Today, more than ever before, “never fail” operation of emergency and critical care equipment is a need, not an option. Devices such as ventilators provide life-maintaining oxygen, and the recent pandemic has increased demand for these devices. Ventilators are mission-critical to sustaining life and require a robust design to ensure they maintain the necessary up-time for patient care.
One factor leading to the necessity for reliable ventilators and other critical care devices is the growing market demand. In general, global critical care, emergency equipment, and diagnostic medical equipment market are growing at an annual rate of about 6 percent. This growth rate does not include the recent spike in demand for ventilators to address the coronavirus pandemic. Population growth, alone, necessitates growth in patient care, emergency care, and diagnostic products. The aging of the population in the developed world is boosting growth in patient care, emergency care, and diagnostic products. Pollution, poor diets, and less-active lifestyles also contribute to a greater need for critical healthcare equipment.
Technological innovations that provide enhanced capabilities and facilitate new applications contribute to market growth. Highly integrated electronic components, LED displays, and wireless communication advancements allow ventilators to be portable, have wireless communication capability, and provide improved diagnostic analysis capability. These devices can be used in any location enabling a doctor to assist personnel in locations remote from medical facilities.
Designers need to protect their product designs with components that provide overcurrent protection, overvoltage transient protection, and electrostatic discharge (ESD). Also, designers need to utilize the appropriate control components to safely and efficiently manage such circuits as motor controls and high-voltage discharge circuits. Furthermore, designs must incorporate appropriate sensors such as those that detect when an instrument is enabled for activation to protect both users and patients.
This article presents important considerations for overload protection, safe and efficient control, and protective sensing of ventilators. An example ventilator design is used to illustrate where protection, control, and sensing components are needed.
Protection, Sensing, and Control Components for Ventilators
Figure 1 shows an example of a critical care ventilator designed for hospital use. Also listed are protection, control, and sensing components for consideration for the various functional blocks. Figure 2 details a ventilator block diagram; and, the adjacent table details the possible components for specific circuit blocks.
Power Supply. The power supply block connects to the power line, and the circuit is subject to overcurrent and overvoltage conditions. To render it more reliable, the circuit must be protected against both conditions. For overcurrent conditions, the designer can select a conventional fuse or a polymer positive coefficient (PPTC) component, a resettable fuse. Ceramic body, cartridge fuses rated at 250 Vac offer a wide selection of current ratings from 0.125 to 20 A and have cold resistances from around 20 Ω to under 10 mΩ. Based on the application, the designer can select fast-acting or time-lag fuses. The time-lag fuses can avoid nuisance failures from transient overcurrent.
A PPTC component eliminates the need to replace a blown fuse. An overcurrent causes the component to heat up; and, its crystalline structure breaks down, which converts the component into a very high resistance element. When the overcurrent condition ends or is corrected, the PPTC’s resistance returns to a lower value and the device conducts current again. The trade-off is that PPTC devices have higher resistances than conventional fuse elements. PPTC resistances, depending on the current rating, can run from over 30 Ω to around 100 mΩ.
Circuit designers must also protect their power supply circuits from the effects of lightning strikes and large inductive spikes that can occur when motors are energized and de-energized. Metal oxide varistors (MOVs) are the recommended component and are placed across the input line at its entrance. MOVs can clamp the transient voltage to no more than three times the line voltage. The MOVs can absorb peak currents as high as 10 kA that can last as long as 20 µs. In addition, an MOV can absorb up to 400 J of a square wave type of transient that can last over 2 ms. MOVs absorb the transient energy and keep it from entering and damaging the device circuitry.
Designers should not neglect the secondary side of the power supply. On the secondary side or low voltage side of the power supply circuit, typically a switching-topology power supply, multilayer varistors and transient voltage suppressor (TVS) diodes provide transient overvoltage protection for integrated circuits and other circuit components. TVS diodes can absorb substantial power, up to 1500 W from a 10 × 1000 µs (10 µs rise time pulse with a pulse duration of 1 ms) transient event. These diodes respond to a transient overvoltage in under 1 ps and provide bipolar protection to absorb both positive and negative transients.
To maximize efficiency, designers should consider using Schottky diodes for rectification of the AC voltage waveform. Schottky diodes have a low forward voltage drop and low leakage to minimize power loss in rectifier circuitry. Furthermore, these diodes can operate at high frequency, which contributes to less power loss in a switching power supply.
If isolation is required either between the power supply and other circuit blocks or for functions in other circuit blocks, designers should employ solid-state relays. Solid-state relays provide a much longer life than electromechanical relays and need lower drive power. In addition, solid-state relays offer high isolation, up to 3750 VRMS input/output isolation.
Battery Management Unit. The battery management unit monitors and controls a set of lithium-ion batteries. Both the batteries and the charging control and battery output balancing circuitry need to be protected from overcurrent and overvoltage. A PPTC resettable fuse provides protection from either overcurrent in the load or a short in the battery pack.
A combination of TVS diodes and TVS diode arrays provides overvoltage protection and ESD protection for the low voltage control circuits respectively. The TVS diodes can withstand a lightning strike as high as 30 kV and respond to the transient in less than 1 ps. The designer can select either unipolar or bipolar versions. TVS diode arrays provide ESD protection for logic circuits operating at up to 5 V. Models can be bipolar or unipolar; and, with capacitance under 30 pF, the diode arrays have a minimal impact on circuit characteristics.
MCU/MPU/DSP Block. It is essential that the main processing and control circuitry remain at a temperature that prevents an overtemperature condition and a disabling of the ventilator. Negative temperature coefficient (NTC) thermistor sensors can help provide the necessary protection. NTC thermistors are small, reliable components that take up minimal space. They have a nominal diameter of 0.095 in. (2.41 mm) and a thermal time constant of 10 s. Monitoring the temperature of this circuit block will help ensure reliable operation for the critical intelligence of the instrument.
User Interface. The user interface is subject to the external environment and requires protection from ESD, lightning, and other transients. As with other digital circuit blocks, TVS diodes and diode arrays can provide the necessary protection; and, with low capacitance, these components have a minimal impact on circuit performance.
USB Interface. The USB interface, like the user interface, has exposure to the external environment. Designers should protect the USB communication port from overcurrent and overvoltage conditions. A PPTC resettable fuse provides the necessary level of overcurrent protection while consuming a minimal amount of pc board space. A PPTC fuse takes up an area of only 2.2 × 1.5 mm. Designers should consider multilayer MOV components to protect the low voltage USB circuit from overvoltages caused by transient voltages and ESD. The multilayer MOVs offer bipolar clamping, compact surface-mount form factor, and a wide operating temperature range. To protect the USB interface from an overtemperature event, a small (2.0 × 1.2 mm) surface-mount, temperature indicator can provide a fast indication of temperature rise. The use of these design considerations and the use of the appropriate components will help protect the five most susceptible circuit blocks of a ventilator.
Safety Standards for Medical Equipment
Designers need to be aware of the important standards that apply to medical devices so that their designs can be approved by both the Federal Drug Administration (FDA) for connection to patients and other standards bodies for general electrical safety. Table 1 highlights select international safety standards applicable to medical devices. Designers should review and confirm all applicable standards and certifications needed, as each device and each geography is unique. Standard IEC 60601-1-2 defines requirements for medical instruments to ensure protection from ESD and transient disturbances. Standard IEC 60601-1-11 covers overload safety protection requirements for medical instrumentation. Standard IEC 62311-2 is a general standard that applies to all portable products using any batteries with alkaline or other non-acid electrolytes. Thus, this standard applies to lithium battery packs.
To summarize, designers need to consider protection for all circuit blocks in their designs that are susceptible to overcurrent, overvoltage, and ESD. Standards dictate many of these requirements; and, it is essential that new critical care, medical equipment comply with these standards. Attention to the details of both protecting key circuit blocks and controlling and sensing for maximum safety and efficiency will help implement reliable and robust medical instruments.
Due to the critical nature of these medical devices, it is recommended that designers collaborate with component manufacturers to ensure the safety and compatibility of components in their specific designs.
Users must independently evaluate the suitability of and test each product selected for their own specific applications. It is the user’s sole responsibility to determine fitness for a particular system or use based on their own performance criteria, conditions, specific application, compatibility with other components, and environmental conditions.
Note: Users must independently provide appropriate design and operating safeguards to minimize any risks associated with their applications and products. Littelfuse products are not designed for, and may not be used in, all applications. Read the complete Disclaimer Notice here . To learn more, download the company’s Circuit Protection Selection Guide here .