NCS37000

http://onsemi.com

6

RECOMMENDED EXTERNAL COMPONENTS:

Component Type

Instance

Value

Note

SCR

Q1

−

ON−MCR08

Diode

Dx

−

ON−1N4007

Capacitor

Csup

1 − 5 mF

For a full bridge rectifier

Capacitor

Cgn

2 − 10 nF

Low pass corner frequency for GN detection

Capacitor

Cidf

220 nF

1kHz differential corner frequency

Capacitor

Cct

2.2 − 5 nF

Match to current transformer for resonance

Capacitor

Cbias

10 nF

Added for filtering

Resistor

RctresD

40 − 80K

Matched to current transformer

Resistor

RctGN

100 − 400K

Matched to current transformer

Resistor

Rzc

400 − 800K

Zero cross detector shunt resistance

Resistor

Rmains

10 − 20K

For a full bridge rectifier

TVS

T1

−

~250 − 400 V

Filtering

The analog signal capture portion of the IC includes a

single pole filter that can be set externally with Cidf. This

provides an additional layer of protection against false

tripping under steady state noise conditions. High frequency

steady state noise is common with pumps, motors or other

cyclic noise generators.

Cidf = 220 nF = 1 KHz low pass.

Setting Trip Sensitivity

The CTresD resistor sets the threshold for the differential

current fault levels. Increasing CTresD causes the fault

levels to trip at lower differential currents. CT efficiency at

60 Hz must be considered.

CTresD= 200*#Turns – Subject to CT efficiency at 60 Hz

The CTresGN resistor sets the threshold for ground to

neutral fault detection. The Rt parameter is the desired

ground to neutral resistance trip level. Higher CTresGN

values will cause the part to trip at higher ground to neutral

impedances.

CtresGN = #Turns^2 x Rt/2 – Subject to CT efficiency from

3 kHz to 4 kHz

Setting Grounded−Neutral response time

Cgn is used to define the response time of the

grounded−neutral detection circuit. The analog portion

converts the impedance into a DC voltage and a high

frequency (aliased) component. The capacitor is used to

remove the high frequency component leaving just the DC

representation of the impedance.

CTcapGN= 1.8E-4/CTresGN