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PWM Generator (2-Level)

Generate pulses for PWM-controlled 2-level converter

Library

Control and Measurements/Pulse & Signal Generators

Description

The PWM Generator (2-Level) block generates pulses for carrier-based pulse width modulation (PWM) converters using two-level topology. The block can control switching devices (FETs, GTOs, or IGBTs) of three different converter types: single-phase half-bridge (1 arm), single-phase full-bridge (2 arms), or three-phase bridge (3 arms).

The reference signal (Uref input), also called modulating signal, is naturally sampled and compared with a symmetrical triangle carrier. When the reference signal is greater than the carrier, the pulse for the upper switching device is high (1), and the pulse for the lower device is low (0).

The figure shows the pulse generation for a single-phase half-bridge converter. In this case, one reference signal is required to generate the two pulses. For a single-phase full-bridge, a second reference signal is required to generate the two pulses of the second arm. This signal is internally generated by phase-shifting the original reference signal by180 degrees. For a three-phase bridge, three reference signals are required to generate the six pulses.

The reference signals can also be internally generated by the PWM generator. In this case, specify a modulation index, a voltage output frequency, and phase.

Pulse Generation for a Single-Phase Half-Bridge Converter

Dialog Box and Parameters

Generator type

Specify the number of pulses to generate. The number of pulses generated by the block is proportional to the number of bridge arms to fire.

Select Single-phase half-bridge (2 pulses) to fire the self-commutated devices of a single-phase half-bridge converter. Pulse 1 fires the upper device, and pulse 2 fires the lower device.

Select Single-phase full-bridge (4 pulses) to fire the self-commutated devices of a single-phase full-bridge converter. Four pulses are then generated. Pulses 1 and 3 fire the upper devices of the first and second arm. Pulses 2 and 4 fire the lower devices.

Select Three-phase bridge (6 pulses) to fire the self-commutated devices of a three-phase bridge converter. Pulses 1, 3, and 5 fire the upper devices of the first, second, and third arms. Pulses 2, 4, and 6 fire the lower devices.

Mode of operation

When set to Unsynchronized, the frequency of the unsynchronized carrier signal is determined by the Carrier frequency parameter.

When set to Synchronized, the carrier signal is synchronized to an external reference signal (input wt) and the carrier frequency is determined by the Switching ratio parameter.

Carrier frequency (Hz)

Specify the frequency, in hertz, of the triangular carrier signal. This parameter is visible only if the Mode of operation parameter is set to Un-synchronized.

Switching ratio (carrier frequency/output frequency)

Specify the frequency (Fc) of the triangular carrier signal.

This parameter is visible only if the Mode of operation parameter is set to Synchronized.

Internal generation of modulating signal (s)

When selected, the reference signal is generated by the block.

When not selected, external reference signals are used for pulse generation.

The parameter is visible only if the Mode of operation parameter is set to Unsynchronized.

Modulation index

Specify the modulation index to control the amplitude of the fundamental component of the output voltage of the converter. The modulation index must be greater than 0 and lower than or equal to 1. The parameter is visible only when the Internal generation of modulating signal (s) parameter is selected.

Output voltage frequency (Hz)

Specify the output voltage frequency used to control the frequency of the fundamental component of the output voltage of the converter. The parameter is visible only when the Internal generation of modulating signal (s) parameter is selected.

Output voltage phase (degrees)

Specify this parameter to control the phase of the fundamental component of the output voltage of the converter. The parameter is visible only when the Internal generation of modulating signal (s) parameter is selected.

Sample time

Specify the sample time of the block, in seconds. Set to 0 to implement a continuous block.

Inputs and Outputs

Uref

The vectorized reference signal used to generate the output pulses. The input is visible only when the Internal generation of modulating signal (s) is not selected. Connect this input to a single-phase sinusoidal signal when the block is used to control a single-phase half- or full-bridge converter, or to a three-phase sinusoidal signal when the PWM Generator block is controlling a three-phase bridge converter. For linear operation of this block, the magnitude of Uref must be between −1 and +1.

P

The output contains the two, four, or six pulse signals used to fire the self-commutated devices (MOSFETs, GTOs, or IGBTs) of a one-, two- or three-arm converter.

Characteristics

Sample TimeSpecified in the Sample Time parameter
Continuous if Sample Time = 0
Scalar ExpansionNo
DimensionalizedNo

Example

The power_PWMGenerator2Levelpower_PWMGenerator2Level model uses a simple circuit to illustrate the operation of the PWM Generator (2-Level). Run the simulation and use the FFT Analysis tool of the Powergui block to see the harmonics and the THD value of the voltages produced by the three-phase two-level converter.

The model sample time is parameterized by the Ts variable set to a default value of 50e-6 s. Set Ts to 0 in the command window and change the Simulation type parameter of the Powergui block to Continuous to simulate the model in continuous mode.

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