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UCC2818: Synchronization causing instability at load

Thien Huynh
Prodigy 70 points
Part Number: UCC2818

Hello there, I am looking for insight on the problem I am experiencing with the UCC2818.

The problem appears when I put load on the output while synchronizing the controller.

                                  Figure 1: Instability at load with synchronization (1A output)

Channel 1: rectified AC voltage

Channel 2: AC current

Channel 3: inductor current

Channel 4: Ct voltage

Figure 1 shows the response when 1A load is applied. You can see that for some reason there is a peak in the current and voltage waveform.

                                                 Figure 2: Zoomed-in of figure 1

Figure 2 shows a zoomed in picture at the peaking section. It can be seen that there is nothing wrong with the voltage across Ct.

I tried disconnecting the sync and just let the controller free run and there was no problem at the same load as shown in figure 3 below.

                                      Figure 3: Response during free run (No sync, 1A load)

I tried playing around with the capacitance needed for synchronization. By reducing the capacitance, I was able to get a stable output at 1A as shown in figure 4 below

 

                    Figure 4: Response with sync but with lower sync capacitance (1A load)

However, as soon as I increased the load to 2A, the problem resurfaced.

                                       Figure 5: Response with sync (2A load)

I need to mention that this problem does not appear throughout the whole load range if I let the controller free-run. Is there any correlation between the sync capacitor to the operation of the controller? Any insight is appreciated.

Best,

Thien Nhien Huynh

  • 0
    TI__Genius 10755 points

    Hi Thien, what are you using as your sync source?

    I think that the sync circuit may be limiting the maximum duty cycle.  At crossover it needs as close as possible to100%

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 70 points

    Hello Ray,

    I have an oscillator to generate the sync frequency. I guess I don't understand how that can limit the maximum duty cycle. I can disconnect the sync, increase the free-run frequency to the sync frequency and the converter can run just fine. 

    Best,

    Thien Nhien Huynh

  • 0 in reply to Thien Huynh
    TI__Genius 10755 points

    Hi Thien,

    The "instability" seems to occur after the flattened zero crossing, so my thought was that the current loop is saturated at the zero crossing and then when it is able to regulate again, the controller over-compensates for a few cycles until it catches up.  Since this zero-crossing distortion appears to be caused by the added sync circuitry, I am trying to find a cause.

    The CT discharge current is fixed.  While it is being discharged, the gate drive is off so the duty cycle may be limited by the period of the sync signal.

    Perhaps you can check the duty cycle with the non-synced CT at the zero crossing and then again with the sync to see if that is what's happening.

    If so, you can try a smaller CT and adjust RT proportionally to provide a similar switching frequency.  Also try increasing/decreasing your switching frequency and the sync frequency to try to minimize this effect. 

    Re-evaluate your current loop compensation.

    I also noticed the large spikes of current on CT which look to go as high as 9V.  AlthoughI don't see it in the datasheet, I would try to keep the voltage on CT < 7.5V.  Perhaps you can reduce VCC3 or insert a small limiting resistor in series with your sync signal.

    I don't know your purpose of using the sync circuit, but perhaps you can run the UCC2818 in free-running mode and sync your other circuits to it. Just an idea for a solution.

    Regards,

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 70 points

    Hello Ray,

    I did try reducing the CT and readjust RT but that did not seem to have much effect on the response. The only thing seems to have an effect is the coupling capacitor from the sync. I have better response with lower capacitance. I have decided to use 100pF for now since that seems to give me the least distortion at max load.

    What you see here is just one part of the system. I have a 3 phase input which power 3 PFC phase to phase so the sync is there to spread the switching 120 apart to help reduce EMI. I suppose the end game here is whether or not I can pass EMI and THD requirement. I will probably do some comparison with and without sync to see what I can get away with. 

    Thank you for your suggestions

    Thien Nhien Huynh

  • 0 in reply to Thien Huynh
    TI__Genius 10755 points

    Hi Thien,

    I agree, THD (and EMI) will be the litmus test

    It's interesting that the coupling cap is having this effect. If you have waveforms showing the difference in CT signal with different caps, I may be able to discern more from that.

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 70 points

    This is with 2-47pF in parallel. If I zoom in, the peak is just right there before the cap discharge as shown below

    The peak is higher with more capacitance. I think I saw the peak of 7V with 390pF. I also tried putting in 47pF in parallel with 30pF and the response changes quite drastically. The added voltage from the sync is now moved down to around the middle of the ramp as shown in the drawing below. It will still sync because of the shift in voltage but I can't explain the drastic peak shift in the ramp. This is why I think 100pF is as low as I can go. I will try lifting some circuit up to see if there's noise interference

     

    Best

    Thien Nhien Huynh

  • 0 in reply to Thien Huynh
    TI__Genius 10755 points

    Hi Thien,

    The sync signal in purple looks good, so it looks like 100pF is about right on the coupling cap.  You need the sync pulse to exceed 5V to trigger the discharge portion of the cycle. It appears to do that without excessive overshoot. It also appears to have a ramp rising to ramp falling of ~10.80us/(10.8us+0.86us) = 92.6%.  This is important as I was trying to understand how much your sync was affecting the ramp. 

    A little more info on how the sync pulse limits the duty-cycle to < 95%:  The charge and discharge current on CT is 19:1 (internally set). When the oscillator is free running, the rise and fall time will also be 19:1 (so the fall time is 5% of the total cycle time).  When the sync pulse interrupts the rise time, the fall time will still occur in the same timeframe but will now comprise more than 5% of the total charge/discharge cycle.  This explains why decreasing the CT cap while increasing the RT resistor proportionally won't have any effect on the duty-cycle: the RT will change the current, but the 19:1 charge/discharge current ratio on CT still applies.

    What I was considering was that at the zero crossing, the sync-limited duty cycle may be less than what the system is requiring, thus distorting the response and saturating the current loop.  This in turn causes a spike when the current loop comes out of saturation.  If your max duty cycle is almost 93%, this doesn't seem to be a likely cause. 

    It is interesting that the coupling cap improves the response significantly.  I'm wondering if perhaps the extreme overshoot on CT was resulting in current injection into the IC thereby affecting operation of the internal circuitry.

  • 0 in reply to Ray DiCecco
    Prodigy 70 points

    Hello Ray

    This is the response at max load (3.5A out) with 2-47pF cap. The blue trace is the rectified AC voltage. The green trace is the AC current. And the red trace is the inductor current. The response is pretty decent. There is no more peak along the rising of the current anymore. The dead time during zero crossing is also significantly reduced. The only thing now is there some oscillation of 9.3kHz during the rising edge. My switching frequency is 93kHz. I don't know if there is a correlation or not. I have to see if this will become a problem for EMI. Interestingly, the oscillation is not as defined in the AC current.

     

    I think I will stick with this for now and move forward.

    Thank you,

    Thien Nhien Huynh

  • +1 in reply to Thien Huynh
    TI__Genius 10755 points

    Looks a lot better Thien.

    The frequency being 1/10 of your switching frequency is likely just a coincidence.  It could be a harmonic from the difference in the sync freq and the osc freq.  Since it's present on your rectified voltage, it may have more to do with the input filtering.  If you need to resolve it, you can play with the input filter to see if that affects it.  Regardless, I think 9.3kHz is too low to affect your THD.

    Ray