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UCC3895: UCC3895 Peak Current Mode Control method using MCU with DAC

Pierce Shi
Prodigy 10 points
Part Number: UCC3895
Other Parts Discussed in Thread: UCC2895, UCC28950, UCD3138

Dear Sir or Madam, I am currently using UCC3895 for Peak Current Mode control PSFB PWM generator. 

I read the datasheet and I set the current sensor value of 0-2V, the frequency 25K, and deadtime about 1us. 

The thing is, for advanced control algorithm, I need to pass an analog value from my MCU's DAC to UCC3895 wishing to set the Peak current value directly. 

However, I am not sure whether I am using it correctly by connecting the value from MCU to EAP and shorting the EAN and EAOUT. 

Another thing is, I see all reference schematic diagram using a transistor at CT, I am not sure whether it was for chopping off the current at peak current and I just copied the part of circuit 

from one design. Will my design of transistor work as expected? 

Could you help me to check whether there is any other issue of this circuit? 

Thank you very much! 

Best Regards

Pierce Shi Zhejian 

Electronics Engineer

  • 0
    TI__Guru 73475 points

    Hello.

    The current sense transformer is used to sense the primary current of the transformer.  It is measured at the input of the H-Bridge and is still needed in your design.

    You can control the peak current by setting the voltage amplimer up in a follower and controlling the voltage at the EA+ pin.  This seems to be what you are trying do.

    The following link will bring you to an application note on how to design a phase shifted full bridge using the UCC28950/1 devices. I think you will find this application note useful. This application note goes through the power stage and current sensing setup, loop compensation, and slope compensation.  The only thing this application note will not cover is how to setup the UCC2895 controller. 

    https://www.ti.com/lit/pdf/slua560

    The UCC3895 was designed for analog feedback.  When it comes to using a MCU and a DAC to create the feedback there is no applications notes to support this for the UCC2895 as far as I know.

    Texas instruments does have a digital power supply controller the UCD3138 that uses digital control.  It has been used in a phase shifted full bridge.  The following will bring you to a link on the firm ware that is required to make the UCD3138 a phase shifted full bridge controller.

     https://www.ti.com/tool/UCD3138FW-PSFB#:~:text=Overview,optimization%20and%20programmable%20slope%20compensation.

    If you want to look into the UCD3138 further and have questions regarding it.  Please repost in the e2e with the UCD3138 in the thread title so the appropriate applications engineer can support your questions.

    Regards,

     

  • 0 in reply to Mike O'
    Prodigy 10 points

    Thank you for your prompt reply, Mike.

    Firstly, Now I understand that setting the Peak current through EAP is possible and thanks for you info. 

    Secondly, Regarding Current Transformer, I do have a DC current Transformer which I could get a 0-2V pulsed DC output to RAMP and CS in the chip which is as you mentioned.

    Thirdly, Regarding feedback control, I am controlling by means of doing something like PID inside my MCU with either voltage or current feedback to its ADC, Setpoint given through digital interface, and MCU will calculate an output of PEAKCURRENT which will be setting 0-2V to UCC3895's EAP. Thus, I guess I don't need UCC3895 to perform slope compensation. However, I believe CS is a peak current "feedback" towards UCC3895 which could make sure current set was as required by EAP despite the fact that system Current and Voltage was unsure as those are controlled by Digital compensator inside MCU. 

    Lastly, I would still like to ask regarding my 3906 transistor in the circuit, I am not sure whether the Peak Current Control will work when I remove it from the circuit.

    Thank you 

    Best Regards

    Pierce Shi Zhejian

  • 0 in reply to Pierce Shi
    TI__Guru 73475 points

    Hello,

    I reviewed your circuit and believe the 3906 PNP transistor will cause errors with the oscillator timing.  So I would not recommend using that circuit.

    To setup the RAMP pin I suggest you use a NPN transistor (Q1) and resistors RH, RJ and RSLC which are represented below from figure 18 of the UCC3895 data sheet.

    Regards,