Brett Fox - EEWeb

TECHNICAL ARTICLE
Some loads may require different controlled parameters
at different times. Such an example is a battery charger
which might require constant current for battery charge
mode and constant voltage for battery maintenance
mode.
Loads will require the controlled parameter to be within a
certain tolerance band for proper operation. The power
supply must maintain the controlled parameter within
the tolerance band.
These parameters may be expressed in terms of average,
RMS, or a peak value with a duration qualifier.
Dynamic Requirements
Loads also exhibit dynamic characteristics which change
over time.
Time Transients
Many types of loads frequently change their effective
impedance. Such an example might be a computer
printer which exhibits rapid step changes in effective
impedance. For such a device to function properly, the
power supply must be able to rapidly source spurts
of output current while maintaining the output voltage
within a specified band. This means that the power
supply must have enough output capacitance and high
enough control loop bandwidth to maintain the output
Vsource
Power Supply
Zout
Feedback
and
Control
Figure 1: Simplified representation of power supply and load.
voltage within the prescribed limits. Loads which have
this type of behavior must have power supplies specified
to limit the droop on the leading edge of the pulse, and
recover to within a certain band of the steady state output
in a prescribed time interval.
Voltage Dependence
Non-linear loads change impedance as voltage is
increased. One example is a typical solid-state circuit
which might draw very little current at low voltages,
and then begin to draw current with a very rapid and
nonlinear increase as voltage is increased.
A more problematic configuration is cascading a power
supply with a second power supply of a switching
converter design. A switching converter has a nonlinear
negative resistance characteristic. At very low input
voltages, below the turn-on threshold, the current may
be miniscule. When the input voltage is increased to
the turn-on threshold, the input current suddenly draws
very high current. As the input voltage is increased, the
input current decreases, following a constant power
characteristic. If care is not taken in the first power
supply design and cable length, the load (the switching
converter) will cycle on and off because of the voltage
drop in the cable length and/or the output impedance of
the power supply.
EEWeb | Electrical Engineering Community Visit www.eeweb.com 14
Iload
Zload
+
Vload
_
TECHNICAL ARTICLE