In this microelectronic era, high performance, long operating time, compact size and low cost are essential factors of a successful electronic product. For example, it is expected a third generation (3G) cellular phone can process extensive multimedia data with standby
time of a week and size of the palm [8–10]. These requirements challenge traditional power supply control techniques and converter design. In this section, trends of power supply design in microelectronic era are described.
1.2.1 High Efficiency
While transistor numbers inside a single chip continues to double every 18 months as Moore’s Law predicts, energy density of batteries has increased little. Conversion efficiency is particularly important in battery-powered equipments. Low heat production also saves space for heat ventilation. Advanced power management techniques and converter circuits design will help to extend battery life and shrink device size.
1.2.2 Low Output Voltage and Low Noise
In portable and high performance systems, electronic systems are designed to operate at the optimal supply voltage [11, 12]. Low operating voltage of new generation integrated circuits has set tight tolerance of converter’s output. Communication and audio circuits are also sensitive to noise interference. Low output ripple and low noise are essential. Fast transient response is required to prevent large output deviation during step load transient.
In addition, fixed frequency operation of switching-mode converters is favorable, because it is easier to filter out switching noise.
1.2.3 Compact Size
Generally, switching-mode converters are composed of many discrete components that occupy large space. Since physical size minimization is a major design objective in portable devices, reductions of external component counts and size are trends of switching-mode
converter design [13, 14]. Significant energy is dissipated in the parasitic impedances of external interconnection and components [15]. Therefore, integrating external components decreases energy loss. Higher switching frequency can reduce the required sizes of filter inductor and capacitor and also improve efficiency [16]. Further physical size minimization can be made by integrating controller with microprocessor or other circuits [17, 18].
1.2.4 Fast Transient Response
Microprocessors today exhibit much heavier load and faster current slew rate. Advanced power management techniques are usually adopted. When the system is in sleep mode, some circuits are shut down and operating voltage is scaled down in order to minimize standby current. The most challenging issue comes from step load transients when the system transits from sleep mode to full loading mode. These two modes correspond to minimum and maximum loading conditions respectively. The regulator has to maintain output voltage within tight tolerance during this fast slew-rate transient. These power requirements have become new challenges [8, 19–23].
1.2.5 Wide Duty Ratio Range
Many portable devices are powered by multiple input sources such as AC adapter and batteries. Under these conditions, range of duty ratio variation is quite large. Unstable operations are observed when using traditional PWM control especially at very low duty ratio. Wide load range also causes duty ratio to change significantly.
1.2.6 Digital Control
Switching-mode converters are one of the few analog parts in today’s electronic system.
Due to cost and some practical issues, digital control of switching-mode converters has not been widely used. Digital control [24–37] provides versatile functions that enable more powerful features of switching-mode power supply. Sophisticated control schemes, converter status monitoring, and system integration can be easily implemented in the digital way. Fast design cycle of digital circuits is another attractive advantage. It is also less sensitive to noise, process, temperature and component variations. As the cost per transistor continuing drop with the advance of semiconductor process, above advantages have made digital control a more attractive choice for today’s switching-mode power supply.
1.3 Motivation
A desirable controller regulates output voltage tightly in the presence of input voltage and load current variations. Voltage-mode PWM control scheme is commonly used in switching-mode power supplies. Design of this control scheme is simple and straight-forward. The output voltage is controlled by directly changing the PWM duty ratio.
Therefore, this control scheme is also called direct duty control. However, its applica-tion is limited because of slow response. Another popular control scheme is current-mode PWM control, which is also called current-programmed control and current-injected con-trol, was introduced in 1978 [38–43]. This control scheme utilizes dual loops to control both output voltage and inductor current. It effectively eliminates the phase lag of the
fil-ter inductor and makes loop compensation easier. However, current sensing elements not only require additional circuitry but also reduce efficiency. Moreover, switching noise can easily corrupt the sensed current signal. Therefore, instability caused by noise is common in a current-mode system [44, 45]. Instability problem also occurs at very low duty ratio caused by high input voltages and low output voltages, which is encountered in notebook computer systems.
Variable-frequency control, also called free-running control [46, 47], includes constant on-time, constant off-time control [39, 48–51], and ripple control [26, 52–57]. Free-running control is the simplest among all control topologies of switching power supply. It performs tightest control over output voltage (voltage mode) or inductor current (current mode).
Main advantages of the free-running control are fast transient response and wide duty cycle range. It also eliminates the need of external compensation parts, resulting in compact size and low cost. However, the switching frequency depends strongly on the operating conditions and power filters. Thus, the use is limited in noise sensitive devices.
Besides, only few related literatures provide analytical insights into this kind of control.
There exist many subjects to be investigated, such as loop gain transfer function and noise immunity.
Switching-mode converters have shown their advantage in conversion efficiency. Se-vere system requirements for future DC–DC converters demand new control schemes.
Traditional methods will not yield acceptable results. Control circuit design techniques of switching-mode DC–DC converters are main concerns of this dissertation. Free-running control is a strong candidate for a high performance DC–DC converter. Switching
fre-quency stabilization, loop gain compensation, and noise immunizing techniques of free-running control are important topics worthy of further investigation. Digital PWM control is another topic in this dissertation. Digital control of switching-mode converters has be-come a popular topic in these years. The uses of digital controllers are limited by their complex circuits and higher cost. It is desirable to build a simple and integrable digital controller to provide a high performance operation.