4. Design of AC–DC Charge Pump

This chapter discusses circuit theory of AC–DC charge pump circuits. The input is a continuous wave with a single frequency or a multi-sine wave with multiple frequencies. An analytical, closed-form AC–DC charge pump voltage multiplier model is described to show the dependency of output current and input power on circuit and device parameters for continuous wave AC–DC charge pump. Then, it is expanded for multi-sine wave AC–DC charge pump. Analysis enables circuit designers to estimate circuit parameters, such as the number of stages and capacitance per stage, and device parameters such as saturation current (in the case of diodes) or transconductance (in the case of MOSFETs). In addition, design optimizations and the impact of AC power source impedance on output power are investigated.

Even though switched-capacitor voltage multipliers were originated with AC–DC, i.e., AC input and DC output, by Greinacher and Cockcroft–Walton, most voltage multipliers for integrated circuits (ICs) have been DC–DC, i.e., DC input and DC output for decades because almost all ICs work with DC input. Recently, wireless sensing nodes and implantable microelectronic devices have been attracting the interest of researchers and engineers. These devices use AC–DC rectifier voltage multipliers to receive power or to harvest energy in AC form. These applications require low power (typically nothing higher than hundreds of μW) and have small form factors—features that are well-matched with the features of voltage multipliers with no inductor or any magnetic element required. Section 4.1 discusses continuous wave AC–DC charge pump voltage multiplier which operates at a single frequency. Section 4.1.1 provides a circuit model which only includes DC voltage source, output resistance, and internal capacitance. Each parameter is expressed by circuit and device parameters. Section 4.1.2 investigates design and device parameter sensitivity on the pump performance. Section 4.1.3 discusses optimum design for maximizing output current at a given output voltage and for making a balance between circuit area and input power. Section 4.1.4 studies the impact of AC power source impedance on the pump performance and the dependency of design parameters on the AC power source impedance. Section 4.2 expands into multi-sine wave voltage multipliers where the AC signal has multiple frequencies. Section 4.2.1 provides a circuit model. One can estimate output and input power using the model equations when design and device parameters are given. Section 4.2.2 shows design and device parameter sensitivity on the pump performance. Section 4.2.3 investigates the effectiveness of multi-sine waves over continuous waves in terms of power efficiency.