GV4145A_15 Datasheet, PDF (5/7 Pages) Unisonic Technologies – LOW POWER GROUND FAULT INTERRUPTER

GV4145A
 PRINCIPLES OF OPERATION
LINEAR INTEGRATED CIRCUIT
PRINCIPLES OF OPERATION
The 26V shunt regulator voltage generated by the string of zener diodes is divided into three reference
voltages:3/4Vs, 1/2Vs, and 1/4Vs. V_REF is at 1/2Vs and is used as a reference to create an artifical ground of +13V at
the op amp noniverting input.
Figure 1 shows a three-wire 120V AC outlet GFI application using an UTC GV4145A, Fault signals from the sense
transformer are AC coupled into the input and are amplified according to the following equation:
V7=R_SENSE×I_SENSE/N
Where V7 is the RMS voltage at pin7 relative to pin 3, R_SENSE is the value of the feedback resistor connected from
pin 7 to pin 1, I_SENSE 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 formula for V7 is approximate because it does not include the sense transformer characteristics.
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.
SHUNT REGULATOR
R_LINE 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 shunt regulator creates internal reference voltages( see above).
OPERATIONAL AMPLIFIER
R_SENSE is a feedback resistor that sets gain and therefore sensitivity to normal faults. To adjust R_SENSE, follow this
procedure: apply the desired fault current (a difference in current of 5mA is the UL943 standard). Adjust R_SENSE
upward until the SCR activates. A fixed resistor can be used for R_SENSE, since the resultant ±15% variation in
sensitivity will meet UL’s 943 4~6mA specification window.
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).
The sensitivity 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 2Ωor less.
The input to the op amp are protected from overvoltage by back-to back diodes.
SCR DRIVER
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.C_F is a noise filter capacitor that prevents narrow pulses from firing the SCR.
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
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.
TWO-WIRE APPLICATION CIRCUIT
Figure 2 shows the diagram of a 2-wire 120V AC outlet GFI circuit using an UTC GV4145A. 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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
5 of 7
QW-R122-006.C