STAR*TECH Journal
July 1987 (4)
Inside Switching Regulators
(Part 4)
By Janes Beck
Greater southern Distributing Co.
Atlanta, Georgia
Vsuffl!!I
OK, what is control logic? In our case it
is a pulse width IOOdulator (PWM). What's a
pulse.width IOOdulator? Figure 1 is. What
does it do? It responds to a change on its
input by changing the duty cycle of its
output.
ERIIQR .AMPLIFIER
If you will look at figure 1 you will
notice that the circuit is made from two
OP amps. One is used as an error amplifier
and the other is the IOOdulator. The error
amplifier uses an input voltage sampled
from the voltage we want to regulate and a
reference voltage.
As in linear regulators, the reference
voltage is a "rock steady" constant that
is used to conpare to the output voltage,
so corrections can be made. The IOOdulator
is another OP amp that does not have any
negative feedback, this means it is acting
as a conparitor. It takes the output of
the error amp (marked Ve in figure l} and
conpares it to a sawtooth wave applied to
the "-" input.
In summary, a sample of the regulated
output is applied to the error amp. The
error amp then outputs a proportional
control voltage that is then applied to
the IOOdulator. The IOOdulator conpares the
control voltage to a sawtooth wave. When
the amplitude of the control voltage is
greater than the sawtooth wave, the
IOOdulator abruptly pulls its output to the
negative supply rail, and when tbe control
voltage is less than the sawtooth wave,
the IOOdulator swings to the positive rail.
-A_.,....,~__,-A....,co~-\-n,\yoltOl\e
V
's--...,-r-t"
In our situation the negative supply rail
would be ground. Figure 2 shows how the
control and sawtooth wave inputs effect
the output of the IOOdulator. This effect
is exactly what switch IOOde regulation is.
The IOOdulator output can then drive
current amplifiers and ultimately the main
power switching transistors.
That is all fine and dandy, but to get ou~
particular supply up and running we not
only need the pulse width modulator in
figure 1. We need a reference voltage,
sawtooth generator, circuitry to convert
the PWM output to two conplimentary
outputs for our switching transistors, and
current amplifiers.
1~~
1Nv
REF
INPUT INPUT OUT
NON
ll'IV
FEED
INV
INPUT BACK
IN P IJT
CXJTPUT
CONTROL Vee
DEAD
TIME
COITROl.
CT
Fis 3
CZ
E2
El
RT
GND
Ct