ecobee Power Module Designer: Mo Abdelrahim

Transcription

1 ecobee Power Module Designer: Mo Abdelrahim 1

2 1 Introduction ecobee Thermostats, known to many as the best Smart Thermostat available on the market, has 1 main limitation, and that s its incompatibility with 2-wire, heat only systems. As part of the Support Department, we get many frustrated inquiries from customers who bought the ecobee only to realize that they are not compatible out of the box, and that some wiring modifications are required. Most customers end up returning the ecobee and go to a different product that is compatible. To resolve this problem, I decided to design a power module that could be attached to the back of an ecobee Thermostat and provide it with power. This module will be designed with the intention of giving such customers an easy compatibility option that could be purchased separately. 2 System Design Overview The system design will be split into 3 parts. The first part, and the easiest, is the power stealing circuit. The circuit will be designed to steal power from the 24VAC available between the R and W wires. This voltage will be used to charge a 500mAh LiPo battery that will then power the ecobee Thermostat. The second part of the design will be the inversion circuit. The circuit will be designed to invert and boost the battery voltage to an appropriate AC signal that then could be boosted to 24VAC using a step-up transformer. The third and last part will be the step-up transformer design. 3 Part 1: Power Stealing/Charging Circuit 3.1 Building Block 3.2 Key Components MCP16331 The MCP16331 is a highly integrated, high-efficiency, fixed frequency, step-down DC-DC converter. 2

3 Up to 96% Efficiency Input Voltage Range: 4.4V to 50V Output Voltage Range: 2.0V to 24V MCP73831 The MCP73831 device is a highly advanced linear charge management controller High Accuracy Preset Voltage Regulation: % Programmable Charge Current: 15 ma to 500 ma 3.3 Schematic Entry using Autodesk EAGLE The schematic entry for this design was done using Autodesk EAGLE AC-DC Rectification and Current Limiting The first circuit in the design will be the rectification circuit. This will convert the 24VAC available between the R and W wires into DC voltage and current that could be stepped down and used to charge the LiPo battery. This was done using a full-bridge rectifier with a current limiting fuse to ensure the equipment will not be turned on during the power stealing process. Figure 2. Power Stealing & Current Limiting Schematic 3

4 3.3.2 DC-DC Voltage Step Down using Buck Converter The next step would be stepping down the rectified voltage to the 5V needed to charge the LiPo battery. This will be done using the MCP The output voltage is set by the resistors R1 and R2 (voltage divider). Figure 3. DC-DC Step Down Schematic LiPo Battery Charging Finally, the voltage stepped down using the Buck converter is then used to charge the 500mAh LiPo battery using the MCP73831 IC. The charging current was limited to a maximum of 100mA using resistor R4. Figure 4. LiPo Battery Charging Schematic 4

5 Figure 4. Full Power Stealing and Charging Schematic 5

6 4 Part 2: DC-AC Inversion Circuit 4.1 Building Block 4.2 Key Components AP3012 The AP3012 is a high power, constant frequency, current mode PWM, step-up (boost) converter. High Efficiency up to 81% Adjustable Output Voltage up to 29V PAM2423 The PAM2423 is a high-performance, fixed frequency, current-mode PWM step-up DC-DC converter that incorporate internal power MOSFET. It also includes an integrated power MOSFET that supports peak current of 5.5A. Greater than 90% Efficiency Adjustable Output Voltage Up to 24V Built-in Over-Voltage Protection (OVP) Thermal Shutdown Over-Current Protection TC4427A The TC4427A is a 1.5A dual high-speed power MOSFET driver with a wide input supply voltage operating range (4.5V to 18V). 6

7 4.2.4 ATtiny85 The ATiny85 is a high performance, low power AVR 8-Bit Microcontroller. It has 2 high frequency PWM outputs with separate Output Compare registers, ideal for generating the SPWM signals for the H-bridge ZXMHC6A07N8 60V complementary enhancement mode MOSFET H-Bridge N-Channel: V BR(DSS) = 60V, R DS(ON) = V GS= 10V, I D = 1.8A P-Channel: V BR(DSS) = -60V, R DS(ON) = V GS= -10V, I D = 1.4A 4.3 Schematic Entry using Autodesk EAGLE DC-DC Voltage Step Up to 5V The battery voltage will be stepped up to 2 different voltages, 5V for the ATtiny85 and 12V for the H-bridge inverter. The AP3012 will be tasked with providing the 5V required by the ATiny85. Since the ATiny85 has very low power requirements 8MHz), which makes the lowcost AP3012 boost converter more than adequate for the task. The output voltage was set using the resistors R1 and R2 Figure 6. DC-DC Step-Up Schematic (5V) 7

8 4.3.2 DC-DC Voltage Step Up to 12V The PAM2423 was chosen to step up the battery voltage to 12V due to its capability of providing higher currents at higher voltage boosts. It can provide up to 800mA at 12V. This made it ideal for the inversion purposes. Figure 7. DC-DC Step-Up Schematic (12V) 8