Extremely stable! You must know about this highly efficient synchronous SEPIC controller

Analog Devices has a number of synchronous buck and boost converters and controllers, but not many support the synchronous SEPIC topology. The SEPIC topology is actually very useful because it can provide a stable output voltage level regardless of whether the input voltage is much lower or much higher than the output voltage.

This feature is critical in some situations, especially in automotive applications, when electronic products are cold-starting and dumping loads, and in industrial applications, when the power supply lines in the factory are long or suddenly lost. Oil and gas equipment can improve system reliability by connecting multiple different power sources to the load through the SEPIC converter. If one of the power sources fails, the SEPIC converter can power the load through another power source even if the input voltage is different.

Figure 1. Schematic of the LT8711 for SEPIC and buck applications.

The power control system schematic diagram shown in Figure 1 is a synchronous SEPIC converter based on the LT8711, which includes:

• Two uncoupled inductors L1 and L2

• N-channel modulation MOSFET tube MN1, driven by pin BG

• Two P-channel synchronous MOSFETs MP1 and MP2 are driven through pin TG.

• Decoupling capacitors C1, C2, and C3

• Input and output filters

Figure 2. Conversion efficiency of the LT8711 for a SEPIC application.

Figure 2 shows the efficiency of the converter when the input voltage is 14V. The synchronous structure can ensure high conversion efficiency, with a peak efficiency of 93.4%. Figures 3 and 4 show that when the input voltage drops below or rises above the output voltage, the converter output can still remain stable. The standard demonstration circuit DC2493A has been redesigned, and the output current has been increased from 4A to 6A. MOSFET tubes MN1 and MP1, and inductor L2 are replaced by the parts shown in Figure 1.

Figure 3. Cold crank condition. When the input voltage V _IN drops from 15V to 6V, V _OUT remains stable at 12V.

Figure 4. Load dump condition. V _out remains stable when the input voltage increases from 10V to 20V.

This design was evaluated using a reworked demonstration circuit, the DC2493A, whose thermal image is shown in Figure 5. A similar solution can be found in the LTspice ^® ​​model for the LT8711 demonstration circuit, and the data sheet details how to select the SEPIC circuit parameters. Below are basic expressions for peak voltage and current to help understand the operation of this topology.

Figure 5. Thermal image of DC2493A and SEPIC during operation (V _IN =14V, V _OUT =12V, I _OUT =6A). The hottest part is MNI 77°C.

The LT8711 is a versatile and flexible controller that can be designed for synchronous buck, boost, SEPIC, ZETA, and non-synchronous buck-boost converters. It should be noted that the synchronous SEPIC topology allows the output voltage to fall into the middle of the input voltage range and still produce high conversion efficiency, which is particularly suitable for automotive and industrial applications .

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