Power Components:
The inverter is similar in concept to my previous inverter, but it has a boost stage on the front end to get the peak voltage output of the ac sine wave (~330V). The boost stage was actually built as two boost converters in parallel mostly because I already had one inductor wound that could handle 15 amps or so. Also, this stage would see higher currents (5HP is 25A from a 150VDC pack) and it worked out well with the chosen IGBT module.
The boost stage is followed by an H-bridge which is used to turn the high voltage DC into high voltage AC. Right now, the top IGBT on one side turns completely on while the lower IGBT on the other side PWMs one half of a sine wave. Then, the process is repeated with the other two IGBTs to get the other half of the sine wave.
In hindsight, I would've done this a little differently if I had to do it again. The more traditional way would be to use 'dead time' to alternately turn on and off the upper and lower IGBTs of each side of the bridge with each upper and lower IGBT never being on at the same time. Then, 50% duty cycle on each side of the bridge would be 0 voltage. A 40/60 ratio would be 20% voltage in one direction and a 60/40 ratio would be 20% voltage in the other direction. So, the duty cycle ratio just has to follow a sine wave and the output will follow suit. The advantage here is that switching losses get spread evenly among the four IGBTs at the expense of a little more complex circuitry and setup (mostly getting the dead time correct)
The IGBTs used are Microsemi H-bridge modules. There were two types available on Digikey - one optimized for
low switching losses and higher switching frequency at the expense of higher conduction losses, and the other just the opposite -
lower conduction losses but higher switching losses. Not sure the schematic reflects this, but the inverter ended up using one of each - the module optimized for lower switching losses for the boost converter (since the IGBT only conducts roughly half the time) and the other module for the h-bridge (since one of the top IGBT, in the current setup, conducts nearly all the time). Even though the module for the boost stage is designed as an h-bridge module, it works as a boost converter by using the middle rail as the input from the inductors and the V+ rail as the high voltage output. The upper IGBT gates are shorted so they never conduct. Both stages use a 15.6 khz switching frequency with 10 ohm gate resistors.
Each IGBT module has two film caps in parallel directly on top to serve as a low inductance path to absorb voltage spikes at turn off. The spikes on Vce were measured to be about 40V or so, putting the peak voltage more than 200V from the 600V rating.
In between the two stages is a large 2200 uF 450V electrolytic cap to serve as a buffer between the two stages. Despite this, I still measure a current from the batteries that oscillates with each AC half cycle, but that's no big deal.
Here's a closeup from the side of one of hte IGBT modules and the large cap:
The boost inductors are sendust cores (two stacked for each one) bought from
cwsbytemark each hand wound with two strands of 18ga magnet wire. Talk about time consuming! There's some literature on there about estimating the inductance based on the chosen core, number of turns, and operating current. While these inductors may be about 1mH with no current, they're probably closer to 500 uH at operating current. so far, with 11A average for each inductor (22A total), they only get about 10C above ambient.
Here's a closeup of one of the cores. Ignore some of the wiring since this pic was taken during some early low voltage testing.
The IGBT drivers are avagado 2.5A combined opto-isolator/drivers. With 10 ohm gate resistors, these drivers were satisfactory and could handle the 24V output from the DCDC's and send +15V and -9V to the IGBT gates.
The DCDC's are powerex 15V input, +15V, -9V output. They work down to 12V input and since my vehicle's DCDC outputs 14.2V, I just feed that directly to the input of the DCDCs.