Using MOSFET or IGBT Chopper for 100kWh 680VDC Battery Charging from 750VDC Solar Bus

Hi everyone,

We want to charge a static battery pack (around 100kWh, 680VDC nominal) from our solar array (40kWc 750VDC line bus controlled by the inverter which communicates to the power optimizers).

So one way to do it is to have a DC/DC on the 750VDC bus and make the voltage go down appropriately while controlling the current.
While that seems the right thing to do, we were wondering if just chopping the voltage with a MOSFET (Si, SiC) or IGBT would be an option?

Excuse my ignorance but what would be the problems involved? Current spikes at each "chop"? Voltage spikes?

If we also control the current (with a hall effect sensor for example), can we even think of doing a current control?

Thanks for the education! 

Marc

P.S : we've actually done it on a small scale with a current limiting resistor and at small power (<10A @ 750VDC) and well... current did flow in the right direction but that's all we can say for now.

Just chopping the voltage source is not going to be a very good solution. Measuring average current is going to be a challenge, and with lead inductance you are going to get enough ringing in the output (and possibly input) lines that the switch will see significantly higher voltages than your 750V input. When your switch is off, the input will also swing up if the solar panels are unloaded.

Rather than just chopping the voltage source, you probably want to build a buck converter. At 750K it will be a moderately complex task, but you can get accurate current and overvoltage control this way. A digital power solution is probably ideal as you can implement current/temperature control, MPPT control on the input side and isolate everything keeping the HV out of the controller portion of the buck converter.

This will require someone with significant experience in HV/isolated switching power supply design.

Either an IGBT or MOSFET would work for the switch in the buck converter.