All products for medical applications require high reliability while still providing the new technologies and features that end users want. As competition between medical device companies and their end applications intensifies, functionality has increased dramatically, but another factor that could lead to product failure has not been considered. All of these factors are power related, and it is important that we use the latest technology to minimise risk.
Smart MOSFETs are one of these factors driving technological progress, and their popularity is increasing. Due to its simple drive requirements, standard P-channel FETs are often used to switch power distribution nodes, connect charging paths, hot-plug connectors, DC current, and more. Because these components are in the critical path and their failure can disable downstream sensors or processors, it makes sense to invest in reliable power switches. Compared to the equivalent P-channel/N-channel combination approach, the Intellimax FET integrates a P-channel FET and logic-level driver for simple control of this reduced Rdson FET. For increased reliability, these components incorporate ESD protection, thermal protection, overcurrent protection, overvoltage protection, and reverse current blocking. All of this brings higher value and higher reliability to medical applications.
The following sections describe the technology of load switching and the reasons for its existence in current power supply architectures. Its use cases will be presented on a laboratory scale. We’ll discuss less than 6V applications where rechargeable portable medical applications should benefit. This article will also discuss the new 40V smart FET applications enabled by the latest technological advancements of Fast semiconductor, and will provide valuable analysis results showing how smart FETs are becoming the trend of smart development in the medical industry.
Evolution of Load Switches in Battery Applications
The need for power isolation has existed since batteries were introduced into electronics. Introducing the battery as a mobile power source means that the battery will be continuously charged and discharged during use. Obviously, the energy-saving features of a design directly affect the time between normal use and charging. In recent years, battery technology has not seen any significant improvement, and there is no significant breakthrough in the future. Therefore, it is necessary to rely on integrated circuit (IC) technology to adhere to strict power consumption specifications to extend the operating time of the device.
Before we discuss load switching, we need to review battery technology, the load on the battery, and the requirements for load switching. Under fixed charging conditions, estimating battery life can be relatively simple if all current consumption paths are known. It is common to see that it is not the controlled duty cycle sensor of 100mA current that affects the power consumption alone, but many less than 1mA, always connected leakage sinks that are slowly dissipating power. These leakage slots must be roughly added to the power equation, however, more difficult, transient spikes can occur when a given function or sensor is enabled. The magnitude and period of these spikes are monitored and used as an energy calculation, usually as a result of a spike multiplied by the number of spikes.
After all regular loads are known, it is straightforward to calculate working hours. Batteries are now measured on the mAh scale, not the previous coulomb, where a 1000mAh battery can deliver 1A for one hour or 100mA for 10 hours at its nominal battery voltage.
Battery operating time (h) = battery rating (mAh) ∕ overall current consumption (mA)
When the operating current is distributed for 100ms of operation with inrush current (eg 1500mA) and the remaining time with continuous current (eg 20mA LED indicator), the average current for this period can be calculated linearly.
Average current per hour=(1.5A×0.100s∕3600s)+(0.020A×3599.9s∕3600s)=20.04mA
Looking at the concept of power consumption in the time domain, one can quickly see that a load switch can be used to isolate continuous, but smaller current consumption. Short-duration sharp pulses are not the culprit, hundreds of uA-level current draws can add up to mA levels if not isolated. This transition will bring the importance of soft power ramps, especially when power is used to downstream ICs to reduce unwanted large voltage spikes on fragile mAh battery ratings.
Regarding the impact of surge and stable power consumption, we can discuss them independently. The effects of these on the battery can vary widely with battery chemistry and the time between surge power consumption. It is a general notion that a reasonably proportioned surge can result in longer battery life than a light, sustained load. For details on this, please consult your battery supplier. The voltage drop of the battery pack with power consumption is also not discussed. In the above formula based on pure current, we assume that the voltage Vbatt is constant. Also, it depends on the technology used by the battery. For alkaline alkaline batteries (non-rechargeable), Vmax is 1.5V, and in most cases, Vmin is assumed to be 0.9V. The nominal state voltage of a rechargeable single-cell Li-ion battery is 3.7V, however, it can be charged to a maximum of 4.2V, and it can still drop to a minimum voltage Vmin of 2.5 to 3V, which has a greater impact on actual charging.
Understanding how the actual current draw drains the battery level, we can now investigate different ways to isolate the downstream draw. Terms such as high side and low side switches will be used. High side means that the switch will be in a working level (rail) circuit and actually current flows from the source to the load and back through the ground circuit. The low-side switch is on the opposite side of the load and directs current to the ground circuit.
Applying this simple switching principle to common FET types, Figure 1 shows the performance of basic N-channel and P-channel MOSFETs for load isolation, each with advantages and disadvantages. Starting from the PN junction cross-section image, we can quickly illustrate that cross-section b is like a high-side P-channel. N-channel is used to drive the gate to simplify logic input control. The disadvantage of diagram b is the ability to forward bias the body diode if the load voltage is higher than the battery voltage. Figure c addresses this shortcoming by using dual P-channel FETs on the high side, a very common method of battery isolation for the mains.
Why can’t N-channel FETs be used for high-side switching? The textbook property of an N-channel FET is to be able to turn on the switch and keep it in the linear region, where the gate voltage must exceed the drain voltage according to the threshold voltage in the Datasheet. Since the master level in battery applications is usually the highest level available, a bootstrap or isolated drive method must be used. This incurs additional cost, however, this N-channel high-side switching method is necessary for higher current applications. Depending on the voltage range, the N-channel Rdson can be reduced by 20 to 50%. In addition to losses due to Rdson, higher voltages, ie above 200V, make P-channel FETs either expensive or simply unavailable due to technical constraints.
Smart MOSFET Technology
For most applications, traditional load switches are effective, but the discussion in this article will only focus on medical applications. These devices require extreme reliability and in most cases are not rechargeable, so power consumption and isolation are carefully studied.
Express Semiconductor’s Intellimax product portfolio meets the functional requirements of smart MOSFETs. Figure 2 shows its standard internal block diagram, although it will vary by device based on the required features. This diagram is based on the P-channel, with the high-side circuit between Vin and Vout. The pin count has been minimized to keep the package size as small as possible. When it comes to packaging, these components can be packaged as small as 1mm × 1mm chip-scale packaging (Chip Scale Packaging, CSP), or the widely used leadless uPak package, also known as MLP. For designs with prototype needs and less space constraints, SC70, SOT23 and SO8 are also available.
The operating voltage Vin of smart MOSFETs varies according to their manufacturing process. For Express Semiconductor’s Intellimax product line, the recommended operating voltage range is from 0.8V to 5.5V. High voltage smart FETs will be discussed later in this article. It is important to note the difference between the input voltage and the control voltage. The input voltage Vin is the actual rating for the high-side load switch. The control voltage level, labeled ON in Figure 2, is the amount of voltage required to turn on the load switch. Figure 3, taken from the Intellimax FPF1039 data sheet, shows the actual Von voltage required to turn on the integrated P-channel FET as it is related to the Vin supply voltage.
The specs in the datasheet add buffers for process, voltage, and temperature variations, indicating that Von must exceed 1.0V to turn on the switch, and must fall below 0.4V to turn off the switch. This results in a very simple driver circuit that can be connected directly to a microprocessor. This Von specification varies by component and may not necessarily be as flat as Figure 3. Don’t stop at the row of the data sheet that shows static threshold levels; refer to the curves for full details.
As mentioned above, this logic level Von makes the functional interface easy to interface to the microprocessor, but thermal shutdown and over current protection (OCP) can also be interfaced well through the Flag pin. This feature is not integrated into the smallest Intellimax solution like the FPF1039, so we switched to the FPF2303. This dual output load switch is capable of driving a 1.3A load and has all the features previously mentioned, but also includes the Flag feature and reverse current blocking. Flag is an open-drain logic level that can be connected directly to the status pins on the processor. Reverse current blocking is shown in conventional load switch diagrams, but requires a dual MOSFET approach. Express Semiconductor’s proprietary method integrates this into the P-channel and acts as an additional function within the IC without the need for external components. If a situation occurs where the potential on the load side of the switch is higher than that on the battery side, reverse current blocking characteristics are required. This can happen when the system has multiple cells with the same initial voltage, or during voltage spikes. Bulk capacitors also tend to provide delta values.
For load switches, an often overlooked specification is the ESD rating, as most MOSFETs in the past have not integrated ESD protection. More recently, ESD protection has been added to discrete P-channel MOSFETs, where they simply act as cost-effective load switches. This comes in the form of a back-to-back zener diode clamp on the FET gate. This increases the capacitance of the gate, making it an unlikely candidate for switching applications (motor drives, power supplies, etc., but with the addition of a 2K HBM (Human Body Model) zener diode , which can make the gate stronger. Intellimax goes even further and integrates the ESD structure in the smart FET, which can double the ESD rating to 4KV HBM. ESD can be further improved in the future. For medical applications, ESD is Important feature because boards are often shipped unpackaged between assembly rooms to complete placement in plastic housings as well as hermetically sealed enclosures. With ESD-related failures, each shipping point has potential risks, especially during lead times. When connecting pins and connectors from the board to the battery or mezzanine.
The characteristic of the next generation of smart FETs that we should delve further into is what happens when the switch is turned off? Traditional load switches using discrete P-channels can be completely turned off and connect the input to the output, whether heavy loads or large capacitors are loaded on the output pins. If this happens, usually the primary side input level will show a voltage dip, which can affect the precision analog-to-digital converter (ADC) or sensor associated with the bias level. In the past, a resistor/capacitor (R/C) network was added to the gate to reduce turn-on speed, but this increased the design time and size of the project. Intellimax supports a slew rate control feature that minimizes level interruptions by limiting inrush current at the input. Figure 4 shows an illustration of this protocol in a laboratory test of an empirical study. Note that on the left is the effect of the conventional P-channel approach on the Vin level, and on the right is the effect of the Intellimax device.
Smart MOSFETs increase reliability
Requiring the load to be disconnected from the input to prevent further damage in the event of an adverse event is a major consideration in solving reliability problems. Traditional load switches of the past were very simple and did not provide current or thermal protection. Current protection can be added, but this will add some external components and require more precise selection tolerances for passive components. In conclusion, are passive approaches able to react in a short enough time to prevent downstream damage? Thermal sensing is applied on a similar basis of comparison.
The details of overcurrent and thermal shutdown events vary by device. While some shutdowns are immediate and require a power cycle to reconnect to the load, other conditions are repeated attempts to reconnect via a retry mode, with confidence that the temperature and current levels are safe. After a careful review of the data sheet, any confusion over device selection can be cleared up. For thermal shutdown of Intellimax devices, generally most ICs do not rely on this feature as a general practice. That said, in normal use, if a thermal event is expected, the normal practice of separate temperature sensing should be used. Relying on continuous thermal shutdown may degrade IC performance.
If overcurrent is detected, the threshold level can be preset in the IC factory. In some smart load switches, this level can also be set externally by using a resistive ground method. While most are short-circuit protected, the latest addition is a significantly improved tolerance on specific current disconnects, ranging from 100mA to 2A. In just a few years, current sensing tolerances have dropped from 30% to 10% accuracy. When choosing threshold levels, note that minimum and maximum specifications can vary depending on process, voltage, and temperature. The dynamic range of the current is relatively large, making it difficult to provide precise and consistent transition points. It is also difficult to react to very slow current ramps when approaching the detection point. If accurate current sensing and load disconnection are important, it is possible to add a small amount of inductance to the output. This will “buffer” changes in the current di/dt, allowing the smart FET to sense the delta value more accurately. The size of the inductance will directly reflect the sensitivity of the current transition. Each family of smart MOSFETs reacts differently after an overcurrent event. Some are completely disconnected, others use preset steps to ramp down the current, and some even provide a fixed voltage output at the safest tolerable current limit. Please pay close attention to this specification when selecting components.
Smart MOSFET Specification Comparison
After discussing the advantages, what are the possible disadvantages or sensitive specifications that must be closely evaluated when selecting a smart MOSFET? The key is the smart functionality within the smart FET. Of course, the power supply must be used to sense the current and drive the high-side switch. This is written in the data sheet’s quiescent current specification, which is the effective current used within the IC to verify and drive the load switch. For Express Semiconductor’s Intellimax product line, this specification is less than 1µA minimum. For those applications seeking maximum battery life, the listed leakage current must also be strictly compared.
When comparing smart FETs, perhaps the most commonly used data on the datasheets evaluated are the same datasheets that are of equal interest to common discrete MOSFET datasheets. The on-resistance of the high-side FET, known as Rdson, is a key number used to calculate losses across the load switch. This Rdson will vary based on the input voltage, since the same Vin is used to drive the high-side FET, so it is practical to use Ron as the target data for a specific application. Vin is often used to calculate the lowest Ron when the application will actually work at 50%, so do not compare the absolute lowest Rdson in two data sheets. Based on this value of Ron, if the current required by the load is known, the losses across the FET can be calculated. For Intellimax, Rdson can range from 20 ohms to 200 ohms, depending on features and package size.
Another datasheet detail that is sometimes overlooked is the maximum voltage for the high-side FET. To keep Rdson as low as possible, the Intellimax product line limits the input voltage to 6V. This is perfect for battery powered applications, be it a 3.7V rechargeable battery or an AA battery pack. Due to the widespread use of mobile phones, 3.7V single-cell Li-ion battery packs are becoming very common in portable medical applications. However, medical applications may also require hydraulic pumps or fans to operate at a voltage separate from the core battery pack. The most common batteries here are double or triple stacked rechargeable batteries that make 8V to 12V. In the past, discrete MOSFETs were used at these voltage levels. New developments have enabled smart FETs to reach higher voltages.
Express Semiconductor’s AccuPower series of integrated load switches are based on an absolute maximum of 40V, with a recommended 36V process, which is a big technological leap for medium voltage applications. The first ICs will use 100 Ohm technology, with the same features supported by the Intellimax series, but will also include adjustable current limit and a power-good (Pgood) pin. Because of the longer voltage ramp, the load should be at 36V, and the Pgood function will indicate the acceptable level at the output of the microprocessor. Adjustable current limit opens up medical applications. AccuPower devices can be used to drive DC solenoid valves, fans, pumps, and more. Even if the battery voltage is at 12V, the L di/dt voltage spike across the dynamic winding load will easily exceed the 12V breakdown voltage or even the 20V breakdown voltage of discrete FETs. The 36V breakdown voltage supports these load types with 12V and possibly 24V battery voltage. FPF2700 devices supporting these voltage levels are available now.
Medical Smart MOSFET
After reviewing the latest in battery technology and the transition from traditional load switches to smart FET load switches, we can see how medical applications can benefit, however the perceived value may vary. Portable medical devices place emphasis on disconnecting power and loads in order to extend battery life. However, as we discussed, exactly what happens after the switch is turned off is just as important, if not more important. Power regulation adds immediate reliability to higher current applications in the event of inrush current or overcurrent.
Regardless of the application, the trend toward load isolation points continues to evolve, and smart MOSFETs can help achieve higher performance and higher reliability. Maintaining an edge over competitors in medical applications requires the rapid implementation of a range of capabilities. Traditional P-channel FETs will continue to be used for simple switching, but when reliability and time-to-market are key metrics in product design, recent advances in smart MOSFET technology cannot be ignored.
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