FM2141 Shanghai Fudan Microelectronics Co., Ltd., FM2141 Datasheet - Page 6

• Manufacturer Part Number: FM2141
• Description: Fm2141 Gfci Low Power Ground Fault Interrupter
• Manufacturer: Shanghai Fudan Microelectronics Co., Ltd.

GFCI Application
FM2141 Low Power Ground Fault Interrupter
The GFCI detects a ground fault by sensing a difference current in the line and neutral wires. The
difference current is assumed to be a fault current creating a potentially hazardous path from line
to ground. Since the line and neutral wires pass through the center of the sense transformer, only
the differential primary current is transferred to the secondary. Assuming the turns ratio is 1:1000,
the secondary current is 1/1000
secondary current to a voltage which is compared with either of the two window detector reference
voltages. If the fault current exceeds the design value for the duration of the programmed time
delay, FM2141 will send a current pulse to the gate of the SCR.
Detecting ground to neutral faults is more difficult. RB represents a normal ground fault resistance,
RN is the wire resistance of the electrical circuit between load/neutral and earth ground. RG
represents the ground to neutral fault condition. According to UL 943, the GFCI must trip when
RN=0.4Ω, RG=1.6Ωand the normal ground fault is 6 mA.
Assuming the ground fault to be 5mA, 1mA and 4mA will go through RG and RN, respectively,
causing an effective 1mA fault current. This current is detected by the sense amplifier and
amplifier by the sense amplifier. Now the ground/neutral and sense transformers are mutually
coupled by RG, RN and neutral wire ground loop, producing a positive feedback loop through the
sense amplifier. The newly created feedback loop causes the sense amplifier to oscillate at a
frequency determined by ground/neutral transformer secondary inductance and C4. Typically it
occurs at 8 KHz.
C2 is used to program the time required for the fault to be present before the SCR is triggered.
Refer to the equation below for calculating the value of C2.
Unit of C2 is nF, Unit of T is ms.
RSET is used to set the fault current at which the GFCI trips.
When used with a 1:1000 sense transformer, its typical value is 1MΩ for a GFCI designed to trip
at 5mA.
C2 = 6 x T
R
SET
=
I
FAULT
×
COS180
4
6 .
×
N
T (
/
) P
th
the fault current. FM2141’s sense amplifier converts the
Ver 1.0
Specification
6