The invention relates to battery circuits, and more particularly,
to a battery protection circuit using a sequential blowing fuse circuit.
Rechargeable batteries are used in a variety of portable electronics
devices such as mobile phones. The use of rechargeable batteries brings several
challenges to the area of circuit protection. It is necessary to protect both the
electronic device and the battery from over current and voltage conditions. Further,
it is necessary to protect the battery cell from over current for both the forward,
or discharging, direction and for the reverse, or charging, direction. Failure
to adequately protect the battery during recharging can result in the battery exploding.
Referring now to Fig. 1, a prior art battery protection scheme for
a mobile phone battery is illustrated. A battery, VBATT 10, is rated in this example
at about 4.2 Volts. The battery output OUT 52 drives the LOAD 56 through a protection
network 20. The LOAD is coupled to the mobile phone circuit 30 and to a battery
charging source ICHARGE 40. The protection circuit 20 comprises a fuse
F1 44 and a switch S1 48. The protection circuit 20 works by simply turning on
the switch S1 48 to allow current flow. If the current is larger than the rating
on the fuse F1 44, then the fuse will blow, or be destroyed, to open the circuit
A problem with the prior art circuit is that fuse F1 44 must have
a high rating so that it does not blow inadvertently. However, if there is a fault
on the LOAD 56 node, such as a large over voltage condition, the current can be
quite large. If the fuse does not blow at a low enough value, the reverse current
into the battery may cause the battery to explode.
Several prior art inventions describe battery protection circuits
using fuses. U. S. Patent 4,698,736 to Higa teaches a protection circuit for a
power converter apparatus. The circuit comprises plural series-parallel connected
elements. Fuses are connected in series with semiconductor elements and in parallel
with nonlinear resistors. U.S. Patent 5,754,384 to Ashley describes an over discharge
current protection circuit for a battery. U.S. Patent 6,049,144 to Frannhagen et
al describes a rechargeable battery with a built-in safety circuit. An active
device is connected in parallel with the battery cell while the fuse is connected
in series. U.S. Patent 6,172,482 to Eguchi describes a battery protection circuit.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide an effective
and very manufacturable protection circuit for a battery.
A further object of the present invention is to provide battery protection
circuit where the fuse apparatus is tripped at a lower value for a reverse current
than for a forward current.
In accordance with the objects of this invention, a battery protection
circuit for use between a battery output and a load is achieved. The circuit comprises,
first, a plurality of fused cells coupled in parallel between the battery output
and the load. Each fused cell comprises, first, a fuse having first and second
terminals where the first terminal is coupled to a battery output. Second, a means
having zener effect has a p terminal and an n terminal. The p terminal is coupled
to the second terminal of the fuse. Finally, a cell switch having first and second
terminals completes each fused cell. The cell switch first terminal is coupled
to the second terminal of the fuse, and the cell switch second terminal is coupled
to the n terminal of the diode to form a cell output. Finally, the battery protection
circuit comprises a shorting switch, that may comprise a MOS transistor that exhibits
punch through, that is coupled between the load and each fused cell output. The
plurality of fused cells forms a large current rating fuse that can be blown at
a small current rating during error conditions using a sequential blowing technique.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings forming a material part of this description,
there is shown:
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Fig. 1 illustrates a prior art battery protection scheme for a mobile phone
- Fig. 2 illustrates the preferred embodiment of the present invention.
- Fig. 3 illustrates a first preferred embodiment of the means of zener effect
of the circuit of the present invention.
- Fig. 4 illustrates a second preferred embodiment of the means of zener effect
of the circuit of the present invention.
The preferred embodiments disclose a novel circuit for protecting
a battery from over current conditions both in the reverse and in the positive
direction. It should be clear to those experienced in the art that the present
invention can be applied and extended without deviating from the scope of the
Referring now to Fig. 2, the preferred embodiment of the present
invention is illustrated. Several important features of the present invention are
shown. As in the prior art example, a battery VBATT 70 drives a LOAD 144 through
a protection circuit 80. The protection circuit 80 comprises a plurality of fused
cells 81, 82, and 83, that are coupled in parallel between the battery negative
terminal NEG 142 and the load ground GND 143. Note that the plurality of fused
cells can also be coupled between the positive battery terminal VBATT 140 and
the load. Each fused cell, such as cell 81, comprises, first, a fuse F1 104 having
first and second terminals. The first terminal is coupled to the battery output
Preferably, the fuses F1 104, F2 108, through F3 112 comprise on-chip
fuses and, more preferably, silicon on-chip fuses. The novel design of the circuit
uses relatively low current rating fuses, such as on-chip fuses, to create a large
effective value, combined fuse. This effective fuse exhibits a large effective
value for normal operation yet maintains a small blowing current value during error
conditions. This is an important feature of the present invention.
As an important feature, a means of zener effect DZ1 116 has a p
terminal and an n terminal. The means of zener effect may be an actual zener diode
formed by a p-n junction, it may be a MOS transistor exhibiting a punch-through
effect during certain operating conditions such as is shown in Fig. 4, or it may
be a more complex implementation as shown in Fig. 3. Referring again to Fig. 2,
the means of zener effect DZ1 for each fused cell must exhibit the key zener operation
of conducting current when the reverse bias exceeds a reverse breakdown voltage.
The means of zener effect p terminal is coupled to the second terminal of the fuse
F1 104. Each fused cell contains a switch S1 120 that has first and second terminals.
The switch first terminal is coupled to the second terminal of the fuse F1 104,
and the switch second terminal is coupled to the n terminal of the means of zener
effect DZ1 116 to form a cell output.
Finally, the protection circuit is completed by a shorting switch
144 that is coupled between the fused cell outputs and the positive batter terminal
VBATT 140. The shorting switch 144 preferably comprises a MOS transistor that
is standard for the process. This transistor 144 may comprise either a NMOS or
a PMOS device. The MOS transistor 144 exhibits punch-through when a high voltage,
during an over voltage event, is present across the battery terminals VBATT 140
and GND 143. For example, a MOS transistor having a channel length of about 0.5
microns may exhibit the punch-through breakdown at about 9 Volts. In this case,
the device will respond like a zener diode to an over voltage in excess of 9 Volts.
In addition, the breakdown voltage may be tailored by selecting the appropriate
transistor length. The same transistor may, for example, exhibit about a 7 Volt
punch-through, or zener effect, if the gate length is limited to only about 0.4
The key concept of the circuit is that, first, a relatively large
fuse is formed by the parallel combination of the fused cells 81, 82, and 83. Therefore,
during normal forward operation, the circuit can use fuses (F1, F2, F3) with a
small breakdown rating to achieve a large rating in the forward direction. During
normal operation, all of the cell switches S1-SN are ON and the shorting switch
SSHORT is OFF. Secondly, the circuit exhibits a smaller fuse value when
an error voltage (over charging voltage or under discharging voltage) is detected.
During these error conditions, all of the cell switches S1-SN are turned OFF and
the shorting switch SSHORT is turned ON. SSHORT conducts
in an attempt to protect the fuses. However, if the current passes the maximum
level, then SSHORT is shut OFF and the cell switches are turned ON
sequentially to cause sequential blowing of the fuses at a relatively low current
level. In this way, relatively small fuses, such as about 4 Amp on-chip fuses,
may be used to protect the circuit up to about 40 Amps for forward, discharing
operation. Yet, during charging, a protection level of about 4 Amps is advantageously
achieved. The use of the means of zener effect and the shorting switch create this
For example, each individual fuse may comprise a fuse current of
between about 200 mA and about 5 Amp. A very wide fuse current range may exist.
However a fuse with a 100 mA fuse current may be made. If 100 of these fuses are
placed in parallel, then the blowing current will be given by the rating of one
fuse multiplied by N, or about 50 Amp. Such a fuse is able to withstand a 25 Amp
peak current without any problems. However, for a charging over voltage condition,
the fuse current rating is given by the rating of just one of the fuses, or about
100 mA. Therefore, the circuit is well-suited to protect the battery from over
voltage during charging while not overly limiting discharging current performance.
Finally, it is further preferred that the fuses comprise on-chip silicon fuses.
Referring now to Fig. 3, the first preferred embodiment of the means
of zener effect is shown. The means of zener effect 150 preferably comprises, first,
a plurality of diode-connected MOS transistors M2 154, M3 158, and M4 162, coupled
in series and having an input 194 and an output 175. The input IN 194 is coupled
to the second fuse F1 terminal 194. The output OUT 175 is coupled to the switch
M1 input 175. A tri-stateable digital logic gate G1 has an input CONTROL 178 and
an output 173. A resistor R1 166 is coupled between the digital logic gate output
173 and the plurality of diode-connected MOS transistors output OUT 175 to thereby
control the switch M1. A switch S1 202 is coupled between the output of the plurality
of diode-connected MOS transistors M2 154, M3 158, and M4 162 and a current sink
I1 206 to ground.
Note that transistor M1 198 is the equivalent of the cell switch
(for example S1) of Fig. 2. Referring again to Fig. 3, diode-connected MOS devices
M2, M3, and M4, create between about 3 Volts and about 4 Volts drop. Further, to
turn OFF M1 an additional about 1 Volt is needed. Therefore, the total voltage
drop of this circuit is greater than about 5 Volts. By adjusting the number of
diode-connect MOS transistors, the voltage drop can be adjusted.
If the gate-to-source voltage of M1 198 is about 5 Volts, then the
transistor is turned ON. This can be done using the tri-stateable logic gate 174.
When M2-M4 are flowing current, the gate of M1 198 goes high and switches ON M1
by the Vth
shift of the transistors. The logic gate 174 is then tri-stated
and only a small current flows from the gate to ground. Under normal operation
voltages and currents, S1 202 is OFF and the tri-state is disabled. During a voltage
error (such as an over voltage) S1 is turned ON and the tri-state is activated.
Therefore, a voltage drop is created by the current (I1) flowing through
M2-M4. When a current error creates a condition where the fuse F1 190 must be blown,
then the tri-state is disabled and M1 is switched ON totally.
Referring now to Fig. 4, a second preferred embodiment of the means
of zener effect is shown. In this embodiment, a standard MOS transistor 300 is
coupled between the fuse 304 and ground 308. The MOS transistor 300 gate length
is designed such that the device exhibits punch-through breakdown from drain-to-source
when the drain-to-source voltage exceeds a specific value. For example, the punch-through
voltage of 0.5 micron device ay be about 9 Volts. Note that, once the device goes
into punch-through, the lifetime of the device is no longer an issue. The device
begins to blow the fuse and, after a few milliseconds, the device is 100% dead.
The present invention provides an effective and very manufacturable
protection circuit for a battery. The battery protection circuit uses a fuse apparatus
that is tripped at a lower value for a reverse current than for a forward current.
The circuit uses relatively small rating, on-chip fuses to create a parallel combined
fuse with a large effective rating. Yet, during an error condition, the fuses are
sequentially blown using a small blowing current.
As shown in the preferred embodiments, the novel method and structure
provide an effective and manufacturable alternative to the prior art.
While the invention has been particularly shown and described with
reference to the preferred embodiments thereof, it will be understood by those
skilled in the art that various changes in form and details may be made without
departing from the spirit and scope of the invention.