By Tony Armstrong and Steve knoth
ADI / adeno semiconductor
In today’s continuous operation world, no matter what the external environment or operating conditions are, the continuous operation of many electronic systems is a common phenomenon. In other words, any fault of system power supply, whether it is instantaneous, in seconds or in minutes, must be considered in the design process. The most common way to deal with this kind of situation is to use uninterruptible power supply (UPS) to make up for these short downtime, so as to ensure the continuous operation of the system with high reliability. Similarly, there are many emergency and backup systems today to provide backup power to building systems to ensure that security systems and critical equipment remain operational during power outages, regardless of the root cause.
We can easily find some obvious examples in the ubiquitous handheld electronic devices used in our daily life. Due to the importance of reliability, handheld devices are designed with portable power supply, which can be used reliably under general conditions. However, no careful design can prevent people from misoperation. For example, a portable handheld scanning device fell from a factory worker, causing its battery to fall out. These events are unpredictable in electronics, and without some form of safety net – some kind of short-term power holding system in which enough energy is stored to provide standby power until the battery is replaced or the data is stored in permanent memory – important data stored in volatile memory will be lost.
This example clearly shows that the electronic system needs other forms of power supply to be available when the main power supply is interrupted.
In the automotive electronic system, there are many applications that need continuous power supply, even if the car is in the parking state (the engine is not running), such as remote keyless entry, security, and even personal information entertainment system. These systems usually include navigation, GPS positioning and eCall functions. It’s easy to understand why these systems must remain on even when the car is not in motion, because the GPS of these systems must always be online for emergency and safety purposes. This is a necessary requirement so that basic control can be activated by an external operator if necessary.
Consider eCall system (take OnStar of General Motors Corporation of America as an example) ® The system is becoming more and more popular in new cars all over the world, and many manufacturers have already equipped this system on various models. In fact, it is mandatory in Europe that all new cars and light trucks sold after March 31, 2018 must be equipped with such systems. This is a fairly simple technology: when there is a collision and the car’s airbag is turned on, eCall will automatically contact the emergency service. It transmits the time, location, vehicle type and fuel type to relevant organizations through GPS. At the same time, after the system is activated, you can use the microphone in the car to communicate directly with the call handler. ECall system can tell you which direction you are driving when an accident occurs, so that the relevant authorities know which side of the road you need to enter the scene of the accident. All this allows ambulances, police and firefighters to arrive at the scene as soon as possible after the incident and have as much information as possible. Individuals can also activate eCall by pressing the button, so if someone is sick (or injured in a collision, but the airbag is not turned on), they can still easily call for help.
After understanding that many systems need standby power supply, the following question is: what are the storage media options for this kind of standby power supply? Traditional choices are capacitors and batteries.
It can be said that capacitor technology has played an important role in the application of power transmission and distribution for decades. For example, the traditional design of thin film and oil-based capacitors can achieve many functions, including power factor correction and voltage balance. However, a lot of research and development have been carried out in the past decade, which has made significant progress in capacitor design and capacity. These advanced capacitors, known as supercapacitors, are very suitable for battery energy storage and standby power systems. The total energy storage of supercapacitor is limited, but its energy density is very high. In addition, supercapacitors have the ability to quickly release high energy and charge.
Super capacitor is not only compact in structure, but also robust and reliable, which can meet the requirements of standby power supply system and deal with the short-term power loss event mentioned above. In addition, super capacitors are easy to be stacked in parallel or in series, or even in series parallel combination, providing the necessary voltage and current for the final application. However, supercapacitors are not just capacitors with very large capacitance. Compared with standard ceramic capacitors, tantalum capacitors or electrolytic capacitors, supercapacitors of the same size and weight have higher energy density and larger capacitance. Although super capacitors require special maintenance, they can even replace batteries in data storage applications requiring high current / short-term backup power supply.
In addition, supercapacitors can be used in a variety of peak power and portable applications requiring high burst current or short battery backup, such as UPS systems. Compared with batteries, supercapacitors provide higher burst peak power with smaller size, more charging cycles and wider operating temperature range. By reducing the upper cut-off voltage of the supercapacitor and avoiding high temperature (> 50 ° C), the service life of the supercapacitor can be extended to the maximum.
Batteries， on the other hand， can store a lot of energy， but are limited in terms of power density and delivery.
On the other hand, batteries can store a lot of energy, but have limitations in power density and delivery. The number of charging cycles is limited due to the chemical reaction inside the battery. Therefore, if we want to deliver the right amount of power in a long time, then the battery is the most effective, and let the battery output large current very quickly, it will seriously shorten its effective service life. Table 1 summarizes the advantages and disadvantages of supercapacitors, ordinary capacitors and batteries.
Table 1. Characteristics comparison of super capacitor and ordinary capacitor and battery
New standby manager power solution
Now that it is clear that supercapacitors, batteries and / or a combination of the two can be used as backup power for almost all electronic systems, what solutions are available?
First of all, any IC solution will need a complete lithium-ion battery backup power management system, which must be able to keep the 3.5 V to 5 V power rail powered in case of main power failure. Batteries provide much more energy than supercapacitors, so batteries are more suitable for applications that require long-term backup power supply. Accordingly, any IC solution needs an on-chip bi-directional synchronous converter to charge the backup battery efficiently; If the main power rail is interrupted, it can also provide high current backup power to the downstream load. Therefore, when the external power supply is available, the device will be used as a step-down battery charger for a single lithium-ion or LiFePO4 battery, while giving priority to the system load. However, if the input power suddenly drops below the adjustable power fail input (PFI) threshold, the IC will need to act as a boost regulator to provide several amperes of current from the backup battery to the system output. Therefore, if a power failure occurs, the IC will need to perform power path control to provide reverse blocking and seamless switching between input power and standby power. Typical applications of this IC include fleet and asset tracking, automobile GPS data recorder, automobile remote information processing system, charging system, security system, communication system, industrial standby power supply and USB power supply equipment. Figure 1 shows using ADI’s power by linear ™ Typical application schematic diagram of ltc4040 lithium ion battery backup manager.
Figure 1. Standby power supply with ltc4040 and user set PFI threshold.
The ltc4040 also has an optional over-voltage protection (OVP) function, which protects the IC from input voltages higher than 60 V through an external FET. The adjustable input current limiting function supports the use of current limiting power supply, and the system load current is prior to the battery charging current. The external disconnect switch isolates the main input power from the system during standby power supply. Ltc4040’s 2.5 a battery charger offers eight optional charging voltages optimized for Li ion and LiFePO4 batteries. The device also has input current monitoring function, input power loss indicator and system power loss indicator.
Similar to batteries are supercapacitors. However, super capacitor does not support the situation where the main power supply is lost for a long time, but it is an excellent choice for the system requiring high power and short time standby power supply. Therefore, any IC supporting this kind of application usually needs to be able to support 2.9 V to 5.5 V power rails during main power interruption. As we all know, the power density of super capacitor is higher than that of battery, which makes it an ideal choice for systems requiring peak power backup power in a short time. For example, the ltc4041 in ADI’s power by linear product line uses an on-chip bi-directional synchronous converter to provide high efficiency step-down super capacitor charging, as well as high current and high efficiency step-up standby power supply. When an external power supply is available, the device is used as a step-down battery charger for one or two super capacitor units, while giving priority to the system load. When the input power supply falls below the adjustable PFI threshold, ltc4041 switches to the boost mode, which can provide a maximum current of 2.5 A from the super capacitor to the system load. During the period of power failure, the device’s performance is stable ™ The control function provides reverse blocking and seamless switching from input power to standby power. Typical applications of ltc4041 include passing through “dying gasp” power supply, high current passing through 3 V to 5 V UPS, power meter, industrial alarm, server and solid state drive. Figure 2 shows a schematic diagram of a typical ltc4041 application.
Figure 2. Application schematic diagram of ltc4041 super capacitor standby power supply.
The ltc4041 has an optional OVP function that uses an external FET to protect the IC from input voltages higher than 60 v. The internal super capacitor balancing circuit keeps the voltage on each super capacitor equal and limits the maximum voltage of each super capacitor to a predetermined value. The adjustable input current limiting function supports the use of current limiting power supply, and the system load current is prior to the battery charging current. The external disconnect switch isolates the main input power from the system during standby power supply. The device also has input current monitoring function, input power failure indicator and system power failure indicator.
If the system is required to be continuously available, even if the main power failure or short interruption can not stop, then providing backup power is always a wise choice. Fortunately, there are many IC options for designers to consider to meet specific requirements, including the ltc4040 / ltc4041 standby manager. When the main power supply is interrupted or lost, this kind of IC is easy to make the standby power supply work, whether its storage medium is super capacitor, electrolytic capacitor or battery. Ltc4040 and / or ltc4041 have the function of providing standby power supply for terminal system, whether it is instantaneous power supply or long-term power supply. Therefore, make sure that your system has the right backup power available when needed. See?
About the author
Tony Armstrong is the product marketing director of ADI’s power by linear product division. He is responsible for power conversion and management of products from launch to shutdown. Prior to joining ADI, Tony held different positions in marketing, sales and operations at linear technology (now part of ADI), SILICONIX Inc., Semtech Corp., Fairchild semiconductors and Intel. He graduated from the University of Manchester in England with a B.A. (Hons) in Applied Mathematics. Contact: Anthony [email protected]
Steve knoth is a senior product marketing engineer in ADI’s power by linear division. He is responsible for all power management integrated circuit (PMIC) products, low dropout regulators (LDOS), battery chargers, charge pumps, charge pump based LED drivers, supercapacitor chargers, low voltage monolithic switching regulators and ideal diode devices. Steve has held various marketing and product engineering positions at micro power systems, ADI and Micrel semiconductor since 1990, and then joined linear technology (now part of ADI) in 2004. He received a bachelor’s degree in electrical engineering from San Jose State University in 1988 and a master’s degree in physics from the University in 1995. In 2000, Steve also received an MBA from the University of Phoenix. In addition to spending time with children, Steve also enjoys playing Pinball / arcade games or muscle cars, as well as buying, selling and collecting antique toys and movie / sports / car souvenirs.