60 September/October 2025 | E-Mobility Engineering In many of the control schemes for a Vienna rectifier a rotating reference frame type of control structure is used. These schemes rely heavily on computation of sine and cosine values. With the faster switching frequencies in Vienna rectifiers, it’s even more important to compute these routines quickly. For example, compared with MCUs that only have a floating-point unit, the sine instruction with the TMU instruction set can execute a sine instruction in four pipelined cycles, compared with up to 41 cycles. The chip also includes a Control Law Accelerator (CLA) as a secondary core that is available to offload Interrupt Service Routine (ISR) control tasks from the main controller core. This can be used as a parallel processing unit to run the control loop faster, thereby enabling higher switching frequency control of the Vienna rectifier. The CLA can enable control loops for Vienna rectifiers of up to 200 kHz using parallel processing on 100 MHz devices such as F280049. With a device rated at over 200 MHz, up to 400 kHz control loop frequencies are feasible with control algorithm code still written in the C high-level language. With assembly code, even higher control loop frequencies can be achieved with C2000 real-time MCUs. Other blocks also help to simplify the fast-charger design. The Analogue to Digital converter (ADC) enables highly accurate sampling and measurement resulting in better THD, while an integrated comparator subsystem (CMPSS) integrates protection for overcurrent and overvoltage without the use of any external circuitry, thus making the board smaller and lower in cost. A crossbar architecture in the chip means trip events from multiple sources such as the CMPSS and I/O ports are available to flag gate driver faults in a quick and easy manner, again without need for external logic or circuitry. As AC signals are sensed for the voltage and current, the first step in using these values for control is removal of the DC-offset. Even for non-AC signals, removal of DC-offset can be an important first step to meet system voltage regulation requirements. An ADC Post Processing Block (PPB) enables automatic removal of the offset from the sensed signal in hardware to allow the control loop to directly read a signed register value. This saves cycles used to load offsets and subtract offsets from the critical path in the Interrupt Service Routine (ISR). Furthermore, the ADC PPB block can save cycles by incorporating offset subtraction in hardware. For example, on the Vienna rectifier, a minimum of eight signals are needed to be sensed with offsets subtracted (three voltages for AC input, three currents for AC input and two output bus voltages). Operations to read a single signal require reading the ADC, reading the offset, subtracting the offset, scaling the result and storing in memory. This can take up to 12 cycles, even with optimised assembly code. With the PPB, it can be reduced to eight cycles, marking a 33% improvement in execution of these operations. The CPU load for computing a Vienna rectifier control loop can also be extensive involving reading of eight signals (in addition, oversampling may require even more CPU bandwidth), execution of four controllers, and updating the PWM. For example, even with the TMU and the ADC PPB for a 50 kHz control loop on a 100 MHz device, the CPU load is ~47%, out of which 37% is for the main control ISR (50 kHz) and ~8.5% is for the instrumentation ISR at 10 kHz. The Vienna rectifier is a three-phase power topology; therefore, it requires multiple comparators and references. For example, just to implement current protection, two op-amps, six comparators, and additional resistors and capacitors may be needed to implement a simple overcurrent trip for all three phases. The CMPSS is connected to the PWM module and can enable fast tripping of the PWM. The TMS320F280049 saves board space because the rectifier design can take up to five CMPSS windows on the TMS320F280049 MCU: three for current sensing each of the three phases and two for sensing on the DC bus. A Vienna rectifier (Y-connection) topology for EV charging stations (Image courtesy of Texas Instruments)
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