc is not coming up, is it likely I killed the 754? 

2) What else would cause Vc not to come up?  A shorted gate?

3) Any guesses as to what I killed?

And for later, here are some questions related to the design:

1) Is there a design guide for this system like SLVA305C for the tps23753, or is it valid for the 754?

2) If I calculate the bias resistor for the optocoupler to work in the 2 ma current range, I get about a 2x greater value than on the schematic of SLVA475.  Seems wrong that it's running at 2x the current suggested in SLVA305C

3) What is the purpose of R26, 49.9 Ohms, in the design?  I accidentally left it off my prototype schematic, and if I calculate the currents and bias points of the circuit, they seem OK.  Is it for stability?

Just to note, I removed the diode from the gate of the primary transistor since my schematic shows it backwards.

Thanks for any hints or pointers to other documents.

  • 0 in reply to John Ford
    TI__Expert 3700 points

    Hello,

    Thank you for your question. 

    The PoE applications engineering team is currently out of the office and will respond to your question by end of day Monday, 7/6. 

    Best Regards,

    Brett Colteaux

    PoE Product Marketing Engineer

  • 0
    TI__Mastermind 21320 points

    Hello,

    If VDD_RTN is 48V but the VC is not going above 1V, then the part is probably damaged. I would switch out the IC. 

    1.) No there is not a design guide, but the design guide is for a diode rectified flyback. It can be independent of the TPS23753A, but it is only for diode recitified flybacks. There are papers on ACF, but not synchronous driven flybacks. Some of the same principles apply, but not everything is the same (especially when it comes to the secondary side). 

    2.) I notice there is a difference of optocouplers, so they might require different currents. Typically the 5mA range is acceptable. 

    3.) This is there to introduce an AC signal to test the feedback circuitry (bode machine).

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Intellectual 530 points

    Hi Michael.  Thanks for the quick reply.  I'll get the part replaced and get back to testing.

    I found more design information for this particular part in  "Under the Hood of Flyback SMPS Designs".

    From the schematic and the fact that it was ramping up to about 2.5V and then stopping, and repeating that at ~75 Hz rate, can you point me to what might be causing this behavior?  It seemed like it was going into shutdown for some reason or another until I killed it with my probe,  Could this be due to improper biasing of the optocoupler?  The one I used has about the same  CTR as the original, I think, but I'll recheck it.

    So the lack of the 50 ohm resistor should not hurt the operation, just won't let me do a loop test.  I'll add it to the next rev, but for now I'll continue without it.

    John

  • 0 in reply to John Ford
    Intellectual 530 points

    I think I have figured out the issue.  My pinout of the tlv431A is wrong, and it isn't providing correct bias to the optocoupler.