current limit circuit for SMPS design

Posted by: bmesser on 03/01/2010

Greetings. I have implemented the following power supply design and as shown in the attached schematic. The supply is for a universal 24VDC output that needs to be limited to 2.5A in order to keep thermal footprint to acceptable UL levels.
My present design runs upto 75C ambient however I am using the external current limit resistor to limit output current. While this works the tolerance is too sloppy as my supply outputs range from 2.2A upto 2.8A. Is there a more accurate current limit circuit it can implement using the some control on the feedback opto? I am seeking around 1% tol. for the 2.5A limit.

Files

ClassicDC_PWR(120and240).pdf (457.03 KB)

ClassicDC_PWR(120_240).pdf (89.75 KB)

Comments

Hi,

Yes, you can achieve a tighter output power limit across line voltage range by connecting a resistor from the X-pin to the rectified DC Bus positive rail. Please refer Figure.23 in our datasheet for reference. This configuration artifically reduces current limit as the input voltage rises.

http://www.powerint.com/sites/default/files/product-docs/tophx_family_datasheet.pdf

The value of RLS can be adjusted based on bench results and depending on the value of X-pin resistor used, sometimes a resistor value as high as 10Meg may be necessary. Please see Figure.23, 24 and 26 of the TOPSwitch-HX application note for reference.

http://www.powerint.com/sites/default/files/product-docs/an43.pdf

Regards

PI-Sarek

You can try using a TLVH431 as a reference for your current limit. For a 2.5 Amp current limit, use a 0.499, 5 Watt current sense resistor. You can drive an opto to inject 250uA of current into the V pin.

Z. Cochrane

Would you please provide a little more detail around this recommendation. I am uncertain as to how to connect the recommended current limit circuit modification suggested above.

I have attached a circuit that you can try. This is different than the first one that I proposed. This approach uses the x pin to limit the output current. This may work better than using the V pin. This is because the TOP HX V pin will start a auto-restart cycle. The X pin should disable the switching only during the over current condition.

Z. Cochrane

Thank you for the sketch and suggestion. I am using the TOP261Y package that has the X and V pins returning to the G pin. Is there a problem with taking the emitter of the 2N3709 to the G pin instead of I am assuming from picture the Source pin?

Yes, you want to connect the emitter of the 2N3904 to the G pin.

Z. Cochrane