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LM5026: How do one connects a N-MOSFET for resetting the core of the forward converter?

Frederic Esposito
Prodigy 10 points
Part Number: LM5026
Other Parts Discussed in Thread: UCC2894

LM5026: the datasheet indicates 2 reset possibilities: P-MOSFET & N-MOSFET.

Plenty of examples for the P-MOSFET and it works.

Not a single example for connecting the N-MOSFET and any attempt to use one (in the spirit of the chip) has blown the LM5026.

I was hoping to speak to a Ti application Engineer to know what was intended, but no... just a forum, after 4 days of fight with a registration (error 403!).

How do one connects a N-MOSFET for resetting the core of the forward converter?

  • 0
    TI__Mastermind 26025 points

    Frederic

    Active Clamp Transformer Reset, section 4 shows the proper configuration for a high-side clamp forward converter using the N-channel clamp MOSFET you are describing. Regarding the LM5026, you need to configure the output specifically for driving either the P-channel (low-side) or N-channel (high-side) clamp MOSFET. The configuration for high-side clamp is described in section 7.3.3 of the LM5026 data sheet and specifically pay attention to the figure below. The resistor connected to the TIME pin is referred to as "RSET" and for p-channel (low-side) active clamp, the RSET is connected between the TIME and AGND and this is the configuration you will see in most applications. For your high-side clamp application, you need to connect the RSET resistor between the TIME pin and REF pin and set the desired dead-time according to what I'm showing below.

    Once you have the OUTA, OUTB phasing configured and the dead-time set as you require, you need to level shift the high-side gate drive and this is commonly done with a gate drive transformer as shown below. Details for designing gate drive circuits and gate drive transformer circuits can be seen in section 7.1 here. This video is another source for the fundamental introduction to designing transformer coupled gate drive circuits.

    Attached is a ref design showing the use of a high-side N-channel clamp but this is based on UCC2894. LM5026 will be similar. Below is the gate drive and active clamp portion related to what I mentioned above:

    PMP5425_REVA.pdf

    Regards,

    Steve