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UCC28950: Problem in configuring TMIN pin.

MOHD AFROZ AKHTAR
Prodigy 40 points


I am only using the feature of UCC 28950 controller IC for generating Phase shifted switching pulses for DC-DC converter of 160W using Bridge Rectifier in place of synchronous rectifier.

1. We configured Tmin as 710ns by inserting 120k ohm resitor(using the datasheet equation) but practically Tmin was coming to be 1200ns. We changed that resistor with 22k ohm for 130ns but again it is coming to be 500ns.

2. We are generating Phase shifted switching pulse by using Error amplifier of UCC 28950 as a buffer and providing dc voltage between the range of 0.8V to 2.8V at 4th pin (COMP) due to the fact that ramp also ranges between 0.8V to 2.8V as described in the datasheet.
Following are the observations

1. By providing only 0.7V at COMP pin I am getting phase shifted signal with Tmin as 500ns (i.e. 18 degree).
2. Below 0.7V I am unable to get any switching signal.
3. Only at 1.67V I am getting maximum of 4.825us (174 degree) phase shifted signal.

My doubt is why and how I am getting switching signal even below 0.8V at COMP pin and why are we not able to use the complete range of 0.8V to 2.8V. Please help us for which I am thankful to you all.

Thanks and Regards

  • 0
    TI__Guru* 86286 points

     

    I checked the data sheet and Rtmin is tested with 88.7k ohm Rtmin and has a delay time 525ns typically.  This agrees with the equation.

    In my expirence with working with the device I have noticed that the comp pin is noise sensative.  Figure 50 in the data sheet references this.   When I have tested the device open loop I setup the voltage amplifier as a follower.  FED EA + with a voltage divider to reduce some of the noise issues.  Try this and let me know if you can get a lower tmin.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    You could also try to ratio the divider to EA + to give you more resolution in adjusting the duty cycle.

    Regards,

  • 0
    TI__Mastermind 31580 points

    Hi,

     

    Can you tell me the values for the following components

    1. Rt, used to set the switching frequency,

    2. Rsum, the resistor used to set the slope compensation,and  is the resistor terminated to gnd of Vcc?

    Thanks

  • 0 in reply to Peter Meaney
    Prodigy 40 points

    Hi Peter,

    The values for the following are: -

    Rt = 60k resistor connected to VREF for 100kHz switching signal (Master)

    Rsum = 47k. We are not using cycle by cycle limit feature, therefore we chose value within limit as 47k. It is connected to VREF for configuring it to VCM. We are testing the IC with according to your suggestion and update it here accordingly.

    Thanks and Regards