PWM to mosfet - using isolator or ?

Gerlu · 19th December 2004, 06:11 PM

I try to build a converter, but I am not sure how the connection is made from the output of PWM to mosfet. Would it require an isolator ? Can someone show me in diagram ?

The mosfet I used should be p-channel. And I need some suggestions on what mosfet I need, is RFP30P06 ok ? The incoming supply is 28V with switching frequency 100kHz.

The diagram is as below, any problem with that ?

I am new to this design and here too. Hopefully I am posting in the right place. Appreciate and expecting any suggestions. Thanks.

Gerlu · 22nd December 2004, 06:16 PM

AnyOne ? Plz ..

Petter · 22nd December 2004, 07:33 PM

Nobody is answering because your question is not answerable.

There simply isn't enough information to provide a meaningful answer.

May I suggest you use a reference design from one of the manufacturers instead?

Please note that there is limited competency in SMPS on this board - true some individuals are highly skilled but the traffic on SMPS is very limited.

Petter

Gerlu · 23rd December 2004, 08:17 AM

I see .. thx ! Will try your suggestion

Circlotron · 23rd December 2004, 11:02 AM

Have a look at an IR2121 or an IR2125. Not sure which one but they are similar.
Seeing your converter will have a negative output voltage, will the error amplifier input on the controller chip work ok with this?

Gerlu · 23rd December 2004, 01:29 PM

Quote:

Originally posted by Circlotron
Have a look at an IR2121 or an IR2125. Not sure which one but they are similar.

Ok, will check it out.

Quote:

Originally posted by Circlotron
Seeing your converter will have a negative output voltage, will the error amplifier input on the controller chip work ok with this?

Yup, by first inverting it with another error amplifier (inverting amplifier -- positive input to ground)

raypsi · 14th January 2005, 03:52 PM

Your schematic shows 2 grounds, ya? So either a toroid transformer or opto isolator are required. Usually the feedback reference is isolated too.

sivan_and · 14th January 2005, 04:38 PM

Somewhat confusing circuit.
If u are looking for a step down conv,,,Try a single chip solution like MAX 724.
www.MAXIM-IC.Com
For MAX 724. i/p can go upto 40 Volts and O/p Variable can sink upto 5 Amps.

Or SG3524 (both o/p added for max duty cycle.and IR2110 or any high side driver.

raypsi · 14th January 2005, 07:56 PM

Your circuit looks ok to me. look up an 4n36 here:

http://www.fairchildsemi.com/ds/4N/4N36.pdf

Page 25 has a schematic how to connect an opto coupler:
http://focus.ti.com/lit/ds/slus565f/slus565f.pdf

sivan_and · 15th January 2005, 02:48 AM

Even the outputs can be connected directly to MOSFEt w/o isolator,if u r using SG3524 or TL494.Since the IC output transistors collectors and emitters are easily accessible,can be configured freely...

23rd December 2004, 11:02 AM	#5
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	Have a look at an IR2121 or an IR2125. Not sure which one but they are similar. Seeing your converter will have a negative output voltage, will the error amplifier input on the controller chip work ok with this? __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

14th January 2005, 03:52 PM	#7
raypsi diyAudio Member Join Date: Jan 2005 Location: middle earth	pwm Your schematic shows 2 grounds, ya? So either a toroid transformer or opto isolator are required. Usually the feedback reference is isolated too. __________________ semper fi

14th January 2005, 04:38 PM	#8
sivan_and diyAudio Member Join Date: Dec 2004 Location: Dxb / India	Somewhat confusing circuit. If u are looking for a step down conv,,,Try a single chip solution like MAX 724. www.MAXIM-IC.Com For MAX 724. i/p can go upto 40 Volts and O/p Variable can sink upto 5 Amps. Or SG3524 (both o/p added for max duty cycle.and IR2110 or any high side driver. __________________ SivanandBalan

14th January 2005, 07:56 PM	#9
raypsi diyAudio Member Join Date: Jan 2005 Location: middle earth	Your circuit looks ok to me. look up an 4n36 here: http://www.fairchildsemi.com/ds/4N/4N36.pdf Page 25 has a schematic how to connect an opto coupler: http://focus.ti.com/lit/ds/slus565f/slus565f.pdf __________________ semper fi

15th January 2005, 02:48 AM	#10
sivan_and diyAudio Member Join Date: Dec 2004 Location: Dxb / India	Even the outputs can be connected directly to MOSFEt w/o isolator,if u r using SG3524 or TL494.Since the IC output transistors collectors and emitters are easily accessible,can be configured freely... __________________ SivanandBalan


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22nd December 2004, 06:16 PM	#2
Gerlu diyAudio Member Join Date: Dec 2004 Location: M.H.	AnyOne ? Plz ..

22nd December 2004, 07:33 PM	#3
Petter diyAudio Member Join Date: Jan 2001 Location: Scandinavia	Nobody is answering because your question is not answerable. There simply isn't enough information to provide a meaningful answer. May I suggest you use a reference design from one of the manufacturers instead? Please note that there is limited competency in SMPS on this board - true some individuals are highly skilled but the traffic on SMPS is very limited. Petter

23rd December 2004, 08:17 AM	#4
Gerlu diyAudio Member Join Date: Dec 2004 Location: M.H.	I see .. thx ! Will try your suggestion

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