What are HVAC controls?
The definition of HVAC control systems can range depending on whether it is applied to a very basic residential application or potentially to an extremely complex large-scale application, but in short, HVAC controls consist of devices that control the operations of Heating, Ventilation, and Air Conditioning equipment.
In residences, you’ll typically find a thermostat that is connected to some kind of self-contained A/C unit. By adjusting the temperature (or setpoint), you are controlling the functions of that self-contained unit. For example:
When you set your thermostat to 75 degrees in the summer, you are dictating that your A/C unit should run until your indoor temperature reaches 75 degrees, at which point, it should shut off.
In this simple example, your thermostat is considered an HVAC controller, however, it is also a sensor because it is sensing your indoor temperature. We’ll talk more about sensors as we delve deeper into more complex systems.
When it comes to applying HVAC control systems to larger facilities, the HVAC controls are sometimes a part of a larger system known as a building automation or energy management system.
Building Automation Systems
In large facilities, mechanical systems are often very complex and need to work efficiently in order to be effective and not waste energy. The equipment controlled by building automation systems (BAS) frequently includes that of HVAC, lighting, fire alarm, and access/security systems.
Having these consolidated under one software platform can be advantageous when trying to build sequences of operations to make a building “smart”, however, some building operators view this as a risk having “all their eggs in one basket”. For the purposes of this article, we will stay focused on stand-alone HVAC control systems.
In the interest of keeping things simple, we’ll break down the typical BAS into 4 major categories:
Head-end computer/software
Network Infrastructure
Controllers
End Devices
As you gain a deeper understanding of HVAC controls, you’ll learn there are many pieces of equipment that encompass more than one of these categories, however, we’ll do our best to stay with simple examples for each.
Head-end Computer/Software
The head-end computer/software may also be known as a workstation, the front-end, or energy management software. This is the central user interface that allows an operator to gain insight into field operations and send control functions as needed. Users see the results of the control signals sent based on monitoring the system inputs. For example, if an air duct damper was closed and is commanded open, the zone temperature will start dropping based on that command being sent.
Network Infrastructure
In a modern HVAC controls system, the network infrastructure typically consists of CAT-5/6 Ethernet and RS-485 twisted pair cables. The CAT 5/6 Ethernet cable generally carries messages from the head-end to and from the controllers while the RS-485 wires take care of the communication between the controllers and field devices.
Controllers
A device that is connected to the network infrastructure and provides a means for monitoring and/or controlling end devices.
End Devices
These devices include sensors that measure the value of a variable such as a temperature or humidity, relay switches that open and close circuits, and actuators that convert energy into some kind of mechanical force. There are a multitude of end devices. These are just a few.
Together, these four components make up an HVAC controls system whose primary goal is to ensure occupant comfort and safety while operating as efficiently as possible from an energy consumption standpoint.
To maximize the efficiency and reduce system power consumption of HVAC controllers, we first need to understand the required products and related designs. HVAC controllers are divided into two categories, one is System controller and the other is Unit controller.
The first is the System controller part. Its structure is composed of 26 parts, namely Input rectifier、Input power protection、Non-isolated DC/DC power supply、Isolated DC/DC power supply、AC input monitoring、Wireless interface、Output user interface、Isolation、AC switch、Current sense、AI protection、Analog input、IO protection、Universal I/O、Digital input protection、Digital input interface、Digital input、Digital input isolation、Computing unit、Relay driver、Analog output、Fieldbus interface、Industrial Ethernet interface、AO protection、IF protection、Ethernet IF protection.
The second is Unit controller part. Its structure is composed of 20 parts, namely Input rectifier、Input power protection、Non-isolated DC/DC PoL、Isolated DC/DC power supply、Thermostat supply、Communicating thermostat interface、Thermostat interface、Wireless interface、Output user interface、Fieldbus interface、Controller、Isolation、AC switch、Current sense、Temp sensor interface、Damper isolation、Damper AC switch、Damper current sense、Relay driver、Proportional damper interface.
Each part has its corresponding classification and product model. I will introduce them to you one by one below.
I will introduce System controller first:
Input power protection
E-fuses can be used to get an integrated solution to achieve over voltage, undervoltage, over current, short circuit and reverse polarity protection on the power supply. Input power protection include eFuses & hot swap controllers, the corresponding products are TPS2595、TPS25921A、TPS2662
Non-isolated DC/DC power supply
Multiple voltage rails are generated from the input power source to support all the devices used in the system. Buck converters may be used to step down the input voltage. Linear regulators (LDOs) are a good option for simple designs. PMICs offer integrated multiple rails especially for different processors. Non-isolated DC/DC power supply include Multi-channel ICs (PMICs)、Buck converters (integrated switch)、Buck controllers (external switch)、Supervisor & reset ICs、Linear & low-dropout (LDO) regulators、SR & load share controllers, the corresponding products are TPS65219、LMR33630、LM25145、TPS3813、TLV1117LV、UCC24612
Isolated DC/DC power supply
The wired interface or the solid state relay circuit may need isolated voltage supply to meet signal integrity and safety. Isolated DC/DC power supply include Flyback controllers、SR & load share controllers、Isolated DC/DC converters & modules、PWM controllers、Transformer drivers, the corresponding products are UCC28740、UCC24612、UCC12050、TL2842B、SN6501
AC input monitoring
On board sensing monitors the AC input voltage, helps to identify the undervoltage or over voltage conditions and can give signal to the processor for safe shutdown. AC input monitoring include Shunt voltage references、Series voltage references、Comparators、AND gates、OR gates、Noninverting buffers & drivers、Inverting buffers & drivers, the corresponding products are LM4040、REF3425、LM2903B、SN74AHC1G09、SN74LVC1G32、SN74LVC2G17、SN74LVC1G14
Wireless interface
An integrated microcontroller and radio topology is intended for designs requiring additional resource management. Integrated topologies also allow the application layer, wireless stack and RF radio functionality to be on one chip. A dedicated RF radio is an option for designs requiring 2.4 GHz, Sub-1 GHz, or Wi-Fi wireless connectivity. The dedicated RF radio allows the existing host processor the handle the wireless stack and application. Wireless interface include Wi-Fi products、Other wireless products、Low-power 2.4-GHz products、Sub-1 GHz transceivers、Sub-1 GHz wireless MCUs, the corresponding products are WL1801MOD、CC2530-RF4CE、CC2651R3、CC1120、CC1312R
Output user interface
The output user interface may consist of a simple LED to indicate system status. An LED driver is an option for flashing a single color LED. A GPIO expender. Output user interface include Digital multiplexers & encoders、I2C general-purpose I/Os (GPIOs)、Noninverting buffers & drivers、Backlight LED drivers, the corresponding products are CD74ACT151、TCA6408A、SN74LVC2G17、TPS61165
Analog input
An analog input signal condition circuit is required to interface different sensor or actuator feedback. The input signal could be 0-10V, 2-10V, 0-20mA or 4-20mA. Analog input include General-purpose op amps、Comparators、Precision op amps (Vos<1mV)、4-20mA signal conditioners、Precision ADCs、N-channel MOSFETs、Series voltage references、Analog switches & muxes, the corresponding products are LM2902B、TLV1701、OPA4317、XTR111、TLA2021、CSD18543Q3A、REF3425、TMUX1511
Universal I/O
An input signal condition circuit is required to interface different sensor or actuator feedback. The input signal could be an analog signal (0-10V, 2-10V, 0-20mA or 4-20mA), digital signal, resisive input or dry contact. The same interace may be needed to function as an input or output. Universal I/O include N-channel MOSFETs、Series voltage references、Precision DACs (≤10 MSPS)、Precision ADCs、General-purpose op amps、Comparators、Precision op amps (Vos<1mV)、Analog switches & muxes, the corresponding products are CSD18543Q3A、REF3425、DAC43608、TLA2021、ADS1119、LM2902B、LM2903B、OPA4317、TMUX1119
Digital input interface
The digital isolator can be used to interface any digital signal from the sensors and actuators. Digital input interface include Isolated digital inputs, the corresponding products are ISO1211、ISO1212
Next, I will introduce Unit controller
Input power protection
E-fuses can be used to get an integrated solution to achieve over voltage, undervoltage, over current, short circuit and reverse polarity protection on the power supply. Input power protection include eFuses & hot swap controllers, the corresponding products are TPS2640、TPS2662、TPS24750
Thermostat supply
Sometimes the unit controller provides supply to power up thermostat, and proper inbuild over current and reverse polarity protection are required. Thermostat supply include eFuses & hot swap controllers, the corresponding products are TPS25200、TPS2663
Communicating thermostat interface
The unit controller communicates to the thermostat using an RS-485 interface, and the interface lines need adequate protection against ESD. Communicating thermostat interface include ESD protection diodes、TVS diodes、RS-485 & RS-422 transceivers、Application-specific port protection ICs, the corresponding products are ESDS552、ESD204、TPD4E02B04、TSM24CA、THVD1400、TPD6S300A
Controller
The processor or microcontroller in an HVAC system controller monitors system sensors, controls actuators and heating drives and communicates with other devices in the HVAC system through the wired interfaces. Controller include Arm Cortex-M0+ MCUs、MSP430 microcontrollers、Arm Cortex-M4 MCUs、Inverting buffers & drivers、Fixed-direction voltage translators、Direction-controlled voltage translators、Noninverting buffers & drivers, the corresponding products are MSPM0G1506、MSP430FR2672、TM4C123GH6PM、SN74LVC1G04、SN74LV1T34、SN74LVC1G125、TXS0108E、CC3100MOD
Temp sensor interface
An operational amplifier may be required to interface the temperature sensor output at different nodes of the HVAC system. Temp sensor interface include General-purpose op amps, the corresponding products are TLV9042、OPA4992、TLV9001、TLV9351
Hope our integrated circuits and reference designs can help you create a differentiated heating, ventilation and air conditioning (HVAC) controller design for building management systems with maximum efficiency and lower system power.
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