poor regulation & slow response for load transient at light load

Posted by: bhsohn on 11/15/2012

I have a problem due to poor regulation and slow response for load transient.

My circuit has muliti outputs(3.3V, 5V) and light load.

But basically circut and transformer are designed for high power.

At first there is no problem but a few samples have some resonance on Tvo (not Trz, Trn) during off time.

I think that this duration have to have the same voltage with VIN.

But now I can see resonance and late response for abrupt load transient.

It makes some problem on my board.

How can I remove this resonance?

I don't want to use different transformer.

If possible I don't want to change current circuit diagram.

Comments

I can't offer much advice without seeing additional information about your design.

Please reply with the following:

PI Expert design files

Power supply specs

Input/Output conditions where problems occur

Circuit schematic and PCB layout (PDF is fine)

Any other information you feel is relevant.

Regards,

The Traveler

I attached circuit diagram.

1. power is used for PoE.

2. VIN is from 36 to 57Vdc.

3. Vout is maximum 1.7A@3.3V and 1.1@5V

4 . trans. is 1.76mH. please see the trans. spec.

5. for layout, I don't have the file.

Please advise me about why the resonance is occurred and how to remove.

Can I improve the load transient response?

Best regards

bhsohn -

I'm pretty puzzled with your design. It looks like you're trying to build a forward converter with DPASwitch and synchronous rectification but you're using a flyback transformer that isn't connected correctly.

I would highly recommend downloading our PI Expert software as well as the DPASwitch datasheet and application notes. We also have numerous Reference Designs for you to look at and use as a starting point.

Here are a couple of the ones I found fairly quickly:

http://www.powerint.com/sites/default/files/PDFFiles/der29.pdf

http://www.powerint.com/sites/default/files/PDFFiles/der24.pdf

Good luck with your design,

The Traveler

Thanks for the response.

But I think that you misunderstand the forward transformer.

As far as I know, the dot position of the flyback transformer is opposite

In case of the forward, the dot position has the same.

My transformer also has the same direction.

And the switching pulse is ok in normal operation.

My problem is that it has some resonance in DCM mode at light load.

If I add some more load, the resonance is disappeared.

Instead, I can see the normal DCM pulse(VIN input dc level after trans. reset) in Vdrain pulse.

(I refered AN31 doc. for this)

Could you advise me.

Regards.

B.H.Sohn

I'm attaching our switching pulse.

In there you can see some resonance in DCM.

I don't understand why it is occurred.

Please advise me.

B.H.Sohn

Having the correct dot convention is an important difference between a forward and flyback transformer.

However, the two switching power supply topologies operate in completely different way. A flyback transformer is using the primary inductance to store energy while the MOSFET is ON and then the energy is delivered to the output when the MOSFET turns OFF. In a forward converter, the transformer has very little primary inductance (ideally it should be zero). When the MOSFET turns ON, energy is delivered into a seperate inductor on the secondary side.

Before you proceed any further, you really need to read up on the two power supply topologies and make sure you understand what you're trying to design. The two topologies have similarities but when it comes to the details they are incredibly different.

Regards

The Traveler