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UC2854B: Maximum Supported Switching Frequency

Robert Stowe
Prodigy 50 points
Part Number: UC2854B
Other Parts Discussed in Thread: UCC28180

What is the maximum supported switching frequency of the UC2854B? Datasheet page 11, figure 7 implies 800kHz. But TI web selection tool indicates 200kHz.

I am currently at Fsw=200kHz for a high power 380-665Hz line, would like to increase to at least 300kHz to meet requirements.

What determines the upper frequency limit?

  • 0
    TI__Mastermind 34465 points
    Hello Robert,
    As you say, Figure 7 of the data sheet shows that the oscillator can be run up to about 800kHz.
    At this frequency you should expect a max duty cycle of anywhere between 80% and 90% and this will have a negative impact on the power factor and harmonics that you can obtain.
    Also the device has not been characterized at this high frequency. You can see in the datasheet that the device is characterized at 100kHz and it has a line and temperature variation of 80kHz to 120kHz. So I cannot estimate what variation in switching frequency you can expect at these much higher frequencies. Also you can expect the power loss in the device to rise. Most pfc circuits are operated at 150kHz or less for efficiency and EMI reasons.
    You can also consider using the UCC28180 which has been characterized at up to 250kHz.
    Is there a particular reason you need such a high switching frequency. There are applications with a 400Hz supply that switch at 100kHz.

    Regards
    John Griffin
  • 0 in reply to John Griffin1
    Prodigy 50 points

    Hello John,

    Thanks for your response. Here are my comments to your response.

    1) I will investigate the power factor / harmonic performance as a function of increasing Fsw. At 200kHz, power factor / harmonic performance does not seem be significantly impacted.

    2) In our application we are providing a sync signal, so I do not expect Fsw to be significantly impacted by line and temperature variation.

    3) Can you elaborate on where the main increase in dissipation takes place within the IC as a function of increasing Fsw. I would expect it to be in the output driver circuit. If the loss increase elsewhere is significant, can you elaborate. I have built my own model for this controller for use in SIMPLIS that I would like to improve if needed.

    4) Efficiency is currently not an issue at 200kHz. Still evaluating EMI as compliance is one of the reasons we want to go higher since this design is very space constrained.

    5) I did review all of the available controllers and selected UC2854B as the best choice. I did not select UCC28180 because the IC PFC functionality was not adequately transparent and not reasonably understandable.

    6) This application is very space constrained and has stringent requirements on leading power factor. Therefore, it is advantageous for the Fsw to be higher so that the EMI filter can be small.

    Are there any other parameters of the IC that would be sensitive to higher Fsw?

    Thanks!

  • 0 in reply to Robert Stowe
    TI__Mastermind 34465 points
    Hello Robert,
    The output driver section of the UC2854B is bipolar in nature and it may show an increase in power dissipation at higher frequencies.
    That's really all that I can foresee.
    You may be able to reduce the amount of differential mode filtering by increasing the switching frequency but you may also see an increase in common mode emissions. This is difficult to predict until you prototype your design.

    Regards
    John