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UCC256403: Maximum switching frequency

UCC
Prodigy 55 points
Part Number: UCC256403
Other Parts Discussed in Thread: UCC25660, UCC256404, UCC256402

Tool/software:

I am more or less finished with the design of a UCC256403, but there's one thing that concerns me. 

I've read several places in this forum that the maximum resonance frequency recommended for the UCC256403 is 300 kHz, but it does state anything about the maximum switching frequency. 

I want to use a resonance frequency of 350 kHz due to space limitation on the PCB, with a maximum switching frequency is 490 kHz. Is this frequency way too large?
What is the risk? And how can one reduce the risk? Is it possible to workaround this by reducing the propagation delay on the PCB?



  • 0
    TI__Genius 16155 points

    Hi,

    I would recommend using UCC25660 LLC controller for such a high frequency application. This has much improved features ex: 750 kHZ resonant frequency, IPPC control algorithm for consistent OPP, Burst mode operating levels, Hand switching avoidance during startup, adaptive soft start etc. 

    Let me know if you are okay to change the IC? 

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 55 points

    Hi,

    I cannot use this controller due to the input voltage range. The input voltage range I need is 30-60 VDC.

    So in other words, do I need to decrease the maximal switching frequency to 350 kHz?

    I cannot find anywhere in the datasheet what the maximum switching frequency is.

  • 0 in reply to UCC
    TI__Genius 16155 points

    Hi,

    I dont see the reason why you cant use the UCC25660 for 30-60V DC? Could you let me know the reason why you cant use the new controller?

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 55 points

    The reason I cannot use UCC25660 instead of UCC256403 is the following:


    1. Higher efficiency in the required power range. 

    2. Ultra-quiet standby operation.


    Can you now please answer me what is the maximum switching frequency that can be used for UCC256403?

  • 0 in reply to UCC
    TI__Genius 12795 points

    You can use UCC256403 above 300kHz. You may not find a paring SR controller at that frequency though.

    You don't bias the IC Vcc pin with input voltage. Aux winding is typically used to provide the bias. The Spec you list is not relevant here.

  • 0 in reply to Ning Tan
    Prodigy 55 points

    Thanks for the answer. 

    So it is okay to use a resonance frequency of 350 kHz, and 480 kHz at light load?

    Is it it anything in particular I have to take into account if I am at worst-case scenario up to 480 kHz?

    Making sure that I use HIGH and LOW side MOSFETS that have a low enough input capacitance? And propagation delay?

  • 0 in reply to UCC
    TI__Genius 12795 points

    Let me clarify, UCC256404 is capable up to 1MHz switching frequency during startup and in theory you could push the normal switching freq above 300kHz, but we typically recommend keeping the resonant frequency to 300kHz and below, because:

    1. the thermal challenge at higher switching freq as the high side and low side drivers are inside the IC

    2. if the switching frequency is high, the resonant current at turn off event of HO/LO is small, which could trigger ZCS protection.

    3. the propagation delays is independent of the switching freq. 

  • 0 in reply to Ning Tan
    Prodigy 55 points

    Let us summarize. 

    Ning Tan said:
    1. the thermal challenge at higher switching freq as the high side and low side drivers are inside the IC

    This is okay. If there's a problem with thermal due to the drivers for the LOW and HIGH side MOSFETs, we will manage some cooling.

    Also, we use MOSFETs with quite small die, so the input capacitance is not that HIGH. I assume this might be a problem with larger power mosfets.

    Ning Tan said:
    if the switching frequency is high, the resonant current at turn off event of HO/LO is small, which could trigger ZCS protection

    Let us try to break down this statement into something more useful which gives context. 

    The resonant capacitor voltage sense pin does not measure any current, but it actually measures the voltage. Also, ZCS protection usually happens when the controller/LLC is in the capacitive region. This happens when you have a heavy load and a low frequency. Which is exactly the opposite as the scenario I am describing. We have a high frequency during light load. 

    I assume that high frequency with a light load is only a problem if the high switching frequency can cause disturbance on the resonant capacitor voltage sense pin. I assume this will be mostly radiated noise that can affect this pin. If we can eliminate the switching noise to manifest itself on the resonant capacitor voltage sense pin, it will not be a problem. 

    Ning Tan said:
    the propagation delays is independent of the switching freq. 

    We need to clearify this simplified statement.

    Of course, propagation delay is independent of the switching frequency, but if you have large MOSFETS with a high input capacitance, and you also have a high frequency, in the worst-case scenario the dead-time can be too low. This due to the fact that for large MOSFETs you have a high input capacitance, so the drivers can not charge the capacitance fast enough, and if you also take into account a large propagation delay, in the worst-case scenario the HIGH-SIDE mosfet is not charged fast enough, before it should be turned off again. 

    The proagation delay could be relevant if the MOSFETs are far away from the controller, which is not the case for our application, and could affect in the corner case scenarios. 

    Last thing: We have also done extensive simulations, and we have not had any problems so far. 

  • 0 in reply to UCC
    TI__Genius 12795 points

    The main challenge you will probably facing is the second one. As the Switching freq gets higher, there may not be enough current build up within each switching cycle at ISNS pin. False tripping of the ZCS protection mechanism may occur.

  • 0 in reply to Ning Tan
    Prodigy 55 points

    Then it would be a fix to change the RISNS value or avoid switching noise on ISNS pin?

  • 0 in reply to UCC
    TI__Genius 16155 points

    Hi, 

    Sorry I was out of office last week. Let me pick it up where I left off.

    UCC said:

    The reason I cannot use UCC25660 instead of UCC256403 is the following:


    1. Higher efficiency in the required power range. 

    2. Ultra-quiet standby operation.

    As far as efficiency is concerned during the high power range, its purely based on the power stage design. So, you won't lose efficiency using UCC25660.

    Coming to the standby operation, UCC25660 does have the burst mode operation. So, I would still recommend using this new controller at such a high switching frequency.

    As Ning suggested, due to the propagation delay of UCc256402, we won't recommend using the controller at such a high frequency. 

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 55 points
    Manikanta Pallantla said:
    Coming to the standby operation, UCC25660 does have the burst mode operation. So, I would still recommend using this new controller at such a high switching frequency.

    I cannot use burst-mode due to EMI.

    Manikanta Pallantla said:
    As Ning suggested, due to the propagation delay of UCc256402, we won't recommend using the controller at such a high frequency. 

    As Ning suggested, propagation delay has nothing to do with frequency. It only matters if you use HIGH power MOSFETs as we earlier discussed.

  • 0 in reply to UCC
    TI__Genius 16155 points

    Hi, 

    We never used the 40x at such a high resonant frequency. You can give it a try and let us know if you see any issues.

    We did operate the UCC25660 close to 500kHz and haven't found any issues.

     

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 55 points
    Manikanta Pallantla said:

    We never used the 40x at such a high resonant frequency. You can give it a try and let us know if you see any issues.

    We did operate the UCC25660 close to 500kHz and haven't found any issues.

    But that's probably because you tested with applications with much higher power? The maximum power in our case is 15 W. 

    I will let you know if it works or not. I might be an idiot for not listening to you guys, but I think there are several reasons why it should work.

    1. We've seen no issue in the simulation model.

    2. We use low power and small mosfets. 

    3. We are not expecting any noise, and if we do we will filter it away. Since the sense pins are high impedance inputs.

    4. We do not have a large board where propagation might be a problem.

    5. If we face problems with a high resonance frequency of 350 kHz, we can adjust the frequency by adjusting the leakage inductance and resonance capacitor.