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UCC21551: Inquiry Regarding External Circuit Design for the UCC21551

철하 강
Prodigy 20 points
Part Number: UCC21551
Other Parts Discussed in Thread: UCC21520

Tool/software:

Hello, I'm Cheol-ha kang.

I am a hardware engineer based in South Korea, specializing in the development of converters for the automotive industry.

First, before I ask about the UCC21551, let me briefly describe our topology as follows:

1. Topology : PSFB Converter 

2. Swtiching Frequency : 96kHz 

3. Dead Time : 150ns 

4. Circuit

Now for my main question: Will there be any problem if I design the INA/INB input filter like the attached circuit, which is different from the design recommendation in the datasheet below?

we've implemented an RC filter with 1.6kΩ and 10pF, which results in a cut-off frequency of about 10MHz.

My question is, are we allowed to use values other than those specified in the datasheet's recommendation? I would like an answer on whether it is acceptable to apply our current filter values, even though they deviate from the recommendation, and if this will cause any problems.

I look forward to your prompt response.

thank you.

  • 0
    TI__Mastermind 25980 points

    Hi,

    The team is out of office today on U.S. holiday. We will get back to you soon.

    Thank you for your patience.

    Regards,

    Max Verboncoeur

  • 0
    TI__Mastermind 23206 points

    Hi Cheol,

    Yes, this will be a problem. I do not recommend more than 100 ohms of filter resistance for your input filter. Could you try a 1.6nF, 10 ohm filter instead? Even though this the RC filter frequency is the same 10MHz as for a 1.6kΩ and 10pF, its source impedance is 160x lower vs frequency, and it will absorb transient noise injection with a much lower voltage rise. Consider a simulation example of a 1A transient injection vs. frequency: 

    Best regards,

    Sean 

  • 0 in reply to Sean Cashin
    Prodigy 20 points

    The PWM source (UCC28950QPWRQ1) in the circuit I designed has a 12V output. Therefore, I used high-value resistors to create a voltage divider. Considering both resistor power loss and the source current, resistors in the range of several kilohms seem to be the optimal choice for my circuit. If you can, please tell me the applicable C (capacitor) values when using resistors in the several kilohm range. and in my opinion, a large RC filter value will just act as a signal delay, and as long as I account for that, it shouldn't be a problem for the design

  • 0 in reply to 철하 강
    TI__Mastermind 23206 points

    Hi Cheol,

    I see. We have a device called UCC21520 that accepts a 12V input power supply and a 12V input signalvoltage. It is similar to the UCC21551, but it contains an internal LDO to allow >5V inputs. Otherwise, it is P2P compatible. Have you considered using that version instead?

    The recommended max input resistor is 100 ohms. with a 12V input, you would have to use a 240 Ohm resistor in series with a 171 ohm resistor to get a 100 ohm parallel resistance at the input. That is 30mA of current from 12V, which is too much to justify.

    It is just a noise consideration. If you do not have high current noise injection, then >1k resistors and a ~100pF capacitor should be sufficient to prevent input logic errors due to voltage spikes that exceed the input threshold. Hopefully your target input frequency is not >100kHz.

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 20 points

    I changed the filter resistor from 1.6 kΩ to 1 kΩ, while keeping the capacitor at 10 pF, and measured the waveform to check for high-frequency ringing and its peak amplitude. The measurement was taken right at the PWM input pin of the gate driver, and I confirmed this under 100% load condition on the actual converter.I don't see any potential for signal distortion or any issues that could damage the chip. Wouldn't the current design values be acceptable?

  • 0 in reply to 철하 강
    TI__Mastermind 23206 points

    Hi Cheol,

    10pF is not much, especially with a 1k Ohm source impedance. How much power is running through this PSFB? 

    But if it works in your circuit at 100% load then it works! The ringing in your measurement is not bad. Just make sure there are no positive transients above 1V on this input when the other channels are switching. It seems you understand the potential noise issue well enough to make the judgement.

    Best regards,

    Sean 

  • 0 in reply to Sean Cashin
    Prodigy 20 points

    The maximum power rating is 2.3 kW.

    As you can see in the waveform captured with a wider time scale below, no surge voltage exceeding 1V is observed, even while the other channels are switching.Thank you for your excellent feedback. Lastly, here is the wide-view waveform.

  • +1 in reply to 철하 강
    TI__Mastermind 23206 points

    You are welcome and good luck on the rest of your design.

    Best regards,

    Sean