CV/CC mode

Hi,

If you are using a TOPSwitch part for your power supply design, CC can be achieved by sensing the output current and adding the current feedback to the output regulator such that the optocoupler current will change so as to keep the current constant. The circuit used in DER-136 is a good example as you have pointed out.

For some of our products designed for charger application, the CC transition happens automatically beyond a certain load level. You may refer to Link-II family datasheet for reference.

Please provide us the PIExpert design that you made so that we can review your file and provide answers to your questions.

Regards

PI-Sarek

Here's the PIexpert design. Oops. I cannot attach .uds file or .zip file.

I would like to implement current sense using opamp. The ckt in DER136 uses voltage drop across an LED and 3 transistors. But what if I don't have an LED.
Frankly, the 3 transistor design is a bit hard to understand.
I would like to use a sense resistor(0.1 ohm) and a difference amplifier to sense the overcurrent and increase the current to the opto.
Something like the attached ckt.
But how will this work along with the TL431 feedback ckt?

Hi,

You can use simpler circuits that can help you to achieve CC operation.

Please review DI-012 using the link below:

http://www.powerint.com/sites/default/files/PDFFiles/di12.pdf

This design idea shows how the CV and CC feedback can be combined.

Regards

PI-Sarek

Sarek, Di12 uses low side sensing. correct?
But why 2 resistors - R4 is enough, why R5?.
My present ckt is attached. Can I implement CC without changing the transformer?
I don't need battery monitoring, so I can get rid of R11, R12 and Q2.
I don't need voltage as a function of temperature, so I can get rid of R7, R8, R9 and RT and have just 2 resistors.
Correct?

Hi,

The DI-12 uses low side sensing.

The function and need for each of the resistors is described in the DI-12 text. I request you to go through the DI-12 description in detail.

Yes, you can use CC without changing the transformer for cases where initial current limit of the part is not violated when the power supply operates with a low voltage at the output.If your design already has a low KP, the transformer may have to be redesigned.

It is best to verify this through testing accounting for tolerances.

The initial current limit characteristic of each of our parts is provided in the data sheet of the respective part.

Regards

PI-Sarek

What exactly counts as low KP?
My KP is 0.92. Can I still use the same transformer?

If the load tries to suck more current than the TOP switch initial current limit, won't the constant current circuit kick in?

How to determine the output voltage when my PS is operating in CC mode?