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Number 3185
Ensuring that Power Supply
Application Note Performance Meets Your Requirements
Series Details beyond the specifications that can impact how well the
power supply meets your requirements
Most engineers feel that they understand power supplies
very well. Power supplies are considered one of the simplest
V
instruments since they tend to be a one-function, DC device VMAX
which outputs a controlled voltage. But of course there is much
more to power supplies. While a power supply's specifications
adequately describe performance for most applications, cost,
time, and human resource limitations prevent the specification
of all aspects of a power supply's (or any instrument's)
performance. You will want to understand the power envelope I
IMAX
of the power supply so that it will be able to deliver the power
at the voltage and current parameters that you need for your Figure 1a. Rectangular power supply envelope. Any current level can be
work. For development, characterization, and testing of circuits delivered at any voltage.
that generate or measure low level signals, then the selection of
the design topology of the power supply and investigation into
V
its common mode current can be a necessity so that the power
supply does not interfere with circuit performance. Similarly, if V1MAX
you are developing a device with multiple isolated circuits, then
you will want your power supply not to degrade the isolation
of your device. When you use your power supply as an accurate V2MAX
voltage source for testing your circuit over its operating voltage
range or as a calibrating source, you will want to ensure that you
can obtain the specified accuracy of the supply at the input to I
I1MAX I2MAX
the circuit-under-test. Applications such as these will require a
more detailed investigation into a power supply's characteristics. Figure 1b. Multi-range output. This characteristic allows higher voltages at
lower currents and higher currents at lower voltages.
Understand the Power Envelope
of the Power Supply
V
The most significant decision is ensuring that sufficient power is
available to energize the device-under-test (DUT). While this is VMAX
obvious, it is important to be aware that different types of power
supplies and sources have different power envelopes. One type
of power supply has a rectangular power envelope in which
any current can be supplied to the load at any voltage level. See
Figure 1a. This is certainly the most versatile power envelope.
A second type of supply can have multiple-rectangular envelopes
for multiple ranges (such as the two-rectangular envelope shown I
in Figure 1b). The value of this type of power envelope is that IMAX
it permits higher values of one parameter at the expense of the Figure 1c. Hyperbolic output characteristic. Maximum voltage and current
other parameter. For example a supply with this type of envelope follow a curve.
can output a higher level of current, but at a lower maximum
voltage. Some supplies output a hyperbolic envelope, a more your application requires, so that the supply you select delivers
continuous transition than a multiple range power supply. With the required power at the levels of voltage and current at which
this power envelope, one parameter is inversely proportional to you will need to test.
the other (Figure 1c). High power output supplies tend to have
the multi-range or hyperbolic envelope. Be cognizant of what
The Type of Design Determines
AC Mains Power Supply Load
Noise Performance Input
Common Mode
If you are powering a circuit that operates Current
at a very low voltage or a circuit that uses
or measures very low currents such as
a transducer detector that must pick-up
millivolts or microamp current signals, AC-DC
Conversion
then noise from external sources may Power and RLoad
cause problems. The power supply, itself, Control
Circuitry
is one noise source, and this noise can
be broken into two components: normal Normal
Mode ZCM
mode noise, and common mode noise. Noise
Normal mode noise is noise generated
across the power supply's output
terminals due to the power supply's
Figure 2. Normal mode and common mode noise currents.
internal circuitry. Common mode noise is
earth-referenced noise originating from to the primary (secondary) in order your power supply for low common
the power line and stray capacitance to complete the circuit. Whenever this mode performance. See the sidebar on
across the main transformer. For sensitive current flows through an impedance, a measuring a power supply's common
circuits, linear power supplies provide noise voltage is generated, which under mode current. Keithley Series 2200 power
much lower normal mode output some circumstances, can degrade load supplies have common mode currents
noise than supplies designed using (or device-under-test, DUT) performance under 10