sr087 Supertex, Inc., sr087 Datasheet - Page 6

• Manufacturer Part Number: sr087
• Description: Adjustable Off-line Inductorless Switching Regulator
• Manufacturer: Supertex, Inc.

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 fi lters.
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
AC Line
AC Line
ground
earth
SR08x &
Circuitry
common
OUT
circuit
SR08x &
Circuitry
+ I
REG
).
6
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
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
rectifi er. The reason for this is to minimize capacitance to
allow the rectifi ed 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
rectifi er 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 ‘modifi ed sine wave’
LOAD
OUT
LOAD
(I
= I
= V
OUT
OUT
) and, since the linear regulator is supplied from V
OUT
+ I
(I
REG
OUT
+ I
REG
)
RMS
OUT
RMS
.
SR086/SR087
.
2
REG
R, the fuse must be
(I
REG
RMS
). Total load is
, and for a
NR040306
OUT
,