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Li’l
PowerHouse
A new 40V/1A switchmode
power supply with LCD readout
Is your old power supply so old it has
germanium transistors? Maybe it has Dymo®
labels on the front panel and a fabric-covered
power cord? Er, does it use a copper oxide
rectifier? If you have an ancient power
supply, now is the time to give it the heave-ho
and get this up to the minute design.
By PETER SMITH & LEO SIMPSON
56 Silicon Chip
T
HIS NEW POWER SUPPLY has
a big power output for its size.
It can give DC voltages up to
40V and the output current can be
as much as 1.2A, depending on the
voltage setting. And it can be varied
right down to less than 1.5V while
still giving out 1.2A. This is great for
testing battery circuits that operate at
1.5V, 3V or whatever.
In times past when we have looked
at doing a compact power supply, the
natural approach would be to produce
an analog design with an analog meter
on the front panel. An example of this
was our dual tracking 18.5V supply
published in the January 1988 issue.
The analog approach has the virtue of
simplicity and it gives good results.
But that’s in the past. And it is
boring. Nowadays we can do a lot
better with a switchmode design. It
is more efficient so big heatsinks are
unnecessary and you can get a higher
maximum DC voltage output for a given secondary voltage from the power
transformer. And you can get a lower
minimum DC voltage at full current
with having problems with the high
power dissipation of a conventional
series regulator.
In fact, this circuit will have
losses of less than 10W (including
transformer losses) under worst case
conditions, meaning that it does not
need any heatsink apart from that
provided by the back panel of the
case. By contrast, if we had gone to
a conventional regulator using the
same power transformer, it would
have losses (ie, heat) of around 30W
when delivering 1.5V at 1.2A and it
would need a fairly substantial finned
heatsink on the rear of the case.
Another good feature of the design
is the low level of ripple and hash
in the output, and this is not always
the case with switchmode designs.
We have achieved this with critical
attention to the circuit layout and two
stages of LC filtering.
Digital panel meter
The new supply has a 3.5-digit
LCD panel to monitor the voltage or
current, as selected by a toggle switch.
As well, you can set the current limit
by pressing a button on the front panel
and then rotating the knob closest to
the LCD panel.
A 10-turn potentiometer lets you
precisely set the output voltage which
can be done before you connect the
output load by means of the load
switch. In addition, there is a LED
on the front panel, just next to the
“set current” knob, to indicate when
an overload occurs. The supply is
protected against short circuits by
the way.
The voltage vs current characteristic is shown in the graph of Fig.1. As
it shows, the supply will deliver 1.2A
over the range from less than 1.5V to
30V. At higher voltages, the available
current drops off because the transformer is only rated at 30VA which
means that it could only deliver 1A
at 30V if the circuit had no losses
at all. In fact, we are over-rating the
transformer to get 1.2A at 30V but it
Fig.1: the voltage vs current characteristic of the supply. It is capable of
delivering 1.2A over the range from 1.23V to 30V. Beyond that, the current
falls off due to the transformer regulation.
quite good too. In fact, apart from the
output noise and ripple, the perfor
mance is actually a little better than
our previous 40V 3A power supply
published in the January & February
1994 issues.
Based on switcher
Fig.2: how a switching regulator
operates. When S1 is closed and
S2 is open, current flows to the
load via L1 which stores energy.
When S1 opens and S2 closes, the
energy stored in L1 maintains the
current through the load until the
switches toggle again.
does not appear to be a problem – the
transformer appears to be conservatively rated at 30VA.
At 40V, the output current from the
prototype supply was around 160mA
which is pretty respectable for a small
supply.
As shown in the specifications panel, the load regulation of the circuit
is excellent and the line regulation is
The circuit is based on the National Semiconductor LM2575HVT
high voltage adjustable switchmode
voltage regulator. This is almost
identical to the switcher chip used in
the 40V/3A power supply mentioned
above. However, this new 40V/1A circuit is not simply a cut-down version
of the 1994 design; apart from the use
of the switcher chip, it is different in
a number of aspects.
Let’s have a brief look at how the
switcher works. Fig.2 shows how
a switching regulator operates. In
operation, S1 and S2 operate at high
speed and are alternately closed and
opened. These two switches control
the current flowing in inductor L1.
When S1 is closed and S2 is open, the
Main Features
•
•
•
•
•
•
•
•
•
•
•
Output voltage continuously variable from 1.23V to 40V
Output current of 1.2A from 1.23V to 30V
LCD panel meter for voltage & current
10-turn pot for precise voltage adjustment (optional)
Adjustable current limit
LED current overload indication
Output fully floating with respect to mains earth
Load switch
Low output ripple
Short circuit and thermal overload protection
Minimal heatsinking
JUNE 2000 57
Fig.3: a basic regulator
using the LM2575 switcher
IC. In this circuit, a
switching transistor takes
the place of S1 in Fig.1 and
diode D1 takes the place of
S2. The output voltage is
set by the ratio of R2 & R1
which feed a sample of the
output voltage back to an
internal comparator.
current flows to the load via inductor
L1 which stores up energy. When S1
subsequently opens and S2 closes,
the energy stored in the inductor
maintains the load current until S1
closes again.
The output voltage is set by adjusting the switch duty cycle – the
longer S1 is closed and the current
flows through it, the higher will be
the output voltage.
Fig.3 shows a complete voltage regulator based on the LM2575 IC. It is a
5-pin device which requires just five
extra components to produce a basic
working circuit. Its mode of operation
is the same as that described in Fig.2
except that here an internal switching
transistor is used for S1, while an
external diode (D1) is used for S2.
What happens in this case is that
when the transistor is on, the current
flows to the load via inductor L1 as
before and diode D1 is reverse-biased.
When the transistor subsequently
turns off, the input to the inductor
swings negative (ie, below ground).
D1 is now forward-biased and so the
current now flows via L1, through the
load and back through D1.
The output voltage is set by the ratio
of R2 and R1 which form a voltage
divider across the output (Vout). The
sampled voltage from the divider is
fed to pin 4 of the switcher IC and
then to an internal comparator where
it is compared with a 1.23V reference.
This sets Vout so that the voltage
produced by the divider is the same
as the reference voltage (ie, 1.23V).
Apart from the comparator and the
switching transistor, the regulator IC
Specifications
Minimum no load output voltage........................................................ 1.23V
Maximum no load output voltage.......................................................... 40V
Output current............................................................................... see Fig.1
Current limit range.................................................................. 10mA to 1.2A
Current limit resolution........................................................................10mA
Line regulation..............................0.1% for a 10% change in mains voltage
Voltmeter resolution..........................................................................100mV
Current meter resolution.......................................................................1mA
Meter accuracy......................................................................2% plus 1 digit
Load regulation
no load to 1A <at> 24V.......................................................................1.2%
no load to 1A <at> 12V.......................................................................1.5%
no load to 1A <at> 6V.........................................................................1.8%
no load to 1A <at> 3V.........................................................................3.3%
Output noise and ripple
3V to 24V <at> 1A............................................................ 25mV p-p (max)
58 Silicon Chip
also contains an oscillator, a reset
circuit, an on/off circuit and a driver
stage with thermal shutdown and
current limiting circuitry.
The incoming supply rail is applied
to pin 1 of the IC and connects to the
collector of the internal switching
transistor. It also supplies an internal regulator stage for the rest of the
regulator circuit.
In essence, the LM2575 uses pulse
width modulation (PWM) to set the
output voltage. If the output voltage
rises above the preset level, the duty
cycle from the driver stage decreases
and throttles back the switching transistor to bring the output voltage back
to the correct level. Conversely, if the
output voltage falls, the duty cycle is
increased and the switching transistor
conducts for longer periods.
The internal oscillator operates
at 52kHz ±10% and this sets the
switching frequency. In theory, this
frequency is well beyond the limit
of audibility but in practice, a faint
ticking noise may be audible due to
magnetostrictive effects in the cores
of the external inductors.
One very useful feature of the
LM2575 is the On/Off control input at
pin 5. This allows the regulator to be
switched on or off using an external
voltage signal and we have used this
to provide the adjustable current limiting feature, as we shall see later on.
Circuit details
Fig.4 shows the full circuit of the
new power supply.
Transformer T1 is supplied with
mains power via fuse F1 and power
switch S1. Its 30VAC secondary is
JUNE 2000 59
2000
10F
16VW
+5.1V
C-
1
5
-5.1V
10F
16VW
LED
A
K
-5.1V -5.1V
4 x 0.1F
GND
3
LM2575
4
IC5
ICL7660
8
V+
2
5
C+
OUT
0V
30V
D1-D4
1N4002
100
100
2
3
4
VR3
10k
6
1
4
100k
100k
1
2
1
OUT
1k
100k
2
2
1
6
5
2
3
5
METER ZERO
VR5
100k
1
4
IC4
TL071
7
+5.1V
-5.1V
6
S4: 1 - MEASURE CURRENT
2 - SET CURRENT
1M
100k
100k
1M
330pF
5
8
7
2
300
RFH
9
10
ROH
11
DP3
.8.8.8
6
1M
27k
VOLTS CAL
VR4
5k
0.1F
63VW
13
2
1
V-
V+
S3b
2
1
+
EARTH
_
OUTPUT
1.23-40V
1A
+5.1V
CASE
0.1F
250VAC
0.33F
63VW
LOAD
S2
DP1
Q0570 DIGITAL PANEL METER
1
INLO
COM
RFL
INHI
4
IC3b
LM393
470
8
2 x 47F
63VW
+
LM336-2.5
REF1
_
D6
1N4148
3
*SEE TEXT
*R1
0.005
L2
47H
OVERLOAD
LED1
3 x 470F
63VW
1k
S3: 1 - MONITOR CURRENT
2 - MONITOR VOLTAGE
7
4.7k
680
5W
L1
470H
D5
MBR360
IC3a
LM393
+2.5V
+5.1V
5
100k
S4a
2
S3a
3
FB
ON/
GND OFF
IN
IC1
LM2575HVT-ADJ
CURRENT
LIMIT
VR2
1k
S4b
OP77GP
0.1F
1.5k
CURRENT
CAL
-5.1V
IC2
7
330F
63VW
15k
2 x 2200F
50VW
VOLTAGE
ADJUST
VR1
50k
40V/1A ADJUSTABLE POWER SUPPLY
_ + ADJ
LM336Z
10F
16VW
+5.1V
100F
16VW
ZD1
5.1V
1W
1k
5W
CASE
POWER
S1
250VAC
T1
M6672L
Fig.4: the circuit is based on IC1, the LM2575 switcher controller. It runs at around 50kHz and the resultant DC
output is filtered with inductors L1, L2 and the associated capacitors. IC2 & IC3 provide the current limit feature
while IC4 drives the LCD panel meter.
SC
E
N
240VAC
A
F1
500mA
Parts List
1 PC board, code 04106001, 171
x 127mm
1 M6672L 30V 30VA mains
transformer
1 DPDT 250VAC 6A plastic rocker
switch with neon indicator (S1)
2 S1345 DPDT miniature toggle
switches (S2,S3)
1 DPDT momentary pushbutton
switch (S4)
1 LCD panel meter (Altronics
Q-0570)
1 M205 panel-mount safety
fuse-holder (F1)
1 500mA M205 fuse
1 470µH toroid inductor (L1)
1 47µH toroid inductor (L2)
1 TO-220 insulating bush and
washer
2 15mm knobs
3 captive binding post terminals
(1 red, 1 green, 1 black)
1 cordgrip grommet for mains
cable
1 13-way 2.54mm SIL header plug
(for connection to panel meter)
4 3.2mm solder lugs
22 PC stakes
Hardware for pre-punched
metal case
1 pre-punched metal case
3 M4 x 10mm screws
4 M4 nuts
2 M4 internal star washers
8 M3 x 6mm screws
1 M3 nut
1 M3 flat washers
4 10mm tapped spacers
Hardware for plastic
instrument case
1 plastic case, 200 x 155 x 65
mm (W x D x H) with metal front
and rear panels (Altronics Cat.
H-0481F & H0484F)
2 M3 x 10mm screws
1 M3 x 15mm countersunk screw
5 M3 nuts
1 M3 flat washer
4 M3 internal star washers
2 M4 x 10mm screws
2 M4 nuts
2 M4 flat washers
4 self-tapping screws (to mount
PC board)
Semiconductors
4 1N4002 1A 100V diodes (D1-D4)
60 Silicon Chip
1 MBR360 3A Schottky diode (D5)
(SR306 or 31DQ06 also
suitable)
1 1N4148 small signal diode (D6)
1 LM2575HVT-ADJ high voltage
switchmode controller (IC1)
1 OP77GP op amp (IC2)
1 LM393 dual comparator (IC3)
1 TL071 op amp (IC4)
1 ICL7660 switched capacitor
voltage inverter (IC5)
1 LM336Z-2.5 voltage reference
(REF1)
1 1N4733 5.1V 1W zener diode
(ZD1)
1 3mm red LED with bezel (LED1)
Resistors (0.25W, 1%)
3 1MΩ
2 1kΩ
6 100kΩ
1 1kΩ 5W
1 27kΩ
1 680Ω 5W
1 15kΩ
1 470Ω
1 4.7kΩ
1 300Ω
1 1.5kΩ
2 100Ω
Potentiometers
1 50kΩ 16mm linear pot (VR1)
OR 1 50kΩ multi-turn linear pot
1 1kΩ 16mm linear pot (VR2)
1 10kΩ horizontal trimpot (VR3)
1 5kΩ horizontal trimpot (VR4)
1 100kΩ horizontal trimpot (VR5)
Capacitors
2 2200µF 50VW PC electrolytic
3 470µF 63VW PC electrolytic
1 330µF 63VW PC electrolytic
1 100µF 16VW PC electrolytic
2 47µF 63VW PC electrolytic
3 10µF 16VW PC electrolytic
1 0.33µF 63VW MKT polyester
6 0.1µF 63VW MKT polyester
1 0.1µF 250VAC MKT polyester
1 330pF MKT polyester
Wire and cable
1 2-metre 250VAC mains lead with
3-pin plug
1 600mm length of green/yellow
mains wire
1 200mm length of 13 way ribbon
cable
1 60mm length of 0.4mm
enamelled copper wire
Miscellaneous
Cable ties, heatshrink tubing, heatsink compound, solder, hook-up
wire.
full-wave rectified using diodes D1D4 and filtered using two paralleled
2200µF 50VW elec
trolytic capacitors. The resulting 42V DC supply
is applied to the switching regulator
(IC1). The additional 330µF capacitor
connected between pins 1 & 3 of IC1
is included to prevent circuit instability and is mounted as close to the
IC as possible.
Diode D5, inductor L1, the three
470µF capacitors and potentiometer VR1 form the basic switchmode
power supply block (see Fig.4). D5 is
a 3A Schottky diode which has been
specified instead of a conventional
fast recovery diode because of its low
forward voltage drop. As a result,
there is very little heat dissipation
within the diode and this leads to
increased efficiency.
The output from IC1 feeds directly into L1, a 470µH induc
tor. The
10-turn potentiometer VR1 and its
associated 1.5kΩ resistor provide
voltage feedback to pin 4 of IC1, to
set the output level. When VR1’s
resistance is at 0Ω, the output from
the regulator (pin 2) is equal to 1.23V.
This output voltage increases as the
resistance of VR1 is increased. The
680Ω 5W resistor connected across
the regulator output discharges the
three 470µF capacitors to the required
level when a lower output voltage is
selected.
2nd filter circuit
Inductor L2 and its associated 47µF
and 0.1µF capacitors provide a second
stage of filtering to further attenuate
the switching frequency ripple. The
resulting filtered voltage is then
applied to the output terminals via
load switch S2. Additional filtering is
applied at this point using a 0.33µF
capacitor across the terminals and a
0.1µF capacitor between the negative
terminal and the case.
Current limiting
The current sense resistor (R1) is
wired into the negative supply rail
adjacent to inductor L2 and consists
of a short length of 0.4mm enamelled
copper wire. The voltage developed
across it is multiplied by 200 using op
amp IC2, so that IC2’s output delivers
1V per amp of load current.
IC2 is specified as an OP77GP
which has the required low input
offset voltage (typically 50µV) and a
very low input bias current (typically
Despite the relative circuit
complexity, the power supply
is easy to build. This view
shows the prototype PC board
for the supply, with all parts
in place. The full assembly
details will be published in
next month’s issue.
1.2nA – that’s nanoamps!) This is
necessary to ensure that IC2’s output
is at 0V when no current is flowing
through R1.
Because its inputs operate at close
to ground potential (ie, 0V), IC2 must
be powered from balanced positive
and negative supply rails. The +5.1V
rail for IC2 (and for the remaining
ICs) is derived from the output of
the bridge rectifier via a 1kΩ resistor
and 5.1V zener diode ZD1. For the
negative rail we use IC6, an ICL7660
switched capacitor voltage converter
which operates at 10kHz to provide a
-5.1V supply.
Comparator stage IC3a monitors the
output voltage from IC2 and compares
this with the voltage on its inverting
input, as set by the current limit
control VR2. This 1kΩ potentiometer
and its associated 1kΩ resistor form
a voltage divider network which is
connected across the 2.5V reference,
REF1.
In operation, VR2 sets the voltage
on pin 6 of IC2 at between 0V and
1.25V, corresponding to current limit
settings of 0-1.25A. Because IC3a is
an open collector device, its output
at pin 7 is connected to the +5.1V rail
via a 4.7kΩ pull-up resistor. If the
voltage at the output of IC2 is greater
than that set by VR2, pin 2 of IC3a is
pulled high by this resistor. This also
pulls pin 5 of IC1 high and switches
off the regulator to provide current
limiting.
At the same time, pin 2 of IC3b is
pulled high via diode D6 and so pin
1 switches low and LED1 lights to
indicate current limiting or an overload condition.
When the current subsequently
falls below the preset limit, pin 7
of IC3a switches low again and the
regulator turns back on. Thus, IC3a
switches the regulator on and off at a
rapid rate to provide current limiting.
The 1MΩ resistor and 330pF capacitor at pin 2 of IC3b provide a small
time delay so that LED1 is powered
continuously during current limiting.
Digital panel meter
The LCD panel meter we’re using
for this circuit has simpler interfacing
requirements than those we have used
in the past. It requires a +5V supply
which comes from ZD1 and the resistors across its pins 5,6,7 & 8 configure
it to read 2V full scale (or 1.999V to
be precise). Op amp IC4 is connected
as a unity gain amplifier with level
shifting to provide for an offset at the
input of the LCD panel meter.
The non-inverting input (pin 3) of
IC4 takes its DC input from switch S3
and S4 (current limit set) to monitor
the current or voltage output. The
current monitoring is simple because
the output of op amp IC2 is 1V per
amp, as already discussed. For the
voltage output, we use a voltage divider consisting of a 27kΩ and 300Ω
resistors in series with 5kΩ trimpot
VR4 which is used for calibration.
The second pole of switch S3 selects the decimal points on the LCD
panel meter so that it can read up to
1.999A in current mode and 199.9V
in voltage mode. In practice, the
maximum reading will be around
1.2A in current mode and 40.0V in
voltage mode.
This means that we have more meter resolution in current mode than in
voltage mode. More resolution could
be obtained by range switching for the
panel meter but we wanted to keep the
circuit as simple as possible.
Next month we will present the full
constructional details of the power
SC
supply.
JUNE 2000 61
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