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Picking out FETs for hot-swap source connection

Posted: 19 Nov 2013 ?? ?Print Version ?Bookmark and Share

Keywords:FET? hot insertion? capacitor? resistor? Transient Thermal Impedance?

In this article, we tackle how to choose an FET to avoid damage to contacts during hot insertion. This one seems simple. Read on... It's more complex than you would expect, but the design procedure is very straightforward. All you need is the FET data sheet. We show how to do it in four steps.

First, understand the problem
A highly capacitive load must be connected to a very stiff source, like a DC bus. This occurs when hot plugging anything C a disc drive, a telecom module, a laptop battery... Very high current can flow, possibly damaging the connector, or causing transients that disturb the system.

Figure 1: The simple schematic.

With the capacitor discharged, the switch is closed. Current is limited by the resistance, the capacitor charges. The battery delivers energy to the capacitor at a rate determined by the resistance. After some time, the capacitor is charged, and no more current flows. The resistance has dissipated the same energy as is stored in the capacitor.

The resistance might be only the switch contact resistance and ESRs of the battery and capacitor. Very high current can flow. Contact points can weld together. Plating damage in the connectors exposes the base metal to corrosion. Very bad C time bombs waiting to ruin your reputation.

Some simple circuits that get around the high current transients are shown below.

Short power pin, resistor precharge

An extended pin pre-charges the capacitance on a module as it is plugged in. The input current has 3 spikes C contact of the pre-charge pin, contact of the power pin, and converter start-up.

Figure 2: Long pin pre-charge.

The scheme works fine if:
1. The module is fully inserted, at a controlled rate. The resistor absorbs most of the pre-charge energy.
2. If the module is not fully inserted, the resistor must be able to dissipate the power due to converter input current safely.

Resistor and FET source connect
An FET can be added across the pre-charge resistor. The extended pre-charge pin can be eliminated; the FET carries the converter input current.

Figure 3: FET and resistor pre-charge.

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