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UCC28250: ILIM and CS inputs

Thomas Mandal Nilsen
Prodigy 90 points
Part Number: UCC28250
Other Parts Discussed in Thread: UCC28950

Hi,

The application circuit in the datasheet shows an implementation of a voltage mode control converter. If I want to make a current mode converter controlled from the primary side, how should I configure the ILIM and CS inputs? Could I just use two different sensing resistors (Rs for ILIM, Rb for CS) in series to scale the sensing voltages for ILIM and CS as required? Do you have an example of a push-pull converter with this or a similar circuit where current mode control is used?

  • 0
    TI__Mastermind 25570 points
    Hello Thomas

    I think you already know this but the first thing to consider is that the half bridge circuit shown in the application is inherently unstable in Peak Current Mode control and the controller must be configured in Voltage Mode Control.

    Having said that - a push-pull circuit can be controlled in Peak Current Mode. Section 7.3.7.2 gives some details of how this can be done. You need to generate a CS signal. If you want to use resistors, then you will need on in the source connection of each of the two switching transformers and then find a way to add the two signals. Another problem with resistors is that the ramp amplitude you should use is about 1.4V/Dmax (eq6) so either you accept significant power loss in the resistors of you add an amplifier.
    The better solution is to use a single current transformer between the centre tap of the power transformer and the input so that the current in either switch is measured. This also solves the amplitude problem. You can see how a CT might be used to do this in the User's guides for the EVMs for this device and also the same idea is used in the EVM for the UCC28950.

    The CS amplitude at 100% load should be scaled to be around 2V peak. The ILIM comparator trips at 500mV so you will need to feed an attenuated version of the CS signal into the ILIM pin. Assuming you want to place current limit at 125% of full load then 500mV at ILIM corresponds to 2.5V at CS so a 5:1 attenuator will do this.

    Hope this helps - please re-post if you have any further questions
    Regards
    Colin
  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points
    Hi Thomas

    I'm going to close this thread. Please re-open it or open a new thread if you need further help.

    Regards
    Colin