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UC1525B for Active clamp forward application

Mahesh KC03
Intellectual 1360 points
Other Parts Discussed in Thread: UC1525B, UCC2897A, UC1823, LM5026, UCC2891, LM5025, LM5034, UC1825-SP, UC1825, UC1843-HIREL

Hi,

One of my customer wants to use the UC1525B for an active clamp forward application. Given that it has 2 PWM outputs with adjustable dead time, can it be used in such an application? Are there any other considerations to be taken care of?

  • 0
    TI__Expert 4015 points
    Because of the required orientation of the body diode of the Active clamp FET, either a high side N-FET is required, or more typically, a P-FET is used. See this link for a description of active clamp operation. www.ti.com/.../snva591.pdf .
    For this reason the outputs from an active clamp controller are typically non-complimentary. I would recommend looking at the UCC2897A.

    Hope this helps,

    Thanks

    Billy
  • 0 in reply to Billy Long
    TI__Intellectual 1360 points

    Hi Bill,

    Thank you for the answer.

    If we use an N-FET with a high side gate driver circuit/IC, this device can still be used for driving the active clamp. Am I right?

    As shown in the figure shown below, the high side N-FET offers other advantages like a lower clamp capacitor rating and lower RDson. 

    Instead of UCC2897A, can we use UC1823 which has 2 in-phase outputs with programmable dead time to drive a P-FET based active clamp?

  • 0 in reply to Mahesh KC03
    TI__Expert 4015 points

    Hi Mahesh, if you use a high side driver you can use the UC1525B to drive an active clamp I believe, but as each output is only active half the time, you will have a maximum duty cycle of 40% which is probably not ideal. You would probably be better off using a single ended controller with a higher duty cycle and generating the compliment of the gate drive externally.

    The UC1823 does not look like a 2 output controller to me. It appears to be single output.

  • 0 in reply to Billy Long
    TI__Genius 9130 points

    Mahesh, Billy,

    Maybe I can add more info here to help.

    The datasheet for UC1525B says that it's not recommended for new designs, in big red letters across the top of every page - so really, it's best avoided.

    The UC1525B is a very old bipolar IC (well over 20 years!), so it has very high bias power consumption 14 mA (not including gate drive current). It's also voltage-mode only, and has very low output drive capability (200 mA).

    Finally, the 2 outputs are driven to the same duty cycle D (D < 50% max) but phase-shifted 180 degrees - it's a push-pull or half-bridge controller, so not suitable for active-clamp forward, where the outputs need to be D and (1-D) with appropriate dead-time.

    You should consider one of TI's dedicated active-clamp forward PWM IC's, most of them are current-mode, some voltage-mode, all have the necessary dead-time control built-in, and can readily drive an external PFET clamp. Some can be configured to drive an NFET clamp via external high-side driver.

    LM5026 - active-clamp, current-mode, single channel, integrated startup

    UCC2897A - active-clamp, current-mode, single channel, integrated startup

    UCC2891/2/3/4 - active-clamp, current-mode, single channel, integrated startup (1/3 versions only)

    LM5025/A/B/C/D - active-clamp, voltage-mode, single channel, integrated startup to (various feature differences)

    LM5027A - active-clamp, voltage-mode, single channel, integrated startup, SR drive

    LM5034 - active-clamp, current-mode, dual channel interleaved, integrated startup

    I attach a spreadsheet which compares the above IC's by key features and parameters, which may be helpful.

    active_clamp_IC_parameter-data.xlsx

    I hope this helps answer your question, if so please click the "verify answer" button.

    Thanks,

    Bernard

  • 0 in reply to Bernard Keogh
    TI__Intellectual 1360 points
    Hi Bernard,
    Thank you for the comprehensive comparison table.
    The customer needs a space/mil qualified device for his application. So, the LM and UCC series will not be suitable for this application. I went through UC1825-SP which is a dual output controller with adjustable dead time. The duty cycle can be varied up to 85%. Using a high sde drive, we can drive an N-FET clamp.

    Would this be a good substitute?
  • 0 in reply to Mahesh KC03
    TI__Genius 9130 points
    Mahesh

    The UC1825 would not be suitable - again this is a dual output push-pull controller - OUTA/B are driven with the same duty cycle (up to50% max) and phase-shifted 180 degrees wrt each other - see the design example on page 12 of the datasheet, where the IC is used to control a push-pull power stage.

    If there is no suitable hi-rel/rad-hard active-clamp controller available, then you can use any single-ended controller to drive the main FET, and add external circuitry to drive the clamp FET with the complement of the main drive signal, with appropriate dead-time. One possible controller would be UC1843-HIREL.

    I hope this helps answer your question, if so please click the "verify answer" button.


    Thanks,
    Bernard