No Heatsink Needed for 200-W Buck-Boost Supply

Jul 1, 2008 12:00 PM
By Henry Zhang, Applications Engineering Manager, Linear Technology, Milpitas, Calif.

A common problem power-supply designers face is how to generate an output that, at any time, can be above or below the input, particularly in battery-powered systems. Many conventional solutions — such as two-stage power converters, single-ended primary inductor converters (SEPIC) or flyback converters — suffer from bulky component requirements and low efficiency. A synchronous four-switch buck/boost controller, the LTC3780 avoids these pitfalls by using a high-efficiency single-inductor topology.^[1]

The LTC3780 has four sets of integrated FET drivers for a 4-V to 30-V (36-V max) input- and output-voltage range. However, some applications, such as 48-V telecom systems and automotive systems, require even higher input or output voltage. To extend the input- and output-voltage range, a high-voltage, high-side FET driver should be used as shown in Fig. 1.

Fig. 1 shows the schematic of a design with 36-V to 72-V telecom input voltage and tightly regulated 48-V output with a maximum 4-A load, or approximately 200 W of output power. In this design, because of the high input voltage, the 100-V-rated LTC4440 high-side driver is used to drive the buck-side top FET (Q_A). Internally, the LTC3780 has about 50-ns dead time between the top-gate and bottom-gate signals to avoid the bridge FET short-through.

Because the LTC4440 gate driver has a typical 30-ns propagation delay, as shown in Fig. 2, the dead time between Q_A turn-off and bottom FET (Q_B) turn-on is reduced from 50 ns to a marginal 20 ns with the LTC4440. To compensate for the additional Q_A turn-off delay caused by the LTC4440, a simple R-C-D turn-on delay timing circuit is added to the gate of Q_B.

When the bottom-gate signal (BG2) goes up, R14 and C28 in Fig. 1 add additional delay. When the BG2 signal turns off, diode D7 can still discharge the gate of Q_B quickly. As shown in Fig. 3 with the R-C-D delay timing circuit, the dead time between Q_A turn-off and Q_B turn-on is increased from 20 ns to 93 ns.

It is necessary to point out that bottom synchronous FET Q_B is always operated in the zero-voltage-switching mode; therefore, the R-C-D delay circuit does not increase the switching loss of Q_B significantly.

On the output boost side, synchronous boost switch D (not shown in Fig. 1) can be replaced with 60-V-rated Schottky diode D1. As a result, neither an external high-side driver nor a delay-time circuit are needed. Because the output voltage is 48 V, the impact on the converter efficiency by using the Schottky boost diode is less than 1%.

Unfortunately, if switch D is needed with LTC4440 for better efficiency, the R-C-D delay circuit is not desirable for boost switch Q_C, because of the increased switching loss. In this case, an LTC4440 high-voltage dual FET driver is needed on the boost side.

With three Si7852 SO-8 PowerPAK MOSFETs, one 5-A/60-V Schottky diode PSD560, a 14-mm × 14-mm inductor, the converter has higher than 95% efficiency over a wide load range and a 36-V to 72-V input range. At 48-V input and 200-W output, the efficiency is as high as 97.2%. A graph of the measured efficiency of this 48-V buck-boost converter appears in Figure A.

Thermal measurements taken on power MOSFETs Q_A, Q_B, Q_C and boost-side Schottky diode D1 are also shown in Figure B. The pictures are taken at 36-V, 48-V and 72-V input with 48-V/200-W output. The ambient temperature is 25°C.

Without any heatsink or forced airflow, the maximum FET/diode case temperature rise is just 47.6°C at 48-V input and 54.5°C at 72-V input worst-case condition. Because of its high efficiency and fast overcurrent protection, the LTC3780 supply has low thermal stresses, which enhance reliability.

References

1. “LTC3780 High Efficiency, Synchronous, 4-Switch Buck-Boost Converter Data Sheet,” www.linear.com.

2. Philips, Theo, and Zhou, Wilson, “Breakthrough Buck-Boost Controller Provides up to 10A from a Wide 4V-36V Input Range,” Linear Technology Magazine, Volume XV, No. 3, September 2005.
3. Mathews, Kurk, “Using the LTC4440 Driver to Extend the Voltage Range of LTC3780 Supplies,” Linear Technology Design Article, 2005.

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