AN3276 datasheet(7/22 Pages) STMICROELECTRONICS

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AN3276

The new ST solution - BC2: energy recovery circuit

Doc ID 17975 Rev 1

7/22

The new ST solution - BC2: energy recovery circuit

The innovative circuit has been designed [see Section 5: References, 1.] to respect the five

soft switching criteria in Section 1.2. Figure 6 shows that two additional diodes D1 and D2

and two auxiliary windings NS1 and NS2 wound around the main boost inductor LB are

designed to reset the energy stored in the small inductor L.

Figure 6.

Novel energy recovery circuit: BC2

2.1

Concept description

The winding NS1 allows the IRM current from the boost diode DB to be recovered in the main

boost inductor when the transistor turns on. Since the mains input voltage modulates the LB

voltage, it also modulates the reflected voltage across NS1. This input voltage also

modulates the boost diode current IDB and its associated recovery current IRM. These

combined modulations allow the extra current IRM flowing in the inductor L to be reset into

the winding NS1 even in the worst case. The winding NS2 allows the extra current of L to be

injected into the output capacitor when the transistor turns off. The reflected voltage across

NS2 is also a function of the input voltage. This reflected voltage reaches its maximum when

the AC line voltage is low, corresponding to the maximum value of the inductor L current.

These combined variations allow the current flowing in the inductor L to be cancelled in the

bulk capacitor through the diode D2 even in the worst case. The benefits of these two

additional windings NS1 and NS2 are to switch off the diodes D1 and D2 with a low dI/dt

(about 10 A/µs) as in a discontinuous mode switching converter. Their recovery currents do

not affect the behavior of the BC2 circuit.

m2 =NS2 /NP

m1 =NS1 /NP

NS1

VOUT

Vmains

AN3276 数据表(PDF) 7 Page - STMicroelectronics