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Registered Member #1960
Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48
Hello,
Being somewhat experienced with buck and boost converters I have now encountered a challenge where these topologies are far from ideal.
Basically I'm charging a big li-ion battery (20 kWh 360 Vnom) from a 36 V bus (Solar/generator/4kWh li-ion stabilizing battery).
The specifications: Input voltage: 30 to 42 V Output voltage: 280 to 430 V Power: 3 kW
The easy: Both the load and source are low impedance batteries. This makes control loops less critical.
The hard: Input side will see quite high currents. Parallelled modules might be a solution. Safety. Lethal voltage. When not connected, converter should not be hot. If transformer topology, making a efficient transformer.
I'm thinking I have to choose between flyback, push-pull or half bridge. Flyback seems easy, but lacks the power unless very big magnetics is used. Push-pull needs more attention regarding flux imbalance, but my understanding is that with current control - this will be fine. I know half bridge from SR-Buck converters and one 3PH ebike inverter I built some time ago, but connected to a transformer - I can't readily grasp the challenges.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Given that your input voltage is so low, a full-bridge would be my choice.
Since the output voltage is rather high, a passive set of rectifiers + filter inductance (as seen in most "push-pull" converters) would be pretty straight forward. SiC schottky diodes might be of some benefit.
Push-pull gets harder at higher power levels as leakage inductance naturally grows with size and consequently voltage spikes across the switches become annoying to deal with. Full-bridge is pretty much the "go-to" for high power conversions.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I would try to split up that task to 2 U-core transformers, and not try to get one transformer to handle the full load. The U-cores are fairly inexpensive on ebay, and with 2 transformers you should be able to get just enough of a winding window to handle 4x 90V windings. I think you will need 16 AWG teflon wire to handle 10A though. Don't bother with copper magnet wire, it will have more losses.
On my small converter (18W) I needed maximum efficiency possible because its portable, and I ran into a problem where I could not source enough current, so I have to use 2 identical windings and rectify them center-tap grounded full wave.
If you do a small-scale experiment and find you cannot source the current you need reliably, you might also need to double-up each secondary for more current.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I have done almost exactly that. (mine had a 400 V output voltage and a 30 V input voltage) I used a phase shifted full bridge with one large ETD-59 transformer. The UCC28950 provided my control, with the synchronous rectifier outputs not connected. SiC diodes were used for the rectification, and IRFB3077 MOSFETs (two in parallel) for the primary drive. I haven't tested it at above 3 kW but it has been over-designed to perform well at very high powers. The only issues are the parasitics in the transformer, made worse by having four parallel primary windings.
Push-pull is impossible with that specification. The energy stored in the transformer's leakage inductance (in the order of 1 uH) will be dissipated in the 'FETs on every cycle.
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