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FAN4147SX Folha de dados(PDF) 5 Page - Fairchild Semiconductor |
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FAN4147SX Folha de dados(HTML) 5 Page - Fairchild Semiconductor |
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5 / 10 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN4147 • Rev. 1.0.1 5 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 (A 1) non-inverting input (AC ground reference) and supplies the bias for the delay timer (T1), comparators (C1 & C2), and SCR driver. The secondary winding of the sense transformer is connected to pin 4 (VREF) and to a resistor RIN directly DC connected to the inverting input of the sense amplifier at pin 5 (VFB). The feedback resistor (RSET) 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 +/-VTH 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 IFAULT 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 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. RSET Resistor Calculation The AmpOut signal must exceed the window comparator’s V TH threshold voltage for longer than the delay timer. Therefore: VTH=IFAULT x 1.41 x RSET x COS(2 x (t/2P)) / N (1) RSET=(VTH x N) / (1.41 x IFAULT x COS( x t/P)) (2) where: VTH = 3.5V ; IFAULT = 5mA (UL943) ; t = 1ms (timer delay); P = Period of the AC Line (1/60Hz); N = Ratio of secondary to primary turns (1000:1); RSET = 505K511Kstandard 1% value Note: 4. In practice, the transformer is non-ideal, so RSET may need to be adjusted by up to 30% to obtain the desired Ifault trip threshold. VOS Trip Threshold Error Calculation Since the sense coil is DC connected to the feedback of the sense amplifier, the VOS offset introduces an Ifault threshold error. This error can be calculated as: %Error= 100 x (VOS x RSET) / (RIN + RLDC + RLAC) / VTH (3) where: VOS = +/-450 V (worse case); = +/-150 V (typical); RSET = 511K RIN = 470 (typical value); RLDC = 75 (sense coil secondary DC resistance); RLAC = 1.5K (AC (j L) impedance of sense coil) (L= 4H, f= 60Hz); VTH = 3.5V; %Error = +/- 3.2% (worst case); +/-1.1% (typical). |
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