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UCC28220: Interleaved 3 Switch Forward, FET damage

younghyun kim
Prodigy 50 points
Part Number: UCC28220

HI

I designed an Interleaved 3 Switch Forward circuit using UCC28220. (3 Switch Forward circuit referred to TI patent data. See Attachment. File: 04 Overcoming the Duty Cycle Limits of the Two-Switch Forward - EE Times.pdf)

I designed a 1000W (12V, 84A) SMPS with Interleaved 3 Switch Forward. 

However, the output operation of more than 100W does not work and the FET is shorted Damage.

1) Please check if there is a problem with the circuit and transformer design I designed. (frequency setting: 250Khz, Duty 46.79%, For detailed specifications, refer to the attached simulation data)

2) Also, is the duty of 67% of 3 switch forward suggested by ti really possible?


Thank You.
01 SCHEMATIC - Three Switch Forward (1000W, 12V, 84A).pdf
02 TRANSFORMER - Three Switch Forward (600W, 12V, 50A).pdf
04 Overcoming the Duty Cycle Limits of the Two-Switch Forward - EE Times.pdf
03 Simulation.zip


  • 0
    TI__Guru 57825 points

    Hi,

    It looks like there will be some competition between D3 and D4 when Q1, Q2 and Q3 turn off, that is one in conduction the other cannot unless both diodes along with both winding voltages are exactly the same. Then the MOSFET associated to the diode not in conduction would have high Vds until that diode becomes conducting. During the time when diode not conducting the current and the inductance will create high voltage spikes to apply to the associated MOSFETs.

    It sounds to me this is the root cause since at lighter power the current peak is low so possible associated spike peak is low. But along with increasing power, the current gets higher, the diodes conducting time still quite the same, the results spike peaks become higher and higher, when reaches over Vds rating, the FET will break down.

    It sounds like to me you need to have higher voltage rating MOSFETs along with faster turn on speed diodes to avoid the MOSFET damage.

  • 0 in reply to Hong Huang
    TI__Guru 57825 points

    Hi,

    The max duty D = 67% is based on the volt-seconds balance.

    ton x (Vin/2) - toff x Vin = 0

    (ton/T) x (Vin/2) - (toff/T) x Vin = 0

    D x (Vin/2) - (1-D) x Vin = 0

    D/2 -1 +D = 0

    (3/2) D = 1 

    D = 2/3 = 67%

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Thank you for your advice.

    The reset diode I've been using is a US3M(Ultrafast Rectifirer). US3M's trr is 70 ns. 

    Following your advice I changed the diode to a MURS360(Ultrafast Rectifirer). MURS360's trr is 50ns.

    As a result, I think there is about a 70% improvement. Now it works up to 250W output. (However, above that 250W output the FET becomes damaged as the feedback becomes unstable. I think this a different kind of problem. The 3 switch forward circuit is easily damaged if the feedback is unstable. It seems that the reset of the magnetization energy is unstable and the problem is occurring. I'll ask about feedback circuit design later.)

    <Question>

    If the reset diode is replaced with a Schottky diode, can better characteristics be obtained? Are there any side effects? I would like to apply two 300V Schottky diodes in series.

  • 0 in reply to younghyun kim
    TI__Guru 57825 points

    Hi,

    D2 and D3 turn on time instants corresponding to the operations are likely the main concerns - if they are not on in time, the spikes can damage the MOSFETs.

    It looks to me this configuration is a trade off to obtain max duty cycle up to 67% (in theory) from higher voltage ratings of the MOSFETs due to D2 and D3 turn on time cannot be overlapped unless the design is with perfectly same between the associated circuit. If you need to get up to 67% Dmax, then you would have to deal with these additional challenges.

    I do not know how much schottky diodes can help - they might help from their faster speed - but still the MOSFETs voltage ratings still need to be adequately selected. Also it looks it may help from smaller leakage inductance.

  • 0 in reply to Hong Huang
    Prodigy 50 points

    Hi.

    I would like to know the design of the pulse transformer for driving FET in a three-switch forward circuit.

    Although the 3 switch forward circuit can operate up to 67% duty, isn't the pulse transformer only possible up to 50% duty? (also, Current transfomer)

    Can you connect me to john bottrill, the creator of 3 switch forward circuit theory?

    Please share john bottrill's email address with me. (I know that he is not currently an employee of ti company. Please help me if there is any way to contact him privately.)

    Thank you.

  • 0 in reply to younghyun kim
    TI__Guru 57825 points

    Hi,

    Please refer to the below application notes for duty > 50% when using current sense transformer and single ended gate driver. So both of your concerns are ok to resolve.

    https://www.ti.com/lit/an/slua174/slua174.pdf?ts=1691383670048&ref_url=https%253A%252F%252Fwww.google.com%252F#page39

    https://www.ti.com/lit/ml/slua618a/slua618a.pdf?ts=1691372608706#page3

    (section 7 for gate transformer)

    John is no longer with TI.