# DC-DC Converter

Behavioral model of power converter

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• Simscape / Electrical / Semiconductors & Converters / Converters

## 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 Pfixed 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 Rout 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:

Where:

• vref 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 and the temperature.

This equation defines the relationship between losses and efficiency:

$loss\left({i}_{2}\right)=\left[1-eff\left({i}_{2}\right)\right]abs\left({v}_{1}{i}_{1}\right)$

where

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

• v1 is the input voltage.

• i1 is the input current.

• i2 is the output current.

• eff(i2) is the efficiency of the DC-DC converter as a function of the output current, as specified in Percentage efficiency table, eff(I).

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, I parameter, the block uses the Percentage efficiency table, eff(I) 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 expose the thermal ports, the block tabulates the efficiency depending on both the current and temperature. The losses calculation remains the same as the non-thermal variant.

### 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.

### Modeling thermal effects

The block has an optional thermal port, hidden by default. To expose the thermal port, right-click the block in your model, and then from the context menu select Simscape > Block choices > Show thermal port. This action displays the thermal port H on the block icon, and exposes the Thermal Port parameters.

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, vs, and the current, is. For simulation, the solver must be able to uniquely determine vs. To ensure that the solution is unique, the block implements two assertions:

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

• is < imax — 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 is 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, imax, 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

expand all

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

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

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

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

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.

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.

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 expose this port, right-click the block and select Simscape > Block choices > Show thermal port.

## Parameters

expand all

### Main

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

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.

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.

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.

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.

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.

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.

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

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

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

Converter losses parameterization.

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.

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.

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, I.

#### Dependencies

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

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.

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

#### Dependencies

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

### Dynamics

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.

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.

Value of vref at time zero. Normally, vref 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 vref 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

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

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.

Threshold value used to detect a fault.

#### Dependencies

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

### Thermal

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 associated with thermal port H.