Datasheet
C2
470 ?F
100 V
C21
470 ?F
100 V
R21
4.4 M
R15
1 M
0.5 %
R16
1 M
0.5 %
R17
330 k?
0.5 %
R3
0R15
-V
PRI
LINESNS
R78
13 k?
1%
R72
16.875 k?
1%
VBULK
V
INTER
Q9
D1
HV
L6A
EQ25
1
2
V
BULK
UDG-11106
Q1
Vac
Iac
Vbulk
Conduction
Angle
UCC29910A
SLUSAK8A –MAY 2011– REVISED JUNE 2011
www.ti.com
Buck PFC Power Stage
The PFC stage converts the incoming rectified line voltage to a DC voltage on the output capacitors, power
transfer happening during the times when the line voltage is greater than the output voltage. The resulting
conduction angle is a function of both the incoming line and output voltages. In the reference design mentioned
above, the output voltage is set to 84 VDC. This is low enough to allow conduction angles sufficient to achieve a
PF (Power Factor) of at least 0.9 over an input voltage range from 90 VAC to 264 VAC. Other output voltage
levels may be set by altering the voltage sensing network at the VBULK pin. A 4:1 ratio between the VBULK and
LINESNS scaling ensures optimum operation of the SmartStart algorithm, so if the VBULK scaling is altered
significantly, then the LINESNS scaling should be altered too. A high efficiency second stage, controlled by a
UCC29900, can then down-convert to a nominal output of 19.25 V using a transformer with a simple 4:1 turns
ratio, or to any other desired output voltage. The basic Buck PFC power stage is shown in Figure 1. This
low-side switched buck stage has the same performance as the more usual high-side switched buck converter. It
features easy power FET drive, at the expense of requiring output sense through a PNP level shift transistor, Q9.
The incoming AC line is fed through a rectifier and filter stages, not shown here. The resulting unipolar voltage
(V
HV
) is then fed into the power stage. The MOSFET is switched at a constant 100 kHz and the freewheeling
diode function is provided by D1. The output voltage (V
BULK
) is developed across the two large capacitors, C2
and C21.
Figure 1. Buck Power Stage (simplified)
Figure 2. Illustrative Line Current and Voltage
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