title: An Integrated, Electrolyte Capacitor-free DC-DC Converter with Low Ripple for Reliable and High Precision UV LED Applications
Creators: name: Frisch, Malte
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: High energetic UV radiation is very versatile in its application spectrum. It may be used in medical and industrial applications and is able to disinfect surfaces, air and water. With ongoing technological advancements, the UV LED market share is constantly rising and displaces the environmental harmful and fragile UV tubes over time. Since the emitted UV radiation of UV LEDs is direct proportional to the load current, a precise radiation intensity is only achieved with a low current ripple. Especially the fact, that any ripple of the UV LED supply voltage is causing a current ripple which is amplified by a factor of up to 25, makes a highly precise UV LED intensity control very challenging. To operate the UV LEDs in an electrical efficient way, a driver is required to generate the demanded UV LED voltage from a wide range of supply voltages. Due to their efficiency, DC-DC converters are a suitable fit for this application. The state of the art converters use relatively big electrolytic capacitors, to filter the output voltage. Electrolytic capacitors reach large capacitance values, but age fairly quickly, especially at elevated temperatures. Because UV LEDs still have low electrical efficiencies, the generated thermal stress in this application is very high. Since the electrolytic capacitors are statistically the most likely component to fail in the converter, no reliable UV LED driver circuit may be developed with their usage. An interesting alternative are ceramic capacitors, which are significantly more resistant to thermal stress and therefore more long-lasting. Their major drawback is their relatively small achieved capacitance. With the standard LC filter, the desired current ripple is not achieved with these small capacitance values. This is why in this thesis, a LCL filter is designed, which reaches a very low current ripple, even with the ceramic capacitors. With the usage of this LCL filter, a DC-DC step-down converter is developed which filters its output voltage exclusively with ceramic capacitors and inductors. The DC-DC converter is operated by a dedicated circuit which controls the converter and thus the UV LED current reliably and highly precise. To cover a wide range of applications, the miniaturization of the UV LED driver is essential and therefore the prototype is realised as an optimized and highly integrated circuit in a silicon process. This enables mobile disinfection applications or big UV LED arrays with a lot of required UV LEDs and drivers on very little space. The highly accurate shunt, UV LED temperature and UV radiation intensity measurements are fundamental for the precise and adjustable control of the UV LEDs. To use as little external space as possible and make the prototype more independent and versatile, all needed auxiliary blocks are also integrated on the chip. For example, the clock generator, the internal bandgap reference and all biasing circuitries are implemented on-chip. The control of the prototype is done by the digital core, which adjusts the current of the UV LEDs until it matches the desired load current. With a multitude of registers and individual settings, a variety of control modes may be configured for the measurement, test and characterization. Following, PCBs are designed which are necessary for the characterization of the prototype. To verify the sensor measurements of the chip, a photo current and a temperature diode voltage are generated with the help of the shunt voltage. With a developed software plug-in and the automation of the measurements, the prototype is characterized extensively. The measurements of the prototype show a very small current ripple (0.22 % at 500 mA UV LED current) with a maximum efficiency of 90 % and a total area consumption of all required components of less than 1 cm2. The highly precise current loop reaches an accuracy ≥ 98 % with a 240 μV shunt voltage deviation. Therefore, the main intention of an integrated, electrolyte capacitor-free DC-DC converter with low ripple for reliable and high precision UV LED applications is verified.