KK4145A
Principles of Operation
Adjust RSENSE upward until the SCR activates. A fixed
resistor can be used for RSENSE, since the resultant
±15% variation in sensitivity will meet UL’s 943 4-6mA
specification window.
The 26V shunt regulator voltage generated by the string
of zener diodes is divided into three reference voltages:
3/4 VS , 1/2 VS , and 1/4 VS . VREF is at 1/2VS and is
used as a reference to create an artifical ground of +13V
at the op amp noninverting input.
The roll-off frequency is greater than the grounded
neutral fault oscillation frequency, in order to preserve
loop gain for oscillation (which is determined by the
inductance of the 200:1 transformer and C4).
Figure 1 shows a three-wire 120V AC outlet GFI
application using an KK4145AN. Fault signals from the
sense transformer are AC coupled into the input and are
amplified according to the following equation:
The senstivity to grounded neutral faults is adjusted by
changing the frequency of oscillation. Increasing the
frequency reduces the sensitivity by reducing the loop
gain of the positive feedback circuit. As frequency
increases, the signal becomes attenuated and the loop
gain decreases. With the values shown the circuit will
detect a grounded neutral fault having resistance of 2Ω
or less.
V7 = RSENSE x ISENSE/N
Where V7 is the RMS voltage at pin 7 relative to pin 3,
RSENSE is the value of the feedback resistor connected
from pin 7 to pin 1, ISENSE is the fault current in amps
RMS and N is the turns ratio of the transformer. When
V7 exceeds plus or minus 7.2V relative to pin 3 the SCR
Trigger output will go high and fire the external SCR.
The input to the op amp are protected from overvoltage
by back-toback diodes.
SCR Driver
The formula for V7 is approximate because it does not
include the sense transformer characteristics.
The SCR used must have a high dV/dt rating to ensure
that line noise (generated by noisy appliances such as a
drill motor) does not falsely trigger the SCR. Also, the
SCR must have a gate drive requirement of less than
200µA. CF is a noise filter capacitor that prevents narrow
pulses from firing the SCR.
Grounded neutral fault detection is accomplished when a
short or fault closes a magnetic path between the sense
transformer and the grounded neutral transformer. The
resultant AC coupling closes a positive feedback path
around the op amp, and therefore the op amp oscillates.
When the peaks of the oscillation voltage exceed the
SCR trigger comparator thresholds, the SCR output will
go high.
The relay solenoid used should have a 3ms or less
response time in order to meet the UL 943 timing
requirement.
Sense Transformers and Cores
Shunt Regulator
The sense and grounded neutral transformer cores are
usually fabricated using high permeability laminated
steel rings. Their single turn primary is created by
passing the line and neutral wires through the center of
its core. The secondary is usually from 200 to 1500
turns.
RLINE limits the current into the shunt regulator; 220V
applications will require substituting a 47kΩ 2W
resistor. In addition to supplying power to the IC, the
s
h
u
n
t
regulator creates internal reference voltages (see above).
Two-Wire Application Circuit
Operational Amplifier
Figure 2 shows the diagram of a 2-wire 120V AC outlet
GFI circuit using an KK4145AN. This circuit is not
designed to detect grounded neutral faults. Thus, the
grounded neutral transformer and capacitors C3 and C4
of Figure 1 are not used.
RSENSE is a feedback resistor that sets gain and
therefore sensitivity to normal faults. To adjust
RSENSE, follow this procedure: apply the desired fault
current (a difference in current of 5mA is the UL 943
standard).
KODENSHI [ KODENSHI KOREA CORP. ]
