— by Robert Lang
CAN YOU GENERATE 200 VOLTS
DC FROM A FIVE-VOLT DC PIC
POWER SUPPLY? Yes, you can,
and I will tell you how. In this two-part
series, we will build a 200-volt DC boost
power supply driven by an 18F2455
Microchip PIC microprocessor. The
completed power supply (shown in
Figure 1) will generate between five and
200 volts DC from a five-volt, DC input.
This device can be used for any
application where high voltage and
low current is needed, such as
PIC programming ( 13 volts), neon
indicator bulbs (120 volts), or nixie
■ FIGURE 1. Completed boost power
tube displays (170 volts).
In this first part, we’ll cover the theory of how a boost power supply works.
You’ll learn how to use pulse width
modulation (PWM) to get the 18F2455
PIC to output a square wave with a given
period and duty factor, and we’ll look at
how the PWM’s “ON” time affects
voltage. We will cover the important
parameters in designing a boost power
supply and use the free LTSPICE program to come up with a possible design.
Next month, we will implement
this design on a printed circuit board.
We will use a liquid crystal display
(LCD) to show important power-supply parameters. Using a free C
compiler, we’ll write software to drive
A/D conversion, control PWM, and
output information to the LCD.
A boost power supply is a type of
switching power supply that generates
a higher output voltage than the
supply voltage. In addition to stepping
up the voltage, it can also change
the sign of the voltage input. These
tricks are very useful for generating a
local source of other voltages from a
standard + 5 VDC supply.
A boost converter can have as few
as four basic components: a semiconductor switch, a diode, an inductor,
and a capacitor. The semiconductor
switch may be a unipolar device, such
as a MOSFET, or a bipolar transistor.
The benefit of using a unipolar device
is the absence of stored carriers and,
therefore, theoretically instantaneous
switching transients that are limited
only by small parasitic capacitances.
If you are like me, you may not
have a large assortment of inductors in
your parts box and you may not use
them a lot in your circuits. Let’s take a
more detailed look at inductors.
Inductance (measured in Henries) is an
effect which results from the magnetic
field that forms around a current carrying conductor. Inductance can be
increased by looping the conductor into
a coil which causes magnetic flux from
adjacent loops of the conductor to link.
An inductor is usually constructed
as a coil of conducting material, typically copper wire, wrapped around a
core of ferrous material, which is
highly permeable to magnetic flux.
The coils and the core’s permeability
amplify the current’s magnetic field,
increasing inductance. Some small
inductors look similar to resistors and
use a similar color scheme. Other
inductors look like wire-wrapped
donuts (toroids), as shown in Figure 2.
Two (or more) inductors that have
coupled magnetic flux form a
transformer — a fundamental component of every electric power grid. An
inductor is used as the energy-storage
device in the boost power supply. The
inductor is energized for a specific
fraction of the regulator’s switching
frequency — the “ON” time — and
de-energized for the remainder of the
cycle. When we select the inductor to
use in our boost power supply, we
want to make sure the inductor can
handle the peak current. The peak
current in the inductor of the boost
power supply can be written as a
function of the input voltage, semiconductor ON time, and inductance:
Max_Current = Input_Voltage*
The basic circuit is shown in
Figure 3 and operates as follows:
1. The inductor current ramps up while
the MOSFET is conducting (TP voltage
near zero). When the MOSFET is conducting, the source voltage is applied
across the inductor, and the rate of rise
of inductor current depends on the
source voltage Vin and inductance L1.
If the source voltage remains constant,
the rate of rise of inductor current is
positive and remains fixed, so long as
the inductor has not saturated.