Current-Mode Control (CMC) has been a popular and
effective control technique for power converter systems for
many years. Traditional CMC systems employ pure analog
components. With the development of computer technology,
digital implementation of CMC systems is becoming a
practical approach.
In many power converter systems, the output voltage of
the power stage is sensed and sent to the controller. By
adjusting the duty cycle of the switch control signal, the
output voltage is regulated. Ideally, the output voltage is
identical to a reference voltage. This technique is known as
‘Voltage-Mode Control’ (VMC), since the duty cycle is
solely determined by the error between the actual output
voltage and the voltage reference. Another technique to
regulate the power converter systems is called Current Mode
Control (CMC) where the inductor current is directly
controlled and the output voltage is controlled only
indirectly. A CMC power converter is typically a two-loop
system (voltage loop and current loop). The current loop, in
which the inductor current is sensed as the main controlled
variable, monitors and maintains the switch current (or
inductor current) equal to a reference current. This reference
current is obtained from the voltage loop, which compares a
voltage reference to the output voltage of the power
converter.
CMC has been widely used in many high-performance
power supply applications in recent years, because CMC is
considered to be superior to VMC due to the fast inner
current loop. In a VMC power converter, any variation in
input voltage or output load must alter the output voltage
first, and then the controller can sense the change and react
to that change by adjusting the control effort. In a CMC
power converter, on the other hand, any variation in input
voltage or output load can be reflected in the inductor
current instantaneously. For this reason, CMC typically
responds faster than a VMC power converter.
A CMC power converter looks like a current source.
Therefore, voltage variation at the input does not go through
the output, so a CMC power converter is more immune to an
input disturbance than a VMC converter. This current source
characteristic also makes it easier to parallel current sharing
among several power stages. The power stages can be forced
to share the load current equally by simply connecting the
power stages to a common control voltage. This is very
valuable in high power applications. Another advantage is
that CMC converters have simpler dynamics. Their controlto-
output transfer function usually can be simplified to a first
order system, and the system can be stabilized with a simpler
compensation network around the error amplifier. In
addition, CMC provides inherent over-current protection,
since the inductor current is limited on a cycle-by-cycle
basis.
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