PWM Generator (Power System Blockset)

Generate pulses for a carried-based Pulse Width Modulator (PWM).

Library

powerlib_extras/Control Blocks

Description

The PWM Generator block generates pulses for carrier-based Pulse Width Modulation (PWM) systems. The block can be used to fire the self-commuted devices (FETs, GTOs, or IGBTs) of single-phase, two-phase, three-phase, or a combination of two three-phase bridges.

The number of pulses generated by the PWM Generator block is determined by the number of bridge arms you need to control:

Two pulses are generated for a one-arm bridge. The pulse 1 fire the upper device and pulse 2 fire the lower device (shown here for the IGBT device).

Four pulses are generated for a two-arm bridge. Pulses 1 and 3 fire the upper devices of the first and second arm. Pulses 2 and 4 fire the lower devices.

Six pulses are generated for a 3-arm bridge. Pulses 1, 3, and 5 fire the upper devices of the first, second, and third arm. Pulses 2, 4, and 6 fire the lower devices.

Twelve pulses are generated for a double 3-arm bridge. The first six pulses (1 to 6) fire the six devices of the first 3-arm bridge and the last six pulses (7 to 12) fire the six devices of the second 3-arm bridge.

The pulses are generated by comparing a triangular carrier waveform to a reference sinusoidal signal. The reference signal can be generated by the PWM generator itself, or it can be generated from a signal connected at the input of the block. In the second option, the PWM Generator need one reference signal to generate the pulses for a single or a two arm bridge or it need a three phase reference signal to generate the pulses for a 3-phase bridges (single or double bridge).

The amplitude (modulation), phase, and frequency of the reference signals are set to control the output voltage (on the AC terminals) of the bridge connected to the PWM Generator block.

The pulses that fire the two devices of an arm bridge are complementary one to the other, for example the pulse 4 is low (0) when the pulse 3 is high (1). This is illustrated in the next two figures.

The following figure displays the two pulses generated by the PWM Generator block when programmed to control a one-arm bridge.

The triangular carrier signal is compared to the sinusoidal reference signal. Each time the two signals become equal (at each crossing point), the value of the pulses passes from zero to one, or one to zero, depending to their previous value.

The following figure displays the six pulses generated by the PWM Generator block when programmed to control a three-arm bridge.

The pulse 2 is the complement of pulse1, pulse 4 the complement of pulse 3, and pulse 6 the complement of pulse 5. Note that unlike the pulses generated by the Synchronized 6-Pulse Generator block, the pulses generated by the PWM Generator block are of variable width.

Dialog Box and Parameters

Generator mode: Specify the number of pulses to generate. The number of pulses is proportional to the number of bridge arms to fire. Select for example Double 3-arm bridges (12 pulses) to fire the self-commuted devices of two six-pulses bridges connected in a twelve pulse bridge configuration.
Carrier frequency (Hz): The frequency, in Hertz, of the carrier triangular signal.
Internal generation of modulating signal(s): If checked, the modulating signal is generated by the block. Otherwise, external signals are used for pulse generation.
Modulation index (0<m<1): The amplitude of the reference internal signal. The Modulation index must be greater than 0, and lower or equal to 1. This parameter is used to control the amplitude of the output voltage of the controlled bridge. The Modulation index (0<m<1) parameter is visible only if the Internal generation of modulating signal(s) parameter is checked.
Frequency of output voltage (Hz): The frequency, in Hertz, of the reference internal signal. This parameter is used to control the frequency of the output voltage of the controlled bridge. The Frequency of output voltage (Hz) parameter is visible only if the Internal generation of modulating signal(s) parameter is checked.
Phase of output voltage (degrees): The phase, in degrees, of the reference internal signal. This parameter is used to control the phase of the output voltage of the controlled bridge. The Phase of output voltage parameter is visible only if the Internal generation of modulating signal(s) parameter is checked.

Inputs and Outputs

signal(s)

The input is the reference sinusoidal voltage when the Internal generation of modulating signal is not selected. Connect this input to a single phase sinusoidal signal when the block is used to control a single or a two arms bridge, or to a three-phase sinusoidal signal when the PWM Generator block is controlling one or two three-phase bridge. The input can be left unconnected when the internal generation of modulating signal is selected.

Pulses

The output contains the two, four, six, or twelve pulses signals used to fire the self-commuted devices (FETs, GTOs, or IGBTs) of single-phase, two-phase, three-phase, or a combination of two three-phase bridges

Example

See psb1phPWM.mdl and psb3phPWM.mdl demos for application examples of single-phase and three-phase inverters using the discrete version of this block.

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