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Browse Prior Art Database

FLOAT CHARGE BACKUP BATTERY SYSTEM WITH AUTO REVERT

IP.com Disclosure Number: IPCOM000009002D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-30
Document File: 3 page(s) / 126K

Publishing Venue

Motorola

Related People

Bruce Barlow: AUTHOR

Abstract

This design satisfies the requirement for an inte gral three hour minimum battery backup for a ten watt Base Station.

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0 M M-LA

Technical Developments

FLOAT CHARGE BACKUP BATTERY SYSTEM WITH AUTO REVERT

by Bruce Barlow

  This design satisfies the requirement for an inte gral three hour minimum battery backup for a ten watt Base Station.

BACKGROUND

  A new product design for ten watt 8.50 MHz base stations for DataTAC networks includes the requirement for a minimum three hour battery back- up, integral to the unit. Since the base stations are designed for exterior wall or roof top mount, the product specification defined an operating environ- mental temperature range of -30 to +60 degrees C. The high stored capacity and wide temperature range dictated the use of lead acid technology. To reduce the ac power supply requirement during recharge, a 24 hour charge cycle from 10% to 90% was determined to be acceptable for this product. The base station transceiver is designed to operate over an input voltage range of 10 to 18 vDC without degradation of performance.

DESIGN

  For backup service, a constant voltage float charger is the common choice, reference Figures 1 and 2. A p-channel power MosFET, 42, is used as the pass element with a PNP transistor, Ql, sensing the charge current across a reisitor in the source. A Schottky power rectifier, CRl, is in series with the primary DC supply. This provides some temperature compensation to offset the emitterbase variation at Ql and to block the return path when the system is reverted to the battery supply.

  The AC/DC primary converter is nominally 15 vDC but can be trimmed. An NTC thermistor, R6, and resistor, R7, are used at the TRIM input to taper the regulated voltage from 14.6 to 13.3 vDC over the temperature range -30 to +60 degree C. This supplies a Line/Load regulated float voltage with a linear slope that matches the typical total cell varia- tion versus temperature for 12 volt lead acid batteries.

  The series resistor Rl is selected to limit the current to 1.6 amps, which keeps the maximum load on the AC/DC power supply within design limits, while operating the base station at the full 10 watt RF power transmission at extremes. The AC/DC converter supplies an AC OK signal to the MPU in the base station transceiver. The MPU monitors this and performs normal boot at the initial power up of the station. During the initial boot, the MPU outputs a relay control signal to the charger which closes the relay, connecting the battery to the charger. The bat- tery and the AC/DC outputs are OR gated to the transceiver through Schottky power rectifiers, CR2 and CR3, to provide an auto revert on AC power failure.

  The MPU monitors the DC input voltage at the transceiver interface...