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UCC28712 will not go to 100kHz

Sven Teuber
Prodigy 90 points
Other Parts Discussed in Thread: UCC28712, UCC28711

Hi,

I recently set up a power supply for the conversion of 400V...900V DC into 24V DC with a nominal power of 120W.

I am using a UCC28712 IC with a standard flyback configuration.

My Transformer has a primary inductance of 950µH and a turns ratio of 12.

Yesterday I ran the power supply at nominal power at 400V...900V.

Unfortunatelly the IC will not go to 100kHz.

My first configuration is:

Rcs=500mOhms, Peak-Primary-current=1,56A (@Vcst=780mV)

The converter will switch at about 60kHz with Vcst=750...800mV.

The secondary voltage goes only to 20V (Feedback voltage at VS-pin is about 3,5V)

The converter Switches in the 4th or 5th valley of the Drain-Source-Voltage and does not go up.

Then I reduced the Rcs to 330mOhms (Peak-Primary-current=2,36A)

This means more energy is transfered to the Output per pulse.

The converter then brings the Output to 24V as intended.

Feedback-Voltage at VS-pin is about 4,1V.

Peak-Voltage at CS-Pin is about 750...800mV.

The converter runs at a switching frequency as low as 30kHz.

In this working Point my Transformer, Mosfet and Output-Diodes overheat because of the high Peak-currents.

How do I get the converter running at 100kHz as intended?

Kind regards

Sven

  • 0
    TI__Genius 15910 points
    Let me run through the numbers and get right back to you. Although I do recommend that you set the maximum switching frequency at something slightly less than the maximum allowed by the controller (shoot for 90kHz as a max at constant current mode) just to allow the device to respond to transients.
    What is the target application for this design?
  • 0 in reply to Lisa Dinwoodie
    Prodigy 90 points
    No matter what maximum switching frequency I am setting.
    The controller not even comes near....
    It stays at 30...60kHz and does not go higher (and keeps switching in the 4th....10th valley)

    I hope I am reading the control law diagram at page 14 of the data sheet correctly.
    The controller should get to its maximum peak current staying at 33kHz.
    And when more output power is needed, the switching frequency should go up to its maximum.
    But in my case it does not.
    The Controller does not manage to bring the output into regulation (VS voltage should be ~4V).
    But it does not. VS-voltage stays lower than 4V.
  • 0 in reply to Lisa Dinwoodie
    TI__Genius 15910 points
    Another thought I just had while looking at this, please do not put a probe on the VS pin directly, the best way to "look" at this signal is to either just place a probe on the AUX winding (what is the turns ratio for primary to Aux by the way?) and scale it mathematically with the resistor divider on VS; OR split the bottom resistor on the VS divider into two series resistors, one larger one at the VS pin node and a smaller one in series with the larger one connected to ground and put the probe on the node between the series resistors. In this way the probe impedance isn't affecting the VS waveform. It is very important that the ringing on the VS waveform is minimal at the knee (as shown in Figure 15 of the data sheet) but you don't want a probe-dampened view.
  • 0 in reply to Lisa Dinwoodie
    Prodigy 90 points
    Hi,
    I am using a Tektronix TDP0500 Low-Voltage differential probe.
    It works fine directly at the VS-pin.
    With lower output power the insertion of the probe changes the output voltage by about 0,5V.
    I think this is OK, as the power supply keeps running stable without audible jitter or anything else.
    The capacitance of the Probe of about 1pF helps filtering a bit of the ringing of the Aux winding.
    Also the overall response of the power supply does not alter with or without the probe.
    The last measurements were performed without the probe.

    The Prim to Aux turns ratio is 16 (Prim to Sec ratio is 12) which should give a IC supply voltage of 16V.

    My VS resistors are RS1~86k, RS2~27k.
    I tried a small filter capacitor in parallel with RS2 of about 1pF to 10pF.
    With 10pF the VS waveform looks really clean without excessive leakage-ringing.
    But anyway this should not be an issue as the knee looks really clean and the converter operation is stable without jitter.
    It just keeps the VS voltage stable at a voltage lower than the regulation level of 4V.
  • 0 in reply to Lisa Dinwoodie
    TI__Genius 15910 points
    Hi Sven,
    Basically the primary inductance and the peak current are the only major contributors to setting the maximum switching frequency. The peak primary current is set by the CS resistor and should be calculated for Iocc which is the overcurrent threshold, which should be approx. 5 % to 10% higher than the desired normal operation current (so I would use 5.5A for my Rcs calculation) which comes out to 0.343 Ohms with the Nps = 12.

    The ideal turns ratio, Nps, is based upon the maximum duty cycle and this is dependent upon the expected resonant frequency so the time to hit the first valley is accounted for. If we assume the maximum duty cycle is 0.475 then the Turns ratio of your transformer should be higher than 12 (I calculate closer to 17.88 which would actually let you use that Rcs of 0.5 and bring your peak currents down to a less toasty level. With the new turns ratio of 17.88 and the sense resistor of 0.51 Ohms the Lp should be adjusted to 1.2mH for a switching frequency of 90kHz at 5A load and closer to 100kHz at Iocc. Basically change the turns ratio to 17.88 and increase the inductance to 1.2mH with the 0.51 Ohm current sense resistor for higher switching frequency (but really target 90kHz for max at full load, so that it can go higher at in constant current mode).

    A complete schematic, transformer build drawing, and waveforms may be needed for more detailed assistance.


    As a side note, this family of controllers was targeted for 5V, 10W adapters (like USB chargers) so this might not be the best choice for your application, but I am always impressed when customers successfully use a device outside of its defined scope so keep me posted on your progress.
  • 0 in reply to Lisa Dinwoodie
    Prodigy 90 points
    Another measurement:
    -I changed the IC to a UCC28711
    -I changed the transformer to a type with 2,5mH instead of 950µH
    -Rcs is now 650mOhms

    Same effekt.
    The IC keeps regulating the peak current to its maximum (Vcs=750...800mV)
    But the switching frequency goes down to 27kHz
    and the output voltage does only reach 15V (with a VS voltage of about 3V)

    When I reduce the load current the switching frequency goes UP (to about 35kHz)
    and the IC goes into voltage regulation of VS-voltage= 4V.

    To me it looks like the IC performs some kind of over power protection and keeps the output voltage low
    to prevent excessive output power.
  • 0 in reply to Lisa Dinwoodie
    TI__Genius 15910 points
    VS not hitting the 4.05V typical is curious, have you tried decreasing Rvs2 to 25k?
  • 0 in reply to Lisa Dinwoodie
    Prodigy 90 points
    Hi,

    I know that the controller is intended for USB adapters.
    But there is nowhere stated in the datasheet WHY it cannot be used for higher power-levels.
    As far as I know the only limit is the gate-driver current source/sink capability.
    I use an external driver to overcome this limit.
    The IC also features protection features like overcurrent overvoltage and overtemperature,
    which is all you need for converters up to my power-class.
    For up to 5A output current Flyback works quite well.
  • 0 in reply to Lisa Dinwoodie
    TI__Genius 15910 points
    time for a schematic...and what value do you have for the line compensation resistor? The offset added may indeed result in power limit if not selected appropriately
  • 0 in reply to Lisa Dinwoodie
    Prodigy 90 points
    Hi,

    you have helped me a lot! Thanks.
    Going through my calculations I see, that I must be hitting the constant current regulation level.
    I will follow your suggestion to increase the turns ratio to something around 18.

    I think I am beginning to understand how this IC works...