How to break the transient voltage of motor drive? Input common mode voltage range is critical
We can gain insight into these transient voltages by analyzing a motor drive circuit. The circuit shown in Figure 1 uses the ADuM3223 (isolated gate driver) to drive the gates of two MOSFETs in a half-bridge configuration. The inputs of the ADuM3223 are driven with anti-phase pulse width modulation (PWM) signals with a 50% duty cycle, switching between the two MOSFETs.
Figure 1. Motor drive circuit using the ADuM3223 and AD8418.
The node between the emitter of the upper FET and the collector of the lower FET is the half-bridge point of the motor drive circuit. This node becomes the connection between the shunt resistor, RSH, and the motor load, represented as the inductor, M. In this circuit, the AD8418 (current sense amplifier) is used to monitor the differential voltage across the shunt resistor. Since this differential voltage is usually very small, in the millivolt range, the common-mode voltage sensed by the current sense amplifier is actually the voltage at the half-bridge point, represented as VCM in Figure 1.
When the lower FET is turned on, the half-bridge point is pulled down to ground. When the lower FET is turned off and the upper FET is turned on, the half-bridge point switches to the bus voltage VBUS. It is during this instantaneous switching process that transient voltages become significant. These transient voltages are caused by a combination of high switching speeds and the inductive nature of the load.
Figure 2. Common-mode voltage at half-bridge point.
Figure 2 shows the common-mode voltage measured at the half-bridge point with a switching frequency of 10 kHz and a bus voltage of 15 V. A closer look at the schematic reveals transient voltages at both swings of the common-mode voltage. The transient voltage at the rising edge reaches almost 8 V, which is more than 50% higher than the bus voltage. There is a transient voltage of approximately −2.5 V at the falling edge. For applications with higher bus voltages and faster switching frequencies, the transient voltages may actually be higher.
Figures 3 and 4 show the typical response of the AD8418 to common-mode voltage transients from the ADuM3223 motor drive circuit. When the common-mode voltage switches, the AD8418 output deviates from the expected voltage by approximately 30 mV to 40 mV and then settles to the expected output within a few microseconds. The input common-mode voltage range of the current sense amplifier determines its ability to handle these high transient voltages. Other amplifiers may only specify a maximum value, such as the common-mode voltage withstand range or the continuous input common-mode voltage.
Figure 3. AD8418 output on rising edge.
Figure 4. AD8418 output on falling edge.
The ADI family of current sense amplifiers is designed to support a wide input common-mode voltage. For example, the AD8418 has a common-mode voltage range of −4 V to +85 V. For applications with large negative transient voltages, current sense amplifiers such as the AD8202 can tolerate common-mode voltages as low as −8 V.
The choice of amplifier ultimately depends on the requirements of the current sensing application. You must be able to identify common-mode transient voltages and consider these factors when selecting an appropriate amplifier. The Analog Devices family of current sense amplifiers has a wide variety of input common-mode voltage ranges, giving you great flexibility in considering these factors.
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