Guru
Joined: 21/06/2018
Location: AustraliaPosts: 1156
| Posted: 09:34am 13 May 2019 |
The time has come to fully commit to creating my homegrown inverter. My aim is to create a reliable power platform that is relatively simple to reproduce with a minimum of pyrotechnics and a medium skill set.
Converting from a 48V nominal input to a high frequency multi kW 230VAC inverter is a very demanding & challenging task with multiple tradeoffs and often conflicting requirements. It has challenges in the mechanical domain as well as electronic. My only claim to fame so far is a measly sinewave inverter of 250W. This may be the beginning of a long journey......
I was going to create this project quietly in the background so I would not have to fess up to any self induced errors & catastrophies, but decided to create an online diary of the progress and setbacks along the way, maybe it might at least inform others what not to do..
A catalyst to starting now was finding 2 x Toroidal Transformers for sale 2 weeks ago on Ebay, I made an offer of $180 delivered to Adelaide which was accepted. The listing described them as 2 x 5Kw 230V/32V Toroids. They have labelling that suggests they are from a 48V - 230V inverter.
I feel like maybe I have wimped out by doing this, but I quote from Clint Eastwood's "a mans gotta know his limitations". I was not looking forward to winding the toroids, and this seems like a neat solution if they work out. Maybe I will still need to rewind them any way, but hopefully not.
A number of design considerations helped the project to take shape. One was, what is the good of making it work great if no one else can benefit from it because it is just too hard to build/reproduce.
1) No SMD Parts - designed to use all through hole parts. If it works as I hope & anyone wants to also build one, they dont have to contend with SMD parts.
2) Full galvanic isolation between the control PCB and the Power PCB. This includes isolated power supplies for the 2 x high sides, and 1 for the 2 low side bridge legs. Bridge drive is via 2 optocouplers "Warp" style parallel inverse driven to ensure each side cannot be commanded upper and lower on at the same time.
The upper and lower FETs' are driven alternately as one opto is driven off and into reverse to turn the other opto on. Both Fets on is catastrophic - if it ever occurs, we just need to ensure enough dead time to ensure that first FETs are fully off before the next ones turn on.
3) Relatively simple construction mechanically and electrically. In its simplest form with no bells and whistles only 6 wires are required from the Nano board to the FET Power module to drive the power stage completely, 1 x 12V pair and two pairs of Opto drives. If the isolated supplies are powered from the 48V then only 4 wires are required !
4) I incorporated negative bias into the lower bridge FETs to provide better immunity to dv/dt gate issues. This is experimental, I will start with -5V and if required then try -12V, if it proves to make not a twit of difference it can easily be left out/unused by inserting a PCB link.
5) For bus capacitors I decided to house them on 2 x separate PCB's, each PCB will employ 4 x low impedance 10,000uF 63V capacitors. The capacitor PCB's screw onto each end of the main FET PCB using 6 high current standoffs per PCB. They can be easily removed and the inverter run without them as required for testing.
6) For the Gate drive isolated Power supplies, I have created 2 x PCB's. One uses 3 (or 4 for - bias) of the little Ali modules, the other is for a home grown single transformer with 4 simple 20 turn windings and a little IC to drive it.
Either can be used interchangeably. Both isolated modules incorporate an "off" function to turn them off when the Nano commands a halt to the inverter. The home grown version uses 12V from the Nano board to create the isolated supplies. The main PCB can be configured to use the 48V bus to Power the "Ali" version.
7) Keep the gate drives relatively short & local and low impedance. The Power PCB is 100mm wide x 260mm long with 16 FETs. This meant that I could incorporate a local FET drive buffer driven by high speed power opto couplers. The longest trace for the Gate drive is around 100mm.
8) I am not keen on using the heatsinks for the high current connections. The output stages can be configured easily though to do this if desired. Part of my reasoning is that the oscillations sometimes caused by paralleling multiple mosfets can often be fixed by putting a Ferrite bead on the drain leg.
Of course when the mosfets are all bolted to a common heatsink there is no longer an option to fit the ferrite bead. There is much analysing still to be done on this after the prototypes are assembled.
9) Two power boards can be utilised for real thrill seekers, each as a half bridge configuration with 8 x High and 8 x Low side FETs in each LEG. Both half bridges can be further configured into two groups of 4, allowing 2 or 4 smaller chokes that are then combined, instead of 1 massive choke.
10) I chose to use my own version of a Nano1 controller PCB to provide the 1 x 8 way connector for a single Power board or 2 x 8 way Half bridge connectors. It also user different circuitry for voltage control and over AC current shutdown.
I currently have no thermal fan system employed I will probably just employ some cheap Ali modules for Fan Control and Amp & Volt and Watt displays as they are so
cheap.
In a day or so I will post the various schematics and PCB board design layouts I have created for the project to date.
Sorry for the verbose introduction.
If at first you dont succeed, I suggest you avoid sky diving....
Cheers Mike