supply intended for operation directly from a 120/230VAC
line. Two outputs, 12.6V and 5.0V are provided.
The operating principle is to turn on a pass transistor when
the rectified AC input is below the output voltage, and to turn
it off when the output storage capacitor is charged up to the
regulation point. Since the output capacitor charges up on
the rising edge of a sine wave, reasonable efficiencies can
be obtained without the use of magnetics.
Instructions on page 2 and 3 describe how the demoboard
may be modified to better suit applications that are not satis-
fied by the board as provided. For a full description of the
SR087, see the SR087 datasheet.
Value
90 to 270VAC
45Hz to 65Hz
12.6V ± 8%
1
5.0V ± 5%
120VAC
60mA
100mA
52%
62mW
230VAC
60mA
60mA
44%
200mW
Board Layout and Connection Diagram
R8
Do not connect earth-grounded loads
or test instruments. Doing so will short
the AC line, resulting in damage to the
instrument, load, and/or the SR087 circuit.
Use floating instruments or high voltage
differential probes. Do not use a
transformer on the AC line.
WARNING!!!
Supertex SR087DB1
WARNING!!!
Warning! No galvanic isolation.
Dangerous voltages are present.
Do not connect grounded loads
or test equipment. Do not use
a transformer on the AC line.
Q1
C5
C2
C3
EN
GND
V
OUT
V
REG
No galvanic isolation.
Dangerous voltages are
present when connect to
the AC line.
R1
F1
V
IN
R2
V
IN
90 - 270VAC
50/60Hz
R6
C4
J1
Z1
~~
R7
D1
C1
-
R3
R4
IC1
V
OUT
(12.6V)
J2
V
REG
(5.0V)
ENABLE
GND
+
R5
Terminal blocks J1 and J2 may be removed by pulling straight up.
Doc.# DSDB-SR087DB1
A032014
Supertex inc.
www.supertex.com
SR087DB1
Connections:
V
IN
-
Connect
to a 120/230VAC line. The AC input is pro-
tected by a 275V metal oxide varistor (MOV) and a 1.25A
slow-blow fuse.
Do not use a transformer on the input line. The high induc-
tance creates inductive kickback, which can overload the
MOV.
The SR087DB1 will not operate from uninterruptible power
supplies having a square wave output. This type of output is
usually referred to as “modified sine wave”.
regulates the peak output voltage, not the average, thus the
average voltage will decrease under load.
V
OUT
is adjustable from 9.0 to 50V. See the modifications
section on page 3. A source-only external voltage may be
applied to this output.
Do not connect earth-grounded loads or instruments!
vided, connecting this point to an earth-grounded instrument,
(such as an oscilloscope), will short the AC line, resulting in
circuit and/or instrument damage.
Also note that GND may be at a higher potential with respect
to earth ground, even if the AC is switched off.
Use cau-
tion!
GND -
Circuit common. Since galvanic isolation is not pro-
V
REG
is the output of a linear regulator internal to the SR087,
which in turn is supplied by V
OUT
. Thus any load on V
REG
is ef-
fectively a load on V
OUT
. V
REG
requires at least 4.0V of head-
room to operate. Being a linear regulator, the SR087 will dis-
sipate power as I
REG
is increased, up to 460mW at 60mA.
high (>0.75 • V
REG
) disables Q
1
, shutting down V
OUT
and thus
V
REG
. However, if an external voltage is applied to V
OUT
when
disabled, V
REG
will continue to function and provide a 5.0V
output.
The input has 20kΩ pull-down. If unused, Enable should be
left unconnected or connected to ground.
V
REG
-
The regulated output, 5.0V nominal output voltage.
Enable -
A logic low
(<0.2V) enables Q
1
switching. A logic
V
OUT
-
The main output. No-load output voltage is 12.6V.
Because the V
OUT
storage capacitor (C3) recharges twice
per input cycle, this output will have a certain amount of
100/120Hz ripple, depending upon the load. The SR087
Schematic
V
IN
90 to 270VAC
50/60Hz
F
1
1.25A
V
IN
Z
1
275V
50A
D
1
1.0kV, 1.0A
R
1
200kΩ
R
2
200kΩ
R
4
510kΩ
1
VIN
EN
R
7
20kΩ
2
EN
Q
1
STGD5NB120SZ
C
2
1.0µF
C
5
not installed
R
8
0Ω
C
1
100nF
8
GATE
7
VGD
6
VOUT
FB
VREG
5
4
V
REG
R
6
12.4kΩ
C
3
470µF
V
OUT
12.6VDC
R
3
510kΩ
optional EMI
supressor
R
5
113kΩ
GND
do not
connect to
earth
ground!
SR087
GND
3
C
4
5.0VDC
100nF
Doc.# DSDB-SR087DB1
A032014
2
Supertex inc.
www.supertex.com
SR087DB1
Output Voltage
V
OUT
may be adjusted in the range of 9.0 to 50V
1
by chang-
ing feedback resistor R
5
according to the following equation.
Keep in mind that the peak output voltage is regulated, not
the average, therefore the average output voltage will de-
crease (and ripple increase) with increased loads.
R
5
= R
6
• [(V
OUT
/1.25V) - 1]
EQ. 1
Modifications
R
2
and R
4
are employed to ensure adequate creepage dis-
tances and resistor voltage ratings for 230VAC operation.
For 120VAC-only applications they may be omitted (replaced
with shorts), with R
1
= 1.0MΩ
and R
3
= 430kΩ.
Conducted EMI
As supplied, the SR087DB1 meets 47CFR Part 15 Class B
and CISPR 14-1 requirements for conducted emissions at
combined load currents (I
OUT
+ I
REG
) of less than 20mA. For
higher load currents, an RC network (R
8
& C
5
) can be used
before the IGBT gate to slow down the falling edge. This has
the effect of reducing harmonics, lowering conducted EMI.
Suggested values of 22kΩ and 390pF allow the SR087DB1
to meet EMI requirements at combined load currents of
60mA for 120VAC and 40mA for 230VAC.
Slowing down the falling edge results in the output voltage
overshooting the intended regulation point, causing the peak
V
OUT
voltage to be higher than expected. This may be com-
pensated for by entering a slightly lower value for V
OUT
into
equation 1 when selecting feedback resistors. Slowing down
the falling edge also increases switching losses, resulting in
slightly decreased efficiency.
R
3
+ R
4
Equation
√ 2V
IN2
- V
X2
- V
X
cos
-1
(R3 + R4) <
V
X
√2 V
IN
Leave R
6
at 12.4kΩ - it assures a minimum 100µA load re-
quired for the proper operation of the SR087. Change R
3
and R
4
according to the equation 4. It may be necessary to
replace C
2
and C
3
with capacitors having a higher voltage
rating. For C
3
, use a low ESR capacitor with an adequate
ripple current rating (800mArms). Use ceramic for C
2
.
Since V
REG
is a linear regulator supplied from V
OUT
, the maxi-
mum current available from V
REG
is reduced as V
OUT
is in-
creased due to power considerations.
I
REG(MAX)
=
1.5W
V
OUT
- 3.3V
EQ. 2
or 60mA, whichever is less.
Output Ripple
Storage capacitor C
3
was sized to provide about 2V
P-P
ripple
at 100mA combined load (I
OUT
+ I
REG
). For lighter loads, C
3
may be reduced. Conversely, C
3
may be increased for low-
er ripple. Use a low ESR capacitor with an adequate ripple
current rating (measure using a current probe). Efficiency
and output current capability may drop with increased ca-
pacitance because of a smaller conduction angle associated
with lower ripple. Due to feedback hysteresis, ripple cannot
be reduced below 4%.
V
RIPPLE (P - P)
≈
I
OUT
+ I
REG
2f
IN
C
3
EQ. 3
π • 25µA
Eq. 4
where V
X
= V
OUT
+ 15V
Use the minimum anticipated RMS value for V
IN
. Take re-
sistor tolerance into account, selecting the next lower stan-
dard value. Choosing a lower value has no effect other than
slightly higher standby power.
Input Voltage
To reduce standby power for 230VAC-only applications, or
for supply voltages less than 90Vrms, R
1
and R
4
should be
changed according to equation 4 below. R
1
+ R
2
should re-
main at 400kΩ.
Note:
1. V
REG
requires at least 4.0V of headroom to provide 3.3V. Therefore V
OUT
, including ripple and tolerance, must not fall below 7.3V.
Doc.# DSDB-SR087DB1
A032014
3
Supertex inc.
www.supertex.com
SR087DB1
Bill of Materials
Item #
Description
Value
Tol
Rating
Package
Manufacturer
Manufacturer’s Part
Number
F1
Z1
D1
R1, R2
R3, R4
R5
R6
R7
R8
C1
C2
C3
C4
C5
Q1
IC1
Fuse, slow-blow
MOV
Diode bridge
Resistor
Resistor
Resistor
Resistor
Resistor
not installed
Capacitor, cer X7R
Capacitor, cer X7R
Capacitor, alum, lo ESR
Capacitor, cer X7R
not installed
IGBT
Power supply controller
1.25A
275V
-
200kΩ
510kΩ
113kΩ
12.4kΩ
20kΩ
0Ω
100nF
1.0μF
470μF
100μF
-
-
-
-
-
-
5%
5%
1%
1%
1%
-
10%
10%
20%
10%
-
-
-
250V
50A
1.0kV, 1.0A
200V
200V
-
-
-
-
25V
25V
25V, 800mA
1
16V
-
1.2kV, 5.0A
-
5x20mm
14mm
-
1206
1206
0805
0805
0805
0805
0805
1206
-
0805
0805
D-pak
Power
8-Lead SOIC
any
any
any
any
any
any
any
any
any
any
any
Nichicon
any
-
ST
Supertex
-
-
-
-
-
-
-
-
-
-
-
UHE1E471MPD6
-
-
STGD5NB120SZ
SR087SG-G
Note:
1. At 100/120Hz
Supertex inc.
does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.”
Supertex inc.
does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the
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