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Power conversion for trucks, heavy equipment

Posted: 16 Sep 2015     Print Version  Bookmark and Share

Keywords:DC/DC regulators  automotive  over-voltage clamp  transformer  LTC3892 

Truck, automotive and heavy equipment environments are quite demanding for any type of power conversion devices. Wide operating voltage ranges coupled with large transients and wide temperature excursions combine to make reliable electronic system design challenging. To complicate design considerations, some applications require the power conversion device be installed under the hood, requiring a high temperature rating. At the same time, as the number of electronic components is increasing, space requirements are continuing to shrink, making high efficiency conversion critical due to space limitations.

Consequently, reliable truck switching DC/DC regulators need to work over a wide input voltage range. A 60V input rating gives good margin for a nominal 12V system, which is usually clamped in the 36V to 40V range. Double battery applications, commonly found in both trucks and heavy equipment environments require an even higher operating voltage due to their nominal 24V battery voltage. Most of these are clamped to 58V, so a 60V rating is usually sufficient. The onboard automotive and truck over-voltage clamp is required to maintain a maximum transient voltage caused by the inductive kick back voltage from the starter motor, which can cause a much higher transient voltage when left unclamped.

There are several electronic systems that require continuous power even when the vehicle's motor is not running, such as remote keyless entry, GPS and security systems. It is essential for these types of "always-on" systems to have a DC/DC converter with low quiescent current in order to maximise the battery run time when in sleep mode. Under such circumstances, the regulator runs in normal continuous switching mode until the output current drops below a predetermined threshold of around 30-50mA. Below this level, the switching regulator must go into lower quiescent current operation in order to reduce the current draw to tens of micro amps, thereby lowering the power drawn from the battery which in turn extends the battery run time.

With 60V input capable DC/DC converters that have high step-down ratios in short supply, designers have resorted to a transformer-based topology or external high side drivers to operate up to 60V. Others have used an intermediate bus converter, requiring an additional power stage. Both of these alternatives increase the design complexity and, in most cases, reduce the overall efficiency. However, the LTC3892 from Linear Technology, is the latest part in a growing family of 60V input capable step-down switching regulator controllers that addresses many of the key issues required in automotive and truck applications as outlined herein. Figure 1 shows the LTC3892 operating in an application that converts a 4.5V to 60V input into 3.5V/5A and 8.5V/3A outputs.

Figure 1: LTC3892 Schematic with 4.5V to 60V Input to 8.5V/3A & 3.3V/5A Outputs.

The LTC3892/-1 are high voltage dual output synchronous step-down DC/DC controllers that draw only 29µA when one output is active and 34µA when both outputs are enabled, and are in regulation. The 4.5V to 60V input supply range is designed to protect against high voltage transients, ensuring continuous operation during automotive cold crank, load dump and to accommodate a broad range of input sources and battery chemistries. Each output can be set from 0.8V to 99% of VIN at output currents over 20 amps with efficiencies as high as 96%, making it well suited for 12V or 24V automotive, truck and heavy equipment applications.

The LTC3892/-1 operates with a selectable fixed frequency between 50kHz and 900kHz, and can be synchronised to an external clock from 75kHz to 850kHz. The user can select from continuous operation, pulse-skipping and low ripple Burst Mode operation during light loads. The LTC3892/-1's 2-phase operation reduces input filtering and capacitance requirements. Its current mode architecture provides easy loop compensation, fast transient response, fixed frequency operation, excellent line regulation and easy current sharing with paralleled phases for higher current. Output current sensing is accomplished by measuring the voltage drop across the output inductor (DCR) for the highest efficiency or by using an optional sense resistor for high accuracy.

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