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UCC38086: Duty Cycle

Aswathy Kanth
Intellectual 700 points
Part Number: UCC38086
Other Parts Discussed in Thread: TL431, TLV431

Hi,

I have used TI webench to design a pushpull converter using UCC38086. As per the datasheet of UCC38086, the max duty cycle is 50%, but the webbench design shows the duty cycle as 90%. In the webench, I am not able to update the duty cycle information to change the design. Please advise.

Thanks & Regards,

Aswathy

  • 0
    TI__Mastermind 32885 points

    Hi

    Thanks for connecting through E2E. Can you please show waveforms or provide a link to your design? 

    Regards,

    Steve M

  • 0 in reply to Aswathy Kanth
    Intellectual 700 points

    Hope, the duty cycle change will affect only on the transformer design. Or do I need to entirely change my circuit for this 50% duty cycle change. Please advice. 

  • 0 in reply to Aswathy Kanth
    TI__Mastermind 32885 points

    Hi Aswathy,

    The duty cycle of your Webench push-pull is given by graph below as 64.5%<D<90%:

    This is the total dusty cycle seen by the transformer. The individual duty cycles for each of the two gate drives would be half this amount so that each PWM output should appear as 32.25%<D(OUTA/B)<45% and this is less than 50% as you asked about in your post. The push pull is a double-ended power topology (transformer operates in first and third quadrant) and so there is a multiplier of 2 that needs to be considered in the large signal gain equation as:

    Good luck with your design. I hope this clarifies why/how the duty cycle of a push-pull can be "greater than 50%?"

    Steve M

  • 0 in reply to Steven Mappus1
    Intellectual 700 points

    Hi Steve,

    Very useful information for me. Thank you!!!

    So, do I need to change my transformer design for 45% duty cycle? or can i stick with same 90% duty cycle. Please advice. Hope, if I change the duty cycle, then, transformer inductance, current etc. will remain same & only thing that will change is the number of turns.

    Regards,

    aswathy

  • 0 in reply to Aswathy Kanth
    TI__Mastermind 32885 points

    Aswathy,

    The PWM controller OUTA and OUTB are operating at Fs and Dmax less than 50%. The transformer sees the combination of OUTA+OUTB so it is operating at 2*Fs and see a combined Dmax of less than 100%. Ideal waveforms shown below:

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Intellectual 700 points

    Hi Steve, 

    Thanks for the reply. In my circuit, I am getting tl431 reference voltage as 0.8v, because of which I am getting output of 44v(regulated). How can I increase it to 150v? When I checked the datasheet of tl431,the max cathode to anode voltage shall be 36v. But, as per the webench output, it is coming greather than 36v. Will that be an issue, which is resulting in lesser output for me? 

    Regards,

    Aswathy

  • 0 in reply to Aswathy Kanth
    TI__Mastermind 32885 points

    Aswathy,

    Shunt regulators come in many different packages, many different options. Two of the most popular one used for feedback control in switching power supply design are TLV431 (1.25V ref) and TL431 (2.5V ref). For your output voltage of 44 V, you should be using the TL431 variant with 2.5 V reference. If you are seeing 0.8V refence, something does now seem right. Please read the data sheet to become very familiar with this important component necessary for controlling the output voltage of your power converter. Pay attention to the bias current into the cathode and make sure the TL431 is seeing at least 1 mA cathode to anode. Pay attention to the stability curves in the data sheet because these offer guidance about how much capacitance should be seen by the TL431. Also, the 36 V cathode to anode voltage you mentioned - this is the control voltage range and is not directly related to the regulated output voltage of your converter. You can control and regulate 1 kV or more output voltage with a TL431 rated for 36 V. When biased correctly in an isolated converter application, the cathode to anode voltage variation directly controls the opto current so you need to measure and make sure the opto current is varying proportional to the TL431 cathode voltage variation. The opto and the TL431 must each be biased correctly and ultimately the BJT side of the opto must be able to control the feedback/comp of the primary side PWM. Thankfully, when it comes to biasing and using the TL431, there are numerous app notes, blogs and plenty of online help available.

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Intellectual 700 points

    Steve,

    Thanks. My tl431 is a 2.5 ref one. As per the design, 15ma shall be there for 150v out & 10k combination. For 44v & 10k combo, 4.4ma shall be flowing.

    Apart from this, in the webench generated circuit, there was no resistance inserted between ctrl & vdd of ic. When , I inserted a 10k between ctrl & vdd of ic, then I started to get 44v in the output.with out that 10k, the circuit was not working.(no switching, no output). Now, we are trying to correct the output voltage to 150vdc.

  • 0 in reply to Steven Mappus1
    Intellectual 700 points

    Hi Steve,

    Now I am getting 150vdc output, after adding a 120v zener series in the feedback. But, when I load 0.25 A, the input current is gradually increasing and my 4A power supply is going to CC mode. When I checked the gate waveform, the duty cycle is 50% at 0.19 A. And after that it is fixed.i have used etd29 core with 2 & 20 turns. Output inductor used is 220uh. Pen frequency is 200khz. Getting efficiency of 80% at 0.25a load. switching. Can you suggest any solution,for the input current increase problem?

    Regards,

    Aswathy

  • 0 in reply to Aswathy Kanth
    TI__Mastermind 32885 points

    Hi Aswathy,

    The title of this E2E post is "UCC38086: Duty Cycle" and I believe we resolved the interpretation of UCC38086 duty cycle and now your questions are moving toward TL431 and control loop design. Can I suggest we close this thread since the subject of duty cycle is not longer the focus? If adsditional support is needed, can you open a new thread and title it according to your relevant support issue?

    Regards,

    Steve M