Electronic Design

Leapfrog: First Look -- Minimizing Q Improves Converter Efficiency

In a classic zero-current/zero-voltage (ZC/ZV) converter, the amplitude of the resonant circuit’s signal can be hundreds of volts. Compare that to a sine amplitude converter (SAC), where the oscillation is just a few volts peak to peak. In the former case, the high amplitude is required because the classic ZC/ZV converter processes energy serially.

The SAC depends on minimization of transformer leakage inductance to minimize its Q. This may seem counter-intuitive given that oscillator design typically requires high values of Q. However, in the case of the SAC, the goal is to minimize the energy stored in the transformer’s leakage inductance. That’s because the more energy stored in the leakage inductance, the slower the response of the converter and the greater the transformer’s resistance and its losses.

In the SAC, lowering the Q actually leads to higher conversion efficiency, despite the fact that the fraction of energy lost in the tank circuit rises as its Q is lowered. Although tank circuit losses rise in proportion to 1/Q, the total amount of energy stored in the tank circuit drops—more rapidly—in proportion to Q2. In other words, because much less energy is stored in the tank circuit at lower Q values, you can afford to lose a greater percentage of this energy and still achieve a net decrease in converter losses.

But Q can only be lowered to a point. At the very least, Q must have a value greater than 1 or the SAC will not oscillate. In practice, the Q can only be reduced to 2 or 3, otherwise the oscillation will cease to be sinusoidal—a basic requirement for ZC switching.

Comparing Q and Stored Energy
The Q associated with power-transformer leakage inductance and the energy stored in that leakage inductance vary according to the power conversion technology employed. The table indicates the resulting Q and energy storage levels, when three styles of dc-dc converters are used to generate 200 W of output. Although the pulse width modulation (PWM) converter and the SAC exhibit similar levels of Q, there is much less energy stored in the power transformer of the sine amplitude converter.

That reflects the order-of-magnitude difference in switching frequencies between the two styles of converter. Due to switching losses, the PWM converter switches at the lower frequency. That, in turn, requires a larger number of turns in the power transformer. Those extra turns and the lower switching frequency account for greater leakage inductance in the power transformer, which leads to the higher level of energy storage in the PWM converter.

In the traditional resonant forward converter with ZC/ZV switching, the leakage inductance is intentionally made high to maximize the energy stored in the transformer.

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