Isolated Feedback Design

Kekkaishi · 16th February 2010, 03:56 PM

Hi everyone.

Currently I'm designing the feedback for SMPS. I've searched throughout the forum but still didn't find the answer that I one. Hope that everyone out there can help me. I've done the main part, I mean the flyback part but hesitated in the feedback design.

Attached is my design. As all of you can see, I'm using TL494 as my PWM with TL431. I'm using type II compensated network and connect it to one of the error amplifier contain in TL494 and use the resistor divider network to get 2.5V as a reference so that it will be the same as the voltage from TL431. Am I designing it correctly, I mean the calculation and the connection part? Hope somebody can help me on designing this feedback.

My spec design are

Fs = 100kHz,
Vin = 100-240 V
Vo = 20V
Io = 5V
Efficiency - 85%

Sorry for my bad English and thank you for all your help. Please help me a newbie in designing this SMPS.

Corax · 17th February 2010, 09:00 AM

At a first glance two things are obvious:
1. driving the MOSFET with an open-collector output on a 22k pull-up resistor won't work at all. A push-pull configuration is necessary if you ever wanna reach a 100 kHz switching frequency.
2. you cannot leave the second error amplifier inputs floating. Either you use them, i.e. overcurrent protection, or you better 'disable' the functionality by grounding the (+) input and providing a positiv voltage to the (-) input. See also the internal block diagram of the TL494.
For the rest of your schematic I don't have an immediate statement. But I might tell you as soon as you've corrected the design so far.

Kekkaishi · 17th February 2010, 03:40 PM

Thank you Mr. Corax for replying. Ya I also noticed the 22k resistor. Attach is my latest schematic. Could you please have a look on it. Anything else wrong?

Cristi · 18th February 2010, 05:36 AM

consider using a current mode controller instead of voltage mode, UC384x series can be found anywhere. with this kind of controllers you can also make a 110-230V flyback smps without problems. there are a lot of schematics with this ic's.
if you wan a more compact design, use an integrated power switch, Fairchild, Powerint, ST and others offer this solutions.

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17th February 2010, 09:00 AM	#2
Corax diyAudio Member Join Date: Feb 2005 Location: south of lower saxon	At a first glance two things are obvious: 1. driving the MOSFET with an open-collector output on a 22k pull-up resistor won't work at all. A push-pull configuration is necessary if you ever wanna reach a 100 kHz switching frequency. 2. you cannot leave the second error amplifier inputs floating. Either you use them, i.e. overcurrent protection, or you better 'disable' the functionality by grounding the (+) input and providing a positiv voltage to the (-) input. See also the internal block diagram of the TL494. For the rest of your schematic I don't have an immediate statement. But I might tell you as soon as you've corrected the design so far.

18th February 2010, 05:36 AM	#4
Cristi diyAudio Member Join Date: Dec 2008 Location: Shenzhen	consider using a current mode controller instead of voltage mode, UC384x series can be found anywhere. with this kind of controllers you can also make a 110-230V flyback smps without problems. there are a lot of schematics with this ic's. if you wan a more compact design, use an integrated power switch, Fairchild, Powerint, ST and others offer this solutions.