M OTIVATION

As described in section 1.3, a desirable DC-DC converter has high efficiency, compact size, wide operating range, fast transient response and can operate safely.

Fig. 1.1 Voltage-mode control in SMPS

There are many control methods and circuits developed to meet these requirements. But each one has some disadvantages. For example, voltage-mode control is one of the commonly used control method in SMPS (Fig. 1.1). Design of this control scheme is simple and straightforward. It has only one feedback loop and is easier to design and analyze. The output voltage is controlled by directly changing the duty ratio. Therefore, this control scheme is also called direct duty control. It can provide good noise margin if you use a large amplitude ramp in determining the duty ratio. It also has low impedance output and can provide good line regulation, load regulation and even cross regulation for multiple outputs. However, its application is limited because any change in line or load must first be sensed as an output change and then corrected by the feedback loop. This usually means slow response. Another disadvantage is that the output filter adds two poles to the control loop. We need to add a dominate-pole in much lower frequency or add a zero to compensate it. The loop gain varies with the input voltage also makes the compensation difficult.

Fig. 1.2 Current-mode control in SMPS

Another popular method is current-mode control, which is also called current programmed control and current injected control, was introduced in 1978 [26]-[35]

(Fig. 1.2). Since the inductor current rises with a slope determined by input and output voltages, this waveform will respond immediately to line voltage changes, eliminating both the delayed response and gain variation with changes in input voltage. This control scheme utilizes dual loops to control both output voltage and inductor current.

It effectively eliminates the phase lag of the filter inductor and makes loop compensation easier. Since the current information is sensed cycle-by-cycle, we can see an additional benefit of easier cycle-by-cycle current limiting. The current-mode control has shown more attractive characteristics than voltage-mode. 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 [36]-[37].

Instability problem also occurs at very low duty ratio caused by high input voltages and low output voltages.

There are still many other methods of implementing SMPS. All these circuits have their own advantages and disadvantages. We choose current-mode control as our start point and try to investigate the possibility of eliminating the disadvantages. We want to find a way that a high efficiency, compact size and safely operated SMPS can be easily designed for different applications.

1.5 R

G

C

As being power electronics engineers, providing good quality electrical power is our responsibility. How to improve the quality of supplied power with lower cost is

our research goal. Fast response, tight regulation, high efficiency, high stability, compact size and safety operation are the key factors of a high performance power converter. We focus on the integrated circuit implementations. We had implemented a monolithic current-mode buck converter using the new developed control and protection circuits. All the developed techniques can be reused in the SMPS IC design for other applications. The original contributions of this work are in three main points.

First, the on-chip soft-start circuit which occupies small silicon area and do not need extra pin-out. This circuit helps to protect the power source and the converter itself at the very beginning of the power on of the converter. It eliminates the inrush current when the converter is powered on. The inrush current may cause severe voltage drops at the power source and may cause the system faults or other damages to the power source, the circuit directly powered by the power source and the converter. Traditional circuit utilized a big capacitor or other costly circuit to do soft-start. Our approach using a simple circuit and only occupies a very small silicon area to achieve the same function as well as the traditional circuits.

Second, the Dynamic Partial Shutdown Strategy (DPSS), which increases the conversion efficiency especially in light load operation. The power conversion efficiency is very important especially for portable devices that utilize batteries as their power sources. The DPSS successfully reduces the operating current wasted by the control circuits. The efficiency is improved especially in light load operation. This means a longer standby time can be achieved using this strategy.

Third, the slope compensation and the over-current protection circuits which take advantage of the existing current sense method. As described in section 1.4, the current sense circuits usually cause power loss and greatly reduce the conversion

efficiency. We choose a quasi-lossless current sense circuit to improve the efficiency in the current-mode control. At the same time, the current sense circuit helps the development of the new slope compensation and over-current protection circuits.

Although the compensation of current-mode control has more flexibility, it is not easy to implement a stable current-mode SMPS because of its inseparable dual feedback loop. The slope compensation in the current-mode control was developed for stabilize the current-mode converter. Traditional methods of implementing slope compensation are complex and may induce signal distortion in the control loop. We proposed a simple circuit to improve the slope compensation. The new over-current protection circuit also takes advantages of the current sense circuit. This simple circuit provides fast response to ensure safety operation under over-loaded conditions by very low power consumption.