SBAA791 June   2018 AMC1305M25

 

  1.   1
  2.   Summary
  3.   Trademarks
  4. 1Introduction to sampling solution of the high electrical isolation motor drive current
  5. 2Essence and implementation of SINC filter
  6. 3Configuration and calculation of SIGMA-DELTA filter module based on integration of AMC1305M25 and F28377S
    1. 3.1 Hardware configuration
    2. 3.2 F28377S software configuration
    3. 3.3 AMC1305M25 sense voltage versus F28377S digital expression
    4. 3.4 AMC1305M25 and F28377S zero drift and noise measurements
  7. 4References:
  8. 5Appendix:

1 Introduction to sampling solution of the high electrical isolation motor drive current

In high voltage motor drive systems (as shown in Figure 1-1), the closed-loop current control requires phase current information to complete current closed-loop control. Current sampling in practical applications is typically achieved through three methods: Hall sensor; shunt paired with the isolated amplifier; and shunt paired with SIGMA-DELTA modulators. There are advantages and disadvantages to each of the three sampling methods.

Three high-voltage isolated phase current sampling methods are briefly analyzed in Table 1-1. Hall sensor senses the magnetic field generated by the current. This solution features extremely low impedance, minimal losses, and strong immunity to interference, making it common in high power applications (e.g., 380V, 100A and above). Meanwhile, due to limitations in sensor output amplitude and type, signal conditioning is typically required in the post-output stage to meet the input range of the MCU's built-in ADC. The second method to sense current involves connecting shunts in series in the motor drive power stage and sensing the voltage drop across the shunt through an isolated operational amplifier or an isolated SIGMA-DELTA modulator. The primary difference between them is that the isolated operational amplifier output is a voltage signal, typically a differential signal, requiring an additional operational amplifier for amplitude adjustment to meet ADC input range. At this aspect, it is similar to Hall sensor. SIGMA-DELTA modulator outputs a digital bit stream, necessitating digital filtering in the back end. This requires MCU to possess digital filtering capabilities (e.g., TMS320F28x7x series and FPGA). The most prominent feature of this method is the elimination of operational amplifiers and ADC, making it suitable for MCU and FPGA equipped with digital filtering functionality.

Current sampling and drive stage circuit in motor control Figure 1-1 Current sampling and drive stage circuit in motor control
Table 1-1 Comparison of three popular sampling methods for high voltage isolated phase current
Current sampling type Hall sensor Isolated operational amplifier Isolated SIGMA-DELTA modulator
Power supply requirements Power supply on the low voltage side Power supply on the high voltage side and the low voltage side Power supply on the high voltage side and the low voltage side
Sampling needs for the main controller ADC required ADC required SINC filter required
External resistor/shunt Not required Required Required
Isolation ratings Enable reinforced isolation Enable reinforced isolation Enable reinforced isolation
Output mode Single-ended (current/voltage)/differential Differential Digital
Delay Small Medium Medium/High
On-resistance/shunt range Magnetic field loss (Microohm) Milliohm Milliohm
Application power Small/Medium/High Small/Medium Small/Medium
Solution costs High Medium Low

This article uses SDFM (Sigma-Delta Filter Module) integrated in TMS320F28x7x family, pairing with TI's SIGMA-DELTA modulator AMC1305M25 with the reinforced isolation rating. Additionally, it describes how to build a minimum signal chain to complete the basic testing of AMC1305M25.

  1. It describes the basic principles and implementation methods of SINC filter for SIGMA-DELTA modulator.
  2. Analog signal sampling is implemented using F28377S control board and AMC1305M25 EVM.