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Isolated Dual outputs DC/DC Converter

Loc Nguyen Huu
Prodigy 180 points
Other Parts Discussed in Thread: UCC25600

Hello Experts and Members

I need to design a isolated Dc/Dc converter, I need your help to looking for solution and selecting parts
Some information as below
-Input 27Vdc
-Dual Output +15V (at least 3A), -15V (at least 3A)
-Isolated inputs and outputs
-On/off signal to control output voltages
-Pins are used to adjust output voltages
-Board dimension allowed: Length 50mm x wide 52 mm x height 10mm
I have concerns about power consumption on this board, if it is efficiency 90%, power dissipation on this module will be 4.5W + 4.5W = 9W, too hot. I think it needs higher efficiency so no heat-sink is mounted.
Please give me advice. I appreciate your support.
Thanks and best regards,
Nguyen Huu Loc

  • 0
    Genius 3295 points

    I'd try a boundary conduction mode flyback first. You'd have to use a synchronous FET in place of a diode to get above 90% efficiency. That 10 mm height specification is tough. You'd have to use a planar transformer. The planar would be ideal since it has low leakage inductance. The high capacitance is a problem for EMI noise, but you can absorb it in to a snubber.

    I have designed a BCM flyback that measured 94% peak efficiency. But the output was very high, 200 Vs.

    So maybe you need to use an LLC. Extra switches, but you have enough area. Use synchronous FETs on the output and a planar transformer. The parasitic capacitance can be absorbed into the resonant LC tank.

    Use 4 ounce copper if possible and parallel FETs to get the conduction losses down. You have area but no height, so take advantage of the area. You can turn you PCB into a heat sink the same way the LED guys do it. They use 4 oz copper and thermal vias on the drain and source tabs of their FETs. At 27Vs, you should be able to use 60 V FETs. RDS values at that voltage are very low, about 10 milliohm. 

    Sounds like an interesting design. Good luck with it.

  • 0 in reply to Chuck Sampson
    Prodigy 180 points

    Hello Chuck,

    Thanks for your observation. Your knowledge is very useful for me to research. Can you recommend me which controller should be used in this application?

    Best regards,

    Loc

  • 0 in reply to Loc Nguyen Huu
    Genius 3295 points

    Loc,

    I designed a 300W offline LLC half bridge a few year ago. I used the UC2860. The UCC25600 is a good chip to use as well.

    I used the Unitrode app note,U122, "A New Family of Integrated Circuits Controls Resonant Mode Power Converters" by Larry Wofford to learn about the UC2860PWM IC. This is an excellent reference for understanding the details of resonant mode PWM operation. I also recommend "Designing an LLC Resonant Half-Bridge Power Converter", SEM1900, Topic 3 by Hong Huang. This paper covers everything you need to know to get a design up and running.

    Huang uses the first harmonic analysis method for determining his compensation network. I followed his procedure and came up with a good stable design in a short while. Last, but not least, is a practical guideline for measuring the LLC Resonant Power Converter Loop gain: "Feedback Loop Design of an LLC Resonant Power Converter" SLUA582A.

    There is also a reference design available UCC25600EVM-341. It is a high-voltage design, so it wouldn't be much help for your design except to get used to the PWMIC functions.

    LLC Loop gain measurement

    Hong Huang LLC Design

    UCC25600 data sheet

    UC3860

  • 0 in reply to Chuck Sampson
    Prodigy 180 points

    I think I will use PWM current mode controller LM5034 . This controller have 2 independent channels that produce +15V/3A and -15V/1A. I have a little concerns about transformer configuration to create -15V.

    Can I modify the output channel by reversing the polarity of primary side of power transformer?

    Please advise if you have any concerns.

    Thanks,

    Loc