volts. Looking at Figure 6, move across the bottom axis to
the one amp vertical line. Then follow that line up to the
40 volt horizontal line. That puts you into a region marked
220 μH, almost into the 330 μH region. Note that higher
input voltage requires greater inductance. I’m not actually
going up to 40 volts, so a 220 μH should do the job.
(So would 330 μH; it doesn’t hurt to go a bit larger.)
We’re looking for a power inductor; one that will
keep its value with one amp flowing through it. Such an
inductor is sometimes called a choke. You can obtain
suitable power inductors from vendors such as Jameco.
(You can also make them yourself, but that’s another
article!) I used a surplus drum choke from All Electronics
(part #CR-220). The inductor (shown in Figure 7) is 220
μH and was made by Coilcraft (part #TV1363-B).
Diodes
As mentioned before, diode D1 protects against
reverse polarity input voltage. A 1N4001 or similar diode
will do the job. Diode D2 is the commutating (or catch)
diode. As discussed, the catch diode (also called a
freewheeling diode) provides a closed loop for the inductor
current when the switch opens. In a switching regulator,
the switch opens and closes a lot faster than 60 times a
second. The LM2576 switches at 52 kHz; other regulators
switch at frequencies up ove a megahertz so the selection
of catch diode is important.
While diodes like the 1N4001 work fine at 60 Hz,
they don’t work so well at the high frequencies used in
switching regulators. A certain amount of capacitance is
associated with a reverse-biased diode. When going from
off to on (conducting), that capacitance needs to be
discharged. And when going from on to off, that
capacitance needs to be charged. The time required for
a diode to switch from on to off is called the reverse
recovery time (trr). For a 1N4001, trr is about 30 μs.
But at 52 kHz, the cycle time is T = 1/( 52 x 103) which
is about 19 μs. What would happen if we used a 1N4001
as the catch diode in our circuit? With a recovery time
almost twice as long as the cycle time, the diode would
never stop conducting. We might as well replace it with a
piece of wire! Obviously, we need a faster diode. There
are several types of switching diodes designed to be used
as catch diodes; they have a small trr. One type commonly
used is a Schottky diode (also called a Schottky Barrier
rectifier). Not only are they fast, Schottky diodes also drop
less voltage than a
conventional diode
while conducting. In
this project, we will use
a 1N5819 Schottky
diode which has trr less
than 10 nanoseconds
and a forward drop of
0.6 volts at one amp.
For comparison, a
■ FIGURE 6
■ FIGURE 7
1N4001 diode has a forward drop of 1.1 volts at one amp.
Capacitors
Two electrolytic capacitors are required in our regulator,
C1 and C2. The function of C2 is to filter ripple from the
output voltage. Since we are switching at 52 kHz, the value
of C2 is smaller than what would be required to filter out
60 Hz ripple in a linear power supply. Figure 8 shows the
type of ripple C2 is filtering. On the other hand, the function
of C1 is to supply pulses of current as the LM2576 switches
on and off. Figure 9 shows how current would flow into
the LM2576. Note the fast rise time of the waveform.
Without C1, inductance in the wire between Vin and the
LM2576 would cause a voltage drop every time the
device needed current, and the circuit would be unstable.
As with diodes, not all electrolytic caps are the same.
Two important parameters for filter capacitors are ripple
■ FIGURE 8
■ FIGURE 9
June 2008 45
