# Current Limiter

Behavioral model of current limiter

**Library:**Simscape / Electrical / Semiconductors & Converters

## Description

The Current Limiter block provides a behavioral model of a current limiter. Use it to represent current limiting as found in power supplies and motor drives, and also to represent components that are used to limit inrush current.

The current limiting acts for both positive and negative currents. For applications where limiting is required in only one direction, you can augment the Current Limiter block with a series diode (blocks any reverse current) or parallel diode (no limiting in the reverse direction).

The block implements current limiting by using a hyperbolic tangent function:

$$i={i}_{LIM}\mathrm{tanh}\left(\frac{4v}{{v}_{LIM}}\right)+{g}_{LIM}v$$

where:

*i*is the current through the component.*v*is the voltage drop across the component.*i*is the current limit._{LIM}*v*is the approximate voltage drop across the component when the current limit becomes active._{LIM}*g*is the rate of change of current with voltage drop when on the current limit (limit-state conductance)._{LIM}

When *v* =
*v _{LIM}*, then

$$i={i}_{LIM}\mathrm{tanh}\left(4\right)+{g}_{LIM}v=0.9993{i}_{LIM}+{g}_{LIM}v$$

Therefore the current is approximately equal to the limit. Choose the value for
*g _{LIM}* such that

*g*·

_{LIM}*v*is small compared to

*i*for the maximum expected voltage drop. This term is included in the block equation to improve numerical properties during simulation.

_{LIM}When choosing the value of *v _{LIM}*, consider
that making it too small will require tight solver tolerances and small step sizes. In
practice, current limiters can be implemented using a MOSFET and series source resistor,
the gate-source voltage being driven by the series resistor. This implementation does
not produce a sharp limit, similar to the tanh curve used in this block. You can use a datasheet plot of current
against voltage to pick a suitable value for

*v*.

_{LIM}### Thermal Port

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 thermal port model contains a thermal mass. The power dissipated by the current limiter, plus the heat flow into the thermal port, drives the thermal mass differential equation:

$$m\frac{dT}{dt}={P}_{loss}+{Q}_{H}$$

where:

*m*is the thermal mass.*T*is the thermal port temperature.*P*is the electrical loss,_{loss}*v*·*i*.*Q*is the heat flow from the external network into the thermal port._{H}

## Ports

### Conserving

## Parameters

## Extended Capabilities

**Introduced in R2015a**