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Load Regulation Problem

Posted by: edksi on Tue, 07/07/2009

Hi,

I have design 5V/2.5A supply using top256YN. Input voltage for this design is 35-290Vdc and Power supply is working with the bias voltae of the transformer is 15V. The problem I had was the bias voltage increase while the load current increases. If load current became 2.5A, bias voltage increses to 35V and output voltage gets lower down to 4.72V. I was able to limit the bias voltage to 18V by replacing diode with 1n4004 which is standard recovery time diode but output voltage is still goes down to 4.72V.

What can I try to regulate the load voltage and what could effect the feedback signal?

Comments

Submitted by Tim Starr on Thu, 07/09/2009

Unless you are using primary side feedback the bias winding voltage does not determine the output voltage. What does the load regulation look like like overall? At no load? At 50% Load?

Also how are you measuring the output voltage? Make sure no load current is flowing through the leads which the output voltage is being measured.

If you provide a schematic I can be of much more help! Thanks!

Sincerely,
Wesley

Submitted by edksi on Mon, 07/13/2009

In reply to by zedman

Here is the schematic for the power supply Design. At no load, output voltage is 5V and it is linerly decreased vs load current. As I described before, at 2.5A output voltage goes down to 4.72V and transformer gets hot.Also one thing I realized is for the higher input voltage, noise on the output voltage gets higher too.
Is there anything I can do to improve the load regulation?

Submitted by Tim Starr on Thu, 07/16/2009

I would recommend decreasing R83's value down to perhaps 100 ohms as recommended in AN-43 in Figure 15 on page 13. This will increase the amount of bias current the TL431 has and may help regulation. Also a TL431A will have a tighter reference voltage (~30 mV) tolerance.

See if that helps your regulation at all.

-Wesley