Page 194 - The Art and Science of Analog Circuit Design

Tripping the Light Fantastic

Appendix A

Achieving Meaningful Efficiency Measurements

Obtaining reliable efficiency data for the CCFL circuits presents a high
order difficulty measurement problem. Establishing and maintaining
accurate AC measurements is a textbook example of attention to mea-
surement technique. The combination of high frequency, harmonic laden
waveforms and high voltage makes meaningful results difficult to obtain.
The choice, understanding, and use of test instrumentation is crucial,
Clear thinking is needed to avoid unpleasant surprises! 1

Probes

The probes employed must faithfully respond over a variety of conditions.
Measuring across the resistor in series with the CCFL is the most favor-
able circumstance. This low voltage, low impedance measurement allows
use of a standard IX probe. The probe's relatively high input capacitance
does not introduce significant error. A 10X probe may also be used, but
frequency compensation issues (discussion to follow) must be attended to.
The high voltage measurement across the lamp is considerably more
demanding on the probe. The waveform fundamental is at 20kHz to
100kHz, with harmonics into the MHz region. This activity occurs at
peak voltages in the kilovolt range. The probe must have a high fidelity
response under these conditions. Additionally, the probe should have low
input capacitance to avoid loading effects which would corrupt the mea-
surement. The design and construction of such a probe requires signifi-
cant attention. Figure 11-A1 lists some recommended probes along with
their characteristics. As stated in the text, almost all standard oscilloscope
2
probes will fail if used for this measurement. Attempting to circumvent
the probe requirement by resistively dividing the lamp voltage also cre-
ates problems. Large value resistors often have significant voltage coeffi-
cients and their shunt capacitance is high and uncertain. As such, simple
voltage dividing is not recommended. Similarly, common high voltage
probes intended for DC measurement will have large errors because of
AC effects. The P6013A and P6015 are the favored probes; their 100MO
input and small capacitance introduces low loading error. The penalty for
their 1000X attenuation is reduced output, but the recommended volt-
meters (discussion to follow) can accommodate this.
All of the recommended probes are designed to work into an oscillo-
scope input. Such inputs are almost always 1MO paralleled by (typically)

1. It is worth considering that various constructors of Figure 11-18 have reported efficiencies
ranging from 8% to 115%.
2, That's twice I've warned you nicely.

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