Testing Power Supplies

Application Guide
Testing Power Supplies
A general test set-up for power supplies is
illustrated in Figure 1. This setup can be
used for most critical parameters. Some
general conditions apply:
1. The accuracy of voltmeters and
current meters should be approximate ly ten times the resolution
required to measure a parameter.
All meters should be 4½ digit.
Figure 1 General Power Supply Test Set-Up
2. Any oscilloscopes used should have
a resolution approximately ten times the parameter being
1. The input voltage is set to “nominal” value. This is typmeasured. Bandwidth of the oscilloscope should be 20
ically 110 VAC or 220 VAC for AC/DC power supplies;
MHz to 100 MHz.
or 5 VDC, 12 VDC, 24 VDC or 48 VDC for DC/DC
3. The input power source used should be sufficient to
converters. Nominal input values should be included
supply the maximum input power required by the Unit
on the product data sheet.
Under Test (UUT), plus an adequate guard band.
2. The output load is set to the full rated value.
Example Product
3. The ambient temperature should be maintained at 25°C.
For our examples, we have used the specifications of the
A512RW, a 5W DC/DC converter available from MPD.
4. All connections to the power supply should be made with
great care to avoid inducing errors in the measurement.
Nominal Voltage
18 to 36
No Load Current
Full Load Current
Reflected Ripple
Voltage Range
Nom. Input, Full Load
Nom. Input, Full Load
Line Regulation
Low Line to High Line
Load Regulation
Nom. Input, Full Load
Ripple & Noise
Transient Recovery Time
to 1% for IOUT change of 75% to 100%
Nom. Input, Full Load
Isolation Voltage
Input to Output
mV P-P
Power supplies with a remote sensing option should
be connected in local sensing mode (with the sense
leads con nect ed to the
appropriate output pin). All
test equipment should be
al lowed suficient time to
warm-up and stabilize before any testing begins.
To help illustrate some of
the examples used in this
ap pli ca tion note, we will
use the A512RW, a 5W
DIP compatible DC/DC
con vert er avail able from
MicroPower Direct.
Summary specifications are
given in Table 1 at left. A full
data sheet is available on
the MPD website.
MicroPower Di rect
Equipment/Test Set-Up
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As a complex electronic subassembly, a power supply has a high
number of parameters that are routinely
specified by manufacturers. It is important to accurately evaluate the product
initially in engineering to insure it is fit
for use in the application. It is equally
important that production testing insures
an appropriate performance and quality
level is maintained by the vendor.
MicroPower Direct
Output Voltage Accuracy
Output accuracy is the maximum deviation between the measured output of a power supply and
its specified nominal output. Typically given as a
percentage, output voltage accuracy is sometimes
refered to as “output voltage tolerance” or “output
voltage setpoint”. The measurement is made as
1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of
rated value.
2. Measure the output voltage using the 4½
digit voltmeter. This is VOUT.
3. Output accuracy (as a %) equals:
5.00 VDC
regulation as a “%/% change in VIN”. Essentially,
this means the percentage change in input line voltage is factored into the calculation to derive output
regulation. For a unit specified in this manner, line
regulation is derived as follows:
X 100 = 0.4%
5.02 VDC - 3.75 VDC
5.02 VDC
24 VDC - 18 VDC
24 VDC
Line Regulation
Line regulation specifies the change in output voltage caused by varying the input line over a specified range. Expressed as a percentage, the range
is typically low line to high line. The measurement
is made as follows:
1. Set the input voltage to the specified “nominal”
level. Set the output load to 100% of rated
2. Measure the output voltage using a 4½ digit
voltmeter. This is VOUTN
3. Change the input voltage level to low line (or
the specified low value for the line regulation
spec.). Measure the output voltage. This is
X 100
X 100
1. Set the input voltage to the specified “nominal” level. Set the output load to 100% of
rated value.
3. Change the output load to the specified low
value. Measure the output voltage. This is
4. Load regulation (as a %) equals:
For our unit, an output load change of 75% to
100% is specified. At 75% load, an output voltage
measurement of 5.05 VDC yields:
5.02 VDC - 5.05 VDC
5.02 VDC
The output is changing 1% for each 1% change in
input voltage. Obviously, care must be taken when
using units specified this way, that the input line is
very stable or that the application can operate with
potentialy wide output swings.
Load Regulation
Load regulation specifies the change in output
voltage caused by varying the output load over a
specified range. Expressed as a percentage, the
range will vary somewhat, dependent upon the
product design; but is typically given as a condition
in the manufacturers data sheet. The measurement
Also called Periodic And Random Deviation,
(PARD), this is the noise and ripple voltage super
imposed on the output of a power supply. Output
ripple is the periodic AC component and noise refers to the high frequency spikes that are unrelated
to the switching frequency of the power supply.
Due to the high frequencies involved, care must be
taken not to induce errors into the measurement.
An oscilloscope with a minimum bandwidth of 20
MHz so that all significant harmonics of the ripple
spikes are included.
Figure 2 illustrates how to make this measurement.
A standard oscilloscope probe is shown in Figure
2A with a ground clip. When making measurements
5. Line regulation (as a %) equals:
X 100
Where VMDEV is the output voltage measured (VOUTH
or VOUTL) that causes the greatest deviation from
For our example unit, VOUTL equals 5.015 VDC,
while VOUTH equals 5.03 VDC. Using 5.03 VDC,
we derive:
5.02 VDC - 5.03 VDC
5.02 VDC
X 100 = 0.2%
Well within the specified ±0.3%.
Some unregulated power supplies specify line
X 100 = 0.6%
Output Ripple & Noise
X 100 = 1.0%
4. Change the input voltage level to high line (or
the specified high value for the line regulation
spec.). Measure the output voltage. This is
X 100
Well within the specified ±1%.
This in turn equals:
0.25 VDC
Well within the specified ±1%.
X 100
Where ∆VIN % is the change in input line as a percentage and VMDEV is the output voltage measured
that causes the greatest deviation from VOUTN. If
our example unit was specified this way; and an
output of 3.75 VDC was measured at an input of
18 VDC. We could we derive the line regulation
as follows:
is made as follows:
2. Measure the output voltage using a 4½ digit
voltmeter. This is VOUTFL
∆VIN %
X 100
Where VNOM is the nominal output voltage specified
for the power supply. For our unit, a measurement
of 5.02 VDC yields:
5.02 VDC - 5.00 VDC
Testing Power Supplies
Figure 2 Measuring Output Ripple & Noise
in a field of radiated, high frequency energy, the
ground clip acts as an antenna. Since this could
inject unwanted noise into the measurement, the
clip should be disconnected and not used.
The injection of unwanted noise is eliminated by
using the technique shown in Figure 2B. Here the
scope probe is placed directly across the output
terminals of the power supply. The ground band on
the probe is pressed against the output common
terminal, while the probe tip is put in contact with
the voltage output pin. This creates the shortest
possible connection across the pow er supply
output terminals.
Another method for measuring true output noise
and ripple is shown in Figure 2C. A 20 AWG “twisted pair” wire, approximately one foot in length, is
attached between the power supply outputs and
the load. An electrolytic capacitor (typicaly 10 μF
to 50 μF) is connected across the load. The scope
probe, with the ground clip disconnected, measure
the the ripple at the connection of the twisted pair
wires and the load.This technique will eliminate
interference caused by common mode noise.
Transient Recovery Time
The time required for a power supply output to
return to within specified limits following a step
change in output load current. The specified load
change and error band will vary with the product
and/or manufacturer, but should be stated clearly
in the product data sheet.
Using the test set-up shown in Figure 1, measure
transient response as follows:
4. Measure the transient response time as illustrated in
Figure 3.
If the power supply has multiple
outputs, auxiliary outputs should
be set to 100% load whiletransient
recovery time is measured.
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The ratio of total output power
to input power, expressed as a
percentage. Efficiency has become an increasingly important
specification as the power density
of power supplies has increased
and the relative size of electronic
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1. Set the input voltage to the
specified “nominal” level. Set
the output load to 100% of Nearly 2,000
standard models!
rated value.
2. Using the test set-up shown
in Figure 1, measure the
output voltage (VOUT), output
current (IOUT), input voltage
(VIN) and input current (IIN).
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X 100
5.02 VDC
24 VDC
We can then derive the units efficieny as:
1.00 A x 5.05 VDC
0.25 A x 24.0 VDC
Testing power supply isolation requires specialized
equipment (Hipot tester) and can be destructive.
This test should not be attempted without contacting the manufacturer.
Returning to our example unit, if we measure the
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3. Efficiency (as a %) equals:
1. Set the input voltage to the specified “nominal” level.
2. Program the the adjustable load for the step
load change specified by the manuafacturer. In
our case, this is 75% to 100% of rated load.
3. With the oscilloscope triggered
externally, switch the load over
the specified range.
X 100 = 84%
At 84%, this is over the typical specification of 83%.
I/O Isolation
The maximum voltage (ac or dc) that can be continuously applied from the input to output or input
to case of an isolated power supply.
Minimum isolation voltage levels must be maintained to meet most safety regulations. Typical
isolation values specified by manufacturers are:
I/O 3000 VAC
I/C 1500 VAC
I/O 1500 VDC (New Designs)
1000 VDC (Low power designs
500 VDC (Old Designs)
Automatic test equipment (ATE) designed specifically for power supplies is available from a
number of vendors. Systems range from highly
sophisticated (and expensive) stand-alone equipment to lower cost PC based systems. In-circuit
testers (ICT) that are not specifically designed for
power supplies may also be used.
Selecting the right ATE is dependent upon a number of factors including application complexity,
production volume & flow, company procedures,
and the availability of technical expertise.
If problems arise in the automated testing of power
supplies, the vendor should be contacted immediatly. The can help in correlating the data and
verifying the source of any potential problem.
In Summary
Whether power supplies are being bench tested for
an engineering evaluation; sample tested at incoming as part of standard QA screening; or tested in
the application as part of volume production; great
care must be taken to use the correct methods and
end-point specifications.
Use the expertise of the power supply manufacturer
if you have any
Figure 3 Transient Recovery Time
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