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Bose Lifestyle 28 Panduan Pemecahan Masalah
PS18/28/35 Troubleshooting Guide
Theory of Operation
The parameters of capacitor C115 and inductor Llk are selected in such a way that its resonant
frequency is lower than the range of switching frequencies. In other words, the switching frequency
is always above the resonant frequency. The resonant circuit is operated in inductive mode. Two
capacitors C113, C114 are added in parallel with the two MOSFET switches to slow down the
voltage transition across the two switches. Inductive mode operation and a dead time provided by
the IC, shown in Figure 4, allow the voltage across the two switches to transit smoothly from high
to zero and from zero to high. Figure 7 shows gate voltage, drain-source voltage, and drain current
for MOSEFT Q101. As can be seen from Figure 7, the power MOSFET is turned on and turned off
when its drain-source voltage is zero. Zero-voltage switching like this reduces switching loss and
switching noise.
Figure 7. Gate and drain-source voltage, and drain current waveforms for MOSEFT Q101
A characteristic of a resonant converter arrangement shown in Figure 5 is that the ratio of the
output DC voltage to the input DC voltage is determined by the ratio of the switching frequency to
the resonant frequency of capacitor C115 and inductor Llk. Figure 8 shows a plot of voltage
conversion ratio versus normalized switching frequency. Each of the curves represents a different
load resistance that is equivalent to the two loads at the two output rails. For example with load L3,
voltage conversion ratio decreases with the normalized switching frequency if operated above
resonance (inductive mode). Therefore, by moving the switching frequency away from resonance,
the output voltage is lowered.
Figure 8. Plot of voltage conversion ratio versus normalized switching frequency
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