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Mitigate temperature rise in buck-boost converters

08 Jun 2015  | Keith Szolusha

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A single synchronous buck-boost DC/DC converter can deliver high power due to its synchronous switching topology, but eventually the switching and/or conduction losses at higher power can overwhelm a single converter with excessive board heating. Although heat can be mitigated with bulked up heat sinks, additional external gate drivers, and/or forced airflow, it may be better to simply tie together two or more converters in parallel to spread the load.

This can be done with the LT3790, a 4-switch synchronous buck-boost DC/DC converter. The LT3790 regulates both constant voltage and constant current at up to 98.5% efficiency using only a single inductor. It can deliver hundreds of watts and features a 60V input and output rating, making it a suitable DC/DC voltage regulator and battery charger when both step-up and step-down conversion are needed.

A 120W, 24V, 5A output buck-boost voltage regulator
The buck-boost converter shown in figure 1 regulates 24V with 0A–5A load at up to 98.5% efficiency. It operates from an input voltage range of 8V to 56V. Adjustable under-voltage and over-voltage lockout protect the circuit. It has short-circuit protection and the /SHORT output flag indicates when there is a short circuit on the output. It features DCM operation at light load for lowest power consumption and reverse current protection. The sense resistor ROUT sets the output current limit during both a short-circuit and overload situations, making this a robust application.

Figure 1: 120W, 24V, 5A output buck-boost voltage regulator with 8V–56V input has up to 98.5% efficiency and is easy to parallel.

The temperature rise of this 120W board at 12V input is only 20°C on the hottest component (a switching MOSFET) as shown in figure 2a. There is still margin for either higher output power at 12V input, or the same 120W from a lower VIN without excessive component temperature rise—note that higher output power requires a correspondingly increased output current limit. When operated down to 8V input with 120W output, the components on this standard 4-layer LT3790 PCB remain below 97°C (at room temp) without forced airflow or heat sinking. To deliver significantly higher power with the same, limited temperature rise and input voltage range, two or more LT3790 converters can easily be connected in parallel.

Figure 2: Single 24V, 5A converter shown in Figure 1 has a maximum of 20°C temp rise on any component at 12V input (a) & 50°C at 9V input (b). Even at 8V input (c), the hottest component reaches only 96.5°C without forced airflow or heat sinking.

Parallel converters, constant voltage master, constant current slave
Ideally, paralleled switching converters share the load equally throughout the entire output range. The LT3790's ability to run in either constant voltage or constant current operation allows one master converter to control the output voltage, while its current monitor output (ISMON) tells one or more slave converters how much output current to regulate (CTRL input) in order to match its own output level. Current matching between multiple converters is nearly ideal using this technique.

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