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Original question:

UCC28951-Q1: turns ratio selection of power transformer and inductance of inductor

UCC28951-Q1: UCC28951-Q1- not getting pwm from control IC

Rajashekar Koutla
Intellectual 590 points
Part Number: UCC28951-Q1

Hello 

1. What are the minimum conditions to obtain PWM on pins 17-22?

2. I am doing an experiment on this IC and let me explain the setup. I am giving PWM to full bridge inverters by using microcontroller and gate drivers so that I can vary DUTY CYCLE. On the secondary side of the transformer I have placed full bridge diodes instead of Syn Rectifiers for easier understanding. Later I give the main feedback signals of VOUT and Input current via Current sense transformer. Atleast I am expecting PWM on all the pins (17-22). But I observe PWM only on pins 19-22 with 50% duty cycle. The output voltage is well within the design range. Where am I going wrong?

3. Will I get PWMs if I give DC less than 2V on CS channel (pin 15) provided I have given DC less than 2.5V on pin 3?

  • 0
    TI__Mastermind 25570 points

    Hello Rajashekar

    The OUTx signals on the UCC28960 (and all other Phase Shift Full Bridge controllers) always have a 50% duty cycle. The PWM applied to the transformer comes from the phase shift between the signal pairs OUTA/OUTD and OUTC/OUTB. The animation below shows how this works.

    /cfs-file/__key/communityserver-discussions-components-files/196/0714.Animation.pptx

    The phase shift between the signal pairs is proportional to the voltage on the CS pin. If you supply a 0V signal at CS then you will get maximum duty cycle, the duty cycle will reduce as you increase the voltage. If you are doing an open loop test you should use a voltage source with a 1k source resistance.

    If the link is absent then please post your email address here and I can send it directly to you.

    Please let me know if you have any other questions.

    Regards
    Colin