Ever since the introduction of the topology, phase-shifted full bridge rectifiers (PSFB) have gained a wide range of acceptance in industrial applications like power supplies, telecom rectifiers, battery charging systems, renewable energy systems, and so forth. Due to the ease of controllability, the PSFB rectifier is often selected as the go-to method for transferring bulk amounts of power from the high-voltage (HV) DC to the low-voltage (LV) DC side with electrical isolation. One such simple control scheme that can be used in this application is called peak current mode control (PCMC). The power transfer happens from the HV to LV side when the turn ON period of diagonally opposite switches of two legs overlap with each other. In PCMC, this overlapping period is controlled by making the appropriate peak current value setting. On top of this, an outer voltage control loop is implemented to maintain the output voltage at the desired level. In this work, the control mechanism is implemented using ARM R5F core-based SITARA MCU of Texas Instruments (TI) to showcase the real-time control capabilities. In addition to that, a synchronous rectification feature is implemented on the LV side to harness the optimized power output from the converter.
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This document presents how an AM263x MCU can be used for controlling the bidirectional DC-DC converter power stage of the ASIL D Safety Concept-Assessed High-Speed Traction, Bidirectional DC/DC Conversion Reference Design. This hardware is compatible with MCUs in an HSEC controlCARD™ format. While originally designed for the C2000™ MCU product family, this reference design can accept the AM263x controlCARD (TMDSCNCD263) also with minimal modifications. This reference design is useful for implementing a digitally peak current mode controlled (PCMC) phase shifted full bridge (PSFB) DC-DC converter which converts a 400V DC input to a regulated 12V DC output and vice-versa. Although the design is capable of performing bi-directional power conversion, the scope of this application note is limited to illustrate only high voltage to low voltage conversion, that is, step down operation. Novel PCMC waveform generation based on type-5 PWM and internal slope compensation, and simple PCMC implementation are the highlights of this design.
Phase-shifted full bridge (PSFB) DC-DC converters are used frequently to step down high DC bus voltages and provide isolation in medium- to high-power applications like server power supplies, telecom rectifiers, battery charging systems, and renewable energy systems. Traditionally, microcontrollers have been restricted to only performing supervisory or communications tasks in these systems. With the availability of high-performing microcontroller devices, it is now possible to use microcontrollers for closing control loops in these systems, in addition to handling the traditional microcontroller functions. The transition to digital power control means that functions that were previously implemented in hardware are now implemented in software. In addition to the flexibility this adds to the system, this simplifies the system considerably. These systems can implement advanced control strategies to optimally control the power stage under different conditions and also provide system-level intelligence.
A PSFB converter consists of four power electronic switches (like MOSFETs or IGBTs) that form a full-bridge on the primary side of the isolation transformer and diode rectifiers, or MOSFET switches for synchronous rectification (SR) on the secondary side. This topology allows all the switching devices to switch with zero voltage switching (ZVS), resulting in lower switching losses and an efficient converter. In this reference design, ZVS for switches in the one leg of the full bridge, and zero or low-voltage switching for switches in the other leg, is achieved.
For such an isolated topology, signal rectification is required on the secondary side. For systems with low output voltage or high output current ratings, implementing synchronous rectification instead of diode rectification achieves the best possible performance by avoiding diode rectification losses. In this design, current doubler synchronous rectification is implemented on the secondary side with different switching schemes to achieve optimum performance under varying load conditions.
A DC-DC converter system can be controlled in various modes, like voltage mode control (VMC), average current mode control (ACMC), or peak current mode control (PCMC). PCMC is a highly desired control scheme for power converters because of the inherent voltage feed forward, automatic cycle-by-cycle current limiting, flux balancing, and other advantages.