Powering a 12V bus with 24V batteries in automotive applications

24V systems are common in commercial trucks, including heavy-duty pickup trucks, as well as military vehicles that require extra power.

Converting power from 24V batteries to 12V for low-voltage electronics inside a vehicle cockpit necessitates the use of a DC/DC converter with a wide input-voltage range, fast load transient response and complete system protection, as shown in Figure 1.

Figure 1: 24V automotive battery system

24V systems (just like 12V systems) can experience wide input-voltage variations as a result of cold-crank and load-dump events, as shown in Figure 2. These peak load-dump transients can be as high as 60V.

Figure 2: 24V battery input-voltage transient events

A 24V-to-12V buck DC/DC converter has to be able to handle a wide input-voltage variation (up to 60V), provide tight load regulation and system protection, and have strong drivers to enable high conversion efficiency with external metal-oxide semiconductor field-effect transistors (MOSFETs).

The 280W Automotive Dual-Phase Synchronous Buck Reference Design with 24VDC Input, 12VDC Output showcases a 24V-to-12V automotive battery power-conversion design. The resulting 12V output of the converter is available for various electronics inside the cockpit.

The synchronous buck reference design uses the LM5119-Q1 automotive-qualified two-phase pulse-width modulation (PWM) buck controller and is designed to downconvert 24V to 12V at 20A for a total of 240W of output power. This 240W design is 120mm by 70mm and includes all of the components required for the power supply (Figure 3).


Figure 3: Synchronous buck reference design 24V to 12V buck converter for automotive batteries

The reference design will support cranking down to 18V and load-dump transients (input voltage overshoot) to 58V.

Efficiency is >97% from 6A to 20A with a power loss less than 9W, as shown in Figure 4.

Figure 4: Synchronous buck reference design 24V-to-12V buck converter efficiency and power loss

As shown in Figure 5, the load regulation has only 2mV of deviation over the load current range.

Figure 5: Synchronous buck reference design load regulation

Output ripple voltage is less than 10mVpp, with noise at 20mVpp (Figure 6).

Figure 6: Synchronous buck reference design output voltage ripple

If you are designing an automotive battery 24V-to-12V buck DC/DC converter, consider this reference design for your toolkit. It comes with all of the technical documents and files to enable a quick implementation so that you and your customers can start steering the wheel.

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