LM2576T-15FlowLB03 - National Semiconductor

TL/H/12054

High-Efficiency 3A Battery Chargers Use LM2576 Regulators AN-946

National Semiconductor

Application Note 946

Chester Simpson

May 1994

High-Efficiency 3A Battery

Chargers Use LM2576

Regulators

This paper describes two LM2576-based designs which

provide up to 3A of current for battery charging.

3A Battery Charger Has Built-In

Overcharge Protection

This design is a 3A battery charger intended for use with

5-cell Ni-Cd or Ni-MH battery packs (but can be modified to

suit other numbers of cells). The circuit includes automatic

shutoff that occurs when the battery temperature rises 10§C

above ambient.

CIRCUIT CONCEPT

This battery charger shown in

Figure 1

was developed spe-

cifically for applications using either

Nickel-Cadmium

(Ni-Cd)

Nickel-Metal Hydride (Ni-MH)

batteries that will

accept a 3A fast-charge rate, and provides automatic shut-

off of the high-current charge when the battery is full.

After shutoff, a continuous (low level) charge current is used

to ‘‘trickle charge’’ the battery which keeps it topped off and

prevents charge loss due to any internal leakage.

The trickle charge rate used must always be low enough

that the amount of gas developed within the cell is small

enough that it can recombine, thus preventing pressure

build-up and venting (opening of the cell’s internal vent to

release pressure). The maximum safe trickle charging rate

is determined by the size and type of battery (this is covered

later in the paper).

TL/H/12054–1

Notes (Unless Otherwise Specified):

All capacitance values are in mF.

All resistors are 5%, (/4W.

U1 requires small heatsink (RTH k15§C/W).

C1 and C4 must be low ESR aluminum electrolytic.

U5D is not used.

FIGURE 1. 3A Charger with Overcharge Protection

C1995 National Semiconductor Corporation RRD-B30M75/Printed in U. S. A.

The critical specification for a battery is its

Amp-hour (A-hr)

rating,

which is numerically equal to the maximum amount

of current the battery can supply to a load for one hour

before the cell reaches its

end-of-life voltage

(usually tak-

en as 1.0V/cell for Ni-Cd and Ni-MH batteries.

When a battery is charged or discharged at a current that is

equal to its A-hr rating,

this is known as the ‘‘c’’ rate.

Most Ni-Cd and Ni-MH batteries can be safely charged at a

1c rate, as long as they are not overcharged. However, the

battery temperature must be within a range of about 15§Cto

45§C (the reasons are detailed later in this paper).

OVERCHARGING: THE SILENT KILLER

The nemesis of all rechargeable batteries is overcharge . . .

although some battery types tolerate it better than others,

the results of overcharge range from minor damage to cata-

strophic failure.

In the case of Ni-Cd, which is the most popular rechargea-

ble battery type presently in use, sustained overcharge

causes increasing pressure within the battery that eventual-

ly causes the cell’s vent to open and release oxygen. This

has a detrimental effect on the battery, although it may still

retain some useful capacity.

If Ni-MH batteries are overcharged, they will also build up

pressure and release gas: however, the gas released will be

hydrogen, which is extremely explosive near spark or flame.

One battery manufacturer created an interesting euphe-

mism for some of the unfortunate accidents in cases where

Ni-MH batteries were overcharged:

Rapid Spontaneous

Disassembly.

DETECTING END-OF-CHARGE

There are several ways to detect end-of-charge for Ni-Cd or

Ni-MH batteries, but one way that is both simple and reliable

is called a DT detector. It measures both the ambient tem-

perature and the battery temperature and cuts off the high

current charger when the battery rises a pre-set amount

above ambient. This design uses a 10§C rise as the cutoff

point (which is recommended by most battery makers), but

can be easily adjusted by changing resistor values.

Ni-Cd cells are perfectly suited for DT cutoff techniques,

because their charge process is

endothermic

(they get

slightly

cooler

when a discharged battery is being re-

charged). Even at fast charge rates, the battery will not be-

gin to heat until it is nearly fully recharged. At that point, the

battery is no longer converting the electrical current into a

chemical reaction, so it must be dissipated as heat. The

resulting increase in temperature provides a very accurate

indicator that it is time to stop charging.

The Ni-MH battery is not quite as accommodating: the re-

charge cycle is

exothermic

(the battery gets slightly

warm-

during recharge) but still shows a fairly well defined in-

crease in temperature when the battery is fully charged. Us-

inga10

CDT detection point will give good results in most

cases, and is recommended by the battery makers.

NOTE: WARNINGS ABOUT FAST CHARGING NI-MH

AND NI-CD BATTERIES

Since the Ni-MH battery normally gives off heat during re-

charge, the 10§C ‘‘window’’ may have to be adjusted to suit

the characteristics of the specific cell: The window must be

wide enough to prevent premature cutoff from ‘‘normal’’

heating, but narrow enough to detect the temperature rise

which occurs at full charge (and execute appropriate charge

termination).

Any new design that uses Ni-MH batteries should be care-

fully evaluated to verify accurate end-of-charge termination

because of the potential for battery explosion if hydrogen is

released.

IMPORTANT:

With Ni-Cd or Ni-MH cells, the 1c (fast)

charge rate can only be safely used if the battery tem-

perature is in the range of about 15§Cto45

low temperatures,

gas recombination within NiCd and

NiMH batteries does not occur as easily, which limits the

amount of charging current that can be safely used before

venting will occur. If low-temperature (k15§C) recharging is

required, consult the battery maker for safe charging current

levels.

A battery that is recharged at

elevated temperature

will

retain substantially less energy than a battery recharged at

25§C. At high temperatures (l35§C) gas generation within

the cell occurs at a much lower state of charge, meaning

that the cell will not accept as much charge (compared to

25§C) for a given amount of cell temperature rise.

The poor charging efficiency seen at high battery tempera-

tures means that extremely long recharge times (at low

charging currents) are required to deliver full (25§C) capacity

of charge to a ‘‘hot’’ battery.

TRICKLE CHARGE CURRENT

All batteries lose charge internally due to self-discharge,

usually occurring due to leakage paths through the battery

separators (insulators). The amount of leakage is depen-

dent primarily on battery age and usage, with leakage in-

creasing dramatically in batteries that are old or have com-

pleted many cycles of charge and discharge.

Trickle charging is a continuous low-level charging current

that tops off the total charge in the battery, and prevents

any energy loss that would occur due to leakage.

The

maximum safe trickle charging current for a typical

Ni-Cd cell is about 0.1c,

this being the maximum charge

rate at which all of the gas developed internally is able to

recombine (so there is no internal pressure buildup that

would cause venting).

For Ni-MH batteries, the maximum (safe) trickle charge

rate is lower

(one manufacturer specifies c/40). This is an

important difference between Ni-Cd and Ni-MH batteries,

and must not be exceeded for continuous charging.

In this design, the trickle charge current is provided by the

resistor labeled RTR (see

Figure 1

). This current flows any

time VIN is present, regardless of operation of the high-cur-

rent charger. When the high-current charger is operating,

the total charging current is the

sum

of the trickle current

and the current provided by U1.

Once the input voltage VIN and the desired trickle charge

current ITR are known, the value for RTR is found using

Ohm’s Law:

RTR e(VIN b7b0.7)/ITR

The maximum power dissipation in RTR must also be calcu-

lated (when selecting a resistor, make sure the power rating

is greater than the value calculated below):

PMAX (RTR)e(VIN b4b0.7)2/RTR

Note that the power dissipation in the resistor is dependent

on the battery voltage. As the battery voltage increases, the

voltage drop across RTR decreases (causing the power dis-

sipation to decrease).

In the above equation, a battery voltage of 4V is assumed

as a worst-case minimum value for battery operating volt-

age for a five-cell battery pack (which would provide the

maximum power dissipation for RTR).

good

5-cell Ni-Cd or Ni-MH battery which is being trickle

charged (after being fully recharged) will read about 7V,

which will produce the

minimum

power dissipation in resistor

RTR.

DETAILS OF CIRCUIT FUNCTION (REFER TO

FIGURE 1

)

The 3A of charging current provided by the fast-charger is

obtained from an LM2576, which is a buck regulator that

switches at 52 kHz. Because it is a switcher, it allows the

user the option of using a wider input voltage range and still

retaining high power conversion efficiency (about 80% @3A

with VIN in the 10V– 14V range).

The LM2576 IC (U1) is used to provide a charging current

that is independent of the battery voltage. Whenever the

ON/OFF pin is held low, U1 will source current into the

battery through D3. A current-control feedback loop is es-

tablished using U5B, R12, and associated components.

R12 is used as a current shunt, and it provides a voltage to

the input of U5B that is proportional to the charging current.

U5B functions as an amplifier with a gain of 8.5, which caus-

es the output of U5B to be 1.23V when the current through

R12 is about 2.9A. The 1.23V signal on the feedback pin of

U1 will ‘‘lock’’ the loop at this value of charging current.

A fast-charge current value other than 2.9A can be set by

adjusting the values of R7, R9, or R12. These values (which

set the overall gain of the stage) should be adjusted so that

the output of U5B is 1.23V at the desired amount of fast-

charge current.

AUTOMATIC SHUTDOWN AT FULL CHARGE

The crucial part of fast charging a battery (especially if it is

Ni-MH) is

knowing when to stop.

This design uses a DT

detector that measures

both

the battery temperature and

the ambient temperature, and shuts down the fast-charge

current source when the battery is a10§C

above ambient.

This method is superior to techniques which sense only bat-

tery temperature. Single-ended temperature sensing may

not accurately measure charge: a ‘‘cold’’ battery will have to

heat up too much before the detection point is reached

(overcharging it), while a ‘‘hot’’ battery will terminate charge

long before full charge has been delivered to the battery

(because its temperature starts out too near the detection

level).

Two LM35 temperature sensors (U3 and U4) provide output

voltages of 10 mV/§C (proportional to their temperature). U3

is used to measure the ambient, while U4 measures the

battery temperature.

Note: U4 must be in contact with the metal case of the

battery to accurately measure its temperature. The plastic

sleeve around the battery may have to be opened up to

allow flush contact. Best results are obtained if the sensor is

located

between two batteries

(touching both).

Monitoring more than one battery virtually eliminates the

possibility that the sensor happens to be reading a bad

(shorted) cell which will not heat up and provide charge ter-

mination. In some laptops, multiple sensors are used so that

all battery cells are monitored, with charge termination oc-

curring when

any

cell temperature reaches the trip level.

The 78L05 regulator (U2) is used to provide a 5V source to

power the LM35 sensors and also acts as a reference point

for resistive divider R2 and R3. Resistors R1 and R11 are

used to sink current (since the LM35 can not).

CONTROLLING THE FAST-CHARGE CURRENT

SOURCE

U5C acts as a comparator which controls the on/off pin of

the high-current charging source (U1). When the output of

U5C is low, the 3A current source is turned on. When the

output of U5C is high, U1 is turned off and LED1 is lit which

indicates that the charger has completed the high-current

charge phase and is now trickle charging.

Hysteresis is built into U5C (see R13), which effectively

‘‘latches’’ the output of U5C high after it completes the fast-

charge portion of the cycle (it stays latched until the input

power is cycled on and off). Without hysteresis, the charger

would again turn on the 3A charger after the fully-charged

battery had cooled during trickle charging.

DETECTING AN END-OF-CHARGE CONDITION

The signals that are sent to U5C are derived from the tem-

perature sensors. They cannot be compared directly, since

detection must occur when the signal coming from U4 (the

battery sensor) is 100 mV above the signal coming from U3

(the ambient sensor).

In this design, the signal from U3 is DC level shifted up

about 0.1V by U5A and its associated components. R2 and

R3 set a 0.1V reference point for U5A, whose output volt-

age is the voltage at the output of U3 added to the 0.1V

reference.

With the signal from U3 level shifted by an amount that is

equal to 10§C, U5C can be used to compare the level-shift-

ed signal from U3 to the signal from U4. When these two

are equal, the temperature sensed by U4 (the battery) will

be 10§C above the temperature sensed by U3 (the ambient).

This is the point where shutdown of the 3A charger occurs,

and trickle charging continues.

3A Battery Charger Has Logic-

Level Current Controls

This design is a 3A battery charger with logic-level controls,

allowing a logic controller to adjust the battery charging cur-

rent to any one of four rates. The circuit was designed to

implement mP-based charging control in a system that oper-

ates from Ni-Cd or Ni-MH batteries.

GENERAL DESCRIPTION

The circuit shown in

Figure 2

is a 3A (maximum) battery

charger that uses a 52 kHz switching converter to step

down the input DC voltage and regulate the charging cur-

rent flowing into the battery. The switching regulator main-

tains good efficiency over a wide input voltage range, which

allows the use of a cheap, poorly regulated ‘‘DC wall adap-

tor’’ for the input source.

The key feature of this circuit is that it allows the mP control-

ler inside the PC to select from one of four different charg-

ing currents by changing the logic levels at two bits. The

various charge levels are necessary to accommodate both

Ni-Cd and Ni-MH type batteries, as they require slightly dif-

ferent charge methods.

Both Ni-Cd and Ni-MH batteries can be charged at the high-

current ‘‘c’’ rate up until the end-of-charge limit is reached,

but the two batteries must be trickle-charged differently

(trickle charging is a continuous, low-current charging rate

that keeps the battery ‘‘topped off’’ after the high-current

charge cycle has delivered about 95% of the battery’s total

charge capacity).

The recommended trickle-charge rate for a Ni-Cd is about

c/10, but for Ni-MH most manufacturers recommend that

the charge rate not exceed c/40. If a continuous charge

rate greater than c/40 is applied to a Ni-MH battery, the

internal pressure can build up to the point where the battery

will vent hydrogen gas. This is detrimental to the life of the

Ni-MH battery and potentially dangerous for the user (hydro-

gen gas is easily ignited).

The circuit shown in

Figure 2

was designed to charge a 3A-

hr Ni-Cd or Ni-MH battery with high efficiency, using logic-

level signals to control the charging current. The four select-

able charge rates are 3A, 0.75A, 0.3A, and 0.075A which

correspond to charge rates of c, c/4, c/10, and c/40 for the

3A-hr battery used in this application.

CIRCUIT OPERATION (REFER TO

FIGURE 2

)

The unregulated DC input voltage is stepped down using an

LM2576 3A buck regulator, providing up to 3A of current to

charge the battery.

In order to regulate the amount of charging current flowing

into the battery, a current control loop is implemented using

op-amp U2. The voltage drop across the sense resistor R8

provides a voltage to U2 that is proportional to the charging

current.

Note: The 0.05Xvalue for R8 was specified by the custom-

er in this application to minimize the power dissipated in this

resistor. If a higher Ohmic value is used (more resistance), a

larger sense voltage is developed and a less precise

(cheaper) op-amp can be used at U2, since the input offset

voltage would not be as critical (of course, increasing the

value of R8 also increases its power dissipation).

When the current-control loop is operating, the voltage at

the feedback pin of U1 is held at 1.23V. The battery charg-

ing current that corresponds to this voltage is dependent on

the overall gain of U2 and the attenuators made up of Q1,

Q2 and the resistors R10, R11, R2 and R3.

Turning Q1 on (by putting a ‘‘1’’ on logic input ‘‘A’’) provides

an increase of 4:1 in load current. The load current is higher

with Q1 on because R2 and R3 divide down the output of

U2 by 4:1, requiring U2 to output a higher voltage to get the

1.23V on the feedback line of U1. Higher voltage at the

output of U2 means that more charging current is flowing

through R8 (also the battery).

The operation of Q2 is similar to Q1: when Q2 is turned on

by putting a logic ‘‘1’’ on input ‘‘B’’, the load current is in-

creased by a factor of 10:1. This is because when Q2 is on,

the sense voltage coming from R8 is divided down by R10

and R11, requiring ten times as much signal voltage across

R8 to get the same voltage at the non-inverting input of U2.

Although both attenuating dividers could have been placed

on the input side of U2, putting the 4:1 divider at the output

improves the accuracy and noise immunity of the amplifier

U2 (because the voltage applied to the input of U2 is larger,

this reduces the input-offset voltage error and switching

noise degradation).

R5, R6, and D2 are included to provide a voltage-control

loop in the case where the battery is disconnected. These

components prevent the voltage at the cathode side of D3

from rising above about 8V when there is no path for the

charging current to return (and the current control loop

would not be operational).

Capacitor C2 is included to filter some of the 52 kHz noise

present on the control line coming from U2. Adding this

component improved the accuracy of the measured charg-

ing current on the breadboard (compared to the predicted

design values).

TL/H/12054–2

Notes (Unless Otherwise Specified):

Note 1: All resistors are in X, 5% tolerance, (/4W

Note 2: All capacitors are in mF

Note 3: Q1 and Q2 are made by SUPERTEX

Note 4: For 3A current, U1 requires small heatsink (RTH s15§C/W)

BENCH TEST DATA

Logic Logic Nominal Battery Measured Battery Power Conversion

Input ‘‘A’’ Input ‘‘B’’ Charging Charging Current (A) Efficiency (%)

Current (A) with VIN e10V with VIN e10V

1 1 3.0 (C RATE) 3.06 77

0 1 0.75 (C/4 RATE) 0.78 79

1 0 0.30 (C/10 RATE) 0.30

0 0 0.075 (C/40 RATE) 0.077

FIGURE 2. 3A Battery Charger With Logic-Level Current Controls

AN-946 High-Efficiency 3A Battery Chargers Use LM2576 Regulators

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