2 Design Challenges

In-line phase current sensing enables higher performance, continuous measurement, and more precise control of the motor phase current over the entire PWM cycle compared to low-side shunt sensing. In a low-side shunt sensing system, the current is discontinuous and the phase current can only be measured during part of the PWM period, when the low-side power switch is turned on. These systems typically result in a less accurate and lower bandwidth phase current closed-loop control. Therefore, in-line phase current sensing is typically the choice for servo drives and robotic applications. However, the phase voltage is pulse-width modulated and periodically switched between GND and the DC-bus voltage, typically 48V. The microcontroller is referred to GND. This means that the phase current sense subsystem needs to handle a high common-mode voltage and high common-mode transients. The slew rate of the common-mode transients are in the range of 10V/ns. With emerging, fast switching GaN-FETs, slew rates are significantly higher. A digital interface between the microcontroller and current sensor is preferable and improves signal integrity and eliminates issues from ground bouncing during switching.

Figure 2-1 shows a simplified diagram of one of the motor phase currents and the corresponding PWM voltage over one PWM cycle. For closed-loop control, it is sufficient to measure the phase current in the center of the PWM. For small PWM duty cycles the rising or falling edge of the PWM switching falls into the sampling window of the delta-sigma ADC. Duty cycle is defined as the ON-time of the high-side FET relative to the PWM period.

An alternative approach is to continuously sample the phase currents at a sampling rate much higher than the PWM frequency. The individual samples are averaged to get an accurate measurement of the average current and eliminate the inherent current ripple. This method also supports fast short-circuit and over-current detection and sample rates up to 2.5Msps are not uncommon. An advanced use case for continuous oversampling is predictive maintenance. For example, analyzing the phase current spectral signature allows detecting the onset of bearing faults.

For both approaches, PWM switching occurs during phase current sampling. Therefore, it is critical that the phase current sensor is immune to high common mode voltage transients, and PWM switching and does not impact measurement accuracy.

Figure 2-1 Current Sampling for Closed-Loop Phase Current Control and Short-Circuit Detection

A current sense subsystem for a high-performance servo drives shall meet the following requirements:

• High accuracy: better than 1%
• High resolution: better than 12 effective number of bits (ENOB)
• Low propagation delay (latency): <20µs
• High common mode voltage: >60V
• High common mode transient immunity (CMTI): >>10V/ns
• Fast short-circuit detection: <2µs
• A digital interface with high immunity against electromagnetic interferences such as conducted RF and fast transient bursts
• Immunity to external magnetic fields
• Small PCB design size and low profile