Power Sub-system is one of the most important aspects of any embedded system. Obviously, nothing works without electric power. Characteristics of a good power supply includes
- Stable and smooth voltage supply
- Sufficient current for the operation of the device
- Good power efficiency
- Stable performance over operating temperature range
- Thermal performance with limited to no air flow
- Proper filtering of noise (complying with EMI standards)
- Proper decoupling
In this blog, in continuation of the series on embedded system design, we will discuss some power supply design considerations for a system.
Power Supply Design – Models
Considering wall power and batteries as the primary power sources, an embedded system could be powered in any one of the following models:
- Wall powered
- Wall powered with battery backup
- Primarily battery backed up
- Fully battery powered
Wall Powered Devices
These devices operate fully on power supply available from wall power. They typically consume more power and work in tandem with systems that consumes a lot of power, that they are redundant when the underlying system could not be powered on. Many of the devices in use fall under this category including medical devices, industrial systems etc.
Wall Powered with Battery Backup
These classes of devices are very similar to above case but will have a limited power backup using batteries. This backup is useful to properly shutdown the system and to store the system configuration and acquired values safely till full power is back.
Primarily battery backed up
The most common example of these devices is mobile phones. They are designed to work primarily with battery power supply. Whenever needed the system can be charged back. It incorporates a full-fledged battery charging and managing circuitry.
Primarily Battery Backed Up
The most common example of these devices is mobile phones. They are designed to work primarily work with battery power supply and whenever needed, the system can be charged back. It incorporates a full-fledged battery charging and managing circuitry.
Fully Battery Powered
These devices are designed to work only from battery supply that does not have a charging mechanism. These batteries have to be externally charged or non-rechargeable batteries used.
Apart from these, there are many power sources being used in embedded systems including photo-voltaic – solar power, etc. With the upcoming wearable computing becoming a trend, the power supplies include generating from unconventional sources like audio jack of smart phone, human/mechanical movements or even body heat etc.
Power Supply Design – Considerations
In this discussion, we will primarily look into the power supply design for wall power with battery backup devices. The below figure typically explains this case.
The DC power input from the wall socket is used to power the system. If the wall power is absent, the battery powers the system. The Power path controller is used to route the power from preferable source. The power conditioning circuit finally supplies to the load at the required voltage and current. Battery monitoring and charger circuit is necessary for managing the battery.
In the upcoming sections, we will discuss about the major components involved in power supply design.
Wall Power Input
Wall power is obtained from AC wall adapter plugged in the wall socket. It provides constant low voltage DC suitable for running the system from the high level AC source in the wall socket. The AC wall adapters are available in multiple ranges with different ampere current ratings. Adjustable wall adapters are also available were the DC output voltage can be varied suitably. The main factors to be considered on selecting the wall power are voltage and current.
The voltage supplied by the wall power should be more enough to satisfy the input voltage requirement of the power conditioning circuit usually comprising of linear or SMPS regulators. Also if the power supply system incorporates battery charging, then the voltage requirement of the battery charger should be taken into account.
Current consumption of the system should be carefully estimated before selecting the wall adapter. The efficiency of the regulators in the power conditioning circuitry should also be taken into account. The wall adapter with current rating more than the estimated amount should be chosen for proper working to allow unexpected surges. And quality of the COTS power supply is important as many low power supplies inject a large amount of noise as well as not able to provide voltage levels across rated current ratings.
Batteries are devices that store electrical energy in form of chemical energy and reconvert to DC current at the time of usage. The primary design consideration in using a battery in embedded design is its Capacity. The capacity of a battery is defined by the amount of electric charge it can supply at the rated voltage and temperature, generally measured as Ampere/Hour. For example, a 1000mA/h Lithium-Ion battery can deliver 1A current over a period of 1 hour without any drop below 3.6V, at room temperature.
While the capacity defines the amount of charge the battery stores, it does not defines how fast this stored charge can be used. For this, Charging rate or the C-rate is used as the important measure of the charge and discharge current of a battery. A charge rate of 1C implies that the battery can be discharged at the rated current in 1 hour while a 2C implies that the capacity can be utilized in even 30 minutes. So a 1000mA/h battery if rated at 1C, can deliver 1A in 1 hour, at 2C can deliver 2A in 30 min and at 0.5C can only deliver 500mA over 2 hours.
Another factor in battery selection is the self discharge rate. The batteries loose a certain amount of charge even without being used. So charge interval requirements of the application should be considered for the design.
Batteries are available in different chemistries which are suitable for difference operating conditions. The most commonly used chemistries are as follows,
One of the most used technologies, Li-ion is used where high-energy density and lightweight is of prime importance. The technology is fragile and a protection circuit is required to assure safety.
The nominal cell voltage is 3.6V with nominal load current at 1C. No periodic discharge is needed, hence it requires less maintenance. Self-discharge is less than half that of NiCd and NiMH. It is more costly than the other battery technologies. Applications include notebook computers and cellular phones.
NiCd is used where long life, high discharge rate and economical price are important. Has relatively high self-discharge hence they require recharge after storage. The nominal cell voltage is 1.25V. Best result is obtained at load current at 1C. Many standard NiCd batteries allow peak current discharge to 20C. Main applications are two-way radios, biomedical equipment, professional video cameras and power tools. The NiCd contains toxic metals and is environmentally unfriendly.
Nickel Metal hydride
NiMh has a higher energy density compared to the NiCd at the expense of reduced cycle life. It contains no toxic metals. The nominal cell voltage is 1.25V. Best result is obtained at load current at 0.5C. 30 – 40 percent higher capacity over a standard NiCd. About 20 percent more expensive than NiCd. Applications include mobile phones and laptop computers.
Lead acid batteries are most economical for larger power applications where weight is of little concern. Nominal cell voltage available is 2V per cell. Best result is obtained at load current at 0.2C. The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emergency lighting and inverter/UPS systems.
Suitable battery backup voltage corresponding to the regulator requirement can be selected by connecting the batteries in series. For example, if the power conditioning regulator requires at least 6V to provide fixed 5V, then 5 NiCd/NiMh (5*1.25V) or 2 Li-ion (2*3.7V) or 3-cell (3*2V) Lead acid battery can be connected in series.
For a detailed analysis of battery technologies, kindly refer to the Ivan Cowie blog on batteries.
Battery Manager and Charging Circuit
Different charging procedures are required for different battery chemistries. During a period of time, they may be charged in voltage mode and later in current mode. Such complex requirements make it impossible to be implemented using simple components. Separate battery charging IC’s are available for charging application. Using these charge management IC’s, battery can be safely charged to its final full charge voltage. These charge management IC will monitor the voltage at the battery terminals and cut off the charging when the voltage reaches the final full charge voltage. They also provide indication over pins or I2C interface about the current stored capacity, battery health status, charge indications etc.
The charge current should be carefully selected. While a higher charge current may charge the battery faster, it will add to input supply requirement if the main load is operating as well. Slower charge current will need more time that might not suitable for practical purposes. Once again it is a complex math of usage, battery chemistry, BoM costing etc.
Power Path Controller
The power path controller performs the switching of the power to the power conditioning circuit. The circuit powers the load with wall power till it is available. Once it is cut off, the battery back-up power is immediately routed to the load. The response time of these controllers should be very fast that the load should not be under-powered during this wall to battery power transition.
Power Path controllers can be implemented with simple mechanism like two diodes or with specialized controller IC’s . The drop in the voltage due to the presence of this circuit should be factored in the design along with the current ratings.
Power Conditioning Circuit
Power conditioning circuits are used to regulate the high voltage DC from the wall or battery power source to low voltage DC suitable for the embedded system.
An embedded system consists of many different peripherals that can operate from a wide range of power supply. So to power the entire system, multiple DC-DC voltage converters are used. Apart from stepping down the power supply, the regulators also minimizes the power supply noise and provides protection for the embedded system from any possible damages due to fluctuating input voltages/Electro static discharges.
Two types of regulators generally used in power supply systems are
Linear regulators use at least one active component like transistor and require a higher input voltage than the output. Typically they accept the load current at the higher DC input voltage, reduces the voltage and delivers the current to the load at reduced voltage. They are very popular due to its small size, less noise and cheaper. They provide clean output voltage with low noise. But since they dissipate the extra power as heat, they generate a lot of heat which must be dissipated using bulky heat sink. And with conversion to this unusable heat, they have low efficiency when compared to switching regulators.
Unlike the linear regulators, the switching regulator can step up, step down or invert the input voltage. They work by transferring energy in discrete packets from the input voltage source to the output. This is carried out with the help of an electrical switches usually MOSFET. In order to transfer the energy from input to the output they use inductors or capacitors. They deliver the power to the system upon requirement and hence they waste less power. So they are very much efficient than linear regulators and can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they can power useful loads from higher voltage sources. Switch-mode regulators are used in devices like portable phones, video game platforms, robots, digital cameras, and computers.
The drawback is that, they operate in high switching frequency which leads to more noise than linear regulators. Also they require more components which lead to more cost, more space and more complex circuit too.
Power Supply Design – PMIC’s
Nowadays for the stringent requirement of less floor space, all the above mentioned circuits are available in a single package as PMIC – Power Management IC. Silicon vendors often offer PMIC for the processor to be used with. The PMIC is tightly coupled with the processor and offers power scaling features for reducing consumption at the time of less load. Apart of these functionality, they may also incorporate other analog system functionalities like Audio Amplifiers, Touch controllers etc. Using PMIC reduces the number of components drastically as well as the cost at a higher design complexity.
Electromagnetic Interference (EMI) considerations
Electromagnetic energy whether intentionally or non-intentionally generated results in Electromagnetic Interference (EMI) with other equipments. This will affect other equipments by injecting noise into it. Power supplies, especially SMPS based, are primary source of EMI. Suitable filtering mechanisms should be implemented to control the EMI along with proper enclosure design. Sometimes, even a passive LC filter will provide a good solution for filtering the EMI.
Embien Technologies has rich experience in designing various embedded system. Kindly contact us for your requirements of system design or for solving any issues in your design.