Current window detection circuit

A ground fault interrupt circuit with an improved current window detector. The current window detector works by first rectifying the input current in a pair of current mirrors connected in back to back fashion in the feedback path of the input amplifier. The rectified current triggers an output when the rectified current exceeds a predetermined threshold.

BACKGROUND OF THE INVENTION 
This invention relates generally to circuits for measuring current and, 
more particularly, circuits to sense when a current falls outside a 
predetermined range. 
In many instances, it is desirable to know that the level of a current is 
within acceptable limits. One important instance where the value of a 
current is very important is in a ground fault interrupt (GFI) circuit. 
These circuits are used to reduce the chance that a person will receive a 
serious electrical shock from an electrical appliance dropped into water 
or otherwise malfunctioning. 
A GFI circuit operates by sensing the current on the power line leading up 
to the appliance. The power line has two wires. When the electrical 
appliance is operating correctly, the current in each wire is equal since 
all the current supplied in one of the wires returns in the other wire. 
If some of the current supplied to the electrical appliance is flowing out 
of the appliance, a dangerous condition exists. For example, when an 
appliance drops into water, current may flow out of the appliance and 
shock a person. To avoid this unpleasant result, GFI circuits sense that 
more current is flowing into the electrical appliance than is flowing back 
from the appliance. Then the GFI circuit disconnects all power from the 
electrical appliance. 
Circuits to sense currents are commercially available. For example, 
National Semiconductor manufactures and sells an integrated circuit chip 
designated LM1851. The circuit is designed to be connected to the 
secondary of a transformer. The primary of the transformer is made from 
both wires of the power line to the appliance. If there is an imbalance in 
the current in the two wires, there will be a current induced in the 
secondary winding. 
For the transformer to accurately reflect the imbalance of current in the 
primary to the secondary, the impedance in the secondary must be very low. 
Thus, the impedance of the circuit to sense currents must be very low, 
typically less than 100 ohms. Also, the circuit to sense current must be 
able to tell when the magnitude of the current exceeds a threshold, 
regardless of the direction of the current. 
While the LM1851 performs adequately, it is desirable to have a circuit 
which is as simple as possible to sense current. A simpler circuit is less 
expensive to fabricate and less likely to fail. 
Also, the LM1851 contains an amplifier with zener diodes in a feedback 
loop. The zener diodes convert the current at the input of the LM1851 to a 
voltage for sensing. If the voltage exceeds a threshold, a fault condition 
is indicated. However, because the LM1851 operates by converting a current 
to a voltage, its supply voltage must be above a certain level. It would 
be desirable in some circumstances to build a current sense circuit to 
operate at lower voltages, say as low as 3 volts or less. 
SUMMARY OF THE INVENTION 
With the foregoing background in mind, it is an object of this invention to 
provide a current window detection circuit that is simple to construct. 
It is also an object of this invention to provide a current window 
detection circuit that can operate with a very low supply voltage, say 
less than 3 volts. 
It is yet a further object of this invention to provide a GFI circuit with 
an improved current window detection circuit. 
The foregoing and other objects are achieved in a GFI chip with a current 
window detection circuit. The current window detection circuit comprises 
two back-to-back current mirrors in the feedback path of a sense amplifier 
which receives the current input. A third current mirror reflects the 
current through either of the first two current mirrors to the input node 
of a comparator. When the current at the input node of the comparator 
exceeds a threshold current, the output of the comparator signals a fault 
condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a schematic of an electrical appliance 14 protected by a 
Ground Fault Interrupt (GFI) circuit 10. As shown in FIG. 1, GFI circuit 
10 is incorporated into the cord 12 supplying power to electrical 
appliance 14. Electrical appliance 14 may be any electrical appliance such 
as a hair dryer, curling iron, or electric drill. 
GFI circuit 10 includes a GFI chip 18 coupled to power cord 12 through 
transformer T.sub.1. As shown in FIG. 1, power cord 12 consists of a HOT 
wire and a NEUTRAL wire. Both the HOT wire and the NEUTRAL wire pass 
through core 20 of transformer T.sub.1. The HOT and NEUTRAL wires thus 
form the single turn primary of transformer T.sub.1. If the current flow 
through the HOT and NEUTRAL wires becomes unequal, a current will be 
induced in the secondary of transformer T.sub.1. Here, transformer T.sub.1 
has a turns ratio of 1:500. Thus, the secondary current ideally is the 
imbalance of the current through the HOT and NEUTRAL wires divided by 500. 
However, this current relationship only holds if the impedance between 
pins P.sub.2 and P.sub.3 of GFI chip 18 is low. Here, the impedance is on 
the order of 1 ohm, as compared to approximately 1000 of prior art 
devices. 
As in a conventional GFI circuit, an unequal current in wires HOT and 
NEUTRAL may signify a dangerous condition involving electrical appliance 
14. GFI chip 18 senses the current flow in the secondary of transformer 
T.sub.1 and actuates normally closed relay 16 when the current in the 
secondary of transformer T.sub.1 indicates the possibility of a failure 
involving electrical appliance 14. 
Relay 16, when actuated, disconnects power to electrical appliance 14. The 
chance of personal injury as a result of the failure involving electrical 
appliances is therefore greatly reduced. Relay 16 is preferably the type 
of relay conventionally found in circuit breakers which, once open, must 
be manually closed. 
FIG. 1 shows that GFI chip 18 is connected to relay 16 through a silicon 
controlled rectifier SCR. As is known, a silicon control rectifier is used 
as a latching current switch. Capacitor C.sub.3 acts as a filter to 
prevent noise from triggering the silicon controlled rectifier SCR. 
Capacitor C.sub.2 is used as a filter to prevent electrical noise from 
interfering with the operation of GFI circuit 10. 
Capacitor C.sub.1 is used to ac couple the current in the secondary of 
transformer T.sub.1 into GFI chip 18. 
Resistor R.sub.LINE connects GFI chip 18 to the HOT wire. R.sub.LINE 
provides power to GFI chip 18. 
Resistor R.sub.TEST and switch S.sub.1 allow, as in prior devices, a test 
of the operation of GFI circuit 10. The MOV acts, as in prior devices, to 
protect GFI circuit 18 and electrical appliance 14 from voltage spikes. 
As described above, GFI chip 18 senses the current between pins P.sub.2 and 
P.sub.3. If that current is large enough that it signals a fault involving 
electrical appliance 14, GFI chip 18 produces a signal at pin P.sub.7 
which actuates relay 16. The level of current between pins P.sub.2 and 
P.sub.3 which is large enough to cause GFI chip 18 to actuate relay 16 is 
determined by the value of resistor R.sub.SET. Smaller values of R.sub.SET 
require a greater imbalance of the current on lines HOT and NEUTRAL before 
relay 16 is actuated. 
GFI chip 18 differs from prior art devices in its internal circuitry used 
to detect when the current between pins P.sub.2 and P.sub.3 exceeds a 
current threshold. Such a circuit is called a current window detector. The 
current window detector must produce a signal indicating that the 
magnitude of current in the secondary of transformer T.sub.1 exceeds a set 
threshold. It will be appreciated from FIG. 1 that power cord 12 supplies 
alternating current and, consequently, the polarity of the current in the 
secondary of transformer T.sub.1 periodically reverses. Thus, the current 
window detector must sense a current having a magnitude exceeding the 
threshold, regardless of its polarity. 
Turning now to FIG. 2, the current window detector of GFI chip 18 is shown. 
Pins P.sub.2 and P.sub.3 are the inputs to amplifier A.sub.1. As shown, 
pin P.sub.3 is connected to the non-inverting amplifier terminal which is 
grounded or connected to a fixed voltage between V.sub.CC and V.sub.EE. 
The output of amplifier A.sub.1 is connected to the inverting terminal 
through a feedback loop including current mirrors M.sub.1 and M.sub.2. As 
is known from amplifier theory, amplifier A.sub.1 will provide a current 
through current mirrors M.sub.1 and M.sub.2 such that pin P.sub.2 is at 
the same voltage as pin P.sub.3. One of skill in the art will appreciate 
that the current through the feedback loop of amplifier A.sub.1 will equal 
the current in the secondary of transformer T.sub.1 (FIG. 1). 
Current mirrors M.sub.1 and M.sub.2 are of similar construction, each 
fabricated from a pair of transistors. The emitter of transistor Q.sub.8 
is twice the size of the emitter of transistor Q.sub.9, whereas the 
emitters of transistors Q.sub.6 and Q.sub.7 are the same size. 
However, current mirrors are connected in the feedback loop of amplifier 
A.sub.1 in a "back to back" or antiparallel fashion. Thus, current from 
the output of amplifier A.sub.1 will flow through only one of current 
mirrors M.sub.1 and M.sub.2 . If amplifier A.sub.1 sources current, that 
entire current will flow through the input current path, including 
transistor Q.sub.7, of current mirror M.sub.2. If the input has the 
opposite polarity and amplifier A.sub.1 sinks current, the entire current 
will flow through the input current path, including transistor Q.sub.8, of 
current mirror M.sub.1. Therefore, the input current path of each current 
mirror is electrically interconnected when the alternating current of the 
first polarity is passed by the first current mirror and subsequently when 
the alternating current of the second polarity is passed by the second 
current mirror. 
Regardless of the polarity of the input current, and regardless of which of 
the current mirrors M.sub.1 or M.sub.2 the current from amplifier A.sub.1 
flows through, a current will flow in the input current path, including 
leg 2 of current mirror M.sub.1 and the output current path (including 
transistor Q.sub.9 or Q.sub.6) of the one of the current mirrors M.sub.1 
or M.sub.2, respectively, through which the current from amplifier A.sub.1 
flows. The current in leg 22, will be one half the magnitude of the input 
current I.sub.IN. However, the polarity will always be such as to forward 
bias transistor Q.sub.20. It will be appreciated that the current is one 
half I.sub.IN because of the relative size of the emitters of Q.sub.8 and 
Q.sub.9. 
The current in leg 22 is reflected into the output current path, including 
leg 24, of current mirror M.sub.3. Here, current mirror .sub.3 is a Wilson 
type current mirror with three transistors. In comparison to a simple 
current mirror with two transistors, M.sub.3 has a higher output 
impedance. Transistor Q.sub.19 is fabricated with an emitter area twice 
the size of the emitter area of transistor Q.sub.20. This layout of the 
transistors results in the current in leg 24 being twice the current in 
leg 22. Since the current in leg 22 is one half I.sub.IN, the current in 
leg 24 has the same magnitude as I.sub.IN. 
Thus, the current in leg 24 represents the input current at pins P.sub.2 
and P.sub.3 rectified to have a known polarity. In the circuit of FIG. 2, 
the current in leg 24 therefore represents the current in the secondary of 
transformer T.sub.1 (FIG. 1) and, if it is large enough, signals a fault. 
In developing the current in leg 24, current mirrors M.sub.1, M.sub.2, and 
M.sub.3 rectified the input current but did not draw any current away from 
the path between pins P.sub.2 and P.sub.3. Accordingly, the path between 
pins P.sub.2 and P.sub.3 will have a very low impedance, which is 
important for proper operation of GFI circuit 10 (FIG. 1). 
The current in leg 24 of current mirror M.sub.3 is coupled to node N.sub.3. 
Node N.sub.3 is the connection between an input terminal of comparator 
A.sub.2 and current reference 26. Current reference 26 sinks a constant 
current of value I.sub.REF. 
As long as the magnitude of the input current is less than I.sub.REF, no 
current will flow into comparator A.sub.2. The output of comparator 
A.sub.2 will then be low. If the magnitude of the input current exceeds 
I.sub.REF, current will flow into the positive terminal of comparator 
A.sub.2. This current flow causes the output of comparator A.sub.2 to go 
high. The output of comparator A.sub.2 is indirectly coupled through a 
filter and latch circuit (not shown) to pin P.sub.7 of GFI chip 18 (FIG. 
1). Thus, when the magnitude of the input current exceeds I.sub.REF, GFI 
circuit 10 (FIG. 1) disconnects power to electrical appliance 14 (FIG. 1). 
Turning to FIG. 3A, details of amplifier A.sub.1 can be seen. Amplifier 
A.sub.1 has two stages. The first stage is a differential input stage. 
Transistors Q.sub.3 and Q.sub.4 are a standard p-n-p emitter coupled 
transistor pair. Resistors R.sub.1 and R.sub.2 are of equal value and 
provide a dc bias for transistors Q.sub.3 and Q.sub.4. Transistor Q.sub.5 
is biased to provide a constant current of approximately 5 .mu.A to 
transistors Q.sub.3 and Q.sub.4. Transistors Q.sub.10, Q.sub.11, and 
Q.sub.12 are the active load for the transistor pair Q.sub.3 and Q.sub.4. 
The active load provides a high output impedance and a large current gain. 
The first stage also includes transistors Q.sub.1 and Q.sub.2. These 
transistors provide input overload current protection and may not be 
needed for some applications. 
The second stage of amplifier A.sub.1 includes transistors Q.sub.13 and 
Q.sub.15. Transistor Q.sub.14 and resistor R.sub.6 limit the current in 
transistor Q.sub.15 if a large input current is applied. The current is 
limited to n amount, here equal to 200 .mu.A, equal to the base to emitter 
voltage of Q.sub.14 divided by R.sub.6. 
The output of the second stage is the output of amplifier A.sub.1, which is 
the junction of the collectors of transistors Q.sub.15 and Q.sub.16. Here, 
Q.sub.16 is biased to provide a constant current of 60 .mu.A at its 
collector. As the input of amplifier A.sub.1 increases, the current 
through transistor Q.sub.15 will decrease. Thus, as the input to amplifier 
A.sub.1 increases, more of the constant current provided by transistor 
Q.sub.16 is coupled to the output of amplifier A.sub.1. Conversely, as the 
input decreases, less current is coupled to the output of amplifier 
A.sub.1. 
FIG. 3A shows three voltage levels V.sub.CC, V.sub.EE, and a bias voltage. 
These voltage levels are generated by a portion (not shown) of GFI chip 
18. As shown in FIG. 1, pins P.sub.5 and P.sub.6 are connected through 
R.sub.LINE across the wires of power cord 12, which carries AC voltage. 
This voltage can be rectified and level shifted with known circuitry to 
provide the desired voltages. For example, V.sub.CC is typically six 
volts. V.sub.EE is typically ground, and the bias voltage is typically six 
volts minus the voltage drop across the base/emitter junction of a 
transistor. 
Turning to FIG. 3B, additional details of comparator A.sub.2 can be seen. 
Transistor Q.sub.22 establishes a current into node N.sub.30. As long as 
I.sub.REF exceeds the current at pin P.sub.2, base current flows in 
transistor Q.sub.23. Transistor Q.sub.23 turns on and transistor Q.sub.24 
turns off, causing transistors Q26 and Q.sub.27 to be on and Q.sub.25 to 
be off. If, however, current flows from node N.sub.3 into the base of 
Q.sub.23, as when the input current exceeds I.sub.REF, no current will 
flow through Q.sub.26 and Q.sub.27. The current from Q.sub.22 will then 
flow into the base of Q.sub.25, turning transistor Q.sub.25 on. The point 
labeled OUT will thus take on a voltage almost equal to ground, indicating 
a current in excess of I.sub.REF was measured. The point labeled OUT is 
indirectly coupled to silicon controlled rectifier SCR (FIG. 1) causing it 
to be triggered when the input current exceeds I.sub.REF. 
Turning to FIG. 3C, details of current reference 26 are seen. Here, a 
simple current reference which uses a resistor R.sub.SET and a transistor 
Q.sub.REF is used. Here, the resistor R.sub.SET is external to GFI chip 18 
as shown in FIG. 1. 
Turning to FIG. 4, an alternative circuit for amplifier A.sub.1 is shown. 
The embodiment of FIG. 4 can operate with a voltage difference between 
V.sub.CC and V.sub.EE less than three volts. Removing transistors 
Q.sub.10, Q.sub.13, and Q.sub.14 (FIG. 3A) allows amplifier to operate 
with a bias voltage as low as 1 volt. The configuration of FIG. 4 has an 
increased offset voltage, typically 1 millivolt, and lower open loop 
voltage gain than the circuit of FIG. 3A. However, sometimes it may be 
desirable to have the circuit operate at very low voltages. 
Having disclosed embodiments of this invention, various alternative 
embodiments will be apparent to one of skill in the art. For example, 
current mirror M.sub.3 (FIG. 2) is a Wilson type current mirror. This type 
of current mirror has a better output impedance than a basic two 
transistor current mirror. The improved output impedance provides more 
accuracy in matching the current in leg 24 (FIG. 2) to the current at pin 
P.sub.2. If high accuracy is not required, current mirror M.sub.3 could 
even be eliminated entirely. A circuit without current mirror M.sub.3 is 
shown in FIG. 5. In the circuit of FIG. 5, a voltage source V.sub.REF is 
used to establish the threshold at which the current window detector 
indicates an input current exceeding a threshold. Here, V.sub.REF is 
selected to be one half R.sub.SET times the current threshold. It is felt, 
therefore, that this invention should be limited only by the spirit and 
scope of the appended claims.