Theory of operation

AN2389

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2

Theory of operation

You can use this converter as buck-boost converter, as a buck converter or as a boost

converter by selecting different combinations of switches SW1 and SW2 driven by the

PWM1 and PWM2 signals output by the ST7 microcontroller.

2.1

Buck-boost implementation

This converter can be used as a non inverting buck-boost converter by selecting the

operating mode from Table 1 which briefly describes the converter modes.

.

If we look at phase 2 in Table 1, here the switch SW1(PWM1) is OFF and switch SW2

(PWM2) is ON. This condition never occurs either in a buck converter or in boost converter.

So you should always take care in your software that this condition must not happen. To

avoid this, if we assume that initially both switches are in OFF condition then you should use

the following guidelines to manage the PWM signals driving the two switches.

1.

Keep the frequency of both PWM signals the same, to better control when

synchronizing the two PWM signals using the next three guidelines.

2.

The duty cycle D1 of control signal PWM1, must be greater than the duty cycle D2 of

control signal PWM2.

3.

PWM1 should be enabled before the PWM2 signal.

4.

PWM1 should be disabled after the PWM2 signal.

Table 1.

Operating modes based on switch combinations

Phase

SW1 (PWM1)

SW2 (PWM2)

Operating modes

1OFF

OFF

BUCK

2OFF

ON

N/A

3ON

OFF

BUCK-BOOST

4ON

ON

BOOST