AN2300 datasheet(7/21 Pages) STMICROELECTRONICS

7 / 21 page

AN2300

Modified Buck converter

Rev1

7/21

The presence of the zener diode in the free-wheeling path does not affect the basic

operation of the converter, but it could impact on the efficiency. In fact, if there is no load on

Vout2, the whole free-wheeling current will flow through both diodes, D1 and DZ, as shown in

Figure 5.).

As the current drawn from Vout2 increases, the free-wheeling current flows through a

different path, splitting in two components as shown in Figure 6. In this way the power

dissipation in DZ is reduced and the efficiency is increased accordingly. Thus, the converter

performs better if the complementary output is loaded, for a given output current Iout1.

In order to guarantee the proper operation of the converter when Vout1 is in open load

condition, a bleeder resistor has to be connected.

A practical implementation of the circuit is presented in schematic A (see figure Figure 7.),

where R1 is the bleeder resistor; D3, C3 and C4 are needed for VIPer12A biasing; L1, C1,

D1, C2 make up the input filter for EMI compliance; R0 limits the inrush current of the

capacitors.

Figure 3.

Buck basic operation during the

switch TON

Figure 4.

Buck basic operation during the

switch TOFF

Vout2

Vin

Rload

Vin

Rload

Vout2

Figure 5.

Modified Buck current flow at Iout2 = 0 Figure 6.

Modified Buck current flow at Iout2≠0

Iout1

Vout2

Vin

Rload1

Iout2 = 0

Vout1

Iout2

Vin

Rload1

Vout1

Vout2

Rload2

Iout1

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