Other Parts Discussed in Thread: UCC28019, , UCC28180
I am using the UCC28019A for a design that has 185Vac-305Vac input range and 600Watt @ 450Vdc output.
I used the Texas Instruments design calculator SLUC117E to calculate the recommended component values.
The circuit design works perfectly with loads up to 400Watts however in the 500-600Watt load range the AC line current becomes non-sinusoidal.
More Specifically the AC line current waveform appears to be clipped at the peak, however there is no change in output voltage it holds steady at 450Vdc.
Using a current probe I checked the boost inductor current waveform and found that at the peak of the AC line where this phenomenon occurs the switching duty cycle is inconsistent.
When the circuit is working properly the duty cycle will vary gradually in synchronization with the shape of the AC line voltage, however in this case the duty cycle varies significantly from one switching cycle to the next, but only in the area around the peak of the AC line.
Also this issue exists over the entire input voltage range from 185-305Vac.
If I reduce Rsense from the recommended value of 100mOhms to 50mOhms the issue disappears.
However this is not an acceptable solution because it increases the SOC and PCL thresholds by a factor of 2 which makes the boost inductor design a problem.
Using the TI Tina Reference Circuit model for the UCC28019 I found what I believe is the cause, i.e. the voltage on Pin2 (Vicomp) is too high.
At a load of 600Watts the simulation indicates that the voltage on Pin2 reaches its maximum limit and results in clipping of the sinusoidal waveform at the peak exactly where the inconsistent switching duty cycle is occurring in my prototype circuit.
In an attempt to verify the simulation results i added a 4.3Vzener in series with a 5K resistor from Pin2 to Pin1 to prevent the Pin2 voltage from reaching its maximum limit. This did indeed eliminate the inconsistent duty cycle issue at the peak of the AC line however it also resulted in crossover distortion of the AC line current waveform.
Can you please give me some guidance regarding how to make adjustments to component values so that Pin2 voltage (Vicomp) does not max out too soon?
Also upon review of the UCC28019AEVM User's Guide, page5, the schematic shows unpopulated components R17 & C9 which connect to Pin2, would adding these components be a possible solution? If so what are the associated design guidelines.
Best Regards,
Jeff Ferrick