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AN1300 APPLICATION NOTE

Figure 2. The Control-Transfer Function of the Series-Resonant Converter

The damping resistance is the sum of the resistances within the primary circuits and the reflected load resis-

tance. The equivalent resistance is reflected through the transformer. This is given by equation 3.

[Eq. 3]

The typical transfer function of the resonant current within the tank circuit is shown in figure 2. It shows when

the converter provides more output power by lowering its frequency. The amplitude of the current grows signif-

icantly. Conversely, when the load lightens, the control loop increases the frequency away from the tank circuit's

resonance. Simultaneously, the Q of the tank circuit decreases slightly.

Figure 3 shows the converter operating at 110% of rated load and the operating frequency is permitted to go

under the resonant frequency of the tank. This condition may occur when the input voltage is at its minimum

rating and the output is at rated or slightly over-rated load and the frequency lower limit is not set properly.

Period 1 is the deadtime of the power switches when the center node voltage between the MOSFETs swings

from one voltage rail to the opposite rail. After the voltage has transitioned, the primary current flows through

the opposing MOSFET’s antiparallel diode.

The rate of change in the voltage waveform is dictated by the sum of the parasitic capacitances on the node.

These would be primarily the output capacitances of the MOSFETs.

Period 2 begins when the opposing MOSFET turns-on. Current is already flowing through the antiparallel diode

when the MOSFET turns ON. The current will continue its decline driven by the tank circuit and then reverse

direction. During periods 1 and 2, the secondary voltage on the forward winding transitions to the output voltage

and the winding’s current begins a sinusoid-like increase. the peak and decline again towards zero where an-

other deadtime is encoundered and the period repeats in the opposing direction.

When the input voltage is low and the output power draw is at or greater than its rated putput, one begins to

notice a current valley begin to appear just past the peaks of the primary current. The supply may be operating

below resonance. This "inflection" in the primary current waveform increases as the input voltage is further low-

ered or the output load is increased.

Waveforms at light load can be seen in figure 4. Here, the two normal operating periods (1 & 2) are seen.

R

re fl

N1

N2

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2

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