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UCC28600EVM-65W Output Control 15V@3A

Expert 2385 points
Other Parts Discussed in Thread: UCC28600EVM-65W, UCC28600, TL431

Dear Ti expert,

I am exploring the UCC28600EVM-65W and found below questions where need your help to clarify.

1) In my idea by adjusting the PWM duty cycle on Mosfet (Q1) gate is able to change the output of EVM. Can you share the calculation or method so that I am able adjust the PWM duty cycle EVM?

For example: Current output voltage is 18V and 70% duty cycle is applied. Then how am I able to adjust the duty cycle to 90% or 60% to so that it will give an increase or decrease of output voltage?

 

Q2. How to adjust the UCC28600 output PWM frequency in related with Mosfet (Q1)?

 

 

Q3. What is the function of below circuit? And how it work? (from EVM)

 

 

Q4. On the Optocouple feedback (U3), any specific type of optocoupler need to be used in flyback converter? If specific optocoupler is needed, then what is the criteria that we choose this specific optocoupler? 

 

 

Q5. What is the difference between QR mode control and Green-mode Control? 

 

Q6. Do you have any tutorial regards designing this type of power supply? Please share if any.

  • Hello N3t,

    I would like to direct you to peruse the wide array of documents available in the product folder pertaining to the UCC28600 on the TI website. Type “UCC28600” into your web browser’s search box and click on the TI link that goes to the UCC28600/Flyback Controller/ general webpage. (Don’t click on any [PDF] links, which will only bring up a specific document.) Once you are in the UCC28600 product folder page, you can find a series of sub-folder tabs across the middle of the screen. Click on the “Technical Documents” tab and it will bring up a long list of documents related to the part, including application notes and white papers. Information within these documents should help address the concerns in your questions.

    About Q1: Your understanding of direct duty-cycle control to change the output voltage is applicable to a fixed-frequency, forward-mode type of switched-mode power supply. The UCC28600 is a multi-mode quasi-resonant (QR) flyback controller. Although the output voltage can be related to the duty-cycle, it is not a proportional relationship. The paper “Compensation Design with TL431 for UCC28600” discusses the duty-cycle transfer function. For manual variation of the output voltage, the easiest way is to manipulate the resistor-divider ratio (R17, R18) at the TL431 input.

    About Q2: The feedback loop automatically adjusts the UCC28600 frequency to regulate the output. A QR flyback converter does not operate at a fixed frequency (except when clamped to a maximum frequency), and there is no single pin or parameter which gives one total control. The paper “Exposing the Inner Behavior of a Quasi-Resonant Flyback Controller” is a great source of information on how this part works.

    About Q3: The UCC28600EVM design provides several options for experimentation in the feedback control. The EVM is configured to regulate one output at 18 V (OUT1), but it has provisions for a second output (OUT2) and place-holders to populate these components by the user if one so chooses. The unused winding has a 16:1 turns-ratio with the primary, so it is apparently intended for a low voltage output, but I don’t know specifically what level; possibly for 5 V. Feedback components R19, R20 and R24 provide a means to re-direct regulation from the 18-V output to the unused output. R24 would connect the bias current path for the TL431 to OUT2, and R13, D7 and C22 (which provide the TL431 bias from OUT1) would be depopulated.

    About Q4: The specific CNY17-type opto-coupler used in the EVM is not critical to this design. Any general purpose analog opto-coupler (not one designed for digital signals) with sufficient bandwidth can be used. Other popular types include LTV-817 and TCMT1100 series parts, to name a few, but not limited to these only. The previously mentioned paper on “Compensation Design…” discusses the opto-coupler properties to some extent.

    About Q5: QR control is generally accepted in the industry to refer to flyback operation at the boundary between discontinuous and continuous current in the inductor (the transformer in a flyback converter is really a coupled-inductor), alternatively known as Boundary Mode, Transition Mode, or Critical Conduction Mode. The turn-on moment of the next switching cycle occurs at the first valley of the resonant ringing of the previous switching cycle to minimize switching loss.

    Green-mode Control is more a marketing term intended to emphasize a mode of operation that maximizes efficiency at light loads. The word “green” is often associated with eco-friendly features of a vast variety of products by any number of companies, to attract customers looking participate in environmentally sound endeavors. In the case of the UCC28600, Green-mode operation consists of short bursts of switching followed by periods of minimal bias current drawn by the controller. This mode keeps the output in regulation at light or no loads without incurring significant bias-supply losses during the intervening dead periods, which minimizes standby power drawn from the mains.

    About Q6: The UCC28600 datasheet contains an application section which goes through the design procedure, and this is further developed in the paper “Design Considerations for the UCC28600”. Please avail yourself to all of the documents listed in the “Technical Documents” folder, and other blogs and guides concerning the UCC28600. There is a lot of information out there.

    Regards,
    Ulrich