Test results

AN2521

10/33

Figure 5.

EVAL6668-75W PFC output voltage vs. ac input voltage

In Figure 5 we can see that the PFC output voltage variation vs. the ac input voltage (i.e. the

input voltage for the flyback stage) is dependent on the input mains voltage, but its range is

narrower than a wide range input. Thus the design of the flyback converter is not completely

optimized as with a standard PFC delivering a stable 400 V output, but its design is much

simpler than that of a wide range flyback. Additionally, the PFC converter using the TBO,

with its lower differential voltage across the inductor and lower current ripple, will have lower

RMS current and therefore better efficiency at low mains, where normally the efficiency of

typical PFCs is lower. The result is a global efficiency of the circuit that will be higher than

that of a fixed output voltage one circuit, especially at lower mains. Most of the power

dissipation will not be concentrated on the PFC only but will be shared with the flyback.

Therefore, there will not be thermal hotspots and the reliability of the circuit will be improved.

This is confirmed in the diagram in Figure 6, where the efficiency of the PFC has been

measured both with the active TBO function and without it. As shown, at low input mains the

circuit has an efficiency improvement better than 2 percent. As the input mains voltage

increases the switching losses become more significant and the fixed output voltage PFC

appears more efficient.

Using the TBO function even the flyback converter efficiency is very good, as shown in

Figure 7 where it is compared with the efficiency of the same converter powered by a fixed

218

242

351

384

186

219

252

285

318

351

384

417

80

130

180

230

280

Vin [Vrms]

Figure 6.

PFC efficiency with and without

TBO function at full load

Figure 7.

Flyback converter efficiency with

and without TBO function at full

load

90%

91%

92%

93%

94%

95%

96%

97%

98%

99%

100%

90

115

230

265

Vin [Vrms]

WITHOUT TBO

WITH TBO

85%

86%

87%

88%

89%

90%

91%

92%

93%

94%

95%

90

115

230

265

Vin [Vrms]

400 Vdc FIXED I/P VOLTAGE

WITH TBO