Due to more and more electronics application need to be supplied by switching converter, the performances of switching converter have to be considered. The important specifications should be care about are listed as follows. Firstly, the power conversion efficiency of switching regulator is an important topic, how to maintain high efficiency over wide load range will be discussed in the section 2.3.1. The second is excellent load and line regulation, which will be discussed in the section 2.3.2. Final part is the transient response, how to immediately response when the suddenly large output current variation will be shown in the section 2.3.3.
2.3.1 Power Conversion Efficiency
The definition of switching regulator’s power conversion efficiency is the ratio of input power and output power and can be wrote as
OUT OUT OUT100%
The detail power losses and the operation range of control modulator have been shown in the section 2.2. Overall saying, the pulse width modulation (PWM) mode is suitable for operating in heavy load condition, and the pulse frequency modulation (PFM) is suitable for operating in light load condition. If the switching regulators operating only one module mode, take cell phone for example, the cell phone system operates in standby mode and many blocks of system doesn’t work, at light load condition but the converter only have PWM controller, as a result, the battery lifetime will reduce quickly. Hence, the best way to improve the efficiency is including the pulse frequency modulation (PFM) to control at light load condition. By dual mode control can keep high efficiency over wide load range. The comparison of PWM and PFM control technique will be listed in TABLE III.
Table IV. Comparisons between PWM and PFM controls.
Pulse Width Modulation Pulse Frequency Modulation Well Efficiency Range Moderate to heavy load Light load
Frequency Constant Variable
Switching Loss of
Whole Load Constant The heavier load
the larger loss
Output Ripple Smaller Larger
Transient Response Faster Slower
Circuit Complexity Complicated Simple
Quiescent Current Larger Lower
2.3.2 Load and Line Regulation
Switching regulators are powerful system of stepping up or down the desired voltage.
Therefore, to keep the regulated voltage and decrease the steady state error when varying the supply voltage and load condition of DC-DC converter is the most important.
The load regulation is defined as the percentage of steady state error of output voltage when the load condition changes and can be calculated as follows.
_
100 %
OUT
OUT desired load
Load Regulation V V I mA
(12)The line regulation is defined as the percentage of steady state error of output voltage when the input voltage changes and can be calculated as follows.
Line Regulation V V V mA
(13)2.3.3 Transient Response
The transient response is an important specification of DC-DC converter for the system applications. The large load current changes suddenly will cause a voltage fluctuation at output of DC-DC converter. The voltage fluctuation may trigger the logic circuit or affect the analog circuit. Therefore, it’s important to reduce the large voltage changing and the time during voltage variation. The transient response of output voltage relates to load current is shown in Fig. 13. During the first period Δt1, the large current flows into the output load from DC-DC converter, due to the DC-DC converter cannot provide enough energy to maintain the output voltage, the output voltage will drop in this period because the output capacitor discharge the energy to support the load current. The drop voltage is shown in Eq. (14). According to the parameters of Eq. (11), selecting the output capacitor well can reduce the drop in this period.
1
During the second period Δt2, the system senses the output variation by feedback loop then turn on the power PMOSFET to recover the regulated output voltage. The summation of Δt1 and Δt2 is called the recovery time and the second period Δt2 depends on the system bandwidth of the DC-DC converter
The static error ΔVOUT between light load and heavy load is relates to the voltage regulator DC gain, the higher DC gain bringing the better load regulation [19]. Comparing to the Fig. 12(b) and Fig. 12(c), the performance of Fig. 12(b) due to the large DC gain and causes the better load regulation, but the second period Δt2 extends the recovery time.
However, it reduces the time of transient response. The performances of Fig. 12(c) due to the poor DC gain and cause huge static error but reduces the time of second period Δt2 and improve the dynamic performance.
When the load current is decreasing to light load suddenly, the output voltage will jump until the DC-DC converter start to recovery the regulated voltage. The redundant current charges the output capacitor resulting to a peak voltage as shown in shown in Eq.
(15) before the feedback loop of DC-DC converter reacts.
3
During the final period Δt4 the output capacitor discharged the redundant current to feedback resistors. As mention described, the transient response is relates to the bandwidth of DC-DC converter, output capacitor, equivalent series resistance (RESR) of output voltage and the load current.
Fig. 13. The transient response of output voltage relates to load current.
Chapter 3
The Theory of the Dual Modulation Technique in High Switching Buck Converters
In the chapter 3, we present the concept description of the proposed technique in high-switching converters. In the section 3.1, we discuss the drawbacks of the conventional switching converter operating at a high switching frequency. And the section 3.2 describes the analysis of the proposed dual modulation technique. Finally, the loop analysis of conventional V2 control and the proposed system are presented in section 3.3.