July 29, 2010

"True" Bridgeless PFC Claims Power Supply Efficiency Increase

I am going to discuss Dr.Slobodan Cuk article entitled “The True Bridgeless PFC Converter Achieves Over 98% Efficiency, 0.999 Power Factor”. It was featured in the July issue of Power Electronics Technology Magazine. The article describes the latest invention of Dr.Cuk aimed at eliminating input bridge rectifier from the offline PFC power supplies and thus increasing their efficiency and power factor.
The bridgeless PFC circuits were known before. The diagram below compares Dr.Cuk’s circuit with a different circuit described in ST’s App Note 1606.

The Dr.Cuk approach is certainly novel and very interesting, just like probably all of his converter circuits. However, I believe the above article contains some exaggerations more suitable for advertising than for a technical paper.

For example, the author says: “This method also leads to a rather unusual converter topology consisting of three switches only: one controllable switch S and two passive current rectifier switches CR1 and CR2”. He also claims: “…The odd number of switches, three, is a distinctive characteristic of this converter with respect to all conventional switching converters…” However, later he admits: “…At present, a single MOSFET implementation is not possible due to built-in body-diode, so that switch S must be implemented by use of the two MOSFET devices connected in series at their sources and driven by a common floating gate drive circuit…” So, in reality, the switch S must consist of two real switches and overall there are still four switches, not three. When the power switch S in ON, the current flows through three switches: S1, S2 and CR1, which is a drawback of this converter. With respect to the conduction losses during the ON time interval, I see no improvement relative to the known circuit of a “bridgeless PFC”, in which ON-state current flows through only two switches, S1 and S2. For the same reason I would say, it is not more "true" bridgeless than the original circuit. I understand, the reduction of conduction losses in Cuk's circuit comes during OFF state of the switch when the current flows through a single switch CR2 while in the known circuit it flows though two switches, such as CR1 and S2. Therefore the main advantage of this circuit would probably be realized when ON time is low and OFF time is high. Since at low input line the duty cycle and ON-time tend to be high, while at high line they tend to be low, the advantage of the Cuk’s proposed converter would be at high input lines where PFC has higher efficiency anyway. The article claims 98% efficiency, but unfortunately it does not state under what input and output conditions it was achieved. I tend to doubt very much it is achieved at low line.

1 comment:

Anonymous said...

This Cuck`s device has VERY higest series resonant cuurents and High conduction losses of switches!