*3.2.1 Diode rectifiers*

Diode-based rectifier circuits are the most common because they have a lower forward voltage drop compared to the CMOS circuits. In rectenna applications, Schottky barrier diodes are widely used due to offering the best alternative to achieve higher PCE, a diode with a lower forward voltage.

The simplest rectifier circuit consists of a series shown in **Figure 11a** (or parallel in **Figure 11b**) and a parallel (or series) capacitor. The series diode circuit is also known as Villard Rectifier or DC restorer. The waveform produced is shown in **Figure 12a**. The parallel version is the well-known half-wave rectifier. When AC voltage comes through D1, only the positive cycle goes in the output, as shown in **Figure 12b**. Because of the reduction of the input, the full-wave rectifier, as shown in **Figure 11c**, is the most popular circuit. The output voltage sees two capacitors in series (each one is storing a voltage of Vpeak). Thus, Vout is a DC voltage twice the Vpeak, as shown in **Figure 12c**. For this reason this circuit is also known as a single-stage voltage doubler circuit or Cockroft Walton voltage doubler.

**Figure 11.** *The four typical configuration of the rectifier: (a) series, (b) parallel, (c) full-wave (d) bridge rectifier.*

*Wireless Power Transfer DOI: http://dx.doi.org/10.5772/intechopen.99990*

**Figure 12.**

*Voltage waveforms (y-axis) simulated towards time (x-axis). The input test is a 1 Volt peak-to-peak (10 kHz) voltage. For each configuration of the rectifier, the red colour represents the input voltage and the blue represents the output voltage, respectively: (a) series rectifier waveforms, (b) parallel rectifier waveforms, (c) full-wave rectifier waveforms, (d) bridge rectifier waveforms.*

Therefore, this topology is more stable and efficient than the halfwave rectifier. There is also the bridge rectifier shown in **Figure 11d**, which rectifies both positive and negative. The figures in **Figure 12** summarise the waveforms obtained. As we can see, the full-wave and the Bridge rectifier "double" voltage have the highest output voltage, as shown in **Figure 12d**.

Different configuration of circuits that convert AC to DC by increasing the values goes with the name of voltage multiplier. The most fundamental configuration is the Cockcroft–Walton voltage multiplier shown in **Figure 13a**. This circuit's operational principle is similar to the full-wave rectifier but has more stages for higher voltage gain. The Dickson multiplier in **Figure 13b** is a modification of Cockcroft–Walton's configuration with stage capacitors being shunted to reduce parasitic effects. Thus, the Dickson multiplier is preferable for small voltage applications. However, it is challenging to obtain high PCE due to the fact that the high threshold voltage among diodes creates leakage current, thus reducing the overall efficiency. Additionally, for high resistance loads, output voltage drops drastically leading to low current supply to the load.

#### **Figure 13.**

*Most common voltage multiplier configurations: (a) three stages Cockcroft–Walton voltage multiplier, (b) four stages Dickson voltage multiplier, (c) four stage Dickson voltage multiplier using CMOS technology, (d) two stages voltage multiplier comprised of differential drive unit.*

### *3.2.2 MOSFET rectifiers*

Limitation of diodes can be overcome by MOSFET technology. Major advantage of MOSFET is the fast switching speed. Dickson charge pump is also designed using MOSFETs in order to merge it in integrated circuits as shown in **Figure 13c**. Relatively low threshold voltages and high PCEs are features of this design.

Moreover, differential drive voltage multiplier **Figure 13d** is widely used because of its low leakage current and potential for further modification in specific applications. The number of stages in a voltage multiplier has a close relationship with its sensitivity and efficiency. If the number of stages grows, the amount of losses per stage increases. However, the tradeoff consists of a higher voltage multiplication and small threshold voltage at the first stage. On the other hand, a voltage
