ꢀ ꢁꢁꢂ ꢃ ꢄ ꢅ ꢆ ꢀ ꢁ ꢁꢂ ꢃ ꢄ ꢇ ꢆ ꢀꢁ ꢁꢂ ꢃ ꢄ ꢄ
ꢀ ꢁꢁꢈ ꢃ ꢄ ꢅ ꢆ ꢀ ꢁ ꢁꢈ ꢃ ꢄ ꢇ ꢆ ꢀꢁ ꢁꢈ ꢃ ꢄ ꢄ
SLUS499A – NOVEMBER 2001 – REVISED JANUARY 2002
APPLICATION INFORMATION
open lamp shutdown/no-lock operation
Due to the high gain characteristics of the piezoelectric transformer, it is important that the operation of the power
stage is suspended if an open lamp occurs. Figure 19 shows the output voltage of a piezoelectric transformer
with no output load and driven with a 5-V
sinusoid on the primary. The primary frequency is swept through
RMS
the resonant frequency of the piezoelectric transformer. As seen in Figure 19 the output voltage approaches
2 kV with open lamp disabled, a gain of 400! If the input voltage were increased to 10 V , the output would
RMS
reach 4 kV
RMS
and possibly crack the PZT transformer.
RMS
OPEN LAMP
PROTECTION
DISABLED
PZT Resonant
Frequency
Maximum
Frequency
Minimum
Frequency
Figure 19. Damaging Voltages at Piezoelectric Transformer Secondary
In order to prevent damaging voltages at the piezoelectric transformer secondary, a 1.5-V comparator at the
OPEN/SD pin is used to shutdown the converter if an open lamp is detected. The RMS secondary voltage at
which an open lamp fault is triggered can be calculated using equation (17).
ǒ1.5 ) V
Ǔ
R
OPEN
DIODE
HV
V
+
OPEN
Ǹ
2 R
(17)
R
will typically consist of several inexpensive high impedance resistors to minimize current in the divider and
HV
to stand–off high voltage. R
issue, however, impedance of the capacitor over frequency needs to be taken into consideration.
can be replaced with a single high voltage capacitor if component count is an
HV
22
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