Design Con 2015

Selecting FETs for hot-swap source connection

Thomas Stamm, STMicroelectronics -November 05, 2013

Here’s 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. This article shows 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 – a disk 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 – 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 – contact of the pre-charge pin, contact of the power pin, and converter startup.

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


The converter and FET are held OFF until the capacitor is charged, then the FET is turned ON and the converter started. Startup can be controlled by the voltage on the resistor.

The FET can be turned off, the resistor can’t.  If the load is shorted, the resistor can overheat.

 

FET Alone

The best design uses a single FET as the connecting element, taking all the strain off the other parts. The pre-charge resistor can be eliminated if the FET can safely dissipate the capacitor charging energy. Hot-swap drivers can be used to control the FET, adding such features as linear current limiting, end-of-charging signals, and converter startup controls.


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