Summary of Limiting Inrush Current
This article explains inrush current as the high instantaneous input surge drawn by devices like power converters during startup, primarily caused by charging input capacitance. It details problems such as false overload triggers and upstream converter shutdowns when multiple units start simultaneously. The text suggests three passive components—resistors, inductors, and thermistors—as solutions to limit this current, noting specific drawbacks for resistors regarding efficiency and voltage drop.
Parts used in the Inrush Current Reduction Project:
- Resistor
- Inductor
- Thermistor
- Input Capacitance
- Fuses
- Circuit Breakers
Inrush current or input surge current refers to the maximum, instantaneous input current drawn by an electrical device during its initial turn on.
Power converters also feature high inrush currents relative to their steady state currents that may be several times their normal full-load current when first energized.
The inrush current of a power converter is typically the charging current of the input capacitance.
Problems Caused by Inrush Current:
Inrush currents can be problematic in circuits that utilize overload protection devices such as fuses and circuit breakers. The selection of overcurrent protection devices is made more complicated when high inrush currents are present. False overload conditions can trigger unwanted protection events.
In installations where multiple DC/DC converters are powered by a single higher power converter installed “upstream”, multiply simultaneous inrush currents from the downstream converters can cause the upstream converter to go into overload or short circuit protection, resulting in a complete shutdown of the power distribution circuit. See Figure 1 Here the total current drawn from the 5W upstream converter is the sum of the three inrush currents.
Solutions with Passive Devices:
There are three passive components that can be installed at the input to the converter that will help reduce the inrush currents: resistors, inductors, and thermistors.
A resistor in series with the line can be used to limit the current that is charging input capacitors with typical values of between 0.5O to 1 O. This approach is not very efficient, especially in high power devices, since the resistor will have a voltage drop and dissipate some power.
The second way to help reduce the inrush current is by installing an inductor in series with the line with typical value of 47µH will also limit inrush current.
Read more: Limiting Inrush Current
- What is inrush current?
Inrush current refers to the maximum, instantaneous input current drawn by an electrical device during its initial turn on. - Why do power converters have high inrush currents?
The inrush current of a power converter is typically the charging current of the input capacitance. - How can inrush current cause protection devices to fail?
False overload conditions can trigger unwanted protection events in circuits utilizing fuses and circuit breakers. - What happens if multiple DC/DC converters start simultaneously?
Multiply simultaneous inrush currents from downstream converters can cause the upstream converter to go into overload or short circuit protection. - Which passive devices help reduce inrush currents?
There are three passive components that can be installed at the input to the converter: resistors, inductors, and thermistors. - What is the typical value range for a series resistor?
A resistor in series with the line has typical values of between 0.5 Ohm to 1 Ohm. - What is a drawback of using a resistor to limit inrush current?
This approach is not very efficient because the resistor will have a voltage drop and dissipate some power. - What inductor value limits inrush current effectively?
An inductor in series with the line with a typical value of 47 microhenries will also limit inrush current.
