SR037DB1
Inductorless Switching Power Supply
22MAR02
1
Introduction
The Supertex SR037 is an inductorless switching power
supply controller intended for 120/240VAC line operation.
The operating principle is to activate a pass element
between line and load only when the input voltage is lower
than a certain value. Thus conduction only occurs with a
low voltage drop across the pass element, resulting in more
efficient operation compared to a standard linear regulator.
The SR037DB1 demo board contains all the circuitry
needed to supply a low-current (<40mA) load. Included is
input protection circuitry, a full wave bridge rectifier, a pass
element, and output capacitors.
Specifications
AC Input
24VAC to 264VAC
40Hz to 100Hz
Output Voltages
V
UNREG
V
REG
12V to 22V
5.0V ±10%
Output Current
I
UNREG
+ I
REG
40mA @120VAC
30mA @ 240VAC
Efficiency
AC
IN
=120VAC 60Hz, I
U
=40mA, I
R
=0mA
40% typ
Board Layout and Connections
AC
IN
Connect to the AC line. The AC input is protected by a 400V
transient voltage suppressor and a 250mA fuse.
GND
Circuit common.
Note that since galvanic isolation is not provided, connecting
this point to an earth-grounded instrument (such as an
oscilliscope) 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!
Do not connect to earth ground. This ground must
remain floating when supplied from the AC line!
V
UNREG
The unregulated output. Nominal output voltage is 18 volts.
This output will have a certain amount of 120Hz ripple,
depending on load.
Do not connect to earth-grounded loads.
V
REG
The regulated output. 5.0V nominal output voltage.
Do not connect to earth-grounded loads.
Z
1
F
1
D
1
IC
1
C
1
C
2
Q
1
V
REG
V
UNREG
GND
WARNING!!!
Do not connect to scope ground or to the ground of other
earth-grounded instruments. Doing so will short the AC line,
resulting in damage to the circuit and/or instrument. Either
use an isolation transformer on the AC line, use a differential
probe, or use a floating, battery-powered instrument to make
measurements.
WARNING!!!
No galvanic isolation. Dangerous voltages are
present when connected to the AC mains.
AC
IN
SR037DB1
Inductorless Switching Power Supply
22MAR02
2
SR037 Schematic
Parts List
Desig Desc
Part
Number
F
1
Fuse, 3AG, 250mA, 250V
Z
1
Transient voltage suppressor, 400V
P6KE400CA
D
1
Diode bridge, 1A, 600V
IC
1
HV Supply Controller
Supertex SR037SG
Q
1
MOSFET, 600V N-channel
Supertex VN2460N8
C
1
Capacitor,alum, 220DF, 35V
C
2
Capacitor, cer, 1
D
F, 16V
Modifications
The SR037DB1 may be modified to provide higher output current. Remove Q1 and replace with a D-Pak N-channel MOSFET with
a voltage rating appropriate for the input voltage. The on resistance of the replacement FET is not as important as the gate
threshold voltage the lower the threshold voltage, the more efficient the converter.
To reduce ripple at the higher output current, C1 may be replaced with a higher value capacitor.
SR037
VN2460
220DF
1DF
V
UNREG
V
REG
120/240VAC
50/60Hz
250mA
P6KE400CA
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