Gheorghe Pristavu

Variable Off-Time Control Loop for Current-Mode Floating Buck Converters in LED Driving Applications

A versatile controller architecture, used in current-mode floating buck converters for LED driving, is developed. State-of-the-art controllers rely on a fixed switching period and variable duty cycle, focusing on current averaging circuits. Instead, the proposed controller architecture is based on fixed peak current and adaptable off time as the average current control method. The control loop is comprised of an averaging block, transconductance amplifier, and an innovative time modulator. This modulator is intended to provide constant control loop response regardless of input voltage, current storage inductor, and number of LEDs in order to improve converter applicability for LED drivers. Fabricated in a 5 V standard 0.5 μm CMOS technology, the prototype controller is implemented and tested in a current-mode floating buck converter. The converter exhibits sound continuous conduction mode (CCM) operation for input voltages between 11 and 20 V, and a wide inductor range of 100-1000 μH. In all instances, the measured average LED current variation was lower than 10% of the desired value. A maximum conversion efficiency of 91% is obtained when driving 50 mA through four LEDs (with 14 V input voltage and an inductor of 470 μH). A stable CCM converter operation is also proven by simulation for nine LEDs and 45 V input voltage.

Two Packaging Solutions for High Temperature SiC Diode Sensors

A partially electrically isolated package with a gold wire and fully isolated solution with a metallic piston, respectively, are designed and tested for high temperature sensors (400°C) based on SiC Schottky barrier diodes (SBD). Electrical behavior and sensor performance are very close for both packaging solutions. The stress due to contact pressure and higher cost are some disadvantages for pressure contact technology.

Variable off time current-mode floating buck controller - A different approach

Switch-mode power supplies have become the premier choice for LED backlighting applications. This paper introduces a new current-mode floating buck controller architecture, used for driving constant current through a string of LEDs. Unlike state-of-the-art controllers, this architecture is based on fixed peak current and variable OFF time as the current control method. An innovative time modulator is added to provide a constant feedback loop response regardless of external components. The proposed controller architecture is designed, implemented in a 0.5μm CMOS process and its performances are validated by simulations and measurements.

Time modulation — The exponential way

A solution for obtaining an exponential variation of time versus input voltage of a time-modulating circuit is investigated, implemented and its performances are measured. Most time-modulating circuits produce a delay that is linearly dependent on the input voltage. This paper proposes a new method of controlling the time delay output by making it vary exponentially with the input voltage. This solution is especially useful in aplications where the time-delay is constantly modulated by a prior error amplifier.

Temperature behavior of 4H-SIC MOS capacitor used as a gas sensor

Temperature behavior of 4H-SiC MOS capacitor is investigated. A new layout with several active MOS structures is proposed. Measured C-V characteristics show good thermal stability up to 300 C. The main electrical parameters of the MOS structure have been extracted by measurements on wafer and encapsulated samples.

SiO2/4H-SiC interface states reduction by POCl3 post-oxidation annealing

Interface state density (D_it) at SiO₂/4H-SiC interfaces is investigated using capacitance-voltage (C-V) characterization. Two different measurement methods for D_it determination (both C-V at different temperatures in the range of 80-300K and high frequency (HF) vs quasi static (QS) characteristics) have been used. A significant reduction of D_it is observed, almost one order of magnitude, after a post oxidation annealing (POA) in POCl₃ ambient, compared to as-grown dry oxidized sample.

Electrical Characterization of Ni-Silicide Schottky Contacts on SiC for High Performance Temperature Sensor

The electrical behavior and stability of a temperature sensor based on 4H-SiC Schottky diodes using Ni2Si as Schottky contact, are investigated. The ideality factor and the barrier height were found to be strongly dependent on the post-annealing temperature of the Ni contact (which lead to the formation of Ni2Si). A nearly ideal Schottky device, with the barrier height approaching the high value of 1.7eV, and a slight temperature dependence, was obtained after an annealing at TA=800°C. This high barrier height proves that Ni2Si is suitable as Schottky contact for temperature sensors, able to reliably operate up to 450°C. Sensor sensitivity levels between 1.00mV/°C and 2.70 mV/°C have been achieved.

Overcoming leakage current by output precharging in error amplifiers

Initial control loop conditions are essential in switching circuits where fault detection blocks are present, in order to avoid permanent fault triggers. Furthermore, multi-channel switching interference may affect circuit functionality and cause additional fault triggers. Error amplifiers aquire the difference between sensed and reference signals and output a correction voltage. We propose a solution which ensures that the initial correction voltage does not determine a perpetual fault condition. The proposed solution is implemented in a current mode buck control loop, which delivers constant current through a load.

Offset cancellation in bandgap references with CMOS operational amplifiers

Two architectures of a bandgap reference with CMOS operational amplifiers are investigated, implemented and compared. In order to reduce the dependence of the reference voltage with temperature and power supply, the trimming technique is often used. We propose a new method of lowering the reference voltage variation, based on a very low offset voltage amplifier. Two operational amplifier schematics are designed, simulated and measured.

An Investigation of SiC Schottky Contact Barrier Inhomogeneity for Temperature Sensing Applications

This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.