A CMOS MPPT Power Conditioning Circuit for Energy Harvesters Francarl Galea 1 , Owen Casha, Ivan Grech, Edward Gatt and Joseph Micallef Department of Microelectronics and Nanoelectronics University of Malta Msida, Malta 1 E-mail: francarl.galea@um.edu.mt Abstract—This paper presents the design of an ultra-low power consumption power conditioning circuit with an analogue maximum power point tracker to maximize the scavenged power generated by energy harvesting devices. Excess power is stored in a large storage capacitor to be used once the power generated by the harvester falls below the power required by the load. The circuit was designed to work with various types of energy harvesters such as solar cells and piezoelectric devices, thus making it compatible with both AC and DC input voltages. The AC/DC-to-DC converter topology employed, does not make use of any rectifier, in order to maximize the power efficiency of the converter. This power conditioner was designed using the AMS High Voltage 0.35 μm CMOS technology. The power conditioner cold starts with a minimum input voltage of 1 V and can subsequently operate with a wide voltage range of 0.2 V to 50 V to be compatible with various types of harvesters. It consumes 250 nW and can work with harvesters capable of generating power in the microwatt and milliwatt range. Keywords—Low Power, Charge Controller, Power Conditioning Circuit, Energy Harvesting, MPPT, Scavenging Devices. I. INTRODUCTION Energy harvesters are small devices which are capable to extract energy from their surroundings. These harvesters are able to power small devices, thus eliminating their dependence on batteries. Although energy harvesters are a challenging solution, they have the potential to replace batteries in low power applications. Such devices would use harvested electrical energy to power their associated microprocessor, conditioning circuitry and communication interface. There are numerous methods of energy harvesting including solar energy harvesting via solar cells, thermoelectric, radio frequency, piezoelectric and pyroelectric materials. In contrast to batteries, the output characteristics of energy harvesters are highly volatile and generate power which is dependent on their load and ambient conditions. The generated voltage is typically variable and discontinuous. This means that a power conditioning circuit is required to manage the harvested power and generate a clean constant voltage as required by the load. The power conditioning circuit should store any excess power so that when the energy harvester’s output power drops, it will be able to sustain the load requirements. The efficiency of the power conditioner is crucial since the power generated by energy harvesters is very small and any inefficiency will further reduce the net available power. Several publications presenting integrated MPPT circuits are reported in literature. All these circuits were implemented in analogue electronics. Only a few circuits offer a wide input voltage range and power range and they make use of either passive or active rectification leading to a reduction in efficiency. Some MPPT circuits were specifically designed to work for a particular harvester [1-3]. This paper presents the design of a novel generic integrated power conditioner having an input AC/DC voltage range of 0.2 V to 50 V and a cold start voltage of 1 V, to work with various types of energy harvesters within the microwatt and milliwatt power range. All the stages of the power conditioner were designed for minimal energy usage, thus maximizing its efficiency. Rectification is being achieved by implementing a direct AC to DC converter rather than including a rectification circuit thus eliminating power loss. Voltage Regulating DC – DC Converter Load Energy Harvester Storage Capacitor AC/DC – DC CONVERTER MPPT Fig. 1: Block diagram of the proposed power conditioner. II. POWER CONDITIONER ARCHITECTURE The proposed integrated power conditioner is shown in Fig. 1. The first stage, consists of an AC/DC-DC converter being controlled by a maximum power point tracking controller (MPPT), and stores the energy extracted from the harvesting device in a storage capacitor. This paper mainly focuses on this stage. The MPPT is implemented using analog electronics in order to work as efficiently as possible and making it suitable for low power harvesters. The capacitor voltage may reach a maximum voltage of around 50 V. This high voltage can be easily reached since some energy harvesters, such as certain piezoelectric energy harvesters, are able to generate voltages in excess of 15 V at nominal vibration levels [4]. Hence, in order for the MPPT to operate correctly, it was designed to work over a wide input voltage range and was implemented using the AMS High Voltage 0.35 μm CMOS technology having CMOS transistors with a breakdown voltage of 50 V. Another advantage of storing a high voltage in the energy storing The research work disclosed in this publication is funded by the ENDEAVOUR Scholarship Scheme (Malta). The scholarship may be part-financed by the European Union – European Social Fund (ESF) under Operational Programme II – Cohesion Policy 2014-2020, “Investing in human capital to create more opportunities and promote the wellbeing of society”.