Page 201 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

Figure 11-A6.
Typical efficiency
measurement
instrumentation.
RMS voltmeters
{center left) mea-
sure output voltage
and current via
appropriate probes.
Clip-on ammeter
(right) gives low
loss input current
readings, DVM
(upper right) mea-
sures input voltage,
Hand calculator
(lower right) is
used to compute
efficiency,

determines power delivered by the CCFL circuit by measuring its load
temperature rise. As in the thermal RMS voltmeter, a differential approach
eliminates ambient temperature as an error term. The differential ampli-
fier's output, assuming a high degree of matching in the two thermal en-
closures, proportions to load power. The ratio of the two cells* E x I
products yields efficiency information. In a 100% efficient system, the
Figure 11-A7. amplifier's output energy would equal the power supplies' output.
Efficiency Practically it is always less, as the CCFL circuit has losses, This term
determination via represents the desired efficiency information.
calorimetric mea- Figure 11-A8 is similar except that the CCFL circuit board is placed
surement. Ratio within the calorimeter. This arrangement nominally yields the same in-
of power supply
to output energy formation, but is a much more demanding measurement because far less
gives efficiency heat is generated. The signal-to-noise (heat rise above ambient) ratio is
information. unfavorable, requiring almost fanatical attention to thermal and instra-

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