User`s guide

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SMPS AC/DC Reference Design User’s Guide

The output voltage is measured using the analog input AN1. The analog comparator

input CMP1A is connected to the output of the current transformer. The output voltage

is controlled by varying the duty cycle of PWM4.

The PWM4 pair is operated in Complementary mode with dead time. The switching

frequency is approximately 500 kHz. The duty cycle is controlled directly by the built-in

Cycle-by-Cycle Current-Limit mode and the analog comparator.

When the current-sense signal at the input of the analog comparator exceeds the

programmed comparator threshold, the PWM output is immediately terminated for the

remainder of the PWM cycle.

The Single-Phase Buck Converter circuitry is designed to operate in continuous

conduction mode at load currents greater than approximately 3A. If the Single-Phase

Buck Converter is operated in Discontinuous Conduction mode, the freewheeling

MOSFET is disabled through software.

At no load and light load current (< 3A), the PWM output may enter a “burst” mode. This

is caused by a low demand for load current by the converter in this range load current.

The voltage control loop is executed in the ADC ISR every two PWM cycles.

3.4.2 Multi-Phase Buck Converter

3.4.2.1 MULTI-PHASE BUCK CONVERTER CONTROL SCHEME

Voltage mode control is used for controlling the output of the Multi-Phase Buck

Converter on the SMPS AC/DC Reference Design. As shown in Figure 3-11, the

control scheme only implements a single control loop.

FIGURE 3-11: MULTI-PHASE BUCK CONVERTER CONTROL SCHEME

The output voltage is compared with the reference and results in a voltage error, which

is fed as an input to the voltage error compensator. The output of the voltage error

compensator modifies the duty cycle of all phases of the Multi-Phase Buck Converter.

The voltage error compensator is implemented as a PID function that is implemented

in the ADC ISR.

The Multi-Phase converter comprises of three individual phases, but the output is

controlled by a single duty cycle that identically drives the three phases. The PWM

drive signals for each phase are phase shifted by 120 degrees using the built-in PWM

phase-shifting feature available on the dsPIC33FJ16GS504. The PWM drive signals

for the Multi-Phase Buck Converter are shown in Figure 3-12.

Voltage

Reference

OUT

Phase 3

Inductor

Voltage Feedback

PWM

OUT

Sense

Voltage

Compensator

ADC

1001011011

S&H

Phase 2

Inductor

Phase 1

Inductor

Update

PWM

Cycle

PWM

Duty

Error