Riding out automotive transients using buck-boost
In this section we look will look at the buck-boost DC-DC topology for automotive front-end power conversion.
A wide-VIN buck-boost offers single stage conversion resulting in higher efficiency than two stage approaches. A buck-boost converter also offers inherent short circuit protection and inrush current limiting which is not present in a boost converter. Another advantage of using buck-boost compared to a boost converter is that the output voltage is well regulated so downstream components never see the over-voltages present on the input rail. This allows the downstream components to be rated for lower voltages.
In terms of automotive transients on the battery rail, a wide-VIN buck-boost converter can enable the downstream circuits to operate through warm and cold cranks, jump-start, as well as load-dump conditions.
The LM5175-Q1 is a wide-VIN buck-boost controller from Texas Instrument with a wide input voltage range of 3.5V to 42V and integrates many useful features such as optional frequency dithering for reduced EMI, programmable UVLO, clock synchronization capability, and hiccup mode current limit for reducing thermal stress in case of overload.
The 4-switch buck-boost converter provides higher efficiency compared to two stage approaches as well as compared to other topologies used for buck-boost DC-DC conversion. The better efficiency is due to:
- Single stage conversion and single inductor avoids double stage conversion losses.
- Synchronous rectification for both buck and boost legs.
- The special switching pattern results in only one switching evert per cycle.
- Lower circulating currents compared SEPIC and Fly-buck topology.
- Also the input and output legs are decoupled allowing the designer to better optimize the FET selection.
LM5175 based design achieves greater than 98% efficiency when VIN and VOUT are close to 12V.
The video shows the response of a buck-boost converter for a load-dump test (slide titled "Load dump/overvoltage"). The input voltage to the converter represent the battery voltage and is shown in yellow. The output voltage of the dc-dc converter is regulated at 10V and a 3A load is present at the output of the converter. As the test pulse simulating load dump condition is applied the input voltage ramps quickly from 13.5V to 27V with a rise time of 2ms (shown in the expanded time scale on the right side). The output voltage is well regulated throughout the elevated battery voltage. The output voltage also remains well regulated when the battery rails voltage returns to 13.5V
The lab results for a cold-crank test profile as per ISO 16750-2 are also shown (slide "Cold-Crank: ISO 16750-2). The input voltage (shown in yellow) falls from its nominal 12V to close to 3V, in 2 milliseconds, simulating a startup condition. The output voltage of the buck-boost converter remains well regulated even as the input goes through the startup transient profile.
The wide-vin automotive rated buck-boost converters and converters available from Texas Instruments and listed. LM5175 is a 4-switch buck-boost controller rated for 42V maximum operating range that can be designed to provide over 100 W. LM5118 and LM25118 are non-synchronous 2-switch buck-boost controllers for medium power application. Whereas the TPIC74100 family of converters have integrated 1A fets for applications that require 5V output and lower currents.
For more information on these parts please visit the respective product web pages.