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- 19th November 2008, 21:17 #1

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## isolated buck converter

Hi. I am designing a forward converter. i am studying the circuit in this link. (page 9)

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Power%20Electronics/PDF/L-23(DP)(PE)%20((EE)NPTEL).pdf

The circuit must provide 5V DC beteen 0-4A. The literature about SMPS suggests to use continuous mode. But, to supply 5V without reload or with big reload (e.g 20ohm) requires to jump discontinuous mode. Because designing such a big coil is impossible. I am not sure about choosing proper LC filter. Because there will be two different calculations for two different modes. (LC filter and feed back cicuit as well) What do you suggest about the beginning of the calculations? [/img]

- 19th November 2008, 22:48 #2

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## isolated forward converter

Usually the dimenioning is calculated for nominal maximum load with a fair (e. g. 30% of load current) ripple amplitude. The change to discontinous mode has to be accepted. You can use a synchronous rectifier with constant current ripple to avoid it.

- 19th November 2008, 22:48

- 21st November 2008, 16:07 #3

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## isolated buck

Also, when using forward converters, a pre-load is usually used like 5% of the load to be in continuous mode. As you can see when you operate in discontinuous mode, there will be an oscillation at your output.

- 21st November 2008, 16:34 #4

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## dc dc isolated forward converter

Thank you for helping. Actually, I choose proper inductance and capacitor for the LC filter. Then I made my simulations for testing the ripple voltage on the load. The problem for me is the circuit must supply 5V DC output with less ripple even it doesn't have any load resistance. I obtain 1A ripple current and proper output with the circuit (39,4uH and 4,16uF ). But without load resistance it is getting really crazy. I also tried different duty cycles for solving this problem to observe that can the feed back control circuit correct this situation. But it doesn't change this. I really don't know what to do.

- 21st November 2008, 16:34

- 21st November 2008, 19:44 #5

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## isolated buck converter design

The problem may be in using a too small output capacitor with your buck converter. But as no real device or complete circuit parameters have been mentioned, I don't see what has been exactly simulated. You didn't give sufficient information.

With real buck controllers, the behaviour in discontinuous mode may be different in detail, but it wouldn't generally imply a higher ripple than at full load.

- 22nd November 2008, 12:31 #6

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## isolated buck dc/dc

Input voltage is 24V DC

Output is 5V DC

f=100kHz

And I found the parameters for 1A peak to peak ripple on the inductor.

D=0.208

L=39.6uH

C=4.16uF

And I tried bigger capacitances and also different duty cycles in without load resistance condition.(in simulations) But I always get big output voltage ripples such as peak to peak 8-10 Volts. When I changed the capacitances the ripple decreased till 8 Volts. And when I changed the duty cycle the output voltage started to fall. I still don't know that does the circuit give the same responses in the real practice. Because I can not decide the if the parameters are real.

- 22nd November 2008, 14:08 #7

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## dc/dc isolated

Sounds like an unsuitable way of controlling the buck converter.

- 22nd November 2008, 14:26 #8

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## constant current forward converter

You can visit at http://electricalengineeringtour.blogspot.com/

for lots of electronics ebooks links for free..

Thankss

- 22nd November 2008, 17:09 #9

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## isolated load buck converter design

Thank you.

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