UCC2897A: Recommendation for Sync. Rectifier controller of Active-clamp Forward topology

Hi Laisan,

One of the benefits of the forward topology is that you can use self driven SR's with some ease. What output voltage is you design and what concerns do you have with the self driven approach?

Regards

Peter

Hi Peter,

1). As mentioned output voltage is 12Vdc from 42 ~ 57Vdc input.

2). My concern on self-driven approach:

    a. No sensing functionality as ZVS or ZCS

    b. No any protection features as output short, OVP, OCP, brownout....etc

    c. Shoot through risk between high and low side MOSFET

    d. Wider input range

    e. Transition between DCM and CCM mode

    f. Deviation in MOSFET parameters

Above metioned disadvantages that discrest soultion using self driven circuit  are difficult to support it; therefore, would like to find controller solution to replace it.

Hi Laisan,

The Active clamp forward is often operate with very high switching frequency, due to the low switching loss. And it operates in CCM. This gives the challenges to the SR controller based on the VDS control. We observed huge efficiency drop once the converter enters the CCM operation. They will see the SR delivers lower efficiency comparing with diode during this operation mode.

The SR controller based on the frequency prediction actually works better. In this case, UCC24630 would be a better choice, at least for the freewheeling diode. It turns off the SR before the primary side conducts, as long as the switching frequency is fixed.

Another option is to use the UCC24610, which is able to take the signal from primary side so that it minimizes the shoot through, but it has its own bias and driving voltage limitations as it was designed for USB outputs so it is limited to 5V Vcc and gate drive level.

You can always use the isolated gate driver to do the job.

Regards

Peter

Hi Peter,

After viewed UCC24610/630 datasheet, two SR controllers are applicated for Flyback topology, I want Foward topology for my current solution using UCC2897A ACF controller. Would you please double check? if 24610/630 chip can support Active-Clamp-Forward, having reference circuit as evalution-board or datasheet for reference? Even two in one package or two chip drive high and low-side MOSFET are ok to me.

Hi Laisan,

The advice I gave came from the SR application engineers so I am confident that the UCC24630 and UCC24610 can be used for active clamp forward (ACF) topology. The biggest challenge is that the ACF will be in continuous conduction mode, CCM, for much of its load range. CCM causes the Vds sensing based SR controller to keep the mosfet on even though the winding voltage is changing and there will be a momentary short circuit before the SR turns off in CCM mode. The UCC24610 can accept an input signal across the isolation barrier to command the SR controller to turn off the mosfet.

In the ACF application I don't think that the use of SR controllers will add value to the solution, I would try to use the self driven approach if possible.

Regards

Peter

Please see both chips datasheet in your web-site (cover page showed 24630 for Flyback on features & schematic, 24610 for Flyback and LLC on applicatios & schematic), as my mentioned before for if 24610/630 chip can support Active-Clamp-Forward, do you have reference circuit as evalution-board or datasheet for reference?

Hi Laisan,

We do not have any reference designs that use SR controllers for the active clamp forward design, this is due to the fact that the topology can use self driven SR's and the use of SR drivers adds little value to performance.

As I said earlier the UCC24630 and UCC24610 can be used for active clamp forward (ACF) topology but when operating in CCM mode of operation there is a risk of momentary short circuits if the SR controller does not get a turn off command from the primary side.

If you go to TI.com there is a section on power supply training and one on active clamp technology, www.ti.com/.../slup262.pdf

It contains a schematic for a 12V output with self driven SR's as an example.

Regards

Peter

Finally TI doesn't have SR controller to support ACF Forward topology or reference ciruit using UCC24630/10 when operating in CCM mode, I would search the SR solution with other chip maker.