DC-DC Converter

Behavioral model of power converter

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• Simscape / Electrical / Semiconductors & Converters / Converters

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Description

The DC-DC Converter block represents a behavioral model of a power converter. This power converter regulates voltage on the load side. To balance input power, output power, and losses, the required amount of power is drawn from the supply side. Alternatively, the converter can support regenerative power flow from load to supply.

This circuit illustrates the converter's behavior.

The P_fixed component draws a constant power and corresponds to converter losses that are independent of load current. The power drawn is set by the Converter losses at zero output power parameter value. The resistor R_out corresponds to losses that increase with load current, and is determined from the value you specify for the Percentage efficiency at rated output power parameter.

The voltage source is defined by the following equation:

v = v_ref – i _load D + i_load R_out

Where:

• v_ref is the load side voltage set point, as defined by the value you specify for the Output voltage reference demand parameter. Alternatively, you can provide this value as input to the Vref port when the Voltage reference parameter is set to External.

• D is the value you specify for the Output voltage droop with output current parameter. Having a separate value for droop makes control of how output voltage varies with load independent of load-dependent losses. Instead of specifying D directly, you can specify the Percent voltage droop at rated load.

The current source value i is calculated so that the power flowing in to the converter equals the sum of the power flowing out plus the converter losses.

To specify the converter behavior when the voltage presented by the load is higher than the converter output voltage reference demand, use the Power direction parameter:

• Unidirectional power flow from supply to regulated side — Current is blocked by the off-state diode, and the current source current i is zero. Set the conductance of this diode using the Diode off-state conductance parameter.

• Bidirectional power flow — Power is transmitted to the supply side, and i becomes negative.

Optionally, the block can include voltage regulation dynamics. If you select Specify voltage regulation time constant for the Dynamics parameter, then a first-order lag is added to the equation defining the voltage source value. With the dynamics enabled, a load step change results in a transient change in output voltage, the time constant being defined by the Voltage regulation time constant parameter.

Tabulated efficiency

You can tabulate the DC-DC Converter block efficiency depending on the output current, input voltage, and temperature.

This equation defines the relationship between losses and efficiency:

$$\begin{array}{l}\begin{array}{cc}loss\left(i_2,v_1,T\right)=\left[1-\frac{eff\left(i_2,v_1,T\right)}{100}\right]abs\left(v_1{i}_1\right)& if{i}_2<0\left(powerflow1\to \text{2}\right)\end{array}\\ \begin{array}{cc}loss\left(i_2,v_1,T\right)=\left[1-\frac{eff\left(i_2,v_1,T\right)}{100}\right]abs\left(v_2{i}_2\right)& if{i}_2>0\left(powerflow2\to \text{1}\right)\end{array}\end{array}$$

where

• loss(i₂,v₁,T) are the DC-DC converter losses.

• v₁ is the input voltage.

• i₁ is the input current.

• i₂ is the output current.

• T is the device temperature, if you expose the thermal port of this block.

• eff(i₂,v₁,T) is the efficiency of the DC-DC converter as a function of the output current, input voltage, and temperature, as specified in Percentage efficiency table, eff(I2,V1,T).

If the output current is equal to 0, the converter losses are equal to Converter losses at zero output power.

These are the steps to compute the used power losses depending on the output current:

1. If the output current is less than the last negative current point or greater than the first positive current point in the Vector of output currents for tabulated efficiencies, I2 parameter, the block uses the Percentage efficiency table, eff(I2) parameter to find the corresponding efficiency (with linear interpolation or nearest extrapolation) and then convert that efficiency to losses.

2. Else, the block blends the zero output current case to the last negative point or the first positive point in the table, as shown in this figure:

   

If you set the Modeling option parameter to Show thermal port, the block tabulates the efficiency depending on the temperature, in addition to the output current and input voltage. The losses calculation remains the same as the option with hidden thermal ports.

Simulating Faults

You can use the physical signal input port F to simulate both DC supply failure and converter failure. This type of event cannot be simulated by simply disconnecting the DC supply, for example by opening a switch, because the average value model will attempt to increase supply-side current to unrealistic values as supply-side voltage drops.

To expose the fault port, F, in the Faults tab, set the Enable output open-circuit fault parameter to Yes.

You control the behavior in response to the physical signal fault input F by the parameters on the Faults tab of the block dialog box. With the default parameter settings:

• Fault condition is Output open circuit if F >= Fault threshold

• Fault threshold is 0.5

If a signal is connected to port F, then the block operates according to the parameter settings on the Faults tab. For example, if Fault condition is Output open circuit if F >= Fault threshold, then when the signal at port F rises above the Fault threshold value, the converter stops operating. Zero current is taken from the supply side, and zero current is supplied to the load side.

Model thermal effects

This block has one optional thermal port. To expose the thermal port, set the Modeling option parameter to either:

• No thermal port — The block does not contain a thermal port.

• Show thermal port — The block contains one thermal conserving port.

The block transfers heat generated from electrical losses through a Controlled Heat Flow Rate Source to a Thermal Mass block. The electrical properties of the block do not change with temperature. Specify the thermal properties for this block using the parameters Thermal mass and Initial temperature.

Assumptions

• The two electrical networks connected to the supply-side and regulated-side terminals must each have their own Electrical Reference block.

• The supply-side equation defines a power constraint on the product of the voltage, v_s, and the current, i_s. For simulation, the solver must be able to uniquely determine v_s. To ensure that the solution is unique, the block implements two assertions:

  • v_s > 0 — This assertion ensures that the sign of v_s is uniquely defined

  • i_s < i_max — This assertion deals with the case when the voltage supply to the block has a series resistance

  When there is a series resistance, there are two possible steady-state solutions for i_s that satisfy the power constraint, the one with the smaller magnitude being the desired one. You should set the value for the Maximum expected supply-side current parameter, i_max, to be larger than the expected maximum current. This will ensure that when the model is initialized the initial current does not start at the undesired solution.

Ports

Conserving

1+ — Input positive terminal

electrical

Electrical conserving port associated with the positive terminal of the input side.

1- — Input negative terminal

electrical

Electrical conserving port associated with the negative terminal of the input side.

2+ — Output positive terminal

electrical

Electrical conserving port associated with the positive terminal of the output side.

2- — Output negative terminal

electrical

Electrical conserving port associated with the negative terminal of the output side.

Vref — Voltage reference

physical

Physical signal input port that provides the external voltage reference signal.

Dependencies

This port is visible only when the Voltage Reference parameter is set to External.

F — Fault signal

physical

Physical signal input port that provides the external fault trigger signal.

Dependencies

To expose this port, in the Faults tab, set Enable output open-circuit fault to Yes.

H — Thermal mass

thermal

Thermal conserving port that represents the thermal mass. When you expose this port, provide additional parameters to define battery behavior at a second temperature. For more information, see the Thermal parameters.

Dependencies

To enable this port, set Modeling option to Show thermal port.

Parameters

Modeling option — Whether to enable thermal port

No thermal port (default) | Show thermal port

Whether to enable the thermal port of the block and model thermal parameters.

Main

Voltage reference — Voltage reference

Internal (default) | External

Specify if you want to model the voltage reference internally or externally.

Output voltage reference demand — Voltage set point

10 V (default)

The set point for the voltage regulator, and the output voltage value when there is no output current.

Dependencies

This port is visible only when the Voltage Reference parameter is set to Internal.

Rated output power — Rated power

10 W (default)

Output power for which the percentage efficiency value is given. This parameter is also used to calculate droop, D, if droop is specified as a percentage.

Droop parameterization — Droop model

By voltage droop with output current (default) | By percent voltage droop at rated load

Select one of the following methods for droop parameterization:

• By voltage droop with output current — Specify the absolute value of droop, D. This is the default option.

• By percent voltage droop at rated load — Specify droop, D, as a percentage at rated load.

Output voltage droop with output current — Voltage droop at 1 A

0.1 V/A (default)

The number of volts that the output voltage will drop from the set point for an output current of 1 A.

Dependencies

This parameter is visible only if you select By voltage droop with output current for the Droop parameterization parameter.

Percent voltage droop at rated load — Voltage droop at rated load

2 (default)

The percentage by which voltage drops compared to the nominal output voltage when supplying the rated load.

Dependencies

This parameter is visible only if you select By percent voltage droop at rated load for the Droop parameterization parameter.

Power direction — Power flow direction

Unidirectional power flow from supply to regulated side (default) | Bidirectional power flow

Select one of the following methods for the direction of power conversion:

• Unidirectional power flow from supply to regulated side — Most small power regulators are unidirectional. This is the default option.

• Bidirectional power flow — Larger power converters can be bidirectional, for example, converters used in electric vehicles to allow regenerative braking.

Diode off-state conductance — Unidirectional diode

1e-8 1/Ohm (default)

Ideal diode incorporated on the output side to prevent current from being forced into the converter in the unidirectional configuration.

Maximum expected supply-side current — Maximum supply current

2 A (default)

Set this value to a value greater than the maximum expected supply-side current in your model. Using twice the expected maximum current is generally sufficient. For more information, see Assumptions.

Losses

Converter losses at zero output power — Constant losses

1 W (default)

Power drawn by the P_fixed component in the equivalent circuit diagram, which corresponds to converter losses that are independent of load current.

Converter losses — Converter losses

Assume proportional to square of output current (default) | Tabulate efficiency

Converter losses parameterization.

Percentage efficiency at rated output power — Rated efficiency

80 (default)

Efficiency as defined by 100 times the output load power divided by the input supply power.

Dependencies

To enable this parameter, set Converter losses to Assume proportional to square of output current.

Variables for tabulation — Tabulation variables

Output current (default) | Output current and input voltage | Output current and temperature | Output current, input voltage, and temperature

Whether to tabulate efficiency on the output current, the input voltage, and the temperature. To enable the temperature options, you need to expose the thermal ports of the block.

Vector of output currents for tabulated efficiencies, I2 — Output currents for tabulated efficiencies

[-2, -1, 1, 2] A (default) | vector of positive ascending scalars

Vector of output currents for tabulated efficiencies. The size of this parameter must be equal to or greater than 3 and equal to the size of Percentage efficiency table, eff(I).

Dependencies

To enable this parameter, set Converter losses to Tabulate efficiency.

Percentage efficiency table, eff(I2) — Efficiency table in function of current

[66, 76, 80, 74] (default) | vector of positive scalars

Efficiency table in function of the current, in percentage. The size of this parameter must be equal to the size of Vector of output currents for tabulated efficiencies, I2.

Dependencies

To enable this parameter, set Converter losses to Tabulate efficiency and Variables for tabulation to Output current.

Vector of input voltages for tabulated efficiencies, V1 — Input voltages for tabulated efficiencies

[15, 30, 45] V (default) | vector of positive ascending scalars

Vector of input voltages for tabulated efficiencies. The size of this parameter must be equal to or greater than 2.

Dependencies

To enable this parameter, set Converter losses to Tabulate efficiency and Variables for tabulation to Output current and input voltage or Output current, input voltage, and temperature.

Percentage efficiency table, eff(I2,V1) — Efficiency table in function of current and voltage

[66, 66, 66; 76, 76, 76; 80, 80, 80; 74, 74, 74] (default) | vector of positive scalars

Efficiency table in function of the current, in percentage.

Dependencies

To enable this parameter, set Converter losses to Tabulate efficiency and Variables for tabulation to Output current.

Vector of temperatures for tabulated efficiencies, T — Temperatures for tabulated efficiencies

[280, 300, 320] K (default) | vector of positive ascending scalars

Vector of temperatures for tabulated efficiencies. The size of this parameter must be equal to or greater than 2.

Dependencies

To enable this parameter, expose the thermal port of this block and set Converter losses to Tabulate efficiency.

Percentage efficiency table, eff(I2,T) — Efficiency table in function of current and temperature

[66, 66, 66; 76, 76, 76; 80, 80, 80; 74, 74, 74] (default) | vector of scalars

Efficiency table in function of current and temperature, in percentage.

Dependencies

To enable this parameter, expose the thermal port of this block and set Converter losses to Tabulate efficiency and Variables for tabulation to Output current and temperature.

Percentage efficiency table, eff(I2,V1,T) — Efficiency table in function of current, voltage, and temperature

repmat([66; 76; 80; 74], 1, 3, 3) (default) | vector of scalars

Efficiency table in function of current, voltage, and temperature, in percentage.

Dependencies

To enable this parameter, expose the thermal port of this block and set Converter losses to Tabulate efficiency and Variables for tabulation to Output current, input voltage, and temperature.

Dynamics

Dynamics — Dynamics model

No dynamics (default) | Specify voltage regulation time constant

Specify whether to include voltage regulation dynamics:

• No dynamics — Do not consider the voltage regulation dynamics.

• Specify voltage regulation time constant — Add a first-order lag to the equation defining the voltage source value. With the dynamics enabled, a load step change results in a transient change in output voltage.

Voltage regulation time constant — Dynamics time constant

0.02 s (default)

Time constant associated with voltage transients when the load current is stepped.

Dependencies

This parameter is only visible when you select Specify voltage regulation time constant for the Dynamics parameter.

Initial output voltage demand — Initial demand voltage

10 V (default)

Value of v_ref at time zero. Normally, v_ref is defined by the Output voltage reference demand parameter. However, if you want to initialize the model with no transients when delivering a steady-state load current, you can set the initial v_ref value by using this parameter, and increase it accordingly to take account of output resistance and droop.

Dependencies

This parameter is only visible when you select Specify voltage regulation time constant for the Dynamics parameter.

Faults

Enable output open-circuit fault — Option to enable faults

No (default) | Yes

Whether to model output open-circuit faults in the block.

Fault condition — Fault model

Output open circuit if F >= Fault threshold (default) | Output open circuit if F <= Fault threshold

Selects whether the converter is disabled by a signal that is high or low:

• Output open circuit if F >= Fault threshold — Converter is disabled if the signal at port F rises above the threshold value. This is the default option.

• Output open circuit if F <= Fault threshold — Converter is disabled if the signal at port F falls below the threshold value.

Dependencies

To enable this parameter, set Enable output open-circuit fault to Yes.

Fault threshold — Fault limit

0.5 (default)

Threshold value used to detect a fault.

Dependencies

To enable this parameter, set Enable output open-circuit fault to Yes.

Thermal

To enable these parameters, set Modeling option to Show thermal port.

Thermal mass — Thermal mass associated with the thermal port

100 J/K (default)

Thermal mass associated with thermal port H. It represents the energy required to raise the temperature of the thermal port by one degree.

Initial temperature — Initial temperature at the thermal port

298.15 K (default)

Initial temperature associated with thermal port H.

Model Examples

Power Split Hybrid Vehicle Electrical Network

The basic architecture of a power-split hybrid transmission. The planetary gear, along with the motor and generator, acts like a variable ratio gear. In this test, the vehicle accelerates from 15 m/s to 20 m/s, and then decelerates back to 15 m/s. The power management strategy uses just electrical power to perform the maneuver.

Open Model

Ultracapacitor with Converter

How a DC-DC converter can be used to maintain a constant load voltage when drawing power from an ultracapacitor. Initially the converter supplies power to the load, and as it does so, the capacitor voltage drops. The protection circuit disconnects the load when the capacitor voltage drops below a threshold of 4V. At 10 seconds the generator is turned on, and power is supplied to both the load and to the capacitor to recharge it.

Open Model

Electric Vehicle Configured for HIL

An electric vehicle model suitable for Hardware-In-the-Loop (HIL) deployment. Energy-based modeling is used to avoid high-frequency switching, and solvers set for fixed-step simulation.

Open Model

Extended Capabilities

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

Version History

Introduced in R2012b