Providing power for robotic surgical systems
Introduction
Imagine major surgery performed through tiny incisions, with much less pain, a shorter hospital stay, faster return to normal activities and the potential for better clinical outcomes. Until recently, options for surgery have been limited and included traditional open surgery with a large incision or, a procedure that uses a lighted tube put through a small incision called laparoscopic surgery. However, even though laparoscopic surgery is a minimally invasive surgery, it is often limited to very simple procedures due to the instruments required. Thanks to breakthrough technology, there is another category of minimally invasive surgery that incorporates robotic assistance.
These types of surgical systems combine a computer and robotic technologies that create a robotic-assisted laparoscopic, thoracoscopic or endoscopic surgery. By providing surgeons with enhanced capabilities, it is possible to treat a broader range of conditions using a minimally invasive approach with better visualization, precision, dexterity and control than possible through traditional surgical approaches.
Minimally invasive robotic-assisted surgery has been used in everything from heart surgery to cancer surgery, to treat conditions as diverse as prostate cancer, endometrial cancer, morbid obesity and mitral valve regurgitation. These systems combine robotics and surgical technology as never before, enabling surgeons to provide the most effective and least invasive treatment option available for a wide range of complex conditions.
The typical set up is a surgeon’s console, a patient cart with interactive robotic arms and a highly magnified 3D image monitor of the body’s interior. New methods of imaging and image-guidance technology provide surgeons with very accurate three-dimensional information about the location of critical subsurface structures and instrument position.
To operate, the surgeon uses master controls that work like forceps. As the surgeon manipulates the controls, the system responds to the input in real time, translating his or her hand, wrist and finger movements into precise movements of miniaturized instruments. Figure 1 shows a picture of a robotic surgical system.
This type of system is usually designed, using distributed power architecture. Therefore, it operates from the AC mains, consisting of either 110VAC or 220VAC and is converted to an isolated 48V DC voltage that charges a bank of 48V batteries. This 48V bus voltage is routed throughout the system to power downstream point of load regulators for all the subsystems, including the robotic arms, system electronics, instruments and a high resolution display. The battery pack maintains system operation when a loss of AC mains occurs. However, depending on the state of charge of the batteries, the battery pack voltage can be above, below or equal to the 48V input making it a challenge to design a power supply for this application. Linear Technology has recently released the LT8705, an 80V synchronous buck-boost controller that addresses the needs of such a power supply requirement.
Power supply for robots
The LT8705 is a high efficiency (up to 98 percent) synchronous buck-boost DC/DC controller that operates from input voltages above, below or equal to the regulated output voltage. This device has four feedback loops to regulate the input current/voltage, along with the output current/voltage. The output current loop provides a regulated output current for a battery charger or as a current source. The LT8705 operates over a wide 2.8V to 80V input voltage range and produces a 1.3V to 80V output, using a single inductor with 4-switch synchronous rectification. Output power up to 250W can be delivered with a single device. Higher output power can be achieved when multiple circuits are configured in parallel.
Additional features include servo pins to indicate which feedback loops are active, a 3.3V/12mA LDO to power external devices, adjustable soft-start, onboard die temperature monitor and an operating junction temperature range of -40°C to 125°C. The LT8705 is available in a 38-pin 5mm x 7mm QFN, and a 38-lead TSSOP package. An LTspice circuit model for the LT8705 is also available and can be used to evaluate all kinds of creative applications quickly.
The LT8705 contains four error amplifiers, enabling it to regulate or limit the output current, input current, input voltage and output voltage. In a typical application the output voltage might be regulated, while the remaining error amplifiers are monitoring for excessive input or output current or an input undervoltage condition. In other applications, such as a battery charger, the output current regulator can facilitate constant current charging until a predetermined voltage is reached where the output voltage control would take over. The schematic in Figure 2 shows an LT8705 circuit that charges a 48V battery and operates from an input voltage that can vary from 36V to 72V. Multiple circuits can be paralleled for higher power applications. There are four external MOSFETs that enable this circuit to be used as a synchronous buck/boost converter and are configured as a current source to charge four each 12V lead acid batteries in series for this application.
LT8705 circuit
Power switch control
Figure 3 shows a simplified diagram of how the four power switches are connected to the inductor, VIN, VOUT and ground.
When VIN is significantly higher than VOUT, the part will run in buck (step-down) mode. In this region, M3 is always off and M4 is always on unless reverse current is detected while in Burst Mode operation or discontinuous mode. At the start of every cycle, synchronous switch M2 is turned on first and the inductor current is sensed by an internal amplifier. A slope compensation ramp is added to the sensed voltage, which is then compared to a reference voltage. After the sensed inductor current falls below the reference, switch M2 is turned off and M1 (synchronous rectifier) is turned on for the remainder of the cycle. Switches M1 and M2 will alternate, behaving like a typical synchronous buck regulator.
As VIN and VOUT get close to each other, the duty cycle decreases until the minimum duty cycle of the converter in buck mode is reached and the part moves into the buck-boost region and all four MOSFETs are switching.
When VOUT is significantly higher than VIN, the part will run in boost (step-up) mode. In this region M1 is always on and switch M2 is always off. At the start of every cycle, switch M3 is turned on first and the inductor current is sensed by an internal amplifier. After the sensed inductor current rises above the reference voltage, switch M3 is turned off and switch M4 is turned on for the remainder of the cycle. Switches M3and M4 will alternate, behaving like a typical synchronous boost regulator.
Fault conditions
The LT8705 activates a fault sequence under certain operating conditions. If any of these conditions occur, such as an over current or over voltage condition, the internal switching and clock output are disabled. At the same time, a timeout sequence commences where the soft start function needs to be reinitialized. If the fault persist, like during an over current condition, the soft start function will not be allowed to restart the converter. After the fault condition has been removed and a predefined timeout period has ended, the converter will restart at a rate dependent upon the capacitor value assigned to the soft start pin on the LT8705. The timeout period relieves the part and other downstream power components from electrical and thermal stress.
Conclusion
Robotic surgical systems allow for several types of minimally evasive major surgeries, which can reduce the hospital stay time, provide a faster return to normal activities and the potential for better clinical outcome. Powering these types of systems with distributive power architecture enables the use of a 48V nominal battery backed up bus voltage that powers downstream point-of-load regulators for all subsystems. Linear Technology’s LT8705 synchronous buck-boost DC/DC controller can simplify the power supply design with its ability to efficiently charge a battery with a float voltage that can be above, below or equal to the input voltage.
About the author
Bruce Haug, senior product marketing engineer, Linear Technology Corporation
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