AN3276 datasheet(10/22 Pages) STMICROELECTRONICS

10 / 22 page

The new ST solution - BC2: energy recovery circuit

AN3276

10/22

Doc ID 17975 Rev 1

VNS2 voltage is subtracted from VOUT. This behavior provides the BC

2 circuit with a benefit

since under the low output load, power saving occurs in the system and it can be evaluated

using:

Thus, the BC2 reduces switch-off losses too.

2.2.3

Phase t1 to t2

Figure 10.

Equivalent circuit t1 to t2

At t1+, the boost diode DB turns off, and an overcurrent IRM is stored in the small inductor.

This overcurrent discharges the DB junction capacitance linearly. At the same time, the

voltage polarity across the main inductor changes until it reaches D1 diode conduction. At

this time, the overcurrent IRM is reduced by the transformer ratio m1 and is sent to the main

inductor.

Thus, the current flowing through NS1 contributes to charge the internal coil magnetization

LB at the same time as the Np winding biased by the mains voltage. The IRM current flowing

in D1 decreases down to reach 0 A thanks to the reflected voltage VNS1 that it is given by:

and

To guarantee a soft switching behavior in discontinuous mode, the current in D1 should

reach 0 A before time t3. The tD1_ON time trend supports the PFC application since the IRM

current is the largest when the Vmains voltage in the sinusoidal period is the highest. In

addition, to cancel the D1 recovery current diode effect, the dI/dt_D1 is always low thanks to

the low reflected voltage VNS1 and it is given by:

Unfortunately, during this phase a high reverse voltage is applied across the boost diode DB:

switching

mains

out

OSS

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

mains

NS2

NS1

V OUT

mains

−

)

mains

−

≈

(

dI_D

mains

−

≈

)

mains

out

reverse

−

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