Controlled PWM Voltage

Pulse-width modulated voltage source

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Libraries:
Simscape / Electrical / Integrated Circuits

Description

The Controlled PWM Voltage block represents a pulse-width modulated (PWM) voltage source. You can model electrical or physical signal input ports by setting the Modeling option parameter to either:

• Electrical input ports — The block calculates the duty cycle based on the reference voltage across its ref+ and ref- ports. This option is the default.

• PS input — Specify the duty cycle value directly by using an input physical signal port.

If you set Modeling option to Electrical input ports, the demanded duty cycle is

$$100*\frac{V_{ref}-V_{\min }}{V_{\max }-V_{\min }}percent$$

where:

• V_ref is the reference voltage across the ref+ and ref- ports.

• V_min is the minimum reference voltage.

• V_max is the maximum reference voltage.

The value of the Output voltage amplitude parameter determines amplitude of the output voltage.

At time zero, the pulse is initialized as high, unless the Pulse delay time parameter is greater than zero, or the demanded duty cycle is zero.

You can use parameters Pulse delay time and Pulse width offset to add a small turn-on delay and a small turn-off advance. This can be useful when fine-tuning switching times so as to minimize switching losses.

In PWM mode, the block has two options for the type of switching event when moving between output high and output low states:

• Asynchronous – Best for variable-step solvers — Asynchronous events are better suited to variable step solvers, because they require fewer simulation steps for the same level of accuracy. In asynchronous mode the PWM switching events generate zero crossings, and therefore switching times are always determined accurately, regardless of the simulation maximum step size.

• Discrete—time – Best for fixed-step solvers — Discrete-time events are better suited to fixed-step operation, because then the switching events are always synchronized with the simulation step. Using an asynchronous implementation with fixed-step solvers may sometimes result in events being up to one simulation step late. For more information, see Simulating with Fixed Time Step — Local and Global Fixed-Step Solvers.

If you use a fixed-step or local solver and the discrete-time switching event type, the following restrictions apply to the Sample time parameter value:

• The sample time must be a multiple of the simulation step size.

• The sample time must be small compared to the PWM period, to ensure sufficient resolution.

Assumptions and Limitations

The model is based on the following assumptions:

• The REF output of this block is floating, it is not tied to the Electrical Reference. One consequence of this is that if you connect the PWM and REF electrical ports directly to the PWM and REF electrical ports of an H-bridge or a gate driver, you must attach an Electrical Reference block to the REF connection line.

• Do not connect the Controlled PWM block directly to a semiconductor gate, because this omits the gate driver output impedance that determines switching dynamics. Use a Gate Driver or a Half-Bridge Driver block to set the gate-source or the gate-emitter voltage.

• Do not use the Controlled PWM block to drive a motor block directly. A PWM motor driver goes open circuit in between pulses. Use the H-Bridge block to drive a motor block.

• When driving a motor via the H-Bridge block, set the Simulation mode parameter to Averaged to speed up simulations. You must also set the Simulation mode parameter of the H-Bridge block to Averaged mode. This applies the average of the demanded PWM voltage to the motor. The Averaged mode assumes that the impedance of the motor inductive term is small at the PWM frequency. To verify this assumption, run the simulation using the PWM mode and compare the results to those obtained from using the Averaged mode.

• If you are linearizing your model, set the Simulation mode parameter to Averaged and ensure that you have specified the operating point of the block correctly. You can only linearize the block for inputs corresponding to a duty cycle greater than zero and less than 100 percent.

• When you use this block in PWM mode with the Use local solver option selected in the Solver Configuration block, set the Switching event type parameter to Discrete—time – Best for fixed-step solvers. Using the Asynchronous – Best for variable-step solvers option in this situation may produce inaccuracies, because simulation with the local solver implies fixed step, and the PWM events will not always coincide precisely with the simulation steps. This results in PWM events sometimes occurring one simulation step late.

Examples

Control DC Motor with PWM Voltage Source and H-Bridge Driver

Control a DC motor by using the Controlled PWM Voltage and H-Bridge blocks. The DC Motor block delivers a mechanical power of 10 W at 2500 rpm and turns at a no-load speed of 4000 rpm when the DC supply voltage is 12 V. Consequently, if you set the PWM reference voltage to its maximum value of 5 V, the motor runs at 4000 rpm. If you set the PWM reference voltage to 2.5 V, the motor runs at approximately 2000 rpm. To achieve fast simulation, this example sets the Simulation mode parameter of the Controlled PWM Voltage and H-Bridge blocks to Averaged. To validate the averaged behavior, set the Simulation mode parameter to PWM in both the Controlled PWM Voltage and H-Bridge blocks.

• Open Model

Buck Converter

Model a switching power supply that converts a 30V DC supply into a regulated 15V DC supply. Use this model to size the inductance L and the smoothing capacitor C and to design the feedback controller. Select between continuous and discrete controllers to explore the impact of the discretization.

• Open Model

Ports

Input

u — Control signal, unitless

physical signal

Input physical signal that specifies the duty cycle.

Dependencies

To enable this port, set Modeling option to PS input.

Conserving

ref+ — Positive terminal

electrical

Positive electrical reference voltage.

Dependencies

To enable this port, set Modeling option to Electrical input ports.

ref- — Negative terminal

electrical

Negative electrical reference voltage

Dependencies

To enable this port, set Modeling option to Electrical input ports.

PWM — Pulse-width modulated signal

electrical

Electrical conserving port associated with the pulse-width modulated signal.

REF — Floating zero volt reference

electrical

Electrical conserving port associated with the floating zero volt reference.

Parameters

Modeling option — Whether to model electrical or physical signal input ports

Electrical input ports (default) | PS input

Whether to model electrical or physical signal input ports.

PWM

PWM frequency — PWM output signal frequency

1000 Hz (default)

Frequency of the PWM output signal.

Pulse delay time — Turn-on delay

0 s (default)

The pulse train does not start until the simulation time is equal to the Pulse delay time. You can specify a small value for Pulse delay time to fine-tune switching times and ensure that an off-going device is fully off before the on-going device starts to turn on. You can also use larger delay times, for example, if you need the pulse train to start only after a number of cycles. The value you provide must be greater than or equal to zero.

Dependencies

Enabled when the Simulation mode parameter is set to PWM.

Pulse width offset — Lengthens or shortens the pulse

0 s (default)

The demanded pulse width as defined by the product of the demanded duty cycle and one over the pulse frequency can be offset by the value you provide for Pulse width offset. A positive value acts to lengthen the pulse by a fixed amount. A negative value acts to shorten the pulse. You can use this parameter, along with the Pulse delay time, to fine-tune switching times so as to minimize switching losses in some circuits.

Dependencies

Enabled when the Simulation mode parameter is set to PWM.

Minimum pulse width — Minimum length of pulse

0 s (default)

The minimum pulse length, based on the internal clock or defined programmatically, to protect the device being driven. The value you provide must be greater than or equal to zero.

Dependencies

Enabled when the Simulation mode parameter is set to PWM.

Simulation mode — Select type of output voltage

PWM (default) | Averaged

Select the type of output voltage:

• PWM — Produces a pulse-width modulated signal.

• Averaged — The output is a constant whose value is equal to the average value of the PWM signal.

Switching event type — Select type of switching event

Asynchronous – Best for variable-step solvers (default) | Discrete-time – Best for fixed-step solvers

Select the switching event type when moving between output high and output low states:

• Asynchronous – Best for variable-step solvers — This option is more efficient for desktop simulation with variable-step solvers, because it requires fewer simulation steps for the same level of accuracy.

• Discrete-time – Best for fixed-step solvers — Use with fixed-step solvers, including the local solver. For more information, see Simulating with Fixed Time Step — Local and Global Fixed-Step Solvers.

Dependencies

Enabled when the Simulation mode parameter is set to PWM.

Sample time — Discrete sampling time

1e-6 s (default)

The time between updates of the block output state. The sample time must be a multiple of the simulation step size. In order for the PWM control to have sufficient resolution, set the sample time to less than one hundredth of the PWM period. (The PWM period is one over the PWM frequency.)

Dependencies

Enabled when the Switching event type parameter is set to Discrete-time – Best for fixed-step solvers.

Input Scaling

Input voltage for 0% duty cycle — Vmin

0 V (default)

Value of the input voltage at which the PWM signal has a 0% duty cycle.

Dependencies

To enable this parameter, set Modeling option to Electrical input ports.

Input voltage for 100% duty cycle — Vmax

5 V (default)

Value of the input voltage at which the PWM signal has a 100% duty cycle.

Dependencies

To enable this parameter, set Modeling option to Electrical input ports.

Input value for 0% duty cycle — Minimum signal value

0 (default)

Value of the input signal at which the PWM signal has a 0% duty cycle.

Dependencies

To enable this parameter, set Modeling option to PS input.

Input value for 100% duty cycle — Maximum signal value

1 (default)

Value of the input signal at which the PWM signal has a 100% duty cycle

Dependencies

To enable this parameter, set Modeling option to PS input.

Output Voltage

Output voltage amplitude — Signal amplitude for high output

5 V (default)

Amplitude of the PWM signal when the output is high.

Extended Capabilities

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

Version History

Introduced in R2008b