Obtaining accurate measurements of load current is essential when
testing power supplies, and makes it possible to calculate very precise values of efficiency. This, in turn, enables your
quality-assurance engineers to characterize the distribution-of-product accurately during manufacturing. Even minute
shifts in power consumption can help detect incorrect or out-of-spec components.
Significantly, the defects might not cause immediate failure. If an incorrect diode with a higher forward voltage drop
were to permit a large abnormal forward voltage, for example, it could produce a "hot spot" that could lead to a much higher
likelihood of failure later.
Efficiency is a very tight distribution parameter, so accurate current measurement is a must.
Electronic Loads
Back to top
Active electronic loads are often used to load power supplies. Many of these electronic loads include current-monitor
ports. These ports provide a DC output voltage that's proportional to the current flow to the load.
Electronic loads, such as those available from Agilent Technologies and others, are commonly used to perform load tests.
Agilent's Model 6050 DC electronic load offers a 12-bit programming and readout capability. Agilent's recently introduced
N3300A Series loads, shown here, offer a 16-bit programming and monitoring capability. The N3300A mainframe supports a series
of load modules, such as the N3302A with 30 A and 3 A current-monitoring read-back ranges.
Figure 1 - The Agilent Technologies 3300 Electronic Load
can be fitted with
a variety of load modules.
Four values are required to measure efficiency. First, you need the voltage across the output of the DC-to-DC converter
(Vo ). Next, you need the current at the output of the converter
(Io ). You also need to know the input voltage
(Vi ) and the current in series with the input
(Ii ). The efficiency (h) is simply the quotient
 . |
(1) |
If an electronic load feeds its data across an IEEE-488 GPIB
(General-Purpose Interface Bus), read-back resolution requires careful consideration. This is particularly true when it's
necessary to obtain accurate readings at the lowest part of the lowest range. At this point, the measurement accuracy and
precision degrade progressively. This is attributed to the inherent limitations of resolution in digital readouts, as well
as an offset (almost always specified as at least one component of accuracy in virtually any electronic measurement circuit).
The Three Components of Error
Back to top
When making current measurements across an electronic load, there are three components that make up the total error to
consider.
First, consider linear error. Linear error is the error inherent in the electronic load, which is a fixed value for each
range, and is a constant percent, regardless of the current reading.
 |
(2) |
Next, consider offset error. This error has a significant impact on the
total error at the low end of a given range since it depends heavily on the actual reading, which appears in the denominator
as shown in this governing equation.
 |
(3) |
Note that this error varies inversely as the actual reading. Therefore,
its impact on the overall accuracy increases proportionally at lower readings.
Next, you must consider digital error. This type of error depends on the range. In this case, the closer the actual reading
(denominator) approaches the range (numerator), the lower the digital error.
 |
(4) |
The total error as a percentage of the reading is simply the sum of the three
errors just described.
| Total Error = Linear Error + Offset Error + Digital Error |
(5) |
Next
>>
