SR037DB2
Inductorless Switching Power Supply
Introduction
The Supertex SR037 is an inductorless switching power
supply controller intended for operation directly from a
rectified 120/230VAC line. The operating principle is to
activate a pass transistor (operating as a voltage follower)
only when the input voltage is lower than 45V. Thus
conduction only occurs with a low voltage drop across the
pass transistor, resulting in more efficient operation
compared to a standard linear regulator.
The SR037DB2 demo board contains all the circuitry
needed to supply a low-current load. Included is input
protection circuitry, a full wave bridge rectifier, an IGBT pass
transistor, and output capacitors. Simply connect the demo
board to an AC line. Provisions are made for modifying the
demo board for various applications.
Specifications
AC Input
Output Voltages
V
UNREG
V
REG
Output Current
I
REG
I
REG
+ I
UNREG
No-load Input Power
Efficiency
referenced to V
UNREG
24VAC to 270VAC
40Hz to 70Hz
11V to 19V
5.0V ±10%
50mA @ 120VAC
20mA @ 230VAC
90mA @ 120VAC
40mA @ 230VAC
125mW typ @ 120VAC
580mW typ @ 230VAC
38% typ @ 120V/90mA
31% typ @ 230V/40mA
Board Layout and Connections
Q
1
–
C
2
AC
IN
ZNR
1
~
D
1
~
+
F
1
IC
1
C
4
R
1
C
3
V
UNREG
V
REG
GND
C
1
WARNING!!!
Do not connect earth-grounded test instruments or
loads. Doing so will short the AC line, resulting in
damage to the instrument, load, or the SR03
circuit. Use floating instruments or differential
probes. Do not use a transformer on the AC line.
WARNING!!!
No galvanic isolation. Dangerous voltages
are present when connected to the AC line.
AC
IN
Connect to the AC line. The AC input is protected by a 275V
transient voltage suppressor and a 500mA fuse.
Do not use a transformer, either variable of fixed, on the AC
line. The high inductance of a transformer interferes with the
normal operation of the SR037.
V
UNREG
Unregulated output. Nominal output voltage is 17V unloaded.
At full load, output voltage drops to 14V typ at 120VAC and
12V typ at 230VAC. This output will have about 1.3V
P-P
of
100/120Hz ripple at full load.
Do not connect earth-grounded loads.
GND
Circuit common.
Note that since galvanic isolation is not provided, 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 caution!
V
REG
The regulated output, nominally 5 volts. Output current
capability is 50mA at 120VAC and 20mA at 230VAC. This
output is supplied by a linear regulator internal to the SR03,
which in turn is supplied by the unregulated output.
Do not connect earth-grounded loads.
A080504
A080504
Supertex, Inc.
•
1235 Bordeaux Drive, Sunnyvale, CA 94089
•
Tel: (408) 222-8888
•
FAX: (408) 222-4895
•
www.supertex.com
1
SR037DB2
Inductorless Switching Power Supply
SR037DB2 Schematic
F
1
500mA
D
1
ZNR
1
275V
Q
1
GN2470
C
2
1µF
AC
IN
24–270VAC
50/60Hz
V
UNREG
C
1
470µF
C
4
not installed
R
1
0
8
7
EMI suppressor for
MOSFET implementations
Gate
1
HV
IN
Src
SR037
V
OUT 6
Gn d
4
V
RE G
C
3
1µF
trace may be cut and
zener installed for
higher output voltages
GND
Desig
F
1
ZNR
1
D
1
Q
1
R
1
C
1
C
2
C
3
C
4
IC
1
Desc
Fuse, 2AG slo-blo
Metal oxide varistor
Full wave bridge rectifier
IGBT
Zero ohm jumper
Capacitor, X7R
Capacitor, alum
Capacitor, X7R
Not installed
Inductorless power supply
controller
Value
500mA
275V
0Ω
1µF
470µF
1µF
Rating
250V
50A
1A, 800V
700V
--
50V
35V
15V
Mfg
any
any
any
Supertex
any
any
any
any
Supertex
PN
GN2470K4
SR037SG
Higher Output Voltage
The SR037DB2 demo board may be modified to provide
higher output voltages. Cut the ground connection to the
SR037 and install a zener in the 2 holes provided, with the
polarity shown. The zener may be low power (100mW).
zener
Constant Current Output
If a constant-current output is desired, such as for driving
LEDs, the circuit of Figure 9 on the SR03x data sheet may be
implemented by cutting the ground trace to the SR037 and
connecting the current regulator circuit shown in the data
sheet .
–
~
cut trace
D
1
~
+
Lower Output Current
IC
1
Both V
UNREG
and V
REG
will increase by the zener voltage.
Note that it may be necessary to replace the output
capacitors (C
1
– C
3
) with ones having higher voltage ratings.
If combined output current requirements (I
UNREG
+ I
REG
) are
less than 50mA at 120VAC or less than 20mA at 230VAC,
the D-Pak GN2470 IGBT may be replaced with a SOT-89
VN2460 MOSFET (pads provided). Since the VN2460 has a
lower voltage rating (600V vs. 700V), it will be necessary to
replace the transient voltage suppressor (ZNR
1
) with one
having a lower clamping voltage, such as a P6KE400CA.
Since MOSFETs have faster switching speeds than IGBTs,
conducted EMI will be greater. To slow the MOSFET
switching time, thus reducing EMI, replace R
1
with a 180k
Ω
resistor and install a 220pF, 25V capacitor at C
4
.
A080504
2
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