Sunday, October 04, 2015

OpenInverter - An Open Source, Micro-Solar Inverter - Part 3

A 12A 24V motor drive H-bridge pcb repurposed into a 64W micro-solar inverter
Further Thoughts

Welcome to Part 3 of this series of posts regarding the Open Inverter - a micro-solar inverter.

The H-bridge is fundamentally a 2 port, power control device, similar to it's cousin the bridge rectifier. Instead of passive rectifiers the H-bridge consists of 4 semiconductor switches that can be controlled under firmware - and as such is much more versatile than the diode bridge. However, unike the diode bridge, it has electrical symmetry - and so power can flow in both directions - and we can use this unique ability to our advantage. The H-bridge becomes a versatile power transformation device.

Consider the H-bridge to be like a black box, that in it's simplest form has 2 ports A and B, to which power sources - or sinks, can be connected.

In the photo above, dc power  (15.949V 3.97A) from a 150W solar panel enters from the right hand side on the red and black wires.

It is then converted by the H-bridge under firmware control into a pwm 50Hz ac signal, that feeds the mains transformer - connected to the left hand side - by white and black wires, and is then transformed up into 230V ac mains .

Thus a board designed for dc motor control has been repurposed into being a micro-solar inverter.

Depending on how the H-bridge is controlled, it can:

1. Transfer power in either direction between ports A and B,
2. Rectify ac to dc
3. Synthesise ac from dc
4. Transform dc up and down in voltage and current
5. Provide a variable impedance load -  for load matching and peak power tracking.

All from a $20 module - Wow!

As I document the project so far and put my thoughts down into words it's becoming increasingly apparent that a versatile H-bridge controlled by a low cost microcontroller has a multitude of uses amongst the hobbyist community - a few here:

Solar Inverters
Step Up (Boost) dc/dc converters
Step Down (Buck) dc/dc converters
Boost-Buck Converters
Split Pi Converters
Synchronous rectifiers
Solar PV - peak power tracking
Battery chargers - high efficiency charging of consumer electronics and portable PCs
Load balancing
DC Motor Control for pumps, solar trackers, machine tools, vehicles (bikes etc)

However, the basic H-bridge design can then be further extended to 4 or more ports. By adding an extra half-H Bridge,  3 phase applications become practical:

3 phase ac or brushless dc motor drives
Solar boost peak power tracking inverter

Furthermore, each half-bridge can be regarded as a power port - where power may be supplied or removed to/from the system.  This means that the H-bridge can be seen as a 3 port device - and in this mode it has applications in some bi-directional boost-buck dc/dc converter topologies - such as the split-pi converter. 

If it is made in a modular fashion that can be extended to cope with more sophisticated or power hungry applications - then it will be a lot more versatile.

So it sounds like the world is in need of a low cost, open source, versatile H-bridge power converter. If it can be made for under $20, it can appeal to a whole variety of price sensitive applications.

A Modular Approach

An H-Bridge Module like this, connected to an Arduino is very versatile

Today it is time to make some fundamental design decisions regarding the inverter power stage.

There are 2 main options:

1.  Traditional N-FETs with driver ICs.
2.  Half-Bridge ICs - such as the Infineon BTN8962

Whilst there are many mosfet driver ICs, only a few work at 30V, which is essential for a nominal 24V battery supply. - eg Microchip TC4431/32

These are available in DIP for easy self-assembly.  The advantage with option 1 is that you can fit whatever FETs you have available - depending on your maximum voltage and current requirements.

Option 2 uses the BTN8962 or BTS7960 integrated driver and half-bridge ICs from Infineon.  These are available as ready made modules from China, at a price cheaper than they could be made here - and might appeal to some experimenters.

So in order to make a design decision, to further the project- I am going to suggest is a traditional  FET board which is footprint compatible with the Chinese module. 

Regarding the "Arduino" part of the design, this could be built on stripboard sized approximately 5cm x 5cm which is then stacked underneath the FET power module on hexagonal spacers.  Header connectors would connect up to the power module.  I believe that this would be a suitable option for home construction.

Additionally, I am looking at a pcb layout for the inverter's mcu section.  This would use the ATmega328 on a pcb sized about 5cm x 5cm so that it stacks below either the discrete FET board or the Chinese BTS7960 motor drive module.

5 x 5 cm is a good size as it allows for additional circuitry & connectors, a 5V regulator and 5x5 boards are very cheap from Using the standard 5 x 5 or 10 x 10cm boards - these can be made stackable with as many power stages as required. In theory, an inverter could be built up, stage by stage, to allow for possibly 1000W.

Wireless Control and Monitoring.

For several years I have been using designs that incorporate a RFM 12B or RFM 69 wireless module. These modules make use of Jean Claude Wippler's Jee Libs - a wireless protocol devised for communication between low cost wireless nodes.

Jeeibs has been adopted by my friends at Open Energy Monitor - for communication between their wireless sensors and energy monitors - and a base station - often web conected, to their cloud based analysis and energy visualisation package, emonCMS.

By including a wireless module on the Open Inverter MCU board - it ensures emonCMS compatibility - allowing remote monitoring and control of the power transferred by the inverter -  using emonCMS.

As well as the micro-solar inverter, the Open Inverter boards could be used for pv peak power tracking, LiPo battery charging/monitoring, and dc/dc conversion for the various voltage outlets of the "dc ring main". They can be used anywhere that power is generated or converted and report back the individual power transfers to emonCMS. 

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