تکنیک های DPWM در مبدل های DC-DC سازگار با EMI با کنترل دیجیتال DPWM techniques in digitally controlled, EMI compliant DC-DC converters

1. Introduction Applying digital control techniques to switching mode power supplies (SMPS) adds several advantages such as flexibility, precision, scalability with the use of software control. However, pure digital control has also disadvantages such as bandwidth, switching frequency, cost, etc. [1]. Our approach is to describe a hybrid method by using the good dynamic performances of an analog switching regulator controller (SRC) and adding the flexibility of controlling the output voltage with a microcontroller unit (MCU) and taking special care on flattening the electro-magnetic interference (EMI) of the converter by a software controlled, active method. One of the advantages of this hybrid method is the possibility of adding an external control loop with a narrower loop bandwidth in order to counteract temperature effect, offset and aging of a pure analog power supply. The external loop bandwidth depends on typical digital control parameters such as instruction cycle speed of the MCU, analog-to-digital conversion time, filtering. In the digital pulse width modulation (DPWM) method, this loop delay is considerably less, since it is dependent only by the instruction cycle speed of the MCU (duty value changed) and the delay introduced by the output filter dampening the AC component of the DPWM signal. The amplified PWM signals drive the gates of the main switches in the converter, which generates electromagnetic interferences, perturbing other electronic equipment. Beside passive EMI countermeasures (layout design, filtering), a good practice to mitigate the emission power level is to introduce a pseudo-random dithering of control clock timing of the SRC, which can be achieved by software and one available PWM peripheral of the same control MCU. Many papers discuss the EMI level mitigation; this work references a few of them [4], [5], [6]. The main experimental application (Figure 1) consists of a digitally controlled DC-DC converter based on a low power, high-performance USB capable, RISC MCU and on a high efficiency, synchronous, buck-boost controller with external reference, driving four external MOSFET switches. The MCU generates two independent PWM signals: PWM1 is used for controlling the SRC and PWM2 is used for synchronizing the switching logic inside the SRC.