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2KW Design with UCC28950

Mark Walters
Prodigy 100 points
Other Parts Discussed in Thread: UCC28180, UCC28950, UCD3138, UCC28070

We are in the process of designing a 2KW AC/DC power supply with the following specs:

Input Voltage 85-250VAC

Output Voltage 200-350DC.

90%+ efficiency

We plan on using the UCC28180 as the first stage PFC that gives us around 400VDC. For the second stage we are looking to use the UCC28950 for isolation and for stepping-down the DC voltage to our output range.  It looks like the UCC28950 can scale up to our power needs but we are suspicious if the higher output voltage is acceptable.  

A few questions:

1.  The design excel sheet for the UCC28950 (SLUC222C) shows a proposed power transformer turns ratio of 1.  Will this work as expected?

2.  Is there any concerns about running at this power level with the higher output voltage range?

3.  Are there any other converters ICs that may be more appropriate for our needs?

4.  We would like to control the output voltage via a microprocessor.  Is there a recommended method for this?

Thank you

  • 0
    TI__Genius 13780 points
    Hello, Mark,

    About the last question, you can use UCD3138 family digital controller. It can support PSFB PCM. You can get more information from Ti website.

    Regards,
    Sean
  • 0 in reply to Sean Xu
    Prodigy 100 points
    Thanks Sean for the info. However, we will already have a microprocessor on the board. Based on the datasheet app notes, R1/R2 set the voltage amplifier reference voltage (V1) while R3/R4 set the DC output voltage(Vout). Since the formula for Vout uses V1 can't we just put an DAC/Buffer combination on V1 to dynamically change Vout? It looks like a +-1V change to Vref would easily cover the entire desired range of Vout. Any thoughts?
  • 0 in reply to Mark Walters
    TI__Genius 13780 points

    Mark,

    I think you are right. You can use this way to adjust Vout.

    Regards,
    Sean

  • 0 in reply to Sean Xu
    TI__Mastermind 25570 points

    Hi Mark

    Just a few thoughts -

    A single stage PFC at 2kW is at the upper limit of this topology. The normal alternative is to use an interleaved PFC (UCC28070) - each stage transfers 1kW and the interleaving gives you some ripple cancellation so that the output capacitor (bulk cap) has an easier life. The EMI filter design is also easier because of the ripple cancellation at the input to the PFC stages - of course this works only if the two PFC stages are running at 180degrees to each other. Anyhow, you should look at the currents in the PFC stage for both designs and see which gives you the best cost / performance trade off.

    The Phase Shifted Full Bridge (PSFB) is the ideal topology to use at this power level - especially when working  from a 400Vdc input.

    A 1:1 turns ratio on the transformer may not be optimum, Vo is 200V min so D would be 50% and 350/400 = 87%  - given that there will be some ripple (at 2 x line freq) on the 400V bus the duty cycle will likely be even higher than that so ...  I don't know how many turns your transformer will have but assume you have 25T on the primary - then choose the secondary turns to give you a duty cycle range which is more 'centred' - 30% to 60% for example.


    The controller has no direct knowledge of the output voltage of course - it just sees scaled representations so if the scaling is correct they you should be ok.

    At this high voltages you will get very little if any benefit from using Synchronous Rectifiers - I'd suggest that diode rectification would be better.

    I'd agree with Sean about using a microcontroller -

    Regards

    Colin