Digital control of a multi-channel boundary-conduction-mode boost converter for power-factor-correction applications
Ryan, Robert T.
University College Cork
This thesis focuses on the design of digital control schemes for multi-channel boundary-conduction-mode (BCM) boost converters. Multi-channel BCM boost converters are commonly used for the front-end power-factor-corrected (PFC) stage of isolated ac-dc power supplies due to the advantages of being low cost and having high efficiency for a universal line-voltage input. Single-channel and two-channel BCM boost converters using analog control ICs have been commonly used in industry. However, the use of multi-channel BCM boost converters with more than two-channels has been limited as there are no analog control integrated circuits (IC) existing on the market with the ability to control BCM boost converters with more than two channels. Digital microcontrollers are an enabling technology, which can be used to implement a control scheme for a multi-channel BCM boost converter with any number of boost-converter channels. Moreover, digital microcontrollers have the added benefit of reducing the power supply’s overall system cost. For example, in an ac-dc medical power supply, there is typically a dedicated analog control IC for the PFC stage, a dedicated analog control IC for a dcdc isolated stage, and a low-power microcontroller used for safety and house-keeping functions, such as reducing standby power, detecting line-fault conditions, providing external communications, etc. The total system cost is reduced by replacing these three chips with a single microcontroller, which provides all the same functions. This requires the development of digital control algorithms which enable the microcontroller to match the performance of the analog control IC for the PFC stage. These functions include providing a well-regulated output voltage, ensuring the input current has high power quality, and permitting interleaving between the different boost-converter channels. It is difficult to have a well-regulated output voltage for two reasons. Firstly, the controller must provide fast output-voltage dynamics over the universal line-voltage range from 85 Vrms to 265 Vrms. Secondly, the output voltage of PFC rectifiers contains a 2nd harmonic ripple which can be fed into the control loop and distort the line current. In this work, an adaptive notch filter which works over a range of line frequencies, is designed to attenuate the feedback of the 2nd harmonic ripple. The notch filter allows the voltage compensator to be designed at a higher bandwidth, thus ensuring fast output-voltage regulation. Moreover, an adaptive voltage-compensator gain is used to guarantee fast output-voltage regulation at all line voltages. BCM boost converters have a variable switching frequency. Hence, a phase-shift control scheme is used to allow interleaving between the different boost-converter channels. It is important that the phase-shift control scheme requires minimal microcontroller computational resources. This allows a low-cost microcontroller to be used. In this work, a novel phase-shift control scheme is proposed. The phase-shift control algorithm is executed at a fixed frequency much lower than the maximum switching frequency of the converter. This reduces the computational requirements of the algorithm. It is important that the PFC controller provides low input-current distortion. BCM boost PFC rectifiers suffer from a zero-crossing distortion of the line current. Feedforward control is commonly adopted in to overcome this problem, however most digital feedforward control schemes require complicated design procedures or are computationally expensive. In this work, a novel feedforward algorithm is proposed which has a simple design procedure, low computational requirements and provides high power factor. In applications which are not cost sensitive, it can be more preferable to use a more powerful microcontroller and more computationally expensive algorithms. Hence, a digital average-current-mode-control (ACMC) scheme is proposed to regulate the input current of BCM boost converter. The algorithm allows for an even greater improvement in power quality of the input line current compared to feedforward control, but comes at the cost of a more complex controller implementation. The design, implementation and performance of the proposed digital control algorithms have been experimentally verified. Experimental results for the different control schemes are demonstrated on a 2-channel 600 W and a 3-channel 1 kW BCM PFC rectifier.
Power factor correction , Boost converter , Power electronics , Power supply , Boundary conduction mode
Ryan, R. T. 2019. Digital control of a multi-channel boundary-conduction-mode boost converter for power-factor-correction applications. PhD Thesis, University College Cork.