using two same switching transformer with DPA426R

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hello, I want to know could connect two same transformer with DPA426R and use one bias with it ?
i want to have 8 output with PQ20/20 core
Vin 16 - 32v
Vout 8 * 12v - 500mA

and can help me about this notification in Pi Expert

"The estimated clamp voltage it too high and could result in a Drain voltage that exceeds the BVDSS (200 V) of the internal Switch of the PI device."

how can resolve it ?

Hello vahidmizbani,

Thank you for interested in using Power Integrations product.

I believe the idea you have makes sense. While i am not sure how you will implement it. Could you send us or ask our FAE to check your final design schematic?

For the voltage stress, you can either reduce the leakage inductance (like use sandwich winding) or change the clamp to a zener type clamp. In this way, you can limit the voltage below the BV specification.

Best Regards

I think it's simple, a standard DC-DC converter using DPA switch, Connecting first transformer as schematic with bias winding & second transformer, connecting like first one but without bias winding.
but the problem i have is the regulation.I need full regulation, so need to higher voltage & post regulation circuit.
ok need to prototype it and test.
is there any problem when load ?

what is the best bias winding for multi output converter ? is the primary side or secondary ?

Hello vahidmizbani,

i am a little bit confused here. you have 8 outputs. So you need tight regulation for all of them?

It is not recommended to connect 2 parallel transformers to DPA switch as you will get separate current for each winding.

Best Regards

I think you didn't understand it well.
yes, I need regulation for all outputs but with post regulation circuit, not the bias regulation.

I breadboarded the circuit, Its work like a charm with two transformer.
but i need to know about the clamp circuit. is need to change it ? or other component for stabilize the circuit in full load condition ?

Hello vahidmizbani,

you can change R1 and C2 to a zener diode for the clamp circuit.

Best Regards

Hello, thanks u for answering my questions
what is "Selected Value of compensation capacitor is smaller than recommended. This may result in smaller phase margin " ?
what can do for it ?

How use zener clamp in Pi expert ?
i just find the TVS clamp and the software say to me it is not recommend
another question is can i replace SB180 with 1n5819 in this circuit ?

Hello vahidmizbani,

I do not have the transformer design details here. 1n5819 has smaller peak reverse voltage rating than SB180. You need to make sure the peak reverse voltage is not exceeding 40V on the diode if you want to use 1n5819.

For the warning of phase margin, it is recommended you build and test the phase margin to see if it is acceptable to your requirement. Otherwise you need to tune the feedback network values to get more phase margin of the design.

Best Regards

Sorry, forgot Transformer detail
now attach it

TitleSize
Transformer Detail1.25 MB
Schematic805.51 KB

can u help me about zener clamp ?
in Pi expert just find tvs clamp and rcd clamp mode
and i added the transformer detail for my last questions

Hello vahidmizbani,

I think the the TVS clamp in PI expert should work for your design.

Best Regards

Hello PI-YY, thanks you for answering
which TVS and Blocking diode is recommend for my circuit ?

"Compensation capacitor CF1 is too small. This may result in too little loop gain. It is better to oversize this capacitor somewhat to guarantee stability in discontinuous conduction mode."
where is CF1?

Hello vahidmizbani,

for the TVS, 100V-150V, 150A should be fine. when using 150V TVS, you can short R3.

for the diode, 200V voltage rating standard or fast recovery should be fine. You should verify the EMI accordingly.

CF1 should be the capacitor between C pin and F pin which is C3.

Best Regards

Thank you for answering.
You said" You should verify the EMI accordingly.", how can i do this?
Is c3 need to be low ESR and is need to have higher voltage rating as bias voltage ?
i have a DPA426R that does not work with less than C3 - 1000uF and blinking, Why ?
Another DPA426R I have takes 20 seconds to start when I plug in the power source.
is the chips have problem ? I killed 2 DPA426R till now.

I bought ten more DPA426R to test

Hello vahidmizbani,

See my response below:

You said" You should verify the EMI accordingly.", how can i do this?

EMI is a standard test for Electro-Magnetic Immunity Test. You can do this yourself if you have the setup. If not, you can find some test labs in your town or city nearby. If you do not have the specification for EMI, then you can ignore this.

Is c3 need to be low ESR and is need to have higher voltage rating as bias voltage ?

Low ESR is preferred. while normally X7R 0805 should be good enough. the diode in your schematic should be fine.

i have a DPA426R that does not work with less than C3 - 1000uF and blinking, Why ?
Another DPA426R I have takes 20 seconds to start when I plug in the power source.
is the chips have problem ? I killed 2 DPA426R till now.

if you buy the chip from the formal distributor, it should be fine. You can send the parts to our local FAE to double confirm. This issue needs more detailed debug, You can also find our local FAE for support if needed. you can find the contact information in:
https://www.power.com/sales/

1000uF capacitor seems to be too big, you should refer to PI expert recommended value.

You can also refer to DER120 as it also has 3 outputs for the feedback loop debug.
https://ac-dc.power.com/sites/default/files/PDFFiles/der120.pdf

best regards

Hello PI-YY, Merry Christmas
"1000uF capacitor seems to be too big, you should refer to PI expert recommended value."
that my fault, i mean C4 in my schematic not C3.
my circuit work with C4 - 1000uF and with less value just blinking and turned off
is this ok?
another question is How much current is needed at for the input ?