FAN4147SX Datasheet, PDF (5/10 Pages) Fairchild Semiconductor – Ground Fault Interrupter

Functional Description
(Refer to Figure 1 and Figure 2.)
The FAN4147 is a GFCI controller for AC ground-fault
circuit interrupters. The internal rectifier circuit is
supplied from the AC line during the positive half cycle
of the AC line voltage. The internal 12V shunt regulator
uses a precision temperature-compensated bandgap
reference. The combination of precision reference
circuitry and precision sense amplifier provides for an
accurate ground-fault tolerance. This allows for selection
of external components with wider, lower-cost,
parameter variations. Due to the low quiescent current,
a high-value external series resistor (R1) can be used,
which reduces the maximum power wattage required.
The 12V shunt regulator generates the reference
voltage for the sense amplifier’s (A₁) non-inverting input
(AC ground reference) and supplies the bias for the
delay timer (T₁), comparators (C₁ & C₂), and SCR driver.
oscillate. When the peak oscillation voltage exceeds the
SCR trigger threshold, the internal delay timer is
enabled. Since the amplifier’s output signal is crossing
the window comparator’s trip threshold typically at 6KHz,
the delay timer alternates between detection of a
fault/no fault. The ratio of the fault/no-fault detection time
interval determines if the SCR driver is enabled.
The sensitivity of the grounded neutral detection is
changed by the neutral coil turns and C2, C3 values.
R_SET Resistor Calculation
The AmpOut signal must exceed the window
comparator’s V_TH threshold voltage for longer than the
delay timer. Therefore:
The secondary winding of the sense transformer is
connected to pin 4 (VREF) and to a resistor R_IN directly
DC connected to the inverting input of the sense
amplifier at pin 5 (VFB). The feedback resistor (R_SET)
converts the sense transformer’s secondary current to a
voltage at pin 6 (AmpOut). This voltage is compared to
the internal window comparator (C₁ & C₂) and, when the
AmpOut voltage exceeds the +/-V_TH threshold voltage,
the window comparator triggers the internal delay timer.
The output of the window comparator must stay HIGH
for the duration of the t₁ timer. If the window
comparator’s output goes LOW, the internal delay timer
starts a reset cycle. If the window comparator’s output is
still HIGH at the end of the t₁ pulse, the SCR driver
enables the current source I₁ and disables Q₁. The
current source I₁ then enables the external SCR, which
energizes the solenoid, opens the contact switches to
the load, and removes the hazardous ground fault. The
window comparator allows for detection of a positive or
negative I_FAULT signal independent from the phase of the
line voltage.
The sense transformer typically has a toroidal core
made of laminated steel rings or solid ferrite material.
The secondary of the transformer is typically 800 to
1500 turns of #40 wire wound through the toroid. The
primary is typically one to two turns made by passing the
AC hot and neutral wires through the center of the
toroid. When a ground fault exists, a difference exists
between the current flowing in hot and neutral wires.
The primary difference current, divided by the primary-
to-secondary turns ratio, is the current that flows through
the secondary wire of the transformer.
A grounded neutral condition occurs when the neutral-
line is grounded at the neutral-load side.
Depending on the resistance of the grounded neutral
connection, this condition causes the sense coil to
detect a lower ground fault current. The detection of a
ground-to-load-neutral fault relies on the principle of
positive feedback. When this occurs, the sense and
neutral coils are mutually coupled, which produces a
positive feedback path around the sense amplifier. This
positive feedback causes the sense amplifier to
V_TH=I_FAULT x 1.41 x R_SET x C_OS(2 x (t/2P)) / N
(1)
R_SET=(V_TH x N) / (1.41 x I_FAULT x C_OS( x t/P))
(2)
where:
V_TH = 3.5V ;
I_FAULT = 5mA (UL943) ;
t
= 1ms (timer delay);
P = Period of the AC Line (1/60Hz);
N = Ratio of secondary to primary turns (1000:1);
R_SET = 505K511Kstandard 1% value^{}
Note:
4. In practice, the transformer is non-ideal, so R_SET
may need to be adjusted by up to 30% to obtain the
desired I_fault trip threshold.
V_OS Trip Threshold Error Calculation
Since the sense coil is DC connected to the feedback of
the sense amplifier, the V_OS offset introduces an I_fault
threshold error. This error can be calculated as:
%Error=
100 x (V_OS x R_SET) / (R_IN + RL_DC + RL_AC) / V_TH
(3)
where:
V_OS = +/-450V (worse case);
= +/-150V (typical);
R_SET = 511K
R_IN
= 470 (typical value);
RL_DC = 75 (sense coil secondary DC resistance);
RL_AC
=
1.5K (AC_(jL) impedance
(L= 4H, f= 60Hz);
of
sense
coil)
V_TH
= 3.5V;
%Error = +/- 3.2% (worst case); +/-1.1% (typical).
© 2010 Fairchild Semiconductor Corporation
FAN4147 • Rev. 1.0.1
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