Hey - I'm building a power supply for use in a device within the auto industry. I need to achieve a steady 12VDC regardless of the state of the vehicle battery, or stop start systems etc... I'm using SEPIC as typically car provides 13Volts which it regulates down to a nice steady 12VDC - my device (which is essentially a GPS tracking system utilising a 32Bit NXP Arm chip, and a large OLED display) consumes about 0.5 to 1A in normal use. Yet when the I simulate a voltage drop / brown out such as what would be experienced in a stop / start scenario where voltage can drop significantly below 12V the current consumption increase significantly. At 11V supply I measure between 1.1A and 2.2A consumption, further investigation shows the GPS tracker still consumer 0.5 to 1A so the remainder is being consumed / dumped by the boost circuit - is this correct? it seems a very high figure for such a small boost?

 

The input power at 11V is 12.1W to 24.2W.
The output power is 6W to 12W, so the power supply efficiency is only about 50% which is very low.
Something is wrong with your power supply circuit or you need a better design.

 

Since the only time that the voltage is pulled down that low is while the engine is cranking, why not just use a super-capacitor to support the load for those few seconds? Also note that the AC blower should shut off with the engine, because it will draw over 30 amps on high speed. The super-capacitor approach will also be far more reliable.