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Lithium Ion battery charging using bipolar transistors
Khagendra Thapa, Systems Engineer, Zetex Semiconductors
Introduction
Portable hand-held applications such as cell phones, PDAs, etc are becoming increasingly
complex with more and more features designed into every generation. This increasing number
of features combined with a requirement for smaller size and extended battery life has made
Lithium batteries the preferred choice for many of these applications. Lithium batteries have
improved in technology. With advances in electrodes and cell chemistries, Lithium batteries have
provided a flatter discharge characteristic. More stringent requirements are being placed on
manufacturers to improve charge time, maximize battery lifetime and reduce size whilst
improving safety. This application note will discuss linear charge techniques and associated
discrete pass elements, highlighting the dominant discrete parameters and selection criteria.
Lithium Ion (Li-Ion) battery charge cycle
In order to model the main power losses in the charging circuit with a view to select the correct
components we have to understand the charge cycle of the Lithium Ion (Li-Ion) batteries. Figure
1 shows a typical charge cycle for single cell Lithium Ion batteries. The pre-charge voltage
threshold, VPRE, upper battery terminal voltage threshold, VT, and recharge threshold (VRECHG)
depend on types of Lithium Ion batteries and manufacturers. For single cell, pre-charge voltage
threshold is 2.5V or 3V and upper terminal voltage limit is 4.1 or 4.2V. These voltage differences
on pre-charge and upper terminal voltage limit depend on the internal chemistry of the battery.
Charge
Current
Battery
Voltage
typical 4.3V
VHPROT
4.1V or 4.2V
VT
VRECHG
I CHG
0.5C to 1C
for fast charge
2.5V to 3V
VPRE
IPRE = 0.1C
VLPROT
typical 1V to 1.5V
IEND = 0.02C
Time
(0.02C to 0.1C )
Pre charge
at 0.1C
(i)
Figure 1
Fast charge constant
current
(ii)
Fast charge constant
voltage
(iii)
Charge Recharge
complete /top-up
Charge
complete
(iv)
Typical Lithium Ion single cell charge profile
As shown in Figure 1, the typical charge cycle is split into four phases. The typical full charge cycle
for a Li-Ion battery is 3 hours. VLPROT and VHPOT are the low and high protection voltage
thresholds for the batteries with internal protection circuits.
Note: It is essential to have the proper battery specification and charge sequence requirements
from the manufacturer for designing battery chargers.
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i) Pre-charge phase
Deeply discharged cells require initial conditioning charge before normal fast charging can take
place safely and without damage to the battery. This phase provides this initial conditioning
trickle charge. Charging current is typically set to 0.1C (or as recommended by the battery
manufacturer) until the battery's deep cell discharge voltage reaches its pre-charge voltage
threshold, VPRE. The pre-charge threshold for single cell is between 2.5V to 3V (or as
recommended by the battery manufacturer) depending on the type of Li-Ion battery. If the battery
voltage is already above VPRE at the start of the charge cycle, the charge phase moves straight to
fast charge phase.
ii) Fast charge - constant current phase
Once the pre-charge voltage threshold is reached constant current charging begins until the
battery reaches its upper terminal voltage, VT. Li-Ion batteries require a very tight tolerance on VT,
typically 1% over temperature.
For fast charge the charge rate is typically between 0.5C to 1C. The charge rate above 1C is not
recommended. The charge rate above 1C reduces the battery capacity and increases the battery
temperature. The charge rate above 1C does not reduce the total charge time to overall full
capacity. Depending on the protection circuits within the battery, the charge rate well above 1C
may not be possible.
iii) Fast charge - constant voltage phase
The fast charge phase switches constant current to constant voltage when the battery voltage
reaches its upper terminal voltage threshold. The battery voltage is maintained at its upper
terminal voltage threshold while the charging current decays exponentially from 1C to less than
0.1C (typically 0.02C), as a consequence of an increase in the internal impedance of the battery.
This phase takes the majority of time during the batteries charging cycle.
iv) Charge termination
The termination of the charge cycle is typically by either timer or charge current decaying to end
of charge current threshold, IEND. The end of charge current threshold is less than 0.1C, typically
0.02C (or recommended by the manufacturer). If fast charge phase continues longer than set time
irrespective of the charge current, the charging is stopped.
In some linear chargers battery temperature is also monitored. If battery temperature rises above
safe charging/operating temperature (as per manufacturers' specification), the charge cycle is
terminated immediately irrespective of the charge status.
Recharge/top up charge
Li-Ion batteries are unable to absorb continuous over charge and therefore continuous trickle
charge to fully charged battery is not recommended. Instead, float charge should be applied if the
terminal voltage drops by certain amount, typically 100mV (or as recommended by the
manufacturer) to the recharge threshold. The float charge can either initiate only the constant
voltage phase or retrigger the complete charge cycle depending on the charger design and the
recharge voltage threshold, VRECHG.
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Charger
There are three main topologies for charging the batteries, switch-mode, linear and pulse
charging each with advantages and disadvantages for differing applications.
Switch-mode chargers offer the best efficiency and faster charging currents but have the
disadvantage of a more complex design. Consequently they are typically used where higher
charge currents are required, such as notebook computers.
Pulse chargers have seen use in Nickel (NiCd) Cadmium and Nickel-Metal Hydride (NiMH) battery
charging application to reduce 'memory phenomenon' and the crystalline formation within the
battery. Although well designed complex pulse chargers with protection (over voltage,
temperature and current) have been used, pulse charger offer little benefit to Li-Ion battery
charging. Instead, maximum pulse current and voltage peaks due to pulse current may interact
with the protective system within the battery. For this reason pulse chargers are not
recommended for Li-Ion batteries.
We will concentrate on linear battery charger in this application note.
Battery charging using linear chargers
Linear chargers are simple in design, small and have no noise associated with 'switching mode
converter' making them suitable for low power and low noise applications. They use an external
pass element to drop the battery voltage from the input supply to the battery voltage thus power
dissipation can be high. Figure 2 shows a typical linear charger application with external pass
element and reverse blocking Schottky diode.
PNP transitor
Supply voltage
VE
V CE
V IN
Reverse blocking
schottky
VS
VC
Q1
D1
R SENSE
V BE
VB
VCC
Drive
ISNS
Li-Ion
Linear charger
VSNS
controller
C IN
GND
C OUT
Figure 2
Typical linear battery charging application
Pass element Q1 can be either MOSFET or bipolar transistors. MOSFETs require a reverse
blocking Schottky diode in series to prevent current flowing from the batteries to the supply,
through its body diode. Two MOSFETs, one as pass element and the other as reverse blocking
diode can also be used. However Schottky diodes are cheaper than MOSFETs as reverse blocking
devices. Most PNP transistors are able to provide reverse blocking for single cell NiCd and NiMH
batteries but this capability is not specified or guaranteed. Li-Ion batteries (including a single cell
with 4.2V), generally, require a blocking diode in series with standard bipolar transistors. Zetex
provides a range of application specific transistors which guarantee the reverse blocking
capability necessary for single cell Li-Ion battery charging.
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The need for additional Schottky diode for reverse blocking is an issue for linear battery charger
where charging voltage head-room is low especially in the case of USB bus-powered chargers.
USB supply voltage can range from 4.4V to 5.25V.
During the constant current phase the battery voltage rises, reducing the transistor collectoremitter voltage to the point where the transistor approaches the saturation region and gain starts
to fall. The charge controller senses this via the sense resistor and compensates by increasing
base drive, thereby maintaining charge current. The transistor saturation characteristic is
therefore important in delivering the charge current at this point in the charge cycle, and must be
lower than the minimum circuit voltage, taking into account the input and battery voltages, the
sense resistor voltage drop plus the forward voltage of any diode used. Clearly the saturation
characteristics are even more important when input voltages are low, eg 4.4V.
As an example, consider the circuit in Figure 2, where USB port output voltage is 4.75V, (the lower
end of high-power USB port voltage range) and charge current is 500mA. The reverse blocking
Schottky diode forward voltage drop is 0.35V. If the saturation voltage of the transistor is 0.3V, the
voltage at the sense resistor, VS, is 4.1V. Allowing for further voltage drop at sense resistor,
voltage at the battery is below 4.1V and is not adequate to take the Li-Ion cell to full charge
capacity. This situation is worse when supply voltage is down to 4.4V as in low-power USB ports.
Zetex' application specific transistors have very low saturation voltage and do not need a reverse
blocking diode for linear charging of a single cell Li-Ion battery and thus maintain the necessary
headroom.
Figure 3 shows a typical USB bus-powered single cell Li-ion linear battery charger application
where a Zetex low saturation ZXTP25020CFF bipolar PNP transistor also provides the reverse
blocking capability.
USB supply voltage
Zetex bipolar PNP transitor
Q1
V IN
R SENSE
4.4V to
5.25V
IC 1
OUT
VCC
CFLAG
Linear charger
controller
COMP /DIS
GND
ISNS
VSNS
ISEL
Single
cell
Li-Ion
R COMP
R ISEL
C IN
C OUT
C COMP
Figure 3
Charging phases
The IC1 drive capability can be typically 5mA to 50mA and may require quiescent supply current
in the range of 250␮A to 1mA.
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Power loss calculation
The greatest power loss occurs when the charge phase enters the fast charge - constant current
phase when battery is at pre-charge voltage threshold, VPRE. The main power loss areas are
shown with calculation example below for this normal operating highest power loss scenario.
The battery specifications for this applications example are:
Battery: single cell Li-Ion 500mAH
e.g. in portable hand held devices such as cell phones, MP3 players, etc.
Battery pre-charge threshold, VPRE = 3V
Battery upper terminal voltage threshold, VT = 4.2V
Fast charge rate 1C = 500mA.
For the example in Figure 2 component and supply specifications are:
Maximum USB Supply voltage, (VIN MAX) = 5.25V
IC1 input supply current, IIC_SUPPLY (Max) = 1mA
Q1 PNP transistor = ZXTP25020CFF
Base-emitter voltage, VBE = 0.7V
hFE gain of bipolar PNP = 275 typical for collector current of 500mA at 25°C
RSENSE = 0.1⍀
Pd(IC) = VIN MAX x IIC_SUPPLY(Max) = 5.25V x 1mA = 5.25mW
Pd(SENSE) = ICHG2 x RSENSE = (0.5A)2 x 0.1⍀ = 25mW
Pd(BASE) = VBE x IB = 0.7V x 1.8mA = 1.27mW
Where, IB = ICHG / hFE = 0.5A/275 = 1.8mA typical at 25°C.
Pd(CE) = ICHG x (VIN -VBAT -VSENSE) = 0.5A x (5.25V - 3V - 0.05V) = 1.1W
Where, VSENSE = ICHG x RSENSE = 0.5A x 0.1⍀ = 0.05V
Pd(TOTAL) = Pd(IC) + Pd(SENSE) +Pd(BASE) +Pd(CE) = 1.13W
As battery voltage starts increasing, the power loss Pd(CE) reduces and so does the total power
loss.
Similar calculations can be done for the start of pre-charge, fast charge - constant current and the
fast charge - constant voltage phases. For the purpose of selecting the PNP transistor, the highest
power loss scenario shown above has to be used to satisfy the power and thermal handling
requirements. The Microsoft Excel® based linear charger performance evaluation calculator can
be downloaded from www.zetex.com/linearcalculator
The higher the difference between the supply voltage and the battery voltage, the lower is the
charger efficiency.
The maximum allowable charge current depends on the thermal capabilities of the PNP device,
the thermal impedance of the board and the voltage difference between the supply voltage and
the battery. The power loss needs to be matched with the thermal impedance of the PCB copper
area the device is mounted on to maintain the device and the junction temperature within normal
operating range.
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Bipolar PNP transistor selection
For appropriate selection of the bipolar device for linear charger applications following
parameters have to be considered:
•
Collector-emitter break down voltage.
•
Low-drop out voltage (saturation voltage) at the operating ICHG/IB condition.
Low drop out voltage of the bipolar transistor allows batteries to be charged by a supply with
low head-room (i.e. low differential voltage between the supply and the battery voltage).
•
hFE gain.
To allow lower base drive from the charger IC, the bipolar devices should have a high hFE
gain.
•
Reverse blocking voltage capability.
The PNP transistor should provide the reverse blocking capability for a single cell Li-Ion
linear battery charger. This then removes voltage headroom issues discussed above
regarding USB battery charging (or similar supply voltage range) This also reduces solution
cost and size by removing the need for a Shottky diode.
•
The power and thermal handling capabilities of the bipolar device and its packaging.
For portable device transistor package size is important but it still requires good power and
thermal handling capabilities.
The linear charger application is subdivided into:
a) USB charging
b) Charging from low voltage output voltage DC-DC or AC-DC adapter
c) Charging from high voltage, e.g. automotive
a) USB charging
USB hub ports can provide between 100mA to 500mA depending whether the hub is USB buspowered or self-powered. Self-powered hubs can provide up to 500mA per port while buspowered hubs can only provide 100mA per port. The voltage supply from USB compliant port is
between 4.4V to 5.25V for 100mA port and 4.75V to 5.25V for 500mA port.
The linear battery charger from USB port will have to comply with the USB voltage and the
current specifications.
Bipolar PNP transistors for USB linear chargers are shown in tables 1. VBECO is the minimum
rated reverse blocking voltage of the PNP transistor.
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Table 1
Single cell Li-Ion, VPRE = 3V; reverse blocking diode not required
Maximum
charge current
@ 5.25V input
(mA)
hFE @ IC/VCE(sat)
(V)
Min.
rated
BVEC0
(V)
ZXTP25020CFF
-20
7
-600*
100 @ -0.5A/-75mV
SOT23 Flat
1.5
ZXTP25020CMA
-20
7
-600*
100 @ -0.5A/-75mV
3L-DFN
2x2mm
1.5
Part number
VCEO
Package
PD
o
@ 25 C
(W)
* The maximum current the USB hub ports can source is 500mA.
b) Charging from low voltage DC-DC or AC-DC wall adapter
Linear battery charger can be supplied by low voltage AC-DC wall adapters capable of sourcing
up to 700mA. The voltage range for the wall adapter is typically 4.5V to 5.5V but can be as high
as 7V. The tables 2 and 3 show bipolar PNP transistors suitable for wall adapter applications. In
cases where the input voltage is high Schottky diodes can be used to share the power loss
thereby allowing a higher charge current.
Table 2
Single cell Li-Ion, VPRE = 3V; reverse blocking diode not required
Maximum
charge current
@ 6V input
(mA)
hFE @ IC/VCE(sat)
(V)
Min.
rated
BVEC0
(V)
ZXTP25020CFF
-20
7
-500
100 @ -0.5A/-75mV
SOT23 Flat
1.5
ZXTP25020CMA
-20
7
-500
100 @ -0.5A/-75mV
3L-DFN
2x2mm
1.5
Part number
VCEO
Package
PD
o
@ 25 C
(W)
Table 3
Single cell Li-Ion, VPRE = 3V; reverse blocking diode required e.g. Schottky diode ZHCS1000
Maximum
charge current
@ 6V input
(mA)
hFE @ IC/VCE(sat)
(V)
Min.
rated
BVEC0
(V)
ZXTP07012EFF
-12
-
-500
100 @ -0.5A/-60mV
SOT23 Flat
1.5
ZXT2M322
-20
-
-700
100 @ -0.5A/- 150mV
3L-DFN
2x2mm
1.5
Part number
VCEO
@
Package
PD
25oC
(W)
Zetex has a range of reverse blocking Schottky diodes, for instance the ZHCS1000 which has
typical forward drop of 0.35V at 500mA at 25°C.
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c) Charging from high voltage
Linear chargers still provide a simple low cost solution for high voltage up to 36V at low charge
current up to 50mA. One example of such application would be for an external linear charger from
car battery to charge hand held devices.
As the charge current is dependent on the PNP devices power and thermal handling capabilities
and the voltage difference between the supply voltage and the battery, higher than 50mA charge
current can be drawn from a supply lower than 36V.
For automotive application, additional protection against load dump is required.
Table 4
Single cell Li-Ion; reverse blocking diode required, e.g. Schottky diode ZHCS1000
Part number
VCEO
Maximum
charge
current at
36V (mA)
hfe @ IC/VCE(sat)
(V)
Min.
rated
BVEC0
(V)
ZXTP2012Z
-60
-
-50
50 @ -0.05A/-40mV
SOT89
2.1
ZXTP2012G
-60
-
-50
50 @ -0.05A/-40mV
SOT223
3
@
Package
PD
25oC
(W)
Conclusion
From the power dissipation breakdown calculations, the dominant loss in the charger circuit
during all phases of the linear battery charging cycle is the on state loss of the pass element. The
power dissipated in the fast charge phase is most significant reaching its peak when the Li-Ion
battery is at pre-charge voltage threshold and the constant current charge phase is initiated. The
key parameters to consider in the pass element are the power and thermal ratings, saturation
voltage, reverse blocking capability and packaging. Zetex' application specific low saturation
voltage bipolar PNP transistors provide reverse blocking while maintaining the necessary
charging voltage head-room for single cell Li-Ion linear charger application.
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or
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