AN2872 datasheet(6/23 Pages) STMICROELECTRONICS

6 / 23 page

Description

AN2872

6/23

Doc ID 15305 Rev 1

Description

3.1

Theory of operation

The detailed calculation and principles of the buck converter for mains voltage operating

range is described in Section 6: References:1. and References:2. The basic ideas and most

important behaviors of the buck converter for mains applications is described in the chapters

that follow.

3.1.1

The basic principle of the buck converter

The schematic of the buck converter is shown in Figure 2. During the ON time, the control

circuit makes the high side switch T1 conduct. The input DC voltage is connected to L1 and

output capacitor C2 during the ON time. Assuming the voltage across C2 is constant and the

input voltage is constant, the voltage drop over L1 is constant (V1 - V2). The constant voltage

over the inductor causes a linear increase in the current through inductor L1. The slope of

the inductor current is proportional to the inductance of the inductor and the level of voltage

drop over the inductor.

Figure 2.

Buck converter schematic

After T1 is switched OFF, the low side switch (diode D1) conducts. If we assume, for

simplification purposes, the voltage drop over the diode is zero, then the voltage drop over

inductor L1 is equal to the output voltage (voltage drop over C2). Because the voltage across

the inductor is different (compare ON time), the slope of inductor current is also different.

The behavior of a buck converter can be expressed by Equation 1.

Equation 1

∆I

Control

ON time

OFF time

–

AM00359

∆I

–

()t

---------------------------------

2 tOFF

------------------

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