An FPGA Controlled DC/DC Step-Up Converter for Automotive Applications

Piezoelectric injectors allow to inject precise proportional quantities of fuel at extremely high speed. Controlled by pulses from the engine management unit, they work like solenoid valves, opening and closing back very rapidly, spraying the exact quantity of fuel determined by the injection computer into the engine combustion chambers. However, for this type of injectors an impulse of a hundred volts is applied in order to actuate them. This is a big concern in automotive environment, where the 12 V battery voltages is still the main power source available. Therefore, a power converter is needed to interface battery voltage and piezo actuator; for mechanical and electrical reasons (weight, dimensions and power losses) a switching amplifier is preferred with respect to linear amplifier, especially for this type of applications where efficiency and weight are the main issues. Fig. 1 presents a switching amplifier for piezo actuator: it consists of two components, a unidirectional DC/DC converter with a small input power that loads a large buffer capacitor and a bidirectional DC/DC converter, which controls the energy alternating between the buffer capacitor and the piezo actuator. The requirements on the unidirectional DC/DC converter are few. It only needs to compensate for the power losses of the two stages plus the energy dissipated in the actuator and the connected mechanical system [6]. Conventional DC/DC boost converter is not the best solution in piezoelectric based applications where a high step up ratio (more than 20) and high efficiency power conversion is required. Flyback converter could be an alternative topology but presents high switching losses and it has an insulated output, so not compliant with electrical load specification. An efficient high-step up DC/DC converter is presented for interfacing the standard 12 V battery with the high voltage DC-bus used for piezoelectric actuators system. In the proposed converter, two coupled inductor boost converters are interleaved and controlled by an FPGA. Different types of control techniques could be used to limit the input current [7]; the solution presented herein allows fast transient response and best performances with respect to traditional control techniques. Moreover the implemented technique has the advantages of lower electromagnetic emission and higher operating frequency capability, compared to the standard fixed frequency control; on the other hand it is difficult to design the EMI filter due to the intrinsic variable frequency. The high computational load of the control algorithms forces the use of an FPGA implementation in order to achieve the desired accuracy in real time control at the frequency of the PWM signal needs for the DC/DC converter, which will be 200 kHz. Design and analysis of the proposed converter are presented. Finally, experimental results are reported in order to validate the proposed converter.