Open Inverter - Part 6 - Thinking Allowed

130 VA Toroidal Transformer Inverter using BTN8960 H-Bridge ICs

It's been a frenetic week of progress on the Open Inverter project. Both Trystan and I have managed to cobble together inverters from FETs, H-bridge ICs and other random, readily available modules - bought cheaply from China.

I have powered up the inverter with a DC input of 24V close to 5A and successfully powered a couple of 60W 230V lightbulbs.  The efficiency looks promising with neither the H-bridge nor the transformer getting anything more than warm.

In this post, I pause for thought to decide upon the future direction of the project based on our findings, so far in this week of discovery.

Here's the current wish-list:

1. An Open Source Inverter, of modular construction that is scalable in blocks of 125W or 250W.
2. Built from readily available, understandable, low cost electronics.
3. Rugged, robust, reliable - delivering reasonable efficiency and power quality.
4. Grid synchronisable - if required - will synchronise to external source.
5. Built in power monitoring, with wireless communications compatible with emonCMS monitoring
6. "Arduino" or similar microcontroller for hackability.
7. Supports a variety of power conversion topologies, including boost, buck, peak power tracking and split-pi.
8. Uses include micro-solar, LiPo4 battery charging, dc ring main schemes etc.
9. Under $20 for primary building block.
10. Easy to build, easy to repair, extendable, hackable.

Expanding on some of the above points.

The proposed inverter will be built from low cost modules that can be plugged together as required, depending on the application.

The basic design will consist of a microcontroller, one or more H-bridge power boards and a 125VA or 250VA torroidal transformer.

Choice of Microcontroller.

The microcontroller could be an Arduino or derivative, or one of the very low cost, easily breadboarded STM32F103 ARM boards - based on the Maple Mini - which can be programmed with Arduino code - using STM32-Duino.

The breadboardable Baite STM32F103 "Maple  Mini" 

The advantage of using the STM32F103 is that it has more I/O and Flash/RAM than the standard Arduino and runs at about 5 times the speed.  It has more versatile and numerous pwm outputs and higher resolution AD converters. Remarkably these little boards are available very cheaply (<£5) from numerous vendors on ebay.

Using STM32Duino, these boards can be programmed from the Arduino IDE - using an additional board file which caters for the STM32Fxxx range of ARM devices.  This allows sketches developed for Arduino to be readily easily converted so as to run on the much greater performance STM32 range.

Choice of H-Bridge.

This handy power board is one I designed earlier in the year to drive a 100W DC Motor, it uses 2 x BTN8960 ICs

The H-bridge board can either be based on standard n-type FETs with driver ICs, or using the more sophisticated BTN8960 H-bridge ICs.

The FET solution may be more hackable and appplicable to other projects, but the H-bridge based on the BTN8960 is a quick and cheap solution.  I had already designed a power board to drive a low voltage dc motor, and it was very easy to adapt it to drive the secondary of the 120VA toroidal transformer, using an Arduino-like mcu to generate the 50Hz sine waves.

I am currently testing both solutions so that I can give more informed advice based on my findings.

The BTN8960 power board runs very efficiently with a 120W load. However, the IC is limited to 25kHz switching frequency - and this is not an easy frequency to create using the fast pwm options on the standard Arduino - short of running it on an 8MHz or 12MHz crystal.

The tabs of the H- bridge ICs are soldered to large copper areas - approximately 30 square cm each. This allows them to run cool even at 120W power - without additional heatsinking.

Choice of Toroidal Transformer.

This can be any toroidal transformer in the 50VA to 250VA range - with the secondary voltage chosen to be approximately that expected from the solar panel, or the battery.  For convenience I use a 130VA toroid with a 24V secondary.

The toroidal transformer does two things for us, it conveniently steps-up the output voltage from the H-bridge to that of the ac mains, and  effectively isolates the low voltage power electronics from the high voltage mains - so that high voltages are contained in the transformer, and not on the H-bridge board - making for a much safer project.

The toroidal transformer is also fairly efficient at converting the low voltage to mains - depending on its VA rating about 91 - 95% is typical.

In the UK, a suitable Vigortronix  120VA transformer 0-12V 0-12V from Rapid Electronics (88-3814) is £15 or less, on ebay.

Putting it in a box.

As the inverter has mains voltages present, it is recommended that it is put into a plastic or metal enclosure.

The largest, heaviest component is the toroidal transformer, which should be securely mounted to the case.  The 120VA inverter should fit in a case about 100 x 160 x 50mm, some of those extrusions use for Eurocad sized pcbs could be used to advantage.  The Vero 14-1003 or the Hammond cases (Rapid 30-1574 or 30-1535) aluminium extrusion at 105 x 165 x 60 or similar would be ideal.

Using components sourced from the UK, a DIY 120VA inverter in a case could be made for about £50.

A few points on efficiency.

The losses in a toroidal transformer are the sum of the Iron Loss and the Copper Loss. The Iron loss is effectively the magnetising current required to set up the field in the core and lost in the eddy currents. For a 230V 120VA transformer, this magnetising current is about 9mA and the iron loss is 0.98W.  The iron loss remains constant at all loads.

The copper loss is the sum of the I2R losses in both the primary and secondary.

For a 2 x 12V  120VA transformer running with 5A secondary current and 0.5A primary current

Secondary loss = (5 x 5 x 0.24) = 6W

Primary loss  =    (0.5 x 0.5 x 14.6) = 3.65W

The copper losses will rise with temperature because of the increase in the resistance of the windings with temperature.

In total there will be about 11W lost in the transformer when working at its rated power.

Losses in the FETs.

These are outlines in the BTN8960 datasheet.  At 25C, the path loss is 14.2 milliOhms.  With a 5A drain current, the I2R losses in the FETs are (5 x 5 x 0.0142) = 0.355W. There wil also be some switching losses, plus powering of the remainder of the circuit.

I measured the input current to the H-bridge, which also includes the Arduino, a relay and a 24V to 5V simple switcher 5V voltage regulator.  When not driving the transformer the circuit consumed 50mA at 24.25V.  A loss of  1.21W

With no load on the transformer primary, the current into the inverter was 0.23A.  This puts the no load driver losses as (24.25 x 0.23) = 5.58W.

Adding all the losses, the best estimate (unconfirmed) for system losses is

No load losses     5.58   W
FET I2R losses    0.355 W
Iron losses           0.98   W
Copper Losses     9.65  W

Total                    16.565 W

Regarding overall efficiency

Inverter Efficiency   (120-16.565)/120  =  86.2%.

Room for Improvement

The proposed micro-inverter is intended to be used when there is no alternative to using ac mains - for certain low wattage devices.  The second part of this proposal is to use dc for direct charging of consumer electronics and mobile computing devices.

By increasing the system voltage, to say 48V, the currents switched in the FETs is halved, and so these I2R FET losses can be quartered, however a transformer with a 48V secondary will have more winding resistance.

The losses in the toroidal transformer are more or less fixed for a given core size,  however by using a larger core than is actually needed, it will have lower resistance windings in both the primary and secondary - and so the copper losses will be reduced, but the iron losses will be up a little.

For example using a 250VA toroid - but running it at 5A

Secondary loss = (5 x 5 x 0.08) = 2W   (previously 6W)

Primary loss  =    (0.5 x 0.5 x 6.1) = 1.525W  (previousy 3.65W)

Iron Loss      = 1.62W  (previously 0.98W)

No load losses     5.58   W
FET I2R losses    0.355 W
Iron losses           1.62  W
Copper Losses     3.525  W

Total                    11.08 W

Inverter Efficiency   (120-11.08)/120  =  90.76 %.

So a half loaded 250VA transformer will run cooler with about half the total losses of the fully loaded 120VA toroid.  This can increase the overall efficiency of the inverter by about 5%.  It also allows for some extra capacity when other loads are switched in, and the voltage droop, under load will be less.