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UCC28950: What is the best way to limit the current on the output?

Part Number: UCC28950
Other Parts Discussed in Thread: PMP8740, , UCC28951-Q1, TL431

Mine schematic looks like this for the PSFB converter.

PSFB converter schematic.pdf

I saw a reference model from PMP8740, see this schematic. 

PMP8740 2KW Rev_E Full Bridge - Schematic.pdf

I also read the datasheet/document of this model but I still don't understand how much current they are limiting? 

I need to make a design that can charge a battery of 8kWh with 500W-2000W (so with 0.0625-0.25C). So when I connect the PSFB converter with the battery I need to choose a limit with my microcontroller.

What is the best way to limit an adjustable output current with a microcontroller.

More information over system:

Output votlage: 43.8V need to stay constant to charge a battery of 36-43.8V 

Output current max load: 45.6A (2kW)

Battery is 8kWh. 

  • Hi Jason,

    I am asking my colleagues if they have a circuit to share for this application.

    Regards

    Peter

  • Thank you, I would appreciate that

  • Hello Jason

    SO - the current sensor at the top of the H bridge is used to control the peak current on each switching cycle. It's the current sensor used for Peak Current Mode control. It's on the Webench schematic and is also present on the PMP8740 schematic - (T1 on sheet 1)

    The UCC28950 will limit the peak current signal to a maximum of 2V at the CS pin so that provides a 'safety net' which the INPUT current cannot exceed.

    However, you want to have an adjustable OUTPUT current limit and for that you need two output error amplifiers - one sensing Vout (U1A) and the other sensing Iout (U1B). This arrangement is basically the same as the one in this document

    /cfs-file/__key/communityserver-discussions-components-files/196/CI_5F00_CV.pptx.

    The way to turn this into an adjustable constant current is to change the reference voltage used by the I_out Error amplifier

    Regards

    Colin

  • Dear Colin,
    Thank you for your quick responses, I almost done with my design I just need to finish the sensing part. 
    Are these MOSFET's neccesary? with the PWM signal. I want to make it as passive as possible. If not with what can I replace them or how does the schematic look then?
    Also where is the voltage divider for VREF in the amplifier schematics, normally they use 2x 2.37kohm (R1,R2) for 2.5V that goes back to EA+ of the UCC28951-Q1 chip. Just like:
    And is R13,R19 at the V_outamplifier the same voltage divider on the output as R6 and R7? 
    With kind regards,
    Xiao Feng Lin
  • Also can I use this transformer:

    750343382

    https://www.we-online.com/web/en/electronic_components/produkte_pb/products_pbcm/product_spotlight/tida00951.php

    for my application? Because I don't really know how to make a centertap transformer myself. 

  • Hello Feng Lin

    I've had to ask the designer of the circuit to explain the reason for the two small signal MOSFETs - I hope to have an answer in a few days.

    The PMP8740 uses a TL431 to generate a 5V reference for the error amplifier - this is sometimes necessary if the 1.5% accuracy of the on-board reference is not good enough. You can see that the +5VREF signal comes from the cathode of U8 on sheet 3. In your case, I'd suggest you use the on-board reference and the 2 x 2.37k resistors to generate the 2.5V reference for your circuit.

    R13 and R19 have the same function as R6 and R7. There are two small differences - R12 is a small resistor added in series with the voltage sensing loop  as a convenient point to inject a signal during loop gain/phase testing - that's the reason for the 'Voltage Loop' note. R39 does the same for the current loop.

    The designer added R14 as a way to modify the output voltage slightly using the Parallel Bias signal. You won't need this resistor unless you are planning to operate two or more of your design in parallel .

    Regards

    Colin

  • Hello Feng Lin

    I got this answer from the designer -

    "Both MOSFETs Q2 and Q4 form actually a “poor man” DAC: when the microcontroller provides a PWM signal (digital level) to PWM_Vref and PWM_Iref, these two square waveforms are translated to a waveform on each drain with the same frequency (but with duty cycle 1-D) and with a peak value = the reference voltage +5Vref. After low pass filtering of this waveform through the double RC filter (for example R24, C7 and R23, C6), we get a DC reference proportional to +5Vref and the duty cycle of the PWM."

    So - they are basically a way to modify the reference into the error amplifier. If you don't need to do this then you don't need the MOSFETs.

    Regards
    Colin

  • Dear Colin,

    Thank you for the informative insight, can I send you my total PCB schematics for a small review? Maybe I am missing something or maybe I am

    doing something wrong I would appreciate it.

    My total schematic:

    Full Bridge converter and Control Unit.pdf  

    I am planning to order the PCB on wednesday, I am just not 100% sure about the transformer can you also check the if I can use this transformer:

    https://www.we-online.com/web/en/electronic_components/produkte_pb/products_pbcm/product_spotlight/tida00951.php

    750343382 it is used as an bi-directional transformer. Can I also use this transformer as an stepdown transformer instead of a stepup?

    With kind regards,

    Xiao Feng Lin

  • Hello Feng-lin

    I should be able to do that tomorrow - I'm based in Ireland so there's a time difference.

    Regards

    Colin

  • Hello Feng Lin

    I neglected to answer your transformer question -

    Yes - the transformer 750343382 can be used as a stepdown transformer instead of a stepup?

    Regards

    Colin

  • Colin Gillmor said:

    Hello Feng Lin

    I neglected to answer your transformer question -

    Yes - the transformer 750343382 can be used as a stepdown transformer instead of a stepup?

    Regards

    Colin