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UCC28C51-Q1: Flyback Input Current Resonance

Sky Hsieh48
Prodigy 130 points
Part Number: UCC28C51-Q1

Tool/software:

Hi TI,

Webench shows I can achieve 1000V/1W output by UCC28C51-Q1 (input 9~16V). I would like to use flyback IC to control a 10kV/1W output flyback.

First, I build a 10kV open-loop topology as schematic below.

Vin=12V, Fsw=20kHz, Duty=50%.

I have 2 questions now.

1. Why the primary-side current of XFMR is a sine wave when MOS is turned ON?

2. Why Vout doesn't increase when MOS is turned ON in negtive current period?

 Ch1: MOS Vds, Ch2: Vout, Ch3: XFMR Ipri

  • 0
    TI__Guru 74095 points

    Hi,

    1. Check your transformer primary inductance, as in a flyback, when the primary-side switch turns on, the input voltage is crossing the transformer winding, so if the inductance is high, then the transformer behaves like a L-C filter with the input capacitors so the current becomes sine on its positive half cycle. The negative half cycle is to the secondary side. 

    2. You need check the transformer secondary side polarity - which seems wrong in your board physical connections - that is why you see the negative current. You need to check the secondary side voltage to see if it is in flyback, although your circuit drawing show correct on the transformer windings.

  • 0 in reply to Hong Huang
    Prodigy 130 points

    Hi Huang,

    1. I try to replace input cap from 470uF+20uF to 20uF, but the frequency of resonance seems no difference.

    Cin=490uF  Cin=20uF

    2. I change the polarity of second side, only VOUT is higher at light/no load condition, input current are the same.

    Original  Change polarity

    3. Using heavy load, there are some differences. Is the original one looks more like flyback converter?

    Ch1: Vds, Ch2: Vpri, Ch3: Ipri, Ch4: Vsec

    Original  Change polarity

  • 0 in reply to Sky Hsieh48
    TI__Guru 74095 points

    Hi,

    Vds does not seem correct. Can you add Vgs of the MOSFET along with Vds, Vpri and Vsec to help understand why Vds waveform is like this?

    When Vds = 0, Vpri should be = Vin, but your waveform Vpri is not. Where did you measure Vpri?

    Your Vsec only about 50V, so your transformer does not work - you need to see 10kV on the secondary side - it looks you have an in-effect transformer.

  • 0 in reply to Hong Huang
    Prodigy 130 points

    Hi Houng,

    I think Vpri is not a stable 12V because I use too large load (100ohm) and too small Cin (20uF).

    I can't output 10kV maybe the ratio of XFMR only 1:215 is not enough, I will receive a higher ratio XFMR later.

    Please refer to the waveform below, the output voltage is 1500V and load is 30Mohm (100ohm yesterday).

    Is the leakage inductance too large to make amplitude of resonance of Vds become negative?

  • 0 in reply to Sky Hsieh48
    TI__Guru 74095 points

    Hi,

    Vin needs stay constant in a flyback. You need to re-measure the waveforms after you can get constant Vin.

    Vsec is not correct - your transformer does not work properly. Vsec should have similar waveform like a pro. But your Vsec shows nothing like that. You need to fix the transformer and make constant Vin.

  • 0 in reply to Hong Huang
    Prodigy 130 points

    Hi,

    I use 3 high turn ratio, and 1 low turn ratio (Lm=2.1uH, ratio=1:1) XFMR to measure input current di/dt.

    1. Lm=40uH, ratio=1:20

    2. Lm=250uH, ratio=1:215

    3. Lm=26uH, ratio=1:1166

    The same phenomenon occurred in 3 high turn ratio XFMRs, di/dt is several times higher than the calculated.

    But it didn't occurred in 1:1 XFMR, only 10% deviation between measured and calculated value.

    For example, Vin=12V, Fsw=20kHz, Duty=3%, Lm=26uH.

    Ipri_peak should be only 0.7A, but I measured 10A.

    Is there any explanation for this phenomenon?

  • 0 in reply to Sky Hsieh48
    TI__Guru 74095 points

    Hi,

    It looks your transformer is in saturation. That is most likely you see that high current.

    You need to check and test your transformer to see where its saturation starting then make adjustments.

    You should also try at no load to see how your transformer behave. Note 1:1162, even with a small secondary current the primary side current will be 1162x so quite large compared to the secondary side.

    You need to check all these to see the numbers are come together then decide what to do next.