22 HEAT PUMPS https://toshiba.semicon-storage.com/ Issue 1 2025 Power Electronics Europe www.power-mag.com synchronous motors (PMSM) or brushless DC (BLDC) motors can be used to drive additional fans to boost the rate of indoor air distribution. Because heat pumps consume significant amounts of power, efficient rectification of the AC mains supply is an important requirement for the powersupply electronics. Designs will need to perform power factor correction (PFC) to prevent high levels of reactive power that are generated by simple rectification circuits, even with the help of capacitive smoothing. Active PFC circuits deliver better correction capabilities, using a boost-chopper configuration to control the flow of energy across multiple switching operations per AC cycle. This circuitry shapes the current waveform to follow the sine-wave profile of the input voltage. In doing so, it delivers a power factor close to unity. Though designers can choose from several methods of PFC, continuousconduction mode (CCM) will deliver the lowest current ripple. One consequence of this is the potential generation of a reverse-recovery current that is superimposed on the current that passes through the circuit’s inductor when each power transistor is switched off. The addition of a Schottky diode such as Toshiba’s TRS24N65FB helps minimise losses. Its silicon carbide technology leads to lower stored charge and with that low switching losses. A fast-switching insulated-gate bipolar transistor such as the GT30J65MRB further improves efficiency. When optimising power-conversion efficiency in a heat pump, the critical motor is that in the compressor. This motor, typically based on an AC architecture, will encounter the biggest changes in operating conditions, resulted by the alternating gas pressure. This leads to large differences in torque requirements. Field-oriented control The key to efficiency in this motor is to use advanced field-oriented control (FOC) rather than simpler methods, such as volts-per-hertz. FOC continually tunes the current supplied to the three groups of stator winding inside a typical BLDC motor to force the rotor to move from one position to the next. In contrast, simple control methods suffer from different flaws such as high torque ripples, excessive losses and slow reaction to rapid load condition changes among others. FOC considers torque as a central element in its strategy. The technique works by accurately modelling the magnetic field inside the motor. It uses that mathematical model to predict the motor’s state, including rotor position, at any point in time. As load conditions change and alter sensor inputs, FOC reacts to adjust the current and voltage supply to each phase. This responsiveness ensures that the motor operates at its optimal efficiency under varying load and speed requirements. A key benefit of FOC lies in its ability to reduce harmonic distortion in the electrical system. The method ensures the current waveform applied to the motor conforms to a smooth sinusoidal shape. This reduces the losses that are associated with harmonic currents. The motor generates less heat and overall system efficiency improves. The use of a mathematical model to determine the rotor state avoids the need to include position or rotary motion sensors in the system design. The algorithm can estimate rotor angle using just current feedback provided by a simple shunt resistor. High-speed processing The mathematical model demands complex arithmetic, which relies on repeated multiplications combined with sine and cosine trigonometric functions. This poses a problem for traditional MCUs. They rarely feature optimized instructions set for such specific tasks. To make use of these devices, developers need to implement software functions based on lookup tables for the trigonometric functions together with multi-instruction emulation of complex multiplications. The alternative is to employ a digital signal processor (DSP) that has required hardware support for these operations, though this may lack the infrastructure needed for interruptdriven input/output control that is also necessary for motor control. Though the pump motors can use relatively simple motor-control techniques, as they are not subject to the large torque changes of the compressor, there are user requirements that will complicate the design. The torque ripple of simple trapezoidal control signals often used to control BLDC motors results in unwanted noise and vibration. That is a problem in domestic environments where people may sleep close to the heat pump’s external unit. As a result, system designers may feel the need to employ multiple MCUs, possibly in combination with a digital signal processor (DSP), to implement the FOC calculations. However, careful attention to the needs of each of these subsystems leads to a solution where one system-on-chip (SoC) device can manage all the major functions of a heat pump.
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