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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