PWM Generator (2-Level)

Generate pulses for PWM-controlled 2-level converter

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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

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 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

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)

Specify the frequency, in hertz, of the triangular carrier signal. Default is 27*60. This parameter is available only if the Mode of operation parameter is set to Unsynchronized.

Carrier: Initial phase (degrees)

Specify the carrier initial phase, in degrees. Default is 90. 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.

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

Carrier: Minimum and maximum values

Specify the minimum (valley) and maximum (peak) values of the triangular carrier signal. Default is [ -1 1 ].

Switching ratio (carrier frequency/output frequency)

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

$$F_c=SwitchingRatio\times OutputVoltageFrequency$$

Default is 27. This parameter is available only if the Mode of operation parameter is set to Synchronized.

Reference signal: Sampling technique

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

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.

Reference signal: 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. Default is 0.8. This parameter is available only when the Internal generation of reference signal parameter is selected.

Reference signal: Frequency (Hz)

Specify the output voltage frequency used to control the frequency of the fundamental component of the output voltage of the converter. Default is 60. This parameter is available only when the Internal generation of modulating signal (s) parameter is selected.

Reference signal: Phase (degrees)

Specify this parameter to control the phase of the fundamental component of the output voltage of the converter. Default is 0. This parameter is available 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. Default is 0. 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

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. Default is cleared.

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.

wt

The external reference signal used to synchronize the carrier. This Simulink input port is not present when Carrier: Mode of operation is set to Unsynchronized.

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.

m

The measurement output that returns the carrier signal used to determine the output pulses and the sampled reference signal. This port is present only when Show measurement port is selected.

Characteristics

Sample Time	Specified in the Sample Time parameter Continuous if Sample Time = 0
Scalar Expansion	No
Dimensionalized	No

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.

The power_BipolarPWMGenerator model shows a comparison between unipolar PWM and bipolar PWM techniques.

Introduced in R2013a