For high-switching operation, power conversion efficiency decreases drastically when load current changes from heavy to light. The dual modulation technique needs to hop switching frequency to find a trade-off between power conversion efficiency and output voltage ripple when load current decreases [20].
The timing diagram of the dual modulation technique is illustrated in Fig. 15. The original PWM control uses a high- switching signal VPRI to regulate the output voltage to achieve a reduced ripple. Since the conduction loss dominates the whole power consumption at heavy loads, the high-switching signal would not result in a great decrease in efficiency. However, the switching loss drastically deteriorates efficiency from medium- to light-load condition due to the high-switching operation. Therefore, the secondary modulator becomes necessary to reduce the switching numbers as shown by the modulated signal VGATE used to control the power switches. At this time, dual modulation operates to
raise efficiency within an allowable output ripple.
Fig. 14. Waveforms of the buck converter with the dual modulation technique.
The secondary modulator contributes to the decrease in the switching frequency and the increase in the hopping period. Light-load conditions require reduced switching frequency in order to save power. The hopping frequency modulator (HFM) circuit can determine a suitable switching frequency to reduce substantially the switching power loss at the power switches. Meanwhile, dual modulation starts to decrease the switching frequency from the constant fSW(constant) to fSW(dynamic) through the hopping frequency, fHOP, in the secondary modulator. As depicted in Fig. 16, the value of fHOP varies with load current. In addition, the hopping frequency not only reduces switching loss but also always keeps the output ripple within the allowable range.
The decrease in the switching frequency is accompanied with an increase in the hopping period as load current declines continuously. The decrease in switching frequency results in increased efficiency from a medium- to light-load condition. Much power is retrenched due to the switching loss reduction at the power MOSFETs. To further raise efficiency at very light loads, the primary modulator is shut down automatically and only the secondary modulator is employed to regulate the output voltage and to save much power in the quiescent operation loss. Furthermore, to avoid operation in the acoustic region, the hopping frequency is always kept higher than the acoustic frequency, facoustic, even at no load condition.
Fig. 15. Efficiency and hopping frequency versus load current for the proposed converter system.
3.2.2 AC Ripple Detection Technique
The architecture of the proposed dual modulation technique is shown in Fig. 16. The controller is separated into two parts. The primary modulator makes the system operate normally under high-switching frequency, and the secondary modulator can raise power conversion efficiency at light loads. Thus, high power conversion efficiency and a fast transient response under high-switching operations can be achieved.
Fig. 16. The proposed buck converter with the dual modulation technique.
The dual modulation technique should detect the load condition using the proposed loading potential detector (LPD) circuit. In addition, the combination of the LPD circuit and the error signal received by the error amplifier can be viewed as the control signal in the HFM circuit. As a result, the hopping signal generated by the secondary modulator can regulate the original PWM signal to find the trade-off between efficiency and output voltage ripple.
For high-switching converters, the conventional current sensing method may fail to provide accurate sensing load current due to limited bandwidth. Thus, it is better to find a
suitable current sensing method for the high-switching converter. Fig. 17 shows the concept of the proposed AC ripple control. In time domain, the output LC stage of the converter can be considered as a low-pass filter and work as an integrator.
However, the existence of ESR and ESL may deteriorate the accuracy of current sensing signal and the steady state duty cycle. Considering the ESR, Resr, and the equivalent series inductor (ESL), Lesl, on the output capacitor, CO, the output voltage ripple can be evaluated as the summation expressed in (16).
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Briefly, (16) is composed of overshoot voltage, vout|esr, across the ESR; induced voltage, vout|esl, which is the differentiation of the inductor current with ESL; and voltage ripple, vout|Co, which is the integration of the inductor current on the CO. Thus, it is convenient to differentiate vout(t) in order to obtain the AC signal of the inductor current. This can be expressed as
At the right side of (17), the first and second terms represent the effect of the ESR and ESL, respectively. Owing to high-switching operation, the ESR and ESL seriously affect system stability and result in a large output ripple. The inductor current information can be accurately derived through the operation of the proposed AC ripple detector. The AC ripple detector behaves as a differentiator and inserts one low-frequency zero to increase system stability. One low-pass filter is utilized to filter out the high-frequency components contributed by the ESR and ESL. Consequently, the accurate inductor current can be derived as the PWM ramp since the effect of the ESR and ESL can be efficiently removed.
In other words, the cheap MLCC can be selected as the output for low cost.
Fig. 17. The concept of current sensing flow as it utilizes the AC ripple detector.
3.2.3 Summary
To sum up the concept described above, the proposed dual modulation technique can solve the converter’s large switching loss in high switching frequency operating. Hence, the size of the output LC filter is reduced without consuming too much power consumption in the light load condition, it makes high switching converters are more suitable for common commercial applications. And the modified V2 control method can regulate output voltage even in zero-ESR condition, and the reduced output voltage ripple can make the converter more suitable for portable device applications. The V2 control topology contains two voltage feedback paths; the path from output voltage can rapidly react to the output voltage variation to speed up the transient response.
We present a new control technique can operate in high switching frequency, which not
only can speed up the transient response, but also can maintain high power conversion efficiency over a wide load range.