Controlling a higher power load with a lower power control circuit is an extremely common
electrical engineering task.
Some type of switching device, such as a transistor, is required to
control the current flow through the load, but if direct modulation is performed, this creates a
rather inefficient design with the control device, the transistor, often using more electrical power
than the load itself.
A much more power efficient control method is to rapidly switch the control
device on and off at a frequency which is high enough that the load effectively sees only the
average over many cycles.
Since the transistor is either full on or full off, either its current is
zero, or its voltage is small, and therefore the power dissipation of the control device can be
This is a common operating principle of many power electronics circuits.
The most common method for providing this type of switch-mode power control is through pulse
width modulation or PWM.
In PWM, the drive to the control device is a train of pulses whose
frequency is kept constant, but whose pulse width is adjusted.
As shown in the lower part of Fig.
2, T is held constant, but the high-time t
is varied to adjust the effective voltage that is given to
the control device and also the current through the load.
The duty cycle is the fraction of a cycle
that the control device is turned on, D.C. = t
/T, usually expressed as a percent.
If the voltage
present in the ON state is V
, then the effective output from the PWM is a voltage equal to V
times the duty cycle:
The great advantage of this type of control is that it
provides a highly linear transfer function, since pulse widths can be controlled quite precisely in
most electronic circuits.
As can be seen, the scaling factor between output voltage and the pulse
width is V
/T, which can be made a constant for the circuit.
The heart of a PWM system is the modulator which controls the PWM pulse width as a function
of some external control voltage input, V
As the input control voltage is varied over a range
of 0 to V
, it is desired to change the output pulse width from 0 to T, corresponding to duty
cycles of 0% to 100%.
In practice, it is difficult to go fully from 0% to 100%, but most PWM
systems come within a few percent of either extreme limit, still giving a large range over which
the control is linear and predictable, for example, 2% to 98%.
A periodic ramped voltage waveform is usually the most convenient and simplest means to make