Generate pulses for PWM-controlled 2-level converter
Simscape / Electrical / Specialized Power Systems / Control & Measurements / Pulse & Signal Generators
Simscape / Electrical / Specialized Power Systems / Fundamental Blocks / Power Electronics / Pulse & Signal Generators
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.
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.
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.
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
.
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
.
Specify the minimum (valley) and maximum (peak) values of the
triangular carrier signal. Default is [ -1 1 ]
.
Specify the frequency (Fc) of the triangular carrier signal.
Default is 27
. This parameter is available
only if the Mode of operation parameter is set
to Synchronized
.
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.
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
.
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.
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.
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.
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.
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.
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.
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.
Sample Time | Specified in the Sample Time parameter Continuous if Sample Time = 0 |
Scalar Expansion | No |
Dimensionalized | No |
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.