AN-D09

Supertex inc.
AN-D09
Application Note
Battery Back-Up
Utilizing Low Threshold MOSFETs
Introduction
The simple battery backup circuit shown in Figure 1 utilizes
Supertex low threshold DMOS devices to achieve excellent
efficiency.
In fact, one of the main reasons why MOSFETs are gaining popularity is that very low voltage drops, which surpass
the performance of various kinds of diodes and bipolar transistors, can be achieved. Many other benefits of low gate
threshold MOSFETs are explained in the text.
Circuit Description and Operation
The battery backup circuit has two modes:
1) Battery charging, and
2) Battery backup.
1) Battery charging mode
The 120VAC is stepped down via transformer and full-wave
rectified by D1, D2, and C1 to 7.5VDC. This 7.5VDC supplies
power to RL as well as providing the charging current to the
batteries. R1, D3, and D4 generate a 1.2V reference for U1a
and U1b. D5, R2, R3, C2, and COMP2 keep Q1 and Q2 off
when switch S is closed. The battery, consisting of 5 nickel
cadmium cells in series, is being charged with a current set
by R8 and the intrinsic drain to source diode of Q2. For fully
discharged batteries, there will be a high charge current for
a few seconds, rapidly decaying to a slow charge.
As the battery becomes almost fully charged to 6.8V, the
current is reduced to a trickle charge current of a few milliamperes. The trickle charge current is further reduced to
microamperes when VBATT exceeds 7.0V. This is because
the voltage across the diode of Q2 is 0.5V and will allow only
a small amount of current flow. This maintains full charge
of the battery, when not in use, over an extended period of
operation.
2) Battery backup mode
When switch S is opened, simulating power outage, unplugged equipment, or blown fuse, the circuit goes into battery backup mode. U1b turns on Q1 and Q2. As VBATT supplies the 60Ω load, U1a monitors the VBATT voltage keeping
it from fully discharging, as complete discharge and subsequent cell voltage reversal can degrade the performance of
the NiCd battery. The circuit is designed for the U1a to turn
Q1 and Q2 off if VBATT is less than 5.5V and on if greater than
6.5V. The hysteresis is designed to avoid oscillation and is
set by R4, R5, R6, and R7.
Figure 1. Battery Back-up Circuit
0.1μF
S
D1
120VAC
-
7.5V
C1
1000μF
D2
R1
5.1kΩ
+
VREF
D5
R2
100kΩ
D3
R3
150kΩ
C2
0.1µF
D4
R4
8.2kΩ
U1a
3
+
2
-
8
4
1
R6
39kΩ
R5
2.0kΩ
R7
5.1kΩ
+
Eveready
Nicad
500mAH
RL
60Ω
Q1
5
6
VBATT
+
-
U1b
R8
30Ω
7
Q2
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A040213
Supertex inc.
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AN-D09
Design Considerations and Component Selec- current of 125mA are used. Maximum voltage drop across
Q1 and Q2 works out to only 375mV. In actual operation, this
tion
voltage drop is substantially lower because the typical value
of RDS(ON) is 0.8Ω. The voltage drop across Q1 and Q2 was
measured to be 200 mV.
The design of this circuit utilizes standard, readily available
components. The number and different types of components
are minimized. Diodes D1 to D5 are 1N4001. All resistors are
standard 1/4 watt, 5% tolerance. National Semiconductor’s
dual comparator LM393N is used for its low biasing current
for U1. The battery consists of 5 Eveready nickel cadmium
cells in series. The cells are AA size, CH15 with a C rating
of 500 mAH.
Figure 2 is a discharge curve of VBATT vs Time showing battery backup operation of approximately 4 hours. Figure 3 is
a charge curve of the battery.
The component selection ensured that basic charging current guidelines for Nicad cells were not violated. Assuming
the worst case, using fully discharged batteries, the maximum charging current will be 227mA.
The most important factor to be considered in the design is
the selection of the MOSFETs Q1 and Q2, which are configured as an analog switch. In the battery backup mode, the
voltage drop across the MOSFETs must be low to minimize
resistive voltage drop and power loss, consequently enhancing battery life.
Supertex TN0604N3, low threshold N-channel DMOS transistors, are selected for their guaranteed low on-resistance
at low gate drive. Another aspect considered was their costeffective TO-92 package, which saves board space.
Device
TN0604N3
Typical
RDS(ON)
Maximum
RDS(ON)
Test Conditions
0.9Ω
1.5Ω
VGS = 5.0V, ID = 750mA
0.6Ω
0.75Ω
VGS = 10V, ID = 1.5A
Rectified D.C. voltage - diode drop 7.5 - 0.7
=
= 227mA
R8
30
This current will last only for a few seconds, and is completely safe for the battery as well as Q2.
In the charging mode, the battery voltage will be between
6.5 to 6.7V for the majority of the time. The charging current
will be from:
Q1 and Q2 are easily turned on with a simple pull-up resistor, R7. For a “worst case” design, RDS(ON) = 1.5Ω and a load
7.5 - 6.5 - 0.7
= 10mA to 30
VBATT vs Time
6.5
VBATT (volts)
VBATT (volts)
70
5.5
5.0
6.5
6.0
5.5
4.5
0
VBATT vs Time
7.5
6.0
7.5 - 6.7 - 0.7
= 3.3mA
30
0
60
120
180
5.0
240
Figure 2. VBATT Discharge Curve
Doc.# DSAN-AN-D09
A040213
0
4.0
8.0
12.0
16.0
t (hours)
t (minutes)
Figure 3. VBATT Charge Curve
2
Supertex inc.
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Conclusion
The charge rate will be from:
Very low drain to source voltage drops can be achieved
with MOSFETs. Bipolar transistor performance is limited by
VCE(sat) and diodes by VF , depending upon the semiconductor material used. This circuit utilized the following features
of MOSFETs:
10mA
3.3mA
= 0.02C to
= 0.007C
500mAH
500mAH
which is very safe for the Nicad cells.
1.
2.
3.
4.
Low drain to source voltage drop.
Complete turn-on/off of bidirectional currents.
Turn-on with low biasing voltages.
No biasing power compared to base current loss in
bipolar transistors.
5. Utilization of the intrinsic drain to source diode for limiting charging currents to efficient and safe levels.
Optional Features
When space is at a premium, Supertex’s TN2504N8 provides performance almost identical to TN0604N3, in the
SOT-89 (TO-243AA) surface mount package.
Added features such as battery backup mode indicator, low
battery voltage early warning, or battery shutdown indicator
can be incorporated by using one or more of the optional
circuits shown in Figure 4A through 4C. These can be easily
modified to interface with a microprocessor in more complex
systems.
The battery backup circuit described demonstrates the benefits of Supertex N-channel low gate threshold devices.
These are available in either surface mount (TN2504N8)
or TO-92 (TN0604N3) packages. These are ideally suited
for battery powered applications. Very often, circuit designs
require low on resistance to prolong battery life, low gate
drive to meet battery voltage limitations, and small packages
to accommodate board space limitations. The Supertex low
threshold DMOS discrete transistor family were designed to
satisfy such requirements.
Nickel cadmium batteries are quite rugged. However, they
are prone to damage due to cell voltage reversal if fully discharged. Other kinds of batteries are more sensitive, and
may be damaged below a certain voltage per cell, e.g.,
1.75V for lead acid.
The circuits shown can be modified to suit other kinds of rechargeable batteries, e.g. lead acid, lead calcium (gel), lithium, etc. For lead acid, the threshold voltage, to disconnect
the load from the battery can be adjusted to 1.75V per cell.
Figure 4. Optional Circuitry
VBATT
VBATT
300Ω
PIN 6
+
PIN 5
-
C
36kΩ
+
VREF
-
VBATT
300Ω
300Ω
C
PIN 3
VREF
+
-
C
9.1kΩ
A) Battery Back-up Mode Indicator
B) Low Battery Voltage Early
Warning
C) Battery Shutdown Indicator
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com)
Supertex inc.
©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited.
Doc.# DSAN-AN-D09
A040213
3
1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
www.supertex.com
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