COVER FEATURE
AIMED AT CELLULAR PHONES. A COMPLETE NICD
BATTERY CHARGER SQUEEZES INTO AN AC-LINE
PLUG AND ALLOWS TALKING 引THILE CHARGING.
NICD BATTERY CHARGER
HIDES IN AC-LINE PLUG
FRANK GOODENOUGH
i!!;:;
ith portability in vogue for a
host of products from tele. phones. to computers , the
move is on to secondary (rechargeable) batteries like
nickel cadmium (NiC哟， nickel metal hydride
(NiMH) , and lithium cells. That move , in turn ,
has increased the demand for battery chargers. These chargers must be more than just
off-line current sources , though. They must
be able to run off the ac line , as we l1 as charge
the batteries rapidly and sense when the battery is fully charged. Moreover, once sensing
this condition, the chargers must switch to a
trickle-charge mode , being careful not to
pump too much current into the battery. Otherwise , overcharging can damage or even destroy a battery.
The last few years has seen a number of
complex ICs that can simplify the task of designing .a charging system (ELECTRONIC DESÌGN, July 8, p. 4 η. However , with the exception of a chip developed by Motorola Semiconductor, all of these control ICs require an additional off-line power supply, or a powersemÏS!onducωr device betwee川he ac line and
the control circuitry in the charger. That is ,
the designer of the charger must handle all
problems and issues inherent in a product
that connects to the ac power line.
To assist those who rely heavily on cellular
Thus , few secondary battery-powered
products offer a built-in charger. Most must telephones , Burr-Brown created a complete
be plugged into a box-like device that in turn charger that's less than 3 in. Iong within the
connects to a wall outlet with a power cord. cross section of a standard ac waII plug (Fig.
Because of their charging system's "non-er- 1). A power cord from the charger terminates
gonomic" mechanical design , most handheld in a telephone plug compatible with the input
portable products become virtually impossi- to the cellular telephone's battery pac k.
ble to use while their battery is being Dubbed the DC151 , its design is based on two
charged. Such a charger severely limits cellu- patents and aims specifically at charging the
lar-telephone applications.
smal15- and 6-cell NiCd battery packs typicalE L E C T R ONIC
DESIGN 回
AUGUST 19, 1993
MINIATURE CELLULAR-PHONE
BATTERY CHARGER
ly found in these telephones.
tus when the plug was disThe DC151 charges most
connected remains on. The
cellular-telephone battery
charger is now drawing 10
packs in one to two hours. It
mA of current from the batalso can charge any 5- or 6tery , obviously not an opera[(0
cell NiCd stack found in
tional mode to conserve batnotebook or palmtop PCs ,
tery life.
personal digital assistants
(PDAs) , and pen-input comNEGATIVE DELTA V
puters. During the rapidTo determine when the
charge mode , the DC151
battery is fully charged , the
provides 600 mA to a 5-cell
DC151 employs a version of
battery pack or 500 mA to a
a relatively common tech6-cell pack. These currents
nique called "negative delta
drop to 60 and 50 mA, rE•
V ," or simply -ß V. The techspectively, during the tricknique is based on the knowlle-charge mode.
edge that when a NiCd cell
reaches a full charge , its terTo charge the batteries ,
minal voltage , which has
the DC151 is plugged inω
been rising , starts to droop.
the wall outlet and the plug
The DC151 enlists a drop in
on the other end of the line
voltage (a -ß V threshold) of
r viceinto the telephon
just 50 m V, which is signifiversa. Unless a fault exists ,
cantly less than that of most
such as a short or open circhargers.
cuit at the battery , opera- 一一tion starts in the fast-charge 醋自1. THIS TINY WALL PLUG contains a complete
The contròl algorithm is
mode. The charger's two- 噩噩 battery charger that can fast -c harge NiCd ωIIs at 600 mA , and
actually more complex. For
color (yellow and green) 噩噩 trickle-charge them at 60 mA. It can har由 this task despite
starters , the droop-detecLED indicates charge sta- I圃 being only 2.6in.long by 1. 3 in. wide by 1 in. high.
tion circuit , implemented
tus. During trickle chargwith the BCC101 propriing, or if the telephone plug is open , condition occurs , the charger goes etary IC , continuously samples and
the LED turns bright green. If the into trickle mode. If the telephone stores the peak value of the battery
telephone plug is shorted , the LED plug is connected and the wall plug is voltage. When each of 32 s tI ccessive
goes off. However, if either fault not, the LED indicating charge sta- samples (which takes at least 276 ms)
drop a minimum of 50 m V below the
line-voltage
previous peak value , the chip deterisolation barrier
mines that the NiCds are fully
charged and the charger switches to
a trickle charge. The combination of
multiple samples and peak detection
eliminates the chance that transient
electrical noise is mistaken for the
battery actually reaching a fully
Main winding
charged state. Such a transient could
put the charger into the trickle mode
before
the battery is fully charged.
rthulu
11E
1i!
Resona~
If
the
telephone is turned on while
low-voltage
mode
the
battery
is'charging, the battery
power
current
supply
voltage droops significantly more
contro!l er
(BCC100)
than 50 m V due to the telephone's
Charge-status
load-current IR drop across the inindicator LEDs
ternal resistance of the battery. To
川;1f
determine if the battery is powering
片刻
its load , the contro lI er IC actually
looks for a voltage droop in a window
between 50 and 100 m V. That is , if
the droop is greater than 100 m V, the
charger stays in the fast-charge
mode 'Yh iI e powering the telephone
and charging the battery. It w iIl re髓盟 2 阳MAR
E
盯
Y-S
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charger tω01沁00ωsely regulate the current f1 0wing from the se创coωnd臼arηy intωo the battery.
main there when the telephone is
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『
回 ELECTRONIC
AUGUST 19, 1993
D E
s
1 G N
MINIATURE CELLULAR.PHONE
BATIERY CHARGER
i-lil--t
$1
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o
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taken off the line.
The ability to talk while charging
in cellular-telephone applications demands a low-noise , isolated, switch町'
ing power supply between the ac line
and the charging-current line to the
batteries. A typical off-line, pulsewidth幽 modulated (PWM) supply
would be too noisy. Therefore , Tom
Somerville , the DC151's designer,
chose a resonant-mode switching topology (Fig. 2). The circuit reduces
noise and its effects in three ways:
Planar magnetic ferrite core
BCC100
BCC101
Green/yellow lED
• It produces sine waves rather than
square waves, eliminating fast step.
changes in currentand thè attendant
high-frequency harmonics.
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ce flat ferrite core and wi阳
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d ings embedded in a mu
旧i1tiIaye盯r pc board, a 60o- mA , of f.l ine
• It runs at frequencies between 0.5
battery charger can be squeezed into a tioy package.
and 1 MHz , which are above the 45 5kHz IF employed by analog ce l1 ular
such as Underwriters Laboratories the temperature rise in the tiny packtelephones.
• Its zero-voltage switching archi- (UL) and the German Verband age. Tha t' s because during the detecture (switching occurs only when Deutscher Electrotechniker (VDE) , sign process , "getting rid of the
the waveform goes through zero) while still remaining small and easy heat" wasn't considered. As a result ,
eliminates switching noise because to use. Although not required to run i比t'、s worth examining the charger'
the energy (the product of current off batteries , the charger's efficien- overall design and mode of operation1
F、'伊
ig
伊
s.2through η.
and voltage) in the waveform drops cy must be maximizedωmitigate f旷
to zero while the FET switches.
The patented topology employed
is the first to use a resonant-mode
circuit to regulate the current in the
transformer's secondary winding
from the primary winding or input
side. The level of the charger's output current isn't set by feedback control from the secondary to the primarywinding.
Q
BELT AND SUSPENDERS
It's possible that the droop-detector circuits won't catch a -Â V. As a
result, like most sophisticated chargers, a time-out feature was added to
the contro lI er to þack up the -Â V detector. At the end of the time-out period , which is derived from the acline frequency, the charger switches
from fast charging to trickle charging. DC151s must be ordered with
"time-out" periods of either 73 minutes or 146 minutes.
The charger represents an eclectic
mix of technologies, including mechanical packaging , electronic- and
Iιcircuit design, and magnetic-circuitltransformer design. For example , it must meet the specifications
for input-to-output voltage isolation
sanctioned by regulating agencies
回E
W1
CP1/ CP2
BCC101
Sideb
圈 4 川FT川A川 CωOM川TS …
small muωIltωi1 ayer pc board. Traosformer wiodiogs are formed 00 each layer via a pateoted
process.
LECTRONIC
AUGUST 19, 1993
D E
s
G N
MINIATURE CEllUlAR.PHONE
BATTERY CHARGER
Major features of the design revo1ve around the charger妇 pate丑ts
and the two bipo1ar custom ICs designed and fabricated by BurrBrown. The patents cover the use of
a re臼so∞nant converter to control current (rather than vo1tage) and the
transf
臼
orme
盯
r气 p1anar magnetics
f厅7
茧可i切
g. 刀， which includes the printedcircuit transformer windings (Fig 4).
One of the custom ICs , the 8-pin
SOIC BCC100 controller, runs the
resonant-mode switcher. The other
custom IC, the BCC101 , contains the
"full-charge detection" a1gorithm
and the "time-out" circuit (Figs. :J
through5).
Here's how the charger works:
The input line vo1tage is applied to a
full-wave rectifier bridge consisting
of diodes Dl through D4. Its de1iberate1y small 0.1 5-μF fi1ter capacitor
CB 1 10cated under MOSFET switching transistor Ql resu1ts in a highvo1tage dc supp1y with over 50% ripp1e at twice the 1ine frequency (Fig.
6). The ripp1e passes through the
high-voltage resistor R HV to the
BCC100 controller IC , where it
clocks the driver timer (Fig. η.Wh iI e
not c1ean1y filtering the rectified
10w-frequency ac 1ine, capacitor CB 1
provides EMI fi1tering that keeps
the high-frequency switching current flowing through the FET from
getting into the ac 1ine via the p1ug.
If that occurred , it cou1d interfere
with other e1ectronic equipment on
the 1ine.
The p1anar transformer consists
of a mu1ti1ayer pc board sandwiched
between two identica1 gapped pieces
of manganese-zinc (MnZn) ferrite.
Its windings consist of the concentric spira1 copper traces , one on each
1ayer of the board. The top windings
on eachside of the board are portions
of the primary winding W 1. The secondary and bootstrap windings lie
buried between the primary windings on other pc-board layers. Extra
insu1ating 1ayers inserted between
the primary and secondary 飞~indings
ensure that regu1atory-agency , is o1ation-safety requirements are me t.
The high-frequency 10w-1oss MnZn
ferrite combines high efficiency with
the very small size necessary to get
the charger to fit in its tiny 3 .4-in. 3
package.
Primary winding W 1 runs between
the high-vo1tage dc supp1y and
DMOSFET switch Q. 飞Vhen the
MOSFET turns off, primary-winding inductance L and capacitor CR
resonate at a frequency with a period
of 27TìILC, where C =-C R • Tri~ming
the off time of BCC100's gate-drive
circuit to ha1f this period creates a
ha1f-sine-wa ve drain-to-source vo1tage across the MOSFET that returns
to zero just before gating the FET
back on (Fig. 民 α.gaiη). What results
is constant off time , 1-μs ， zero-voltage switching. That is , as noted earlier, there's no vo1tage across Q at the
time it' s gated on. The technique
minimizes MOSFET switching
10sses and keeps high-frequency
harmonics be10w 10 MHz.
The BCC100 IC a1so regulates the
inputcurrent, thereby indirectly regu1ating the output current to the battery. The current in the transformer
primary is sensed by the vo1tage
drop across resistor Rs and applied
to the active integrator formed by
RI' CF , and the op amp in the BCC lO O
IC. The integratωor、 output modulates the on time of the MOSFET
Planar transformer
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Multilayer pC board
8.
cωontroωolle
衍r ， theBCCl00, which operates the zero- voltage switching supply between
and 1 !11 Hz. The supply's output current charges the battery and powers the BCCI0l IC. The chip determines when the battery is
charged and switches the charger from fast-charge to trickle-charge modes.
(:l:J E L E C T R 0 N 1 C
D E S 1 G N
5∞ kHz
AUGUST 19, 1993
MINIATURE CEllUlAR.PHONE
BATTERY CHARGER
driven by the voltage-to-frequency
converter. Thus , as the rectified
high-voltage dc increases in value ,
the switch's on time decreases. This
keeps the input current from the ac
line at a constant level and loosely
regulates the current to the battery ,
in spite of variations in either the ac
line or the battery voltage. In fact ,
the on time is even reduced at the
peak of the rectified sine wave (Fig.
4αgαtη:).
winding drives Schottky rectifier D5'
At startup , the BCCIOO is powered
by the current through RHV ' Once
it' s running , though , it's powered by
the bootstrap winding (Fig. 2,
αgαtη:).
A filter formed from CB2, CB3 , and
inductor L follow the Schottky rectifier. The output current from the filter charges the battery and powers
the BCC lO l IC and the green and yellow charge-status LEDs.
The filter not only attenuat启 s the
high-frequency ripple current, but
also generates line-synchronized
voltage spikes. A hysteresis comparator circuit in the BCCI01 converts
the spikes intοa twice-the-line-frequency cloc k. This clock synchronizes and times the ratchet digital-tc•
analog converter (DAC) , the backup
timer, and the logic circuits contained in the BCCIO l.
During the one-to-two hour charging period, the BCC101's unique 7七it
(b)
duces a trickle charge auto-
Fasl
matically 而切.7， αgain}.
Disabling the MOSFET
gate drive during the valleys in the higl!-voltage dc
supply (the low points in the
ripple) provides additional
efficiency, which reduces internal charger temperature.
During these valleys , even
if the FET is operating at
maximum on time , little
power is transferred to the
battery due to the fixed
turns ratio ofthe transformer. A hysteretic comparator
in the driver-timer gates the
FET off (Fig. 6， αgαin).
Thet
仕r立咀
缸nsf
a
，臼
orm
口ler句 output
Full-wave rectified line voltage
l E)
Trickle
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Due to the lack ofα standaγdfoγ
the battery-end coη nectoγ.s， ini白
J一寸一J
.JLJ Jl一J 二 ;!Jz1:
. tial sales of plug-termin α ted
.
chα rgerswill 0η ly be to OEMs in
(例d创i 时甘吭πm
产户町甘芦户阿叶r
沪…
the U. S. αηd J.αpαn η mini
mum quantities of 1侃)() (European versio旧 ofthe ac plug will
oVMOSF盯 be tooled 仇 the future). Smα II
quantities of the chα俨'ger in α
JLJ
ú \
J\-BV JlZe
平igtαil" co听guratioη (with
out the' battery-eηd connector)
will be available. Bu俨r-Browη ~s
α lso making the chip set
(BCC100/BCC1ρ1)α vailable
with α license for the patented
tγα旧iformer techη ology. ln addition, OEMs cα n contract with
Burr-Browη 旨 subsidia叨" PowE 俨 Convertibles， for custom
chαγ'[Jeγ's
with sophisticα ted coη.
萨 iz卜tR
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iilit--iit
The BCCIOO controller operates
with on times between 700 and 1000
ns during the fast-c harge mode , and
100 and 140 ns during the tricklecharge mode. Consequently, the average trickle-charge currentis about
l/lOth that of the fast-charge current. 引Then fast charging is needed,
the multivibrator in the BCCI01
transmits its output to the BCC100
via a pair of l-pF capacitors , CP j and
CP2• If trickle charging is required ,
no signal is transmitted from
the multivibrator in the
BCCIO l.
Translormer primary liming lor DC151 baHery cha咱er
The sense amplifier in the
BCCIOO detects the presence or absence of the signal
from the multivibrator and
sets a high (fast-charge) or
Inpulline voltage
low (trickle-charge) refer- {a}
ence voltage for the op-amp
integrator , which determines the on time of the
FET switch. Because the
charger output provides the
power for BCC101, shortirη19
the cha町rger妇 out句pu川t elimit heout句
pu
川
t
nates (shorts out) 由
ratchet DAC accurately peak-detects
and holds the rising values of the bat臼ry voltage with essentially zero
droop. A comparator circuit increments the ratchet counter circuit up
one least-significant bit at a time as
the battery voltage rises during fast
charging. A second comparatοr circuit recognizes when the battery voltage droops at least 50 mV from the
peak value stored in the DAC cÏrcuit.
The BCCIOl performs several additional tasks. As noted previously,
to provide noise immunity , a sequence of samples triggered by the
ac-line frequency mustdetect a valid'
droop before the logic switches from
fast to trickle-charge modes. The
backup timer on the chip switches
the charger to the trickle-charge
mode if droop isn't detected. The output of the chip' s hysteresis comparators drive the charge-status LEDs.
The multivibrator tells the
BCCIOO to start up tn the fast-charge
mode and switch to the trickle mode when droop is detected. This occurs if the
timer counter is full , if the
battery is disconnected
from the telephone , or if the
charger output)s shorted.
The BCCIOO controller
switches back to the fastcharge mode if a disconnected battery is reconnected , of
if the plug is removed and
then reconnectedωthe ac
line. Thus , a fully charged
batterystarts offin the fastcharge mode but switches
quickly to the trickle-charge
mode due 协 the sensitivity
MOS
奸Ff
盯T
of the -6, V detector. 口
PRICE AND AVAILABILITY
MINIATURE CELLULAR.PHONE
BATTERY CHARGER
p p
RV
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l.
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Gate
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senses when the battery voltage, which rises during charging, drops by 50 mV (b).
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trol.
ln quantities of 1ω1， the pigtail version
ofthe chαrger， tke DC151.pt, goesfor $66
each and will be av日 ìlable in September.
The price ofthe DC151 (with αbα ttery co π
ηecto γ) will equal the combined cost ofthe
How VALUABLE?
CIRCLE
HIGHLY
MODERATELY
SLIGHTLY
530
531
532
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ELECTRONIC
DESIGN 固
AUGUST 19, 1993