24 V to 12 V 1400 W DC-to-DC converter

There was a need. Then we realized we could do it. Then we made it.

My friend and some of his friends needed a heavy duty DC-to-DC converter for automotive usage. Features they needed:

• 24 V to 12 V conversion (actually 28.8 V to 14.4 V for lead-acid battery usage in cars)
• ability to directly charge 12 volt lead-acid battery (no leaking of any current back to the converter input when input side is disconnected)
• around 100 amps of continuous output current (14.4 volts * 100 amps ~= 1400 watts)
• good protection against converter failures: converter cannot leak input voltage to output in any case, even when the converter breaks
• as low as possible ripple voltage (since they use old-school radios for communication between cars)

This was not as easy task as we originally thought. We got through quite many designs before the device finally worked as expected. But we are very proud of the end result. At lowest the efficiency of our converter is around 93% and  at highest it can go up to 97%. Also we got the ripple voltage down to nominal 50 mV and maximum of 400 mV at worst cases (at a narrow area in current usage when second channel of the converter starts up).

It is not an easy task to find a converter like this anywhere on the market! And it is small! Thermally this design works very well. The converter wont start up the fans until output current is at ~70 amps for quite a few minutes (ambient at ~20 degrees celsius). Lower than 70 amps,.. well, who needs or even likes fans. Peak current you can draw from this converter is, ummm, huge? We don’t really know. We tested a 12 V winch earlier and found out that it takes around 215 amps at startup for around 20 milliseconds (~50 amps after startup under load). Our converter had no problems driving that winch directly. By disabling current limiting I got around 140 amps for 30 seconds out of this device before it heated up too much.

Final performance and features:

• 14.4 volt output (DOH!) (with 22 to 30 volt input)
• 96 amps output current limit
• 93% to 97% efficiency
• ripple voltage:
  • idle: 100 mV
  • nominal: 50 mV
  • maximum: 400 mV
• dual channel synchronous operation
  • second channel starts only at higher currents to make the converter more efficient at lower currents
• can charge 12 volt lead-acid battery directly
• will disconnect output in multiple situations (using dual FET configuration as solid-state relay)
  • when input is lower than 22 volts
    • prevents damaging lead-acid battery with discharging it too much
    • prevents leaking any current from output to input when input has no voltage or when input is lower than output
  • when output (before dual FETs) is higher than 15 volts
    • prevents damaging any devices connected to output
    • this can and has occurred with other DC-to-DC converters when converter has somehow broken and converter switching transistors have short-circuited (input is directly connected to output)
  • overheat etc situations
• overheat protection
  • temperature is measured in two places, between output dual FETs and between switching FETs
  • fans only start at certain threshold in temperature
• remote control (on/off)