Power Management & Industrial Electronics
of the lower inverter legs or with a single shunt in the DC link. The advantage of these approaches is that the shunt signals are all referenced to the power common, but extraction of winding current from the DC link requires the sampling to be synchronised to the PWM switching. There are many topologies that can be used in motor control design to sense motor current with many factors to consider such as cost, power level and performance level. A key objective for most system designers is to improve the current sense feedback in order to improve efficiency within their cost targets. For higher end applications current feedback is critical to other
Reduce shunt resistor sizes From a signal measurement aspect there are some key challenges associated with the shunt resistor selection as there is a trade-off between sensitivity and power dissipation. A large resistor value will ensure the full range, or as much as possible of the analogue input range, of the modulator will be used, thus maximising dynamic range. However a large resistor value also results in a voltage drop and a reduction in the efficiency due to the I2 x R loss of the resistor. Non-linearity through self-heating effects can also be a challenge using larger resistors. As a result, system designers are faced with making trade-offs and further exacerbating this is a common
Figure 3: Current sensing topologies performance spectrum
system performance measures such as dynamic response, acoustic noise or torque ripple, not just efficiency. It is evident that there is a continuum of performance running from low to high across the various topologies available and this is coarsely mapped out in Figure 3 illustrating both lower power and higher power options.
Designer objectives There is a significant trend for system designers to migrate from HE sensors to shunt resistors, with an additional trend to move to the isolated modulator approach versus an isolated amplifier approach. The sensor change alone results in lower bill of materials (BOM) and PCB insertion costs and improved sensor accuracy. The shunt resistor is not sensitive to magnetic fields or mechanical vibration. Quite often system designers replacing HE sensors with shunt resistors may opt for an isolated amplifier and continue using the ADC previously used in the HE sensor based design to limit the level of change in the signal chain, however as already noted, the performance will be limited by that of the isolated amplifier regardless of the ADC performance. Further replacing the isolated amplifier and ADC with an isolated modulator will eliminate the performance bottleneck and greatly improve the design typically taking it from a 9 to 10-bit quality feedback to a 12-bit level. Analogue over current protection (OCP) circuitry can also potentially be eliminated, as the digital filter required to process the modulator output can also be configured to implement a fast OCP loop. Therefore, any BOM analysis should include not only the isolated amplifier, the original ADC and the signal conditioning between these but also OCP devices that may be eliminated.
www.cieonline.co.uk
need to select a shunt size that will service many models and motors at different current levels. Maintaining dynamic range is also challenging in the face of peak currents that can be several times the rated current of the motor and the need to reliably capture both. The ability to control peak currents at system turn on varies greatly depending on the design, varying from tight control of say, 30% above nominal, to as much as a factor of 10 times the nominal current. Peak currents also result from acceleration and load or torque change. However, in general it is common to see the peak current in a system to be in the region of 4 times the nominal current in drive design. In the face of these challenges, system designers are looking for superior modulators with wider dynamic range, or improved signal to noise and distortion ratio (SINAD). Isolated modulator offerings to date have provided 16-bit resolution with up to 12-bit effective number of bits (ENOB) guaranteed performance.
The advent of Mixed-Signal Control
Processors, to which isolated sigma-delta modulators can directly connect, is likely to lead to an increased rate of adoption of the resistive shunt current sensing technique coupled with isolated sigma-delta modulators. This technique was traditionally considered expensive as a result of greater system complexity in the digital domain and associated (FPGA) cost. But manufacturers like ADI are producing cost-effective solutions like the ADSP- CM408F which enable many designers to consider resistive shunt current sensing who were previously constrained by their cost targets.
Analog Devices |
www.analog.com
Nicola O’Byrne is a senior system applications engineer at Analog Devices
Components in Electronics September 2014 33
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52