A comparison of different types of switches used in synchronous buck converter design

A COMPARISON OF DIFFERENT TYPES OF SWITCHES USED IN SYNCHRONOUS BUCK CONVERTER DESIGN SUMMARY Direct current converters in power electronic circuits are being used in a large application area from home appliances, automotive, smart phones&tablets to information technologies. Efficiency has become the most critical parameter in designs according to international quality standards. Depending on this expectations, converters are shaped through low losses and high efficiency. Direct current converters have three types as Buck, Boost and Buck-Boost. This systems regulate the output voltage to lower or higher levels from input voltage. There is a main switch that operates on conduction or cut-off mode in buck converters. Switch is triggered by a pulse width modulated (PWM) signal. The duty cycle of the PWM signal is calculated by the ratio of output and input voltage levels. So, the duty cycle indicates operation mode of the converter in steady state operation. A classical buck converter has an additional filter inductance and capacitor. These components conduct the voltage and current to the output of the circuit. When the switch is in conduction mode, the power is stored in inductance and the desired output voltage obtained by the charged capacitor. In the design, a free wheeling diode is also connected reversly parallel to the load. When the main switch is in cut-off mode, the stored power in the capasitor during previous stage feeds the load. Diode conducts the current and preserves the current direction in this stage. One period cycle is completed after the conduction mode of the main switch begins again. At the instances that conduction or cut-off mode of the switch is started, losses and voltage spikes occur. Spikes also cause resonance on the filter inductance and capacitor. Therefore magnetic field permeability of these components increase. Resonance also affects the output wave form of both the switch and the converter. Another loss in system is caused by the diode related to its internal resistance in conduction mode. In previous studies, low power and low voltage converters were designed firstly. With the improved technological properties, high frequency switching ability of the switches was increased. This paved the way to high power designs. Later on, studies to increase the efficiency began by reducing magnetic field affects and losses. Therefore, synchronous buck converter design for low frequency and high power applicationswere ignited. Synchronous buck converter is more efficient and has less losses than other classical type converters. Synchronous buck converter has two controllable switches (one more switch in place of free wheeling diode) which is different form classical design, a filter inductance and a capacitor as in classical design. In steady state opeariton mode, the circuit is analysed by the assumption that the voltage of inductance and current of the capacitor are equal to zero. The low-side switch of the synchronous buck converter has lower internal resistor than the diode in the classical converter. Therefore, synchronous buck converter has lower conduction losses than that of the classical one. Selection of the switch in a converter design directly affects the performance and the efficiency of the whole system. A change in the low side switch affects the performance of the converter. For example, IGBTs are efficient in relatively lower frequency applications, while MOSFETs are preferred in high frequency operation. New generation switch type, GaN, can operate in both of high frequency and high voltage levels efficiently. Consequently, switch type is a critical parameter on the whole performance of the converter. In this study, simulation of a synchronous buck converter that may include IGBT, MOSFET and GaN switches instead of free wheeling diode is performed and three designs including three different switches are practically compared. For comparison, operatonal uantities like voltage drop, distortion of outputvoltage and current, input current are taken into account under constant high switching frequency and changing output power contraints. In the synchronous buck converter design, operational parameters were chosen 400 V DC as input voltage, 200 V DC as output voltage, 4 kW as output power and 100 kHz as switching frequency. Optimum operation is provided by using a duty ratio of 0.5. A tolerance of ±20% for input voltage, 1% for output voltage and 10% for output current ripple was predicted. Steady state analysis is performed for the synchronous buck converter with the accepted design parameters with a switching frequency of 100 kHz. Limit values for filter inductance and capacitor are calculated by the related equations in steady state operation mode. These values were calculated to provide safe operation even in the worst operational conditions. MATLAB simulations were completed with the calculated inductance and capacitor values. Since the aim of the study is to obtain practical comparison data and simulations were performed to verify the design parameters, only one type of switch (IGBT) is used as low side switch for verification. In simulation stage, different operating conditions for both open and close loop control were analysed and the gate and emetor-collector signals in conduction cut-off intervals were confirmed to be as predicted. Because aiming to get power losses in synchronous buck converter with different type of switches in application, it seems enough that study has only open loop control design. Close loop control is only simulated for analysing of output magnitudes. Genetic algorithm method is used to generate needed PI coefficients in simulation. Physical design of the synchronous buck converter is realized according to the open loop operation results of simulation. Microcontroller is used to drive the switches controllably in the physical design of converter. STM32F103ZCT6 of STMicroelectronics was choosen as processor in the design. In the PWM channel of the processor two conjugate PWM signals were produced. The amplitude of the produced PWM signals were strengthened by gate driver circuits. Triggering of the gate pins were provided. All IGBT, Mosfet and GaN switches were chosen TO-247 type in order to provide the same circuit board to be used for all switches and this type was added to the circuit scheme. Design circuit were tested in the laboratory with the different switches. On the converter circuit board 12 V, 5 V and 3.3 V were produced to supply the processor and the control units. Physical design is manufactured in the way to provide the production of the interrior supply voltages on the same circuit board. All circuits are designed in ALTIUM Designer program and then realized. The open loop control algorithm of the converter circuit is prepared by C programming language in "Keil uVision V5.20". By setting up the initial parameters of the processor timer and pins that produce the PWM signals, triggering signals for low an high side switches were arranged. The comparison of practical test results in open loop operation of the circuit with IGBT, MOSFET and GaN switches. The voltage stability, switching losses and efficiency were made by installing IGBT, MOSFET and GaN switches individually. The most efficient switch is determined by comparing the voltage stability, switching losses and efficiency values for the predicted operational quantities. After practical comparison GaN switches are mentioned to have less switching losses and operates more efficiently.