Long ago, @NotGump laid out an overall topology. To produce AC, it involves stepping battery voltage (48 VDC nominal) up to +/- 200 VDC which would feed the rails of an H-bridge. He mentioned using boost converters for this.
Assuming that [this](https://en.wikipedia.org/wiki/Boost_converter) is the type of converter he had in mind, one concern is the conversion ratio. To go from 48 VDC to 200 VDC, you'd have a duty cycle of D = 1 - (Vi/Vo) = 1 - (48/200) = 0.76. This doesn't leave much headroom, which you need as load changes, and will make it harder to design a control law for the converter to maintain the output voltage.
Usually, when larger conversion ratios (voltage input/output) are needed, a topology with a transformer is used. Two that come to mind are the half-bridge and full-bridge topologies; basically, you use two or four transistors to drive the primary of the transformer, and rectify the secondary to get the DC output. Using a transformer with a tapped secondary, we could even get +/- 200 VDC (essentially 400 VDC) with a single converter. The switching would work at a high frequency, so the transformer can be much smaller than it would need to be for 60 Hz. Here is an example of a full-bridge, which I found in [this paper](https://www.researchgate.net/publication/224101103_Comparison_of_SiC_and_Si_power_semiconductor_devices_to_be_used_in_25_kW_DCDC_converter).
For a half-bridge, two of the MOSFETs in the above would become capacitors. I think that should work for < 500 watts, which may be okay for us.
One challenge with going this route is that the transformer will likely have to be custom. I'm looking around to see if there is a COTS part that could be leveraged, but most of the time these sorts of converters use custom magnetics.
As an aside, an interesting feature about this is that you can replace the diodes on the secondary side with MOSFETs, for synchronous rectification. (That is, you turn them on and off when the diodes would or wouldn't be conducting.) This increases efficiency a little bit, but it also means that the converter can be bidirectional, so it could move power back into the battery. (I should point out that this can be done with a standard buck or boost converter too; you just replace the diode with another transistor.)
If we can get the input rectifier to put out 400 VDC it could drive the output inverter directly, with a full-bridge converter in parallel. When the power goes out, this converter would just take over supplying the bridge.
These are just thoughts; this is more complicated, but I feel it should be considered given the voltage ratio we need. That said, it would be interesting to see if it could be done with the normal boost converter as well.
↧