Solar Power / Panel Inverter – Grid-Intertie Inverter using Attiny45

For the last year I’ve been working on a prototype for a Solar Inverter that can be Grid Intertied. A solar inverter takes the 12V DC (or other voltages) from the solar panels and converts it to 120V AC which is the power that most of your household appliances use. A Grid-Intertied inverter allows you to feed that power back into power grid (your house power) to help power your household appliances.
My goal was to design a small inverter, about 100W, that could be used with one solar panel and could be grid intertied. My second prototype (pictured above) has achieved these design goals. So on these web pages I’m going to document the design of the hardware and the software of my solar inverter. I’m releasing these designs to public without restrictions. All I ask is that if you use any of my design that you credit me and add a link back to this website. I hope these designs will help further the work of other people in this area.

If you are interested in this project there are a couple of things you should consider:
1. This is not a plan to build an inverter for your vacation home. This a work in progress. Although the inverter works it has some problems areas. I’ll try to point them out as I go but I consider the information on these pages more for people who are experimenting in this area.
2. It is probably illegal for you to connect any inverter to your home’s power grid without the approval of your local power company. As far as I know all utilities require that the any inverters connected to their power grid be UL approved, that there is an external disconnect switch and that they inspect the whole installation before approval. What they are concerned about is something called islanding. It is possible if the power grid goes down that your solar powered inverted could backfeed power into the grid and possibly injure a power company work who thinks that the grid is dead. I’ve included anti-islanding safety features in my inverter design and my opinion is that a 100W inverter will shut down way before it can become a safety hazard. That being said it is illegal to connect an inverter to power grid without the utility’s permission so don’t do it.
3. This inverter take a “safe” 12V DC and generates a very dangerous 120V AC. If you don’t know what you’re doing don’t mess with it! This is very dangerous, 120V AC can kill you!
So now that I’m done with all the warnings let’s take a look at the inverter design, remember this a work in progress!
Project Technical Description:
An inverter is basically an electronic device that takes a DC input voltage and converts it to an AC output voltage. My design takes a 12V DC input from my solar panels (two 50w panels in parallel) and converts it 120V AC at 60Hz which is the power that most home appliances use. A grid-intertie inverter connects up to and external AC power system and matches it’s internal AC sinewave to the external AC sinewave to allow the inverter to feed power into the external system. My inverter design also includes a maximum power point tracking algorithm to maximize the power drawn from the solar panel. The inverter uses 4 MOSFETs in a full bridge configuration to convert the input DC to a low voltage AC. The low voltage AC is converted to high voltage AC by a toroidal transformer with a 1:30 ratio. The high voltage AC is rectified by 4 diodes in a full bridge configuration. The resulting waveform is smoothed by an inductor and capacitor filter and then converted to an AC sinewave by the final 4 MOSFET switches. A relay is used to connect the generated AC to the external AC. I used an Atmel AT90PWM3B processor to control the inverter. This processor has some nice feature that make is well suited to digital power control. Please see the hardware description and software comments for a more detailed explanation of the project.
Schematics:

These schematics were created with EagleCAD v4.16r1. I’ve included the Eagle schematics and the pdf version of the schematic and the board layout file. This project is divided into two printed circuit board. The low voltage board (12v DC) is the inverter control board. The high voltage board (120v AC) is the inverter power board.
Inverter Control Board: inverter_control.sch inverter_control.brd inveter_control.pdf inverter_control_parts.txt
Inverter Power Board: inverter_power.sch inverter_power.brd inverter_power.pdf inverter_power_parts.txt
Hardware Description:
Low Voltage Section (inverter_control.sch)
The inverter is divided into two printed circuit board. The inverter_control.sch describes the low voltage circuit board. U4 is the microprocessor. It is an Atmel AT90PWM3B. I choose the AT90PWM3B because it has a very power and flexible PWM control section that they call a Power Stage Controller. The processor has a serial interface to talk to the PC using a MAX232A (IC2) and 9 pin D connector (X5). The ISP programming connector for the JTAGICE mk II is the 6 pin .1″ (SV4). X4 and Q5 are for PWM fan speed control but this feature is not implemented yet. S1 dipswitch is for user software configuration and debugging. U3 is an LM35 analog temperature sensor. This temperature sensor is mounted next to the MOSFETs on the heatsink. It is used by software to make sure the MOSFETs don’t overheat. IC5 is a 74LS32 Quad OR gate and IC1 is a 74LS14 Hex Inverter. These gates are used to read back the zero crossing signals from the high power board. The OR gate is connected to a Timer Input Capture pin so the length of the zero crossing signal can be calculated. Q1, Q2, Q3 and Q4 are the MOSFETs (IRLIZ44 N-fets) that make up the full bridge switcher. This is the heart of the low voltage section. The full bridge switcher takes the DC input voltage from the solar panels and switches it into high frequency AC to pass on to the transformer to generate high voltage AC. The MOSFETs are driven by two IR2104s MOSFET drivers (U1 and U2).

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