Cheating with Boost converters when VAC input falls to 180VAC?

[treez]

This concerns cheating with Boost PFCs and using a smaller inductor than needed to get good PFC across the line input range of 180VAC to 265VAC....

As is known, 240VAC CCM Boost converter PFC’s can be designed to be PFC’d to >0.95 with VAC input of 240VAC, say.

The same Boost converter could also have PFC >0.95 if the VAC input falls to 180VAC. However, as we know, the Boost converter would then have a higher peak current.

However, our contractor tells us that many PSU manufacturers cheat. That is, they don’t tolerate an increased peak current if the VAC falls to 180VAC. They clamp the peak current to that level that it was with 240VAC……this of course then means that the mains input current is much less sinusoidal when at 180VAC, and PF deteriorates. As you know, what would happen is that the Error Amp output would rise, and the Boost inductor current would reach the peak earlier in the cycle, and then simply be switched up to this peak value across the top of the peak of the mains….the result being reduced Power factor…but, one doesn’t need to rate the boost inductor to be able to tolerate the higher peak current that would otherwise be needed when at 180VAC input.

So does this happen?

 

[biswaIITH]

This concerns cheating with Boost PFCs and using a smaller inductor than needed to get good PFC across the line input range of 180VAC to 265VAC....

As is known, 240VAC CCM Boost converter PFC’s can be designed to be PFC’d to >0.95 with VAC input of 240VAC, say.

The same Boost converter could also have PFC >0.95 if the VAC input falls to 180VAC. However, as we know, the Boost converter would then have a higher peak current.

However, our contractor tells us that many PSU manufacturers cheat. That is, they don’t tolerate an increased peak current if the VAC falls to 180VAC. They clamp the peak current to that level that it was with 240VAC……this of course then means that the mains input current is much less sinusoidal when at 180VAC, and PF deteriorates. As you know, what would happen is that the Error Amp output would rise, and the Boost inductor current would reach the peak earlier in the cycle, and then simply be switched up to this peak value across the top of the peak of the mains….the result being reduced Power factor…but, one doesn’t need to rate the boost inductor to be able to tolerate the higher peak current that would otherwise be needed when at 180VAC input.

So does this happen?

I have come across few EV chargers who employs reduced output once the input voltage falls below 200 Vac to reduce the volume of the inductor. A couple of years back when we had designed a 3.3kW On-Board charger(With Interleaved Boost PFC), I used to reduce the power (But effectively keeping the peak current constant till 175Vac to 180Vac) . I have seen some reduction in PF but definitely not less than 0.96-0.97 with increase in line current THD.

 

[Easy peasy]

A lot of PFC take the signal off the mains for the current waveshape - so as the mains waveshape reduces in magnitude - so does the max demand value for the current, e.g. 230Vac = 12A ac rms Iin -> this implies 120Vac = 6Aac rms in max ... - this is a a good and easy way to get current limiting as the mains falls through and below 180Vac. The issue is for systems that span 95 - 265Vac, and need to draw MORE current as the Vin falls, and then current limit below 90Vac ...

The better systems maintain a max sine wave ref for the current, e.g. 12A ac rms, and you can draw this down to 12Vac or less - but of course your power draw by the 2nd stage needs to be reduced else the HV DC bus will fall & fail.

The clipping type peak current limit is less common these days - but as long as it is symmetric on pos and neg cycles - it won't actually harm anything ...

--- Updated Jul 14, 2020 ---

For smaller inductors just go to higher freq's & Sic diodes and well thought out heatsinking and emc ...