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  • TI Thinks Resolved

UCC28950: Problem with Synchronous Rectifier

Intellectual 865 points

Replies: 17

Views: 370

Part Number: UCC28950

Hello

We are developing DC-DC PSU with input voltage range 600-800VDC and output voltage 60VDC@10А using UCC28950. With asinchronous rectifier system works without problems, but power dissipation is too high.

With sinchronous rectifier we got a problem: when SR starting to work we gets a huge noise on all nets - on output, on control nets and even UCC28950 power input. As result, it kills controller. What can cause it?

Oscillograms with enabled SR:

  1. SR transistors gates with SR enabled before stabilization started

2, 3. Output with asynchronous rectifier without load

4, 5. Output with asynchronous rectifier loaded with 1A

6, 7. Output with SR enabled after stabilization started (load 1A)

With input voltage increasing noise level significantly increasing.
If necessary, we can send project files to the email.

Pic1

Pic2

Pic3

Pic4

Pic5

Pic6

Pic7

Regards,

Roman

  • In reply to Roman Zavyalov:

    Sorry, with SR oscillograms I've took second signal from wrong point. Here's correct images for SR.

    Regards,

    Roman

  • In reply to Roman Zavyalov:

    Hello Roman

    Ok - the waveforms now make sense. OUTE and OUTF overlap during the 'freewheeling' interval which is very short when the controller is running at close to Dmax. They are in antiphase and with some minimal overlap. I'd prefer to see a full cycle on these waveforms but what you have sent is fine for now.

    I'd like to see a plot which shows the relationship between the voltage across the primary of the transformer and the SR drains. VT1 drain, VT3 drain, VT6 drain and VT7 drain - ideally 4 or 5 cycles of the weaveforms.
    I'd also like to see a plot showing 4 or 5 cycles of the waveforms at U3 Pin 2 (OUTE), U3 Pin 4 (OUTF), VT6 drain and VT7 Drain

    Do you know why the system is running at Dmax and is this expected ?
    Normally this happens if Vin is too low to generate Vout. This can be a useful diagnostic tool sometimes.

    Regards
    Colin
  • In reply to Colin Gillmor:

    Hello Colin

    That oscillograms was made with low input voltage. We can make previous oscillograms with high input voltage (around 500V, when SR starts to work; above 500V UCC28950 is dying) if needed. Can that plots be usefull?

    Today, when we tried to check one theory, we accidently killed UCC28950, so we will make oscillograms tomorrow. Also, unfortunately, rigth now we can't make oscillograms from primary side because we don't have high voltage probes.

    Regards,

    Roman

  • In reply to Roman Zavyalov:

    Hello Roman

    I think that the first thing I would like to confirm is whether you have the OUTx signals connected correctly. Ideally, I'd like to see a plot showing the voltages I mentioned in the last post.

    However a plot showing OUTA DA1, OUTB DA1,  PIn 7 U3 and Pin 5 U3 would do that. They are all low voltage signals and have the same ground reference. You can take this plot at any input voltage where the SRs are active

    Regards
    Colin

  • In reply to Colin Gillmor:

    Top to down lines:

    • OutA
    • OutB
    • U3-5
    • U3-7

    Vin 0V 4 periods

    Vin 40V 4 periods

    Vin 150V 4 periods

    Vin 250V 4 periods

    Vin 350V 4 periods

    Vin 450V 4 periods

    Vin 530V 4 periods

    Vin 550V 1 period

    Vin 550V 4 periods

    Vin 550V 6 periods

    Regards,

    Roman

  • In reply to Roman Zavyalov:

    Hello Roman
    Thanks - these waveforms are quite informative.

    1/ The OUTA, OUTB, U3-5 and U3-7 waveforms are ok. I assume that you are have corrected the polarity issue I mentioned earlier.
    2/ The output signals from U3-5 and U3-7 get increasingly noisy as Vin is increased. It looks like there may be some false outputs coming from them, especially in the final image.

    For now, I'd suggest you continue to use diode rectification until we can convince ourselves that the SR drives are correct.

    I looked over the layout again and I see that you have a wide separation between the controller and the SR driver IC. This shouldn't be a problem in itself but long lines are prone to noise pickup.

    I'd suggest that you try to get a more stable trigger - perhaps OUTA at the controller - and look at the inputs and outputs from U3 to see if the part is triggering on noise or not. Maybe the outputs are not obeying the inputs correctly in which case we should look at the decoupling around the driver. Maybe the inputs to the driver are very noisy and this is causing the outputs to misbehave. Maybe the controller OUTE and OUTF signals are noisy. The aim would be to follow the signal chain and try to find out the point at which the system is getting disturbed.

    The OUTE and OUTF lines are on the top of the PCB, it may be necessary to put them on an inner layer with ground above and below them to reduce noise pickup - but let's try to understand where the problem is coming from first.

    Regards
    Colin
  • In reply to Colin Gillmor:

    I've marked this thread as resolved. We can continue to work via email or you can open a new thread if you wish.

    Regards
    Colin

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