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A Technique to Provide an Isolated Drive Signal to the Control FET of a Battery Pack for Improved Testability

IP.com Disclosure Number: IPCOM000004821D
Original Publication Date: 2001-Jun-18
Included in the Prior Art Database: 2001-Jun-18
Document File: 3 page(s) / 43K

Publishing Venue

Motorola

Related People

Peter Stack: AUTHOR

Abstract

A Technique to Provide an Isolated Drive Signal to the Control FET of a Battery Pack for Improved Testability

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A Technique to Provide an Isolated Drive Signal to the Control FET of a Battery Pack for Improved Testability

Peter Stack

Introduction

In rechargeable battery packs containing lithium ion cells, circuits are often utilized to guard against overcharging the cells. Generally, there are primary and secondary over voltage protection circuits. This paper focuses on a technique for testing a specific type of secondary over voltage protection circuit commonly found in laptop computer batteries. The aforementioned circuit monitors the pack voltage or the individual cell voltages and will activate a heated fuse if a predetermined voltage level is detected. Under an over voltage condition, activation of the fuse (which is in series with the charge discharge path) prevents any further charging and permanently disables the battery pack. Because the fuse heating element is switched in parallel with the power source for the pack circuitry, comprehensive testing of the secondary over voltage protection circuit can be difficult. The technique described in this paper provides a means to fully test the operation of the safety circuit described above.

Description of Technique

In laptop computer batteries, it is common to use a fuse containing a heating element that is activated when an over voltage condition is detected. When activated, the heating element forces the main fuse link to open. The fuse is activated by a circuit that monitors the voltage of each individual cell or the total battery pack. Because the fuse activation circuit derives its power via the cell connections, a power source capable of blowing the fuse is present during production test. Without a means to isolate the fuse from all power sources, the fuse and activation circuit cannot be tested as one complete functional block.

Figure 1 illustrates the safety circuitry found in a typical laptop computer battery. The battery circuit consists of the control FETs (Q1 Q2) for charge and discharge, a fuse (F1), and the associated circuitry to control the FETs or activate the fuse. Generally, the FET control circuitry (U1) is utilized as the primary method to open the FETs if an over voltage, over current, or under voltage condition is detected. The fuse activation circuitry (U2) is used for secondary over voltage protection.

To test the fuse activation circuit, a technique is required that will isolate the fuse from any power source capable of blowing the fuse when the (secondary) over voltage protection circuit is activated. Referring to figure 1, it can be seen that if the discharge FET is open, then the fuse will be isolated from the cells while power is maintained to the circuitry. Because the ICs used for primary safety only open the discharge FET during conditions of under voltage or over current, a method to override the discharge FET control signal (DCHG) is required in order to open the FET during over voltage te...