6

NR040306

SR086/SR087

Line Transformer

During initial testing it is tempting to use an isolation

transformer or a variable transformer on the AC line.

However, the high inductance of the transformer (frequently

in the mH’s) interferes with the normal operation of the

SR08x and should not be used. This is not a concern with

the normal inductance of the AC line or for AC line filters.

The SR08x draws current from the AC line in short, high

current pulses. The transformer’s high inductance tends to

limit the current pulse. Furthermore, inductive kickback on

the falling edge of the current pulse can create high voltage

spikes which must be absorbed by the transient protector.

EMI Capacitor

The use of a small-value capacitor from circuit common to

earth ground prevents the SR08x from operating and should

not be employed.

EMI

The SR08x circuit as depicted on Page 1, (Typical Application

Circuit), meets FCC Class B and CISPR 14-1 (household

appliances) requirements for conducted emissions, for

combined loads of less than 20mA (I

SR08x &

Circuitry

AC Line

AC Line

SR08x &

Circuitry

earth

ground

circuit

common

Fuse

Although the average current drawn from the AC line is low,

the RMS current is fairly high due to the current being drawn in

short, high-current pulses. Since a fuse is basically a resistor

with a power dissipation given by I

RMS

sized for the RMS current, not the average current. For a 1W

load and 120VAC the RMS current is 700mA

0.5W load at 230VAC it is 360mA

Transient Protection

The transient protector must be located before the bridge

rectifier. The reason for this is to minimize capacitance to

allow the rectified AC to fall below V

Since there is no capacitor to absorb AC line transients,

complete transient protection must be provided by the TVS or

MOV device. Since the recommended IGBT is rated at 1.2kV

and the SRO8x never sees the full input voltage, the bridge

rectifier becomes the limiting element when selection an MOV.

If using a 1.0kV bridge, an MOV having a clamping voltage of

< 1.0kV is recommended.

Load

Total load on the SR08x is the total load current drawn from

V

it also includes the current drawn from V

calculated as follows:

I

P

UPS

The SR08x will not operate from uninterruptable power

supplies having a square wave output. This type of output is

usually referred to as ‘modified sine wave’