AN3276 datasheet(11/22 Pages) STMICROELECTRONICS

11 / 22 page

AN3276

The new ST solution - BC2: energy recovery circuit

Doc ID 17975 Rev 1

11/22

This feature requires a specific diode for this application, so ST has developed an optimized

diode with an accurate trade-off between the IRM current value and its breakdown voltage.

2.2.4

Phase t2 to t3

Figure 11.

Equivalent circuit t2 to t3

At t2, the current in D1 reaches 0 A and then the BC

2 works as a conventional power boost

converter. As the power transistor stays on, the current in the main LB and the small L

inductor increases up to I1 at time t3.

2.2.5

Phase t3 to t4

Figure 12.

Equivalent circuit t3 to t4

At t3, the power transistor turns off. At this time the voltage across the MOSFET increases

linearly as its COSS capacitance is charged by the current stored in the small inductor L to

reach conduction in diode D2. No overvoltage stress occurs on the power switch during the

turn off. At the same time, the voltage polarity across the main inductor changes until it

reaches DB diode conduction. As soon as the diodes conduct together, the output current is

shared as shown in Figure 7. The current in D2 starting at I1, decreases due to the reflected

voltage from NS2 to reach 0 A with a low dI/dt. On the other hand, the current in DB rises to

reach the nominal current at t4. This shared current is a benefit for the BC

2 circuit. In the

PFC application working under a lower mains voltage such as 90 V rms, the highest

mains

NS2

NS1

V OUT

Coss

mains

NS2

NS1

V OUT

AN3276 Datasheet(PDF) 11 Page - STMicroelectronics