externally. Please note that it would be
impossible to label the full net names on
the board and an acronym must be used
instead. For instance: “To Slave Adjust” is
replaced with the acronym TSA. This same
acronym is used on the schematic itself.
Building the Circuit
■ FIGURE 5
The best way to build a switchmode
power supply is by employing a proper
PCB; an example of this is available on
the Nuts & Volts website ( www.nuts
volts.com). This is a combination
SMT/thru-hole board which obtains the
key benefits of both.
You’ll need a good multimeter that can also read
frequency and a power resistor outlined next. An
oscilloscope is not strictly necessary but certainly useful in
observing the overall performance. A variac is also useful.
Since SMT devices are involved, a fine-pointed iron,
tweezers, and a good magnifying glass are required. This
is important as the main reason that a project like this will
not work is incorrect soldering.
Please note that there are a few precision 0.1%
resistors. These are critical for proper current sharing. An
alternative is the more common 1% resistors which could
be sorted with a good 4-1/2 digit multimeter. Additionally,
ensure that the TLC081 is the “A” suffix version, as it has a
lower offset voltage that the standard part at a cost of a
few additional pennies.
The task in assembling this project can be divided into
the following milestones:
1) Wire the main transformer, rectifier bridge, and filter
capacitor. This will be your source. Test it by itself;
it will provide approximately 25 volts.
2) Solder only all the master components to the master
board. All are SMT devices.
3) With the subassembly from Step 1, power up the
master board, make sure that +15V is present, and that
the complementary oscillator sync pulses are present.
Adjust R8 for approximately 120 kHz.
PARALLEL THREE-TERMINAL REGULATORS
Adjustable three-terminal regulators have been around by the same amount.
for many years due to their versatility, high performance, Being a linear supply, it will dissipate heat — lots of it.
and simple — almost foolproof — operation. For audio Mount U1 and U2 in a large common heatsink such that
applications, they are ultra-quiet. However, since they they track thermally. A fan is necessary, as under worst case
dissipate substantial amounts of heat, a high output power conditions the circuit will dissipate close to 40 watts.
level almost always requires paralleling devices. Of course, a single LM338 could have been used, but the
National Semi immediately recognized this fact and techniques learned here may also be used to parallel a pair
designed some circuits to achieve this. The circuit of Figure A of those devices.
comes straight out from National’s
databook, with a couple of changes.
First, I learned a while back that diode
D1 (which was not required in the app
note) is necessary to prevent
catastrophic failure in case one of the
regulators decides to shut down. This
shutdown causes the regulator that
has shut down to sink current from the
output, often leading to a hard failure.
The diode is a dual in a common
package, ensuring tight forward voltage
Secondly, the LM307 op-amp
from the original app note has been
discontinued. I’ve used the LF256
successfully as a replacement. Please
note that if you decide to employ the
cheaper version LF356, its maximum
input voltage is eight volts lower and,
therefore, both the supply’s input and
■ FIGURE A
output voltages have to be scaled down
May 2009 51