DIYOSI notes 426

[Roswell Bob]

Do It Yourself Open Source Inverter.

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DIYOSI Notes 4/26/21

Work has been progressing on the DIYOSI. I have decided that I will build a couple for my own experimentation. There was a question as to would this ever get beyond a paper design. I have answered that question. This initial design is for a LF inverter. I will build the first prototype. I will make board sets available if anybody wants. Maybe heatsinks too. I will have boards assembled at a board house late summer or fall. After LF design done I have a nice push-pull converter design that will output 180vdc with 48VDC in. The HF inverter is the ultimate goal. The performance of a properly designed HF inverter can equal that of a LF inverter for less money. The cost is of course related to the performance. The major costs will be the silicon and the aluminum.

I think the present state of the semiconductor will allow for a good continuous output. 2 switches on Each corner. Because the power board is very small, the switches are close together which makes for a higher performance cooling design. We have a simulator for that. I am going to say inverter will do 10kVA with a 180vdc input. I expect tracks to be good for 200amps +-.

Filters are necessary on both the output and input sides to keep RFI under control. Battery cables will transmit RFI as well as output lines. I would like to keep this unit to a maximum of 8.5 x 11 inches so I can store them in my file cabinet the height is a variable.



There will be 2-3 pc boards(maybe)

Boards will be arranged like a stack of pancakes-one above the other. A two board topology may work, but not sure if the control functions will fit on one board with the flyback on it too. Have to figure that out as soon as Power Board layout is complete. I am about 75% done with power board.

On the bottom is power board

75mm x 125mm- like a 3x5 index card. 4 layers of 4oz or 6oz Cu

Power board will be thermally attached to heatsink to keep copper temperatures reasonable. (Heatsink could run up near 100c.)

Power transistors (8 x TO247) will be under the power board with gate drivers on top side right on top of the switches. It’s important to keep drivers very close to the switches. TI drivers with emitter follower amplifier on each pair of switches.

Switches are dependent on 120v or 240v output / HF or LF design. Si or SiC are both contenders. Not sure if IGBTs will have a place.

Gate drive commands are on a pair of ribbon cables. Each cable runs one pole. A pole is the upper and lower transistor in that leg. This is important if inverters are to be placed in parallel or series. You want to be able to run same gate commands to the pair of bridges to keep them in sync.

A small set of bus caps to take care of reactive currents. Some Ceramic caps on the input side to snub switching transients. Battery array should always be as close to inverter as possible

A DRC snubber on the board. This will snub switching transients and keep switches intact.

Sense resistors in the negative bus isolated with probably a Broadcom part. Maybe same isolator for bus voltage sensing. Bus voltage sensing is good for feedforward output voltage regulation.



Interface board is next.

This will allow any micro board to interface with the power board. Just spin this board to create hooks to adapt to control board. This gets gate drive signals to the switches. This board will send gate commands to parallel slave modules as necessary.

This will have a small flyback power supply to provide gate drive supplies along with analog and micro processor voltages. Flyback will run off of the inverter bus.

Fault detection/ hardware current limit will be handled on the interface board for this first deployment. No need to bother the micro with fault handling. The micro can get stupid and it doesn’t matter. This board will do everything it can to curb a wayward micro.



Control board will be on the top

I have a very simple design that uses an 8 pin PIC to generate the PWM. That is all the PIC does. This is the most basic design with just enough lines of code to get it done. It is easy to understand and I think perfect for an introduction to SPWM.

There is a EGS 002 inverter controller out there that may work. I don’t know much about it. It maybe worth looking at soon.

There are app notes and designs available with code from a few manufacturers. I would expect a boatload of possibilities. For me I will probably stick with the PIC platform as that is what I grew up with.















Open Source Inverter Specification

• Inverter is 48vdc+- to 120vac. 5kW with 100% overload.
• Overload capability will be a function similar to a motor thermal overload. Tjmax <150c in overload. PCB tracks <105c
• A high frequency design will be the goal. This would require a DC-DC converter(s) front end in addition to the basic inverter. Initial design would tend towards a basic low frequency inverter that can drive a 50Hz-60Hz transformer
• Low frequency design is flexible voltage designed to drive many LF transformer ratios. 230v is doable.
• Thermal simulations performed with a crossflow blower seem the likely way to go. A pair of extrusions could be interleaved with converter on one side and inverter on opposite side. One blower would be required to cool both converters.
• Switching frequency at or above 10kHz with possible reduction in carrier frequency in overload condition. (probably little advantage in reducing carrier rate with fets – was good strategy with IGBTs. )
• Output Inverter transistors would be TO-247 style. Silicon Mosfets look to be the best choice with 2 on each corner. (Some interesting SiC fets tho)
• Input DC-DC converter most likely resonant switched open loop Push-Pull design. Two parallel input series output converters with 1:1 transformer may prove best design. Voltage doubler design would provide 1:4 voltage boost.
• DC-DC converters may use toroidal and/or planar transformer sets.
• Topology would be plug-in control boards with 4layer 6oz copper power boards
• Units will be parallel capable with 240v or 120-0-120 volt split phase operation.
• Output frequency selectable 50Hz/60Hz
• On board EMI filters at both input and output.
• Digital display with operation parameters and fault annunciation.
• Auto-restart capable in event of fault.
• Integrated fuse TBD
• Cost target is always zero. We see how close we can get.
• Size target is 250mm x 400mm x 150mm for high frequency design (or smaller)
• No built in chargers of any kind. MPPT chargers are a dime a dozen and a PFC design would complicate and slow effort.
• PCB designs in Altium. Other packages as well. (up to somebody else)
• Thermal modeling is Sauna
• Spice modeling is LT Spice.
• Tenor autem omnino documentum Inverter will be provided. The Magnus Opus design file with everything included – the way it is supposed to be done.