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# UCC28180: AC-Input Current Curve in PFC circuits

Part Number: UCC28180

Tool/software:

Hello Community

For a project we have built a PFC circuit based on UCC28180 with approx. 900W output power and 200Vdc output voltage. The AC input voltage is 90...120Vac.
With an AC input voltage of 90Vac and a load of up to approx. 500W, the AC input current curve is nicely sinusoidal.
The maximum current is approx. 10Apeak.

If I load the circuit with approx. 800W, the AC input current is no longer sinusoidal at all. The curve is more triangular.
What could be the reason for this? Or is this normal in this power range?

(In figure SCR38, yellow is the voltage ripple at the DC output voltage and orange is the AC input current)
(In the SCR38 image, yellow is the DC voltage and orange is the AC input current.

best regards

• Hello John,

For a 900W PFC design running at 800W load, the input current waveshape should be very much like the one on the left (at 500W) except at higher amplitude.  The one on the right is not normal.

The abnormality is not so much a triangular waveform, rather a jump of input current near the peak of the AC line.  At the moment, I can only speculate on the cause of this distortion, however, it does vaguely resemble how the current would appear if the input voltage were to exceed the output voltage by a small amount.
For this to happen, either your AC source voltage is not controlled to 90Vac (not very likely) or your PFC output voltage is drooping well below 200vdc, to graze the peak of the AC line (unusual, but more likely).

Can you monitor both input and output voltage waveforms on the 'scope while increasing your load from 500W to 800W.  See if either of them are moving out of spec.
If not, you may be triggering a peak-current-limit protection (PCL) from the controller, due to an incorrect value for the current-sense resistor.
Check the current in your inductor.

Also, please check the voltage on the ICOMP pin when in the right-side condition, to see if V_ICOMP reaches an internal 6-V clamp limit.   If it does, then the GATE duty cycle cannot reduce further and inductor current can rise similar to as seen.

Regards,
Ulrich

• Hi Ulrich
I have changed the shunt from 14mR to 10mR. The input current is now cleanly sinusoidal. Many thanks for the great support.
I have checked the calculator again. it still says 14mR (see excel). Should I pay attention to something if I simply change the resistance to 10mR? The components such as the coil are now designed for the current at 14mR.

Regards John

• Hello John,

Components in the current path, such as the coil and semiconductors need to be rated for the peak and rms current levels that a 10mR sense resistor can allow.

This controller design is optimized to cover PFC applications using the "universal-line" input range and 390~400V output.
The internal non-linear gain blocks do not handle deviations from this application space, such as high-line only inputs, or low-line only with low-Vout.
The gains skew the VCOMP level low and, in your case, skew the ICOMP amp gain high and Vicomp can hit the 6-V clamp when using what would be considered a reasonable Rsense value based on SOC and PCL thresholds alone.

If you need to meet THDi specs at nominal Vin only (115Vac or 120Vac) and can tolerate distortion as seen in the right-side waveform above at low-line voltages, then it may be possible to increase Rsense value from 10mR to 11mR or 12mR, etc., and accept some current distortion at lower-than-nominal input voltage.  This may be something that you can consider.

Regards,

Ulrich