# An Improved Rectifier Circuit for Piezoelectric Energy Harvesting from Human Motion
this method requires a large inductor, in the order of tens to hundreds of Henry, which is impractical [25]. Another possible way of maximising power from the PD is to directly attach the PD to the storage device/capacitor. The concept of nonlinear circuits or synchronised switch harvesting on inductor (SSHI) has shown significant improvement in power extracted from the PD. A nonlinear technique was proposed by [26], it demonstrated meticulous power extraction and claimed under similar conditions/excitations the proposed circuit has dramatically improved the extracted power by several hundred percent when compared to the FBR circuit. To implement the switching of SSHI circuits, self-powered SSHI circuits have been proposed in [21; 27; 28]. Furthermore, the self-powered SSHI was proposed in [29] with some discrete electronic components. The experimental results showed significant improvement in power extraction from PD, which is 1.6 times higher than the FBR circuit. However, the diodes used in the proposed circuits caused a voltage drop in the conversion process. As a result, the conversion losses are higher than the power generated by the PD. In addition, based on the literature, the losses in the switching process are high. Therefore, there is a need to develop a circuit, which offers low losses and constant output voltage. To minimise the forward voltage loss across the diodes, the diode-less circuit was proposed by [PERSON] [30] as shown in Figure 3.  
The study did not show the output power extracted from the proposed circuit. No comparison was made with conventional circuits. Practical application and appropriate electrical boundary condition were also not considered. [PERSON] et al. [31] has shown that the use of MOSFET bridge rectifier circuit, is able to reduce the power loss during the conversion process by minimising the forward voltage loss across the diodes. The MOSFETs used in the circuit are not self-powered. As a result, the switching losses of MOSFETs are high. Consequently, the power loss in switches is higher than the power generated by the PD. A recent study by [PERSON] et al. [32] improved the above MOSFET bridge rectifier circuit to a self-powered H-Bridge circuit. The proposed circuit was successfully applied for ultra-low voltage and high frequency application. The outcome shows that the proposed circuit notably increases the voltage and the power produced from the PD when compared to traditional FBR circuits. However, this study mainly focused on the rectification process  
Figure 3: The diode-less circuit. Figure 2: (**a**) Bridgeless split capacitor topology-based converter, (**b**) Boost converter having secondary switches. at high frequency; its applicability in low frequency and high amplitude human induced excitation is not considered. So far, much research has been conducted into rectifier circuits for PEH. However, the literature review conducted by the authors also concluded that previous studies mainly adopted a controlled sinusoidal vibration source, usually generated by a mechanical shaker. When human motion is the source of mechanical energy, the AC-DC conversion is more challenging. The AC voltage generated from human motions, such as walking, running, and cycling, is not purely sinusoidal. The generated waveform is often at low frequency, has high amplitude and is relatively unstable due to the complicated nature of such motions [5; 33]. A typical AC waveform generated from a PD with a cantilever beam attached to mechanical shaker is shown in Figure 4.  
Thus far, the suitability of existing rectifier circuits in converting AC voltage generated by PD as a result of low frequency human motion induced non-sinusoidal vibration is unknown. There is thus a need to develop a self-powered rectifier circuit with low losses and capable of providing a stable DC voltage supply under such application. In this paper, a modified H-Bridge circuit is proposed to convert AC into DC for human motion application, which is an extension of works by [PERSON] et al. [32], [PERSON] et al.