Reduction of the EMI in BLDC Motor Drives Based on Delay Compensation

The continues efforts to obtain better performance from power switching circuits as well as reduced size has led to higher switching frequencies, but this has played against the reduction of the electromagnetic pollution they generate [1]. Usually, such kind of problems are addressed including filters, EM shields but also modulating the control signals in the attempt of spreading the interference energy over a wider frequency range. This is usually implemented referring to the spread spectrum technique [2]. This work aims to address the reduction of the EMI delivered by motor drives by a different approach based on the switching edges ' alignment. To this purpose, the system shown in Fig. 1 is considered. It is made up of a three phase inverter driving a BLDC motor and connected to two LISN. The BLDC motor is driven by three trapezoidal waveforms, displaced in phase by 120°, as shown in Fig. 2. Only two legs a time drive the motor, since the rising and the falling edges of the waveforms in Fig. 2 (dashed line) results from keeping the corresponding leg off [3]. The voltage average value is controlled by the duty cycle of the bipolar PWM, which drives the two active legs. In principle, active node voltages should be complementary but actually it is not because of uncontrolled circuit delay, such as those introduced by the MOS drivers. This causes the interference current to flow through the common mode loop, meaning through the LISNs, as shown in Fig.4. Based on that, if the delay is zeroed intentionally, the common mode EM emission can be attenuated [4][5]. The proposed approach has been checked performing SPICE simulations at first. Two configurations have been compared: the first one with the switching output voltages showing a phase shift of 100ns, and the second one with the phase shift almost zeroed. The results of these analyses proving the correctness of what presented so far are shown in Fig. 3. Afterwards, a BLDC driver was designed and prototyped. The three-phase bridge was controlled by a microcontroller that allows one performing a fine adjustment of the PWM duty cycle and phase. The control signals are provided to the full bridge by means of fiber optics, as shown in Fig. 5. The spectra measured are shown in Fig. 6. From these measurements, it has turned out that with the control signals properly phased, the conducted emissions are lowered by 10-15 dB. In conclusion, the investigations carried out within this work have shown that the common mode conducted emissions of three phase BLDC motor drives can be mitigated by compensating the delay introduced by the MOS drivers and the parasitic elements of the test board.