Smart Solar Solution Powers Remote PC/104
By Tom Barnum, PC/104 Consortium
Remote PC/104 systems that depend on solar power can benefit from incorporating intelligence into the power supply and battery management.
The compact yet rugged form-factor of PC/104 makes it ideal for portable equipment installed in remote locations. Such systems often depend on solar power with rechargeable battery back-up. A smart power supply in such applications brings many benefits, including extended battery life, increased reliability and flexibility in system design.
A typical solar-powered system has three main elements to its power source: the solar array, the battery bank and the power supply. While the solar array and battery bank get most of the attention, the power supply represents the real opportunity for system engineering. The supply combines DC-to-DC conversion and battery charger functions as well as managing the energy input sources, placing it in a key position for handling overall system power management. Because it, like all PC/104 modules, connects to the system bus, the supply can become an active partner in system operation rather than an isolated function.
One such supply is shown in Figure 1. This unit has a small microprocessor that connects to the PC/104 bus, allowing it to receive commands from the main system CPU. On the system bus the power supply processor acts as a memory-mapped peripheral, simplifying its configuration and programming. Within the supply, the processor has the ability to sense currents and voltages from the main power source as well as the battery bank. The processor can also control the battery charging circuit and shut down the regulator supplying the PC/104 system.
Match Panel and Battery
The first step in using an intelligent supply to provide active power management is to set its operating framework by choosing the solar panel and battery voltages you need. The panel and battery must be compatible. Most battery charger circuits use a step-down buck design, so the charging source must generate a voltage at least one volt greater than the maximum battery charging voltage. You can’t charge a 20V battery with a solar panel that only generates 17 volts.
Another important aspect to consider is the solar panel’s current generation and the battery’s energy storage capacity requirement, measured in amp-hours. The solar panel typically must supply all the energy the system needs to use when active, including the battery charging current. The battery must have enough capacity to operate the system when solar power is absent, including storms and other inclement weather. Further, the system may not have the opportunity to recharge the battery right away, so the battery capacity may need to be great enough to operate the system without full recharging for as long as several weeks.
This is where active power management can begin to make a difference. Many solar-powered systems only need to be active for short time periods on a scheduled or an event-driven basis. A solar-powered weather buoy, for instance, only needs to be active when it is scheduled to communicate with a passing satellite to provide current weather data.
Without intelligence in the power supply, the CPU in such systems must maintain awareness of time or events, typically by halting system operation while waiting for an interrupt, in order to reactivate as needed. Even though a halted system requires much less current than an active one, it still represents a drain on the system power source. In many cases this standby current represents the bulk of the average system power demand.
Intelligence Saves Battery Power
By providing the power supply with its own intelligence, including programmable timers and a real-time clock, the supply can control system deactivation and reactivation. The supply processor generates a control signal to the main power regulator, allowing it to shut down power to the rest of the PC/104 system. This allows the main system CPU to initiate a controlled shutdown sequence to handle periods of inactivity. The system CPU commands the power supply to shut down following a time delay, gives the supply a wake-up time, then prepares the rest of the system for loss of power. When the supply shuts down system power, it goes into its own standby mode waiting for the reactivation timer event. This reduces the system’s standby current to the needs of the supply’s processor alone, which can be as low as 50 nA. This major reduction in standby current may allow the system to operate with a smaller, less expensive battery bank and solar panel.
The intelligent supply can do more than reduce the capacity requirements on the battery bank; it can provide flexibility in the choice of battery technology. Many types of rechargeable batteries are available, including sealed lead-acid, lithium-ion and nickel metal hydride. Each of these batteries has its own unique charging needs, however.
Consider, for instance, a system that uses a sealed lead-acid battery. It requires a two-stage charging pattern, as shown in Figure 2a. During the initial charging period, the battery receives a constant current. When the battery cell voltage reaches 2.45V, the charger switches over to constant voltage operation, with a set point of 2.3V per cell. This keeps the battery on trickle charge, replenishing energy as it is being drawn out.
The circuit for a lithium-ion cell, on the other hand, needs a three-stage charging profile because such batteries cannot tolerate a trickle charge. As shown in Figure 2b, the initial charge profile also uses a constant current. Its switch-over point is 4.15V per cell, however, and it continues to charge only as long as the charge current exceeds 1.1A. Once the charging current has dropped below that threshold, the charging cycle must terminate to avoid damaging the battery.
Handling Multiple Battery Types
A relatively simple power supply will have a charging circuit designed for only one type of battery. That circuit typically uses analog detectors to monitor the voltage and current set points, triggering hard-wired logic that handles the switch-over. These dedicated circuits require a design change to accommodate a different battery type. An intelligent supply, on the other hand, senses the battery voltage and charging current, then has its CPU control the charging regulator’s set points based on a profile received from the PC/104 system processor. This ability to program the battery charge profile over the PC/104 bus to match any battery allows users to select the battery technology that best suits their application and incorporate new technologies as they become available.
This programmability of the intelligent supply also allows it to control system response to changes in the input power sources. The supply’s internal timer can be used, for instance, to “debounce” transient interruptions in the power before taking action, allowing service personnel to change a battery pack without triggering a “Low Battery” fault. The supply’s processor can use an I2C or SMBus to monitor battery pack conditions to provide early warning of battery failure as well as an estimate of remaining energy capacity so that the system can prepare to shut down gracefully.
The supply can also enhance system reliability. One possibility is to serve as a system watchdog. If the system CPU generates a “heartbeat” signal to the supply, the supply can use that signal to initiate a timer. If the timer reaches the end of its count, the supply then initiates a power shutdown and restart to reboot the system.
A second way of enhancing system reliability is to protect the battery pack from deep discharge. A fully discharged storage battery runs the risk of reversing the polarity of its cells, effectively destroying the battery. If a solar-powered system suffers long periods without sunlight, as in an extended storm, it may completely drain the batteries through normal operation, and then take them to a deep discharge while in standby mode. Only completely disconnecting the battery from the system will prevent deep discharge.
Protecting the Battery
Battery isolation can be built into the battery pack under control of the intelligent supply. The circuit in Figure 3 shows a scheme for isolating the battery if the system needs to shut down completely. Under normal operating conditions, the supply generates a Battery Enable signal that keeps MOSFET transistors in the ground return line active. If main power from the solar panel is absent and the batteries are nearly drained, the supply can initiate a system shutdown sequence. The supply signals the system CPU to prepare for loss of power. Once the system is ready, the supply removes the Battery Enable signal, turning off the MOSFETs and isolating the battery from the system. With the supply shut down, there is no voltage present at the +5C line, so the MOSFETs remain off, protecting the battery from deep discharge.
The system can recover from this mode in two ways. If main power from the solar panels returns, the supply CPU will receive power and reboot, then reinitialize the rest of the system, including reasserting the Battery Enable signal. Since the +5C line will be active, the MOSFETs will turn on and re-engage the battery. An alternative is to use a push button in parallel with the MOSFETs to bypass the isolation and use the battery power to reactivate the system. This is useful when main power is still absent but the battery pack has been replaced.
If using battery isolation with solar panels as the main power source, an additional circuit may be useful. To restart the power supply, the main power source must supply the inrush current needed for the regulator to charge system capacitance and quickly bring the output voltage to its set point. Many solar panels have current limits and are unable to meet the momentary demand, resulting in an inability to start the system from solar power alone. A capacitor-diode network across the sense resistor on the switching regulator can temporarily provide a false indication of the output voltage, slowing the regulator’s response and limiting inrush current to a value the solar panel can produce. The required network and connection depends on the type of regulator in use as well as the power rise time that the system can tolerate.
With an intelligent power supply, then, a PC/104 system can provide robust operation in a variety of situations that can cause simpler systems to fail, sometimes even resulting in system damage. Further, the supply can provide designers with increased options both for the battery technology as well as the sophistication of the system response to power conditions.
Tom Barnum is the current Chairman of the PC/104 Consortium. The author would like to thank Ed Foster, senior engineer at Tri-M Engineering, for his invaluable contributions to this article.
Reprinted from RTC Magazine Copyright 2006
For more information please contact Tri-M Systems' Sales Department.
sales@tri-m.com
1.800.665.5600
By Tom Barnum, PC/104 Consortium
Remote PC/104 systems that depend on solar power can benefit from incorporating intelligence into the power supply and battery management.
The compact yet rugged form-factor of PC/104 makes it ideal for portable equipment installed in remote locations. Such systems often depend on solar power with rechargeable battery back-up. A smart power supply in such applications brings many benefits, including extended battery life, increased reliability and flexibility in system design.
A typical solar-powered system has three main elements to its power source: the solar array, the battery bank and the power supply. While the solar array and battery bank get most of the attention, the power supply represents the real opportunity for system engineering. The supply combines DC-to-DC conversion and battery charger functions as well as managing the energy input sources, placing it in a key position for handling overall system power management. Because it, like all PC/104 modules, connects to the system bus, the supply can become an active partner in system operation rather than an isolated function.
One such supply is shown in Figure 1. This unit has a small microprocessor that connects to the PC/104 bus, allowing it to receive commands from the main system CPU. On the system bus the power supply processor acts as a memory-mapped peripheral, simplifying its configuration and programming. Within the supply, the processor has the ability to sense currents and voltages from the main power source as well as the battery bank. The processor can also control the battery charging circuit and shut down the regulator supplying the PC/104 system.
Match Panel and BatteryThe first step in using an intelligent supply to provide active power management is to set its operating framework by choosing the solar panel and battery voltages you need. The panel and battery must be compatible. Most battery charger circuits use a step-down buck design, so the charging source must generate a voltage at least one volt greater than the maximum battery charging voltage. You can’t charge a 20V battery with a solar panel that only generates 17 volts.
Another important aspect to consider is the solar panel’s current generation and the battery’s energy storage capacity requirement, measured in amp-hours. The solar panel typically must supply all the energy the system needs to use when active, including the battery charging current. The battery must have enough capacity to operate the system when solar power is absent, including storms and other inclement weather. Further, the system may not have the opportunity to recharge the battery right away, so the battery capacity may need to be great enough to operate the system without full recharging for as long as several weeks.
This is where active power management can begin to make a difference. Many solar-powered systems only need to be active for short time periods on a scheduled or an event-driven basis. A solar-powered weather buoy, for instance, only needs to be active when it is scheduled to communicate with a passing satellite to provide current weather data.
Without intelligence in the power supply, the CPU in such systems must maintain awareness of time or events, typically by halting system operation while waiting for an interrupt, in order to reactivate as needed. Even though a halted system requires much less current than an active one, it still represents a drain on the system power source. In many cases this standby current represents the bulk of the average system power demand.
Intelligence Saves Battery Power
By providing the power supply with its own intelligence, including programmable timers and a real-time clock, the supply can control system deactivation and reactivation. The supply processor generates a control signal to the main power regulator, allowing it to shut down power to the rest of the PC/104 system. This allows the main system CPU to initiate a controlled shutdown sequence to handle periods of inactivity. The system CPU commands the power supply to shut down following a time delay, gives the supply a wake-up time, then prepares the rest of the system for loss of power. When the supply shuts down system power, it goes into its own standby mode waiting for the reactivation timer event. This reduces the system’s standby current to the needs of the supply’s processor alone, which can be as low as 50 nA. This major reduction in standby current may allow the system to operate with a smaller, less expensive battery bank and solar panel.
The intelligent supply can do more than reduce the capacity requirements on the battery bank; it can provide flexibility in the choice of battery technology. Many types of rechargeable batteries are available, including sealed lead-acid, lithium-ion and nickel metal hydride. Each of these batteries has its own unique charging needs, however.
Consider, for instance, a system that uses a sealed lead-acid battery. It requires a two-stage charging pattern, as shown in Figure 2a. During the initial charging period, the battery receives a constant current. When the battery cell voltage reaches 2.45V, the charger switches over to constant voltage operation, with a set point of 2.3V per cell. This keeps the battery on trickle charge, replenishing energy as it is being drawn out.
The circuit for a lithium-ion cell, on the other hand, needs a three-stage charging profile because such batteries cannot tolerate a trickle charge. As shown in Figure 2b, the initial charge profile also uses a constant current. Its switch-over point is 4.15V per cell, however, and it continues to charge only as long as the charge current exceeds 1.1A. Once the charging current has dropped below that threshold, the charging cycle must terminate to avoid damaging the battery.Handling Multiple Battery Types
A relatively simple power supply will have a charging circuit designed for only one type of battery. That circuit typically uses analog detectors to monitor the voltage and current set points, triggering hard-wired logic that handles the switch-over. These dedicated circuits require a design change to accommodate a different battery type. An intelligent supply, on the other hand, senses the battery voltage and charging current, then has its CPU control the charging regulator’s set points based on a profile received from the PC/104 system processor. This ability to program the battery charge profile over the PC/104 bus to match any battery allows users to select the battery technology that best suits their application and incorporate new technologies as they become available.
This programmability of the intelligent supply also allows it to control system response to changes in the input power sources. The supply’s internal timer can be used, for instance, to “debounce” transient interruptions in the power before taking action, allowing service personnel to change a battery pack without triggering a “Low Battery” fault. The supply’s processor can use an I2C or SMBus to monitor battery pack conditions to provide early warning of battery failure as well as an estimate of remaining energy capacity so that the system can prepare to shut down gracefully.
The supply can also enhance system reliability. One possibility is to serve as a system watchdog. If the system CPU generates a “heartbeat” signal to the supply, the supply can use that signal to initiate a timer. If the timer reaches the end of its count, the supply then initiates a power shutdown and restart to reboot the system.
A second way of enhancing system reliability is to protect the battery pack from deep discharge. A fully discharged storage battery runs the risk of reversing the polarity of its cells, effectively destroying the battery. If a solar-powered system suffers long periods without sunlight, as in an extended storm, it may completely drain the batteries through normal operation, and then take them to a deep discharge while in standby mode. Only completely disconnecting the battery from the system will prevent deep discharge.
Protecting the Battery
Battery isolation can be built into the battery pack under control of the intelligent supply. The circuit in Figure 3 shows a scheme for isolating the battery if the system needs to shut down completely. Under normal operating conditions, the supply generates a Battery Enable signal that keeps MOSFET transistors in the ground return line active. If main power from the solar panel is absent and the batteries are nearly drained, the supply can initiate a system shutdown sequence. The supply signals the system CPU to prepare for loss of power. Once the system is ready, the supply removes the Battery Enable signal, turning off the MOSFETs and isolating the battery from the system. With the supply shut down, there is no voltage present at the +5C line, so the MOSFETs remain off, protecting the battery from deep discharge.
The system can recover from this mode in two ways. If main power from the solar panels returns, the supply CPU will receive power and reboot, then reinitialize the rest of the system, including reasserting the Battery Enable signal. Since the +5C line will be active, the MOSFETs will turn on and re-engage the battery. An alternative is to use a push button in parallel with the MOSFETs to bypass the isolation and use the battery power to reactivate the system. This is useful when main power is still absent but the battery pack has been replaced.If using battery isolation with solar panels as the main power source, an additional circuit may be useful. To restart the power supply, the main power source must supply the inrush current needed for the regulator to charge system capacitance and quickly bring the output voltage to its set point. Many solar panels have current limits and are unable to meet the momentary demand, resulting in an inability to start the system from solar power alone. A capacitor-diode network across the sense resistor on the switching regulator can temporarily provide a false indication of the output voltage, slowing the regulator’s response and limiting inrush current to a value the solar panel can produce. The required network and connection depends on the type of regulator in use as well as the power rise time that the system can tolerate.
With an intelligent power supply, then, a PC/104 system can provide robust operation in a variety of situations that can cause simpler systems to fail, sometimes even resulting in system damage. Further, the supply can provide designers with increased options both for the battery technology as well as the sophistication of the system response to power conditions.
Tom Barnum is the current Chairman of the PC/104 Consortium. The author would like to thank Ed Foster, senior engineer at Tri-M Engineering, for his invaluable contributions to this article.
Reprinted from RTC Magazine Copyright 2006
For more information please contact Tri-M Systems' Sales Department.
sales@tri-m.com
1.800.665.5600
Copyright© 2005 Tri-M Engineering and Systems Inc. All Rights Reserved.