The Power and Evolution of GaN – Part 4

Bringing Precision Control to Surgical Robots with eGaN^® FETs and ICs

In this series, how the superior switching speed of gallium nitride (GaN)-on-silicon low voltage power devices have enabled many new applications is being discussed.  These applications are transforming industries such as light detection and ranging (LiDAR) for autonomous vehicles, envelope tracking for 5G communications and large surface area wireless power for the home and office. In this article, how GaN power devices are transforming medicine by bringing precision control to surgical robots is examined.

Robotic-assisted surgery allows doctors to perform many types of complex procedures with more precision, flexibility, and control than is possible with conventional techniques. The global surgical robots market is expected to exceed more than US$ 91.5 Billion by 2025 at a compounded average growth rate of 10.4%, as the need for automation in healthcare and the trend toward more advanced robotic surgeries accelerates.

Today, robotic surgery is typically associated with minimally invasive procedures. Minimal invasive surgery using surgical robots gives unprecedented control to surgeons looking to achieve the surgical precision, thereby reducing risk and trauma to the patient and speeding recovery.

Many motors are required to control the various robotic appendages, such as arms, joints, and tool control needed to give the surgical robot the necessary degrees of freedom (DOF) and dexterity to perform extremely delicate surgical tasks. Weight and size of motor control circuitry are thus important factors in the design of such robots since they directly impact the size of the motors that manipulate the robot’s appendages during surgery.

Motors for Robotic Surgery

The 3-phase brushless DC (BLDC) motoristhe motor of choice for robotic-assisted surgery. These motors are compact for their power rating, can be precisely controlled, offer high electro-mechanical efficiency, and can operate with minimal vibration when properly controlled. The choice of motor voltage lies in the range of 24 V to 48 V with balancing power conductor thickness and weight with insulation thickness and stiffness for optimum performance and dexterity being the determining factors.

These BLDC motors are driven by inverter circuits, which in most cases are 3-phase, that traditionally use MOSFETs. Since eGaN FETs switch much faster than MOSFETs, it is now possible to design the inverter to operate at much higher frequencies, which can provide higher electrical efficiency and higher positioning precision.

Being able to run at higher switching frequencies has many benefits including, but not limited to, increased control bandwidth for the motor. This increases the precision at which the motor can be controlled. Also, the higher frequency reduces or even eliminates, mechanical vibration, which is crucial to being able to take advantage of the higher control precision. 

To reduce, or even prevent, heating the motor when operating at higher frequencies, a compact filter can be added. These filters add negligible losses to the system. This combination still yields superior electro-mechanical efficiency and performance over a system designed using MOSFETs. Furthermore, the combination ensures lower EMI generation making the system easier to comply with regulatory standards.

The motor size and power rating will vary depending on the location and assigned task. For example, higher power motors are needed to manipulate an arm, and lower power motors are needed for operating a small precision tool.

eGaN FETs and ICs for Surgical Robot Motor Drives

A typical inverter is comprised of three half bridges with each output connected to a phase of the motor, as shown in figure 2. Ideally, the FETs are switched using a PWM technique and sinusoidally modulated to minimize vibration. Due to weight and size constraints, and the thermal efficiency of the chip-scale eGaN FET package, no heatsinks are needed to cool the FETs of the inverter.

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Figure 2: Typical 3 phase Motor drive with filter

Given that the motor voltage ratings lie between 24 V and 48 V, eGaN FET V_DS ratings can be selected in the range 40 V and 100 V. Depending on the power requirements, continuous current ratings as low as 1 A to as high as 60 A can be encountered. Regardless of current or voltage rating, the basic drive topology and operation, shown in figure 2, remains the same.

eGaN FETs are ideally suited for use in surgical robot motor drives since their superior hard-switching Figure of Merit (FOM) is three to four times lower than that of MOSFETs in the voltage range 40 V to 100 V. This better FOM allows eGaN FET-based drives to operate at higher efficiency and at a higher frequency and therefore with higher precision.

The motor drive can be designed based on a large selection of discrete eGaN FET options, where the motor power rating determines the FET current rating. One additional benefit of eGaN FETs over MOSFETs is the reduced area for the active devices, which includes the monolithic integration of the power stage. This feature is particularly useful for the very small motors that manipulate the surgical tools.

In addition to their superior performance for this application, eGaN FETs have a proven track record of reliability, which is supported with the inclusion of AEC qualified parts.

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Figure 3: eGaN FETs and suitable ICs for surgical robot motor drives

Summary

GaN is transforming medical procedures…bringing precision control to robotic-assisted surgery.  Surgeon directed robots improve surgical precision capability and, in many cases, require minimal invasive access to the patient.   Robotic surgery involves control of multiple compact surgical arms using the ultra-reliable, high-performance brushless (BLDC) class of motor drive systems.  BLDC motors are the best choice for surgical robots due to their small size to power ratio.

These robotic surgical systems require a motor with high efficiency, minimal vibration, and precision control. eGaN FETs and ICs are ideal devices for sinusoidally modulated motor drives given their higher efficiency and higher operating frequency. eGaN FETs and ICs used in the motor control circuit yield higher precision and result in a more compact drive for the motors. This combination allows designers to design surgical robots that are more compact and with superior dexterity over MOSFET equivalent solutions.

Efficient Power Conversion (EPC)