“Imagine you are designing the next power stage for servo, computer numerical control (CNC) or robotics applications. In this case, the power stage is a low-voltage DC-fed three-phase inverter with a voltage range of 12 VDC to 60 VDC and a power rating of less than 1 kW. This voltage rating covers the range of battery voltages typically used in battery powered motor systems or low voltage DC fed motor systems.
By Kristen Mogensen, Texas Instruments
Imagine you are designing the next power stage for servo, computer numerical control (CNC) or robotics applications. In this case, the power stage is a low-voltage DC-fed three-phase inverter with a voltage range of 12 VDCto 60 VDC, the rated power is less than 1 kW. This voltage rating covers the range of battery voltages typically used in battery powered motor systems or low voltage DC fed motor systems. In addition, you may also meet the requirement to design this product without the need for additional cooling of the power stage. It has to be as small as possible to meet the needs of the target application, and of course it needs to be low cost.
Well, in this case, come up with an acceptable solution to design an inverter that satisfies this assumption (albeit demanding) and thus meets the above requirements.
Therefore, it is important to consider the 48V/500W 3-Phase Inverter Reference Design with Smart Gate Drive Servo Drive, which is extremely practical and easy to understand, before starting to define the specified power stage, current sensing and protection circuits.
This reference design achieves the small size requirement with a highly integrated IC, including three half-bridge gate drivers with 100% duty cycle operation. Selectable source/sink currents vary from 50 mA to 2 A. VDSSensing enables overcurrent protection to prevent damage to the power stage and motor. Due to incorrect PWM configuration, VGSA handshake function protects the power stage from shoot-through.
A typical low-voltage DC-fed servo drive power stage can be partitioned as shown in Figure 1. Figure 1 is based on a DC-fed servo drive power stage module. Modules are shown in green boxes.
Figure 1: DC-Fed Servo Power Stage
The covered modules of the low-voltage DC-fed servo drives in Figure 1 have a large impact on system performance and influence design considerations.
By adding fault detection to the half-bridge gate driver to achieve VDSsensing and soft-shutdown to build robust systems. These features allow the gate driver system to detect typical overcurrent or short circuit events. Doing so enables dead-time insertion without adding additional current sensing or hardware circuitry, ensuring that the MCU cannot provide the wrong drive signal, which could cause damage to the power stage or motor due to a short circuit.
One consideration is optimizing efficiency to reduce the cost of heat sinks and radiated emissions (EMI) versus switching speed. Implementing these functions via a 100 V single-bridge or half-bridge field effect transistor (FET) gate driver requires additional active and passive components, which increases bill of materials (BOM) cost and printed circuit board size while typically reducing modification Flexibility in parameters such as gate drive strength.When analyzing system efficiency, the current sensing circuit, with lowDS(on)and low gate charge FETs enable fast switching, which affects system efficiency performance. Typically, system designers want to achieve 99% efficiency in the power stage.
For continuous phase current sensing with minimal losses, a 1mΩ in-line shunt is used in the reference design. The resistor value was chosen as a compromise between accuracy and efficiency. The main challenge for non-isolated in-line amplifiers is the wide common-mode voltage (0V to 80V) used by the system, which takes into account the shunt full-scale voltage of ±30mV in this reference design (designed to be ±30Arms). This is a small signal compared to the common mode voltage of 48V. Therefore, there is a need for a current sense amplifier with a large common-mode voltage range and very high DC and AC common-mode rejection. Amplifiers with additional integrated fixed gain and zero offset further help reduce system cost due to low shunt impedance, while ensuring highly accurate current measurements.
100-VDCA buck regulator generates an intermediate rail from the DC input to power the gate driver and point-of-load. The power stage needs to work efficiently to reduce self-heating so that it can meet the industrial ambient temperature (typically 85°C). With this in mind, this means that the integrated circuits used in the system need to support higher temperatures, as there will always be some temperature rise (self-heating) in electronics.
The reference design of the servo drive was tested at 0 to 500W output power using a PMSM motor. The motor load is controlled by a dynamometer as shown in Figure 2.
Figure 2: Test Setup for Motor Drive Power Stages
The 48V/500W 3-Phase Inverter with Smart Gate Driver Reference Design for Servo Drives shows how to design a compact hardware-protected power stage with low BOM count, in-phase current detection, fault diagnosis capabilities, and high efficiency. This is achieved with Texas Instruments’ DRV8530 100V three-phase smart gate driver featuring a buck regulator and INA240 80V, low/high side, bidirectional, zero-drift, current sense amplifier with enhanced PWM rejection, This enables low level optimization – voltage DC feed power stages. See the reference design guide for more details on system performance and IC usage.