UCC2897A: Secondary Side Rectification - What is D2 doing in this circuit..

Andrew Frazer

Part Number: UCC2897A

This is circuit is a reference design for the UCC2897A. The schematic is below. It is using a VB10150C-M3 to recifiy on the secondary side of the transformer.

As drawn, i've got no idea what the lower diode in the package is doing? Does it do anything at all.. It appears to me its always going to be not conducting?

Any insights would be greatly appreciated.

over 7 years ago

0 Peter Meaney over 7 years ago

TI__Mastermind 31580 points

Hi Andrew,

Unfortunately the image did not come through, the VB10150C-M3 is a dual diode and the original designer may have only used one half of the device. It is more common to connect the two parts in parallel if they are in the same device.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

www.ti.com/.../tidrgw2.pdf sorry

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

The diodes are in parallel and the should be sharing the current equally as their Vf should be very close and they will be running at the same temperature.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

As drawn the Diodes Anodes are not in parallel, one Is attached to either side of the transformer.. This doesn't make snese.

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

Ok I see it now :) It may be a schematic error, I will contact the designer.

Thanks for spotting this.

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

I had a look at the photo of the board in the app notes.. D2 is just to the Right of the transformer. The way its placed its as per the schematic

made

0 Andrew Frazer over 7 years ago in reply to Andrew Frazer

Expert 2305 points

Hi.. I've just released that this is NOT a flyback, but a forward topology and those diodes are where they need to be!

0 Bernd Geck over 7 years ago in reply to Andrew Frazer

TI__Intellectual 2770 points

All, looks as this discussion refers to my PMP10202; that's an Active Clamp FORWARD; so the upper diode is the FORWARD rectifier, the lower diode is the FREEWHEELING rectifier to ground; pretty similar to a buck power stage. Don't leave it out - the upper rectifier won't like the inductor voltage if you cut off inductor current. Please refer to my PMP5711, at "User Guides" you could download my "Active Clamp Technology Presentation" explaining the circuitry, have fun & best regards, Bernd

0 Andrew Frazer over 7 years ago in reply to Bernd Geck

Expert 2305 points

Thanks Bernd thats really helpful. Yes in deed, it now makes sense.. Strangely i found your presentation just this afternoon and it all started making sense. Sometimes you just need a little help to get going.

I do have some questions.. This is nearly what i want.. The output voltage should be 33V, @ 3A. the input range is very similar.. 40-55V DC.. I'm very interested to make it as efficent as possible as the incoming power is quite restricted, and every Watt counts. What needs to be done to change the rectifiers to something active.. And how do i dtermine what the output voltage will be.

0 Bernd Geck over 7 years ago in reply to Andrew Frazer

TI__Intellectual 2770 points

Hello Andrew, performance means cost, here some additional silicon cost to drive the synchronous rectifiers. So if you need max. effcy just see Brian's PMP9127, providing >94% effcy for your load range and input range. At min Vin 40V it'll work for 33Vout. Output voltage is adjusted by output voltage devider. Size is just a quarter brick, hope this helps, best regards, Bernd

0 Andrew Frazer over 7 years ago in reply to Bernd Geck

Expert 2305 points

Thanks. That is really helpful. In this case, the extra complexity is worth paying for. Just to clarifiy R28 and R32 are the resistor divider to modify.

Could you suggest a good reference guide on how to design these kind of supplies.. I have a number of variations to develop.

0 Andrew Frazer over 7 years ago in reply to Bernd Geck

Expert 2305 points

The transformer in this design is somethign that does'tn seem to exisit, any longer.. Do you think htat something like a POE120PL-24 would be suitable?

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

Did you contact Pulse Engineering to inquire about the PA4017NL? Generally these transformers are custom designs and Pulse should be able to re-create them. If not Pulse should be able to recommend the closest equivalent device that they have.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

No, i did'tn. I'd rather not use anything custom, as that often ends up being a problem from a procurement perspectivel

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

I understand. I will ask Bernd to comment on the suitabllity of the other part and he is the original designer but normally the transformer designs are specific to the design. The custom element may be just restricted to the turns ratio used, the bobbin and core are most likely standard off the shelf parts.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

Do you know if there are any calculators/spreadsheets for this Chip? ( its not webbench enabled i know ).. I'm struggling a little with following the calculations, then i could verify this myself..

Is this chip in any SPICE models?

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

I got the following information from the original designer concerning the transformer and output inductor design.

- windings ratio 2:3 to achieve good coupling, results in
- duty cycle 38%..57%, that’s fine
- magnetizing inductance 200uH, results in magnetizing current 450mApk, so around 10%, OK
- windings ratio to auxiliary winding 2:1, auxiliary current <50mA
- output inductor 100uH, inductor ripple in between 20% and 30%, OK

The POE120PL-24 has a primary inductance of 50uH and the designer recommends 200uH so the POE120PL-24 is not a suitable replacement.

There is a PSPICE model on the product webpage, www.ti.com/.../toolssoftware

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

Where does the inductance get specified? I cant' find this in the datasheet.

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

The 50uH is specified in the table on the Coilcraft webpage for the POE120 device.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

Sorry, i mean the 200uH, which teh designer is talking about..

0 Peter Meaney over 7 years ago in reply to Andrew Frazer

TI__Mastermind 31580 points

Hi Andrew,

There is a design guide on the product webpage, www.ti.com/.../slua535.pdf which does run through the design procedure for the power transformer.

Regards

Peter

0 Andrew Frazer over 7 years ago in reply to Peter Meaney

Expert 2305 points

Thanks.. that really helps!

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UCC2897A: Secondary Side Rectification - What is D2 doing in this circuit..