Powering building alarm systems and driving LEDs
Equipping most commercial, residential, or industrial buildings with a fire alarm control system is a legal requirement. It serves a vital role in protecting life, assets, and the fabric of the building. Often placed in the reception area or security office, within close proximity to the building's main entrance, it continuously monitors inputs from a broad range of sensors throughout the facility. Smoke detectors, carbon monoxide, and PIR modules are just some critical sensors that constantly watch for the early warning signs of combustion.
It is imperative that the alarm control system and all its sensors operate reliably 24/7. Typically, the control system is line powered with the provision of a battery backup should the mains supply fail. Similar power arrangements for the sensors are also paramount, although some may operate permanently from a battery source.
This article investigates the technical considerations for powering building alarm control systems, sensors, and indicator panels, emphasising power conversion efficiency and achieving a low power consumption profile.
Investigating the architecture of a building alarm control system
Figure 1 illustrates the typical functional architecture of a building alarm system. Some building alarms incorporate multiple functions, such as a fire alarm, smoke detector, and passive infrared (PIR) movement sensors. Others may be standalone and perform a single operation, and, depending on the sensing complexity, the alarm may incorporate sensor fusion and signal conditioning components.
Many alarm units have a simple human-machine interface (HMI) requirement, typically just an LED to indicate power supply status and an alarm condition. High-volume audible alarm notification is a standard legal requirement.
Figure 1 - The typical functional architecture of a building alarm used in residential or commercial properties (source ST)
Safety standards usually stipulate that alarms be line-powered and equipped with a replaceable battery to maintain operation should the mains supply fail. Some alarm detectors feature a non-replaceable lithium coin cell with an in-service life of seven years, after which the complete unit must be replaced. Safety standards vary between countries and regions, and the regulations are usually stricter for commercial buildings such as offices, factories, and retail premises than residential properties. During the design of a building alarm, the engineering team must carefully review the unit's possible use cases and pay attention to the relevant building controls and safety regulations.
Designing an alarm's power supply has several considerations, including reliability, power conversion efficiency, and low quiescent current. A line-powered AC/DC power supply should be as efficient as possible to minimise waste heat, particularly for compact wall and ceiling-mounted units. Minimising the power consumption profile for a battery-powered alarm is essential to prolong battery life. Also, a legal requirement may stipulate how long the alarm can function reliably.
Some sensors are susceptible to slight variations in the supply voltage and conducted or radiated electromagnetic interference (EMI) from the power conversion circuit. Selecting power conversion and power regulation components requires carefully reviewing the key datasheet parameters.
Power conversion basics
The main methods of AC/DC and DC/DC conversion focus on two different techniques; linear or switching. For an AC/DC power supply, the linear method tends to be ruled out due to the size and weight of the transformer and smoothing capacitors. However, for DC/DC conversion, linear regulators offer a convenient, low-cost, and low component count approach. Switching-based AC/DC and DC/DC conversion techniques employ a pulse width modulated (PWM) signal applied to a switching transistor, such as a MOSFET. DC/DC converters offer an attractive alternative to designing a discrete design, with many low-current devices incorporating the switching MOSFET within a single package. Different conversion methods, termed topologies, are available that store energy in an inductor or a capacitor. Some topologies can deliver an output voltage much higher than the input voltage.
The PWM signal's duty cycle and frequency determine the output voltage, load regulation, and power conversion efficiency. Depending on the topology, the switching MOSFET and the inductor or capacitor store energy during the switch-on cycle. The energy is then released in the off period. Isolation of the output voltage from the input, a safety consideration for some applications, may be achieved by replacing the inductor with a transformer.
Non-isolated buck converter (step-down)
An asynchronous buck converter topology converts an input voltage to a lower output voltage, for example, 24 VDC to 12 VDC and the basic topology is illustrated in Figure 2
Figure 2 - The basic topology of a buck converter
SW1 is usually a MOSFET driven with a PWM signal, and SW2 is a diode. When the MOSFET conducts (ON), energy is stored in the inductor. When SW1 switches OFF, the stored energy flows from the inductor and through the diode SW2 to provide an output voltage. In a synchronous buck converter topology, another switching transistor replaces the diode and is driven with an out-of-phase PWM signal.
Non-isolated boost converter (step-up)
A boost converter topology has a slightly different arrangement - see Figure 3 - that allows it to provide an output voltage higher than its input. How greater the output voltage is above the input voltage depends on PWM duty cycle, the amount of load, switching frequency, and degree of voltage regulation required. For most practical purposes, the limit is a factor of three.
Figure 3 - The basic topology of a boost converter
With minimal components, non-isolated buck and boost converters achieve a high conversion efficiency of up to 95 %. An example of a galvanically isolated topology is the isolated flyback converter. In place of the inductor, a transformer is employed for energy storage and isolation.
Glossary of key terminology
- Common-mode noise: Noise common to the live and neutral input of a power supply with respect to the ground.
- Constant current power supply: A supply that regulates the output current to a specified limit.
- Constant voltage power supply: A power supply that regulates the output voltage to a defined specification.
- Derating: The reduction in operating parameters to improve reliability when used in higher temperatures or load conditions.
- Differential mode noise: Noise measured between the input and output of a DC/DC converter or between the live and neural of an AC/DC power supply.
- Dropout voltage: The lowest point of a converter's input voltage that maintains the specified output voltage.
- Inrush current: The maximum input current instantly consumed by a power supply at switch on.
- Line regulation: The variation of output voltage resulting from input voltage change.
- Load regulation: The variation of output voltage due to a change in the load conditions.
- Linear regulator: A voltage regulation approach achieved by placing a controlling semiconductor in series between the input and the output.
Product showcase
Figure 4 illustrates the internal functional block diagram of an L6981 38 V, 1.5 A synchronous step-down converter IC. With an input voltage range of 3.5 VDC to 38 VDC and an adjustable output voltage from 0.85 VDC to 38 VDC, the L6981 is available in two variants. The -LCM (low current consumption) is optimised for high efficiency when operating from light loads, and the -LNM (low noise mode) for noise-sensitive applications. The -LCM is ideal for battery-powered applications because it skips pulses to conserve energy during light load conditions. To keep EMI from the switching process to a minimum, the -LNM avoids pulse skipping altogether.
Figure 4 - The functional block diagram of the L6981 step-down regulator IC (source ST)
A similar step-down switching regulator is the ST L7983. With a 60 VDC input voltage rating and a maximum output current of 300 mA, the L7983 has an extremely low quiescent characteristic of 10 μA, making it desirable for battery-powered applications.
The L6981 and the L7983 offer a convenient and compact method of designing a DC/DC step-down converter with a minimum of external components. Figure 5 illustrates a typical application circuit using the L7983 with the addition of only six passive components.
Figure 5 - A typical application circuit for the ST L7983 illustrating its minimal external component approach (source ST)
An example of a linear regulator is the STLQ015, a 150 mA ultra-low quiescent current linear regulator – see Figure 6. Accommodating an input voltage from 1.5 VDC to 5.5 VDC and available in ten output voltage variants covering the popular nominals from 1.2 VDC to 3.3 VDC, the STLQ015's quiescent current is only 1.4 uA when delivering 100 % load. The enable pin allows the regulator to be placed into a standby mode where the quiescent current drops to just 1 nA.
Figure 6 - The functional block diagram of the ST STLQ015 150 mA linear voltage regulator (source ST)
A low-dropout (LDO) regulator is a particular type of linear regulator that can operate with an input voltage very close to the output voltage. Like linear regulators, LDOs offer a simple, and low-noise method of providing non-isolated voltage conversion and regulation. An example of an LDO is the 100 mV low-dropout LDK130, capable of supplying up to 300 mA and accommodating an input voltage between 1.9 VDC to 5.5 VDC, a range of fixed output voltages is available. For flexibility, an adjustable version allows configuration of the output voltage using a voltage divider input - see Figure 7.
Figure 7 - An example application use case with the adjustable output voltage variant of the LDK130 (source ST)
As the introduction highlights, building alarms may utilise indicator LEDs to display their operational status and alert occupants of impending danger. An example of a dedicated LED driver IC is the ST LED1202. Capable of driving 12 single LEDs with up to 5 VDC / 20 mA per channel and able to store eight patterns for LED sequencing without MCU intervention, the LED1202 communicates with a host microcontroller via I2C. Figure 8 illustrates a simplified block diagram of the LED1202. Each LED channel permits pre-programmed visual effects, including fade-in, fade-out, and 'breathing.'
Figure 8 - The simplified block diagram of the ST LED1202 12-channel low quiescent current LED driver (source ST)
Power conversion for building alarms
Building alarms are critical to protecting people and buildings. Choosing reliable, efficient, and low-power converters and LED drivers is an essential aspect of the design process. In this short article, we've introduced the basics of power conversion and showcased just some of the many power conversion products available from EBV, an authorised ST distributor.
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