TE Technology TC-48-20 - Manual (Page 11)

11
Pulse‐width modulated power output
The TC‐48‐20 controller regulates the output power to the TE device using a method called pulse‐width
modulation (PWM). With PWM, power to the TE device is switched quickly "ON" and "OFF" at a constant
frequency. This creates a square wave “pulse” of power with a constant time period. The “ON” time, or pulse
width, can be varied to create an average output voltage (Vaverage) that is required by the TE device to maintain
the set temperature.
The important advantage to PWM control is that it does not cause the extreme temperature excursions that are
experienced with a thermostatic control system. This helps to extend the life and reliability of the TE device. At the
same time, PWM control does not generate a large amount of waste heat as compared with most linear control
systems, so large heat sinks are not required with the PWM temperature controller.
Linear control output
The controller also provides an analog proportional output signal. This cannot be used for powering TE devices
directly, but it could be used for a variety of purposes including controlling programmable power supplies. For
example, if the TE device were to require more than 20 A of current, or if true linear control were needed, the TE
device could not be directly connected to the controller. However, the TE device could be connected to a higher
current power supply that accepts an analog input, and the controller could then direct the power supply to apply
its output proportional to the analog output from the controller. Because these power supplies usually have a
linear output signal this method also reduces the electromagnetic noise that might otherwise come from a PWM
type controller.
One such power supply that accepts an analog input is the RKW programmable series switching power supplies
from Kepco (
www.kepcopower.com ). These power supplies accept a 0 to 5 volt input signal which causes the
output voltage of the power supply to vary from 0 V to its full‐scale voltage. These power supplies are available
with full‐scale voltages that range from 3.3 V to 48 V and they are available in 300, 600, and 1500 watt sizes.
Because these are switching power supply there is again the added benefit of little waste heat as compared with
most types of linear control systems. In addition, multiple power supplies can be operated in parallel to boost the
total output power.