AN3276 datasheet(12/22 Pages) STMICROELECTRONICS

12 / 22 page

The new ST solution - BC2: energy recovery circuit

AN3276

12/22

Doc ID 17975 Rev 1

application boost current is shared between DB and D1 diodes. Therefore the conduction

losses in the rectification stage are reduced. The reflected voltage VNS2 and D2 time

conduction are given by:

and

The tD2_ON time trend supports the PFC application since the I1 current is largest when the

Vmains voltage range is lowest. Thus the discontinuous mode could be guaranteed in the

BC2 circuit even under the worst case PFC applications such as high output load current

under the minimum Vmains voltage range. In addition, to cancel the D2 recovery current

diode effect, the dI/dt_D2 is always low due to the low reflected voltage VNS2 given by:

2.2.6

Phase t4 to t5

Figure 13.

Equivalent circuit t4 to t5

At t4, the current in the D2 reaches 0 A and then the BC

2 works like a conventional power

boost converter. Only the boost diode DB conducts. Due to the reflected voltage of NS2, the

voltage across the power switch is lower than VOUT. Thus the COSS capacitor is discharged

in the bulk capacitor and power saving occurs as the transistor turns on at t0.

(

mains

out

−

(

)

mains

out

−

(

mains

out

−

≈

Coss

Vmains

NS2

NS1

V OUT

AN3276 bảng dữ liệu(PDF) 12 Page - STMicroelectronics