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LOW POWER BLOCKING DIODE FOR BATTERY CHARGERS

IP.com Disclosure Number: IPCOM000008741D
Original Publication Date: 1998-Jun-01
Included in the Prior Art Database: 2002-Jul-09
Document File: 2 page(s) / 104K

Publishing Venue

Motorola

Related People

Iilonga Thandiwe: AUTHOR [+3]

Abstract

In charging devices for secondary batteries, there is generally a need for a means to prevent the battery from supplying power to the charger and to keep the heat dissipated by the circuit elements to a minimum. The first issue is typically accomplished by placing a diode (Dl) between the power source (Vs) and the battery to allow current to flow in only one direction, i.e. from the power source to the battery (Figure I). The latter issue is typically addressed with some combination of natural convection elements and/or forced convection elements. Because the blocking diode must also dissipate the power from the charger (this can be :as high as 3 watts or more) careful attention to thermal issues is required, typically necessitating a heat sink on the diode. However, in some applications the size and appropriateness of heat sinks, fans, etc. are either severely restricted or completely undesirable. Moreover, simply paralleling 2 or more diodes together is often neither an attractive nor reasonable alternative.

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MOTOROLA Technical Developments

LOW POWER BLOCKING DIODE FOR BATTERY CHARGERS

by lilonga Thandiwe, Vernon Meadows and James Rey

  In charging devices for secondary batteries, there is generally a need for a means to prevent the battery from supplying power to the charger and to keep the heat dissipated by the circuit elements to a minimum. The first issue is typically accomplished by placing a diode (Dl) between the power source (Vs) and the battery to allow current to flow in only one direction, i.e. from the power source to the battery (Figure I). The latter issue is typically addressed with some combination of natural convection elements and/or forced convection

elements. Because the blocking diode must also dissipate the power from the charger (this can be :as high as 3 watts or more) careful attention to thermal issues is required, typically necessitating a heat sink on the diode. However, in some applications the size and appropriateness of heat sinks, fans, etc. are either severely restricted or completely undesirable. Moreover, simply paralleling 2 or more diodes together is often neither an attractive nor reasonable alternative.

Dl El4

charger

Fig. 1

  By placing a power FET (Ql) across the blocking diode and turning it on and off the thermal require- ments of the blocking diode can be significantly reduced (Figure 2). Ql is pulsed on and off by a control circuit in the charger (Ul). Because of Ql's low Rds (on), the charging current flows almost entirely through it instead of Dl when Ql is on. However, while Ql is on the battery can still supply power to the charger if the charger power source is removed or disabled. When Ql is off while the battery is being charged, the charging current flows

primarily through Dl with a fraction of the current flowing through the intrinsic diode of the FET. (Although Ql's intrinsic diode can function as a blocking diode, for high charge current applications it typically will not be rated high enough to handle the worst case power dissipation.) However, if the charger power source, Vs, is disabled Dl is reversed biased and will prevent the battery from supplying power to the charger. Ul then looses power an...