PWM Generator (2-Level)

Generate pulses for PWM-controlled 2-level converter

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Libraries:
Simscape / Electrical / Specialized Power Systems / Power Electronics / Power Electronics Control

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 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).

To control a single-phase full-bridge device, you can select unipolar or bipolar PWM modulation. Using the unipolar modulation, each arm is controlled independently. A second reference signal is internally generated by phase-shifting the original reference signal by 180 degrees. Using the bipolar modulation, the state of the lower switching device of the second arm is the same as the state of the upper switch of the first arm, and the state of the upper switch of the second arm is the same as the state of the lower switch of the first arm. The unipolar modulation produces better quality AC waveform, but the bipolar modulation produces very low-varying common-mode voltage.

The figure describes the three techniques to sample the reference signal Uref. The natural sampling technique models the behavior of an analog implementation of a PWM generator. Using the two regular sampling techniques, Uref can be sampled twice at both the valley and the peak of the carrier or only once at the valley of the carrier. The former is referred to as asymmetrical sampling or double-edge technique. The latter is called symmetrical sampling or single-edge technique.

Reference Signal Sampling Techniques

Examples

The power_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.

Ports

Input

Uref — Reference signal used to generate output pulses

Vectorized reference signal used to generate the output pulses. 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.

Dependencies

To enable this port, clear the Internal generation of modulating signal (s) parameter.

wt — External reference signal used to synchronize carrier

External reference signal used to synchronize the carrier.

Dependencies

To enable this port, set Carrier: Mode of operation to Unsynchronized.

Output

P — Pulse signals used to fire self-commutated devices

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.

m — Measurement output

Measurement output that returns the carrier signal used to determine the output pulses and the sampled reference signal.

Dependencies

To enable this port, select the Show measurement port parameter.

Parameters

To edit block parameters interactively, use the Property Inspector. From the Simulink^® Toolstrip, on the Simulation tab, in the Prepare gallery, select Property Inspector.

Generator type — Number of pulses to generate

Single-phase half-bridge (2 pulses) (default) | Single-phase full-bridge (4 pulses) | Single-phase full-bridge - Bipolar modulation (4 pulses) | Three-phase bridge (6 pulses)

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 Single-phase full-bridge - Bipolar modulation (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. Pulses 1 and 4 are identical. Pulses 2 and 3 are identical.

Select Three-phase bridge (6 pulses) (default) 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.

Carrier: Mode of operation — Mode of operation of carrier signal

Unsynchronized (default) | Synchronized

When set to Unsynchronized (default), the frequency of the unsynchronized carrier signal is determined by the 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) — Frequency of triangular carrier signal

27*60 (default)

Frequency, in hertz, of the triangular carrier signal.

Dependencies

To enable this parameter, set Mode of operation to Unsynchronized.

Carrier: Initial phase (degrees) — Carrier initial phase

90 (default)

Carrier initial phase, in degrees. A value of 90 degrees means that the triangle carrier initial position is set to midpoint between its minimum and maximum value and the slope is positive.

Dependencies

To enable this parameter, set Mode of operation to Unsynchronized.

Carrier: Minimum and maximum values — Minimum and maximum values of triangular carrier signal

[ -1 1 ] (default)

Minimum (valley) and maximum (peak) values of the triangular carrier signal.

Switching ratio (carrier frequency/output frequency) — Frequency of triangular carrier signal

27 (default)

Frequency (F_c) of the triangular carrier signal.

$$F_c=SwitchingRatio\times OutputVoltageFrequency$$

Dependencies

To enable this parameter, set Mode of operation to Synchronized.

Reference signal: Sampling technique — Sampling technique of reference signal

Natural (default) | Asymmetrical regular (double edge) | Symmetrical regular (single edge)

Specify how the reference signal is sampled: Natural (default), Asymmetrical regular (double edge), or Symmetrical regular (single edge).

If you select a regular sampling technique, the Sample time parameter must be an integer submultiple of the sampling period. The sampling period is equal to 1/Carrier Frequency/2 for asymmetrical sampling and to 1/Carrier Frequency for symmetrical sampling.

Reference signal: Internal generation of reference signal — Internal generation of reference signal

off (default) | on

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

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

This parameter is available only if the Mode of operation parameter is set to Unsynchronized.

Dependencies

To enable this parameter, set Mode of operation to Unsynchronized.

Reference signal: Modulation index — Modulation index

0.8 (default)

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.

Dependencies

To enable this parameter, select the Internal generation of reference signal parameter.

Reference signal: Frequency (Hz) — Frequency of reference signal

60 (default)

Output voltage frequency used to control the frequency of the fundamental component of the output voltage of the converter.

Dependencies

To enable this parameter, select the Internal generation of modulating signal (s) parameter.

Reference signal: Phase (degrees) — Phase of reference signal

0 (default)

Phase of the fundamental component of the output voltage of the converter.

Dependencies

To enable this parameter, select the Internal generation of modulating signal (s) parameter.

Sample time — Sample time

0 (default)

Sample time of the block, in seconds. Set to 0 to implement a continuous block. If you select a regular sampling technique, the Sample time parameter must be an integer submultiple of the sampling period. The sampling period is equal to 1/Carrier Frequency/2 for asymmetrical sampling and to 1/Carrier Frequency for symmetrical sampling.

Show measurement port — Show measurement port

off (default) | on

Select this check box to add a Simulink output to the block. The output returns the carrier signal that is used to determine the output pulses and the sampled reference signal.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2013a