Break through the constraints! Precision bipolar power supply design can be so simple

The most common topology for generating power is the buck converter. However, this topology is limited to generating a positive output from an input voltage that is higher than the output. It cannot be directly used to generate a negative voltage or provide a regulated output when the input voltage is lower than the output voltage. Both aspects of generating the output are important in automotive electronics because negative voltages are needed to power amplifiers or the entire system must continue to function properly during cold cranking when the input voltage rail drops significantly. Today we detail How to use a simple buck controller in SEPIC, Cuk, and boost converters.

Figure 1 shows a bipolar power supply design based on a single buck controller with two outputs.

Figure 1. Electrical schematic of the LTC3892, which generates positive and negative voltages. V _OUT1 is 3.3 V at 10 A and V _OUT2 is –12 V at 3 A.

To get the most out of this chip, one output must be used to generate a positive voltage and the second output to generate a negative voltage. The input voltage range for this circuit is 6 V to 40 V. V _OUT1 generates a positive voltage of 3.3 V at 10 A, and V _{OUT2 generates} a negative voltage of -12 V at 3 A. Both outputs are controlled by U1. The first output, V _OUT1, is a simple buck converter. The second output is a bit more complex. _{V OUT2 is negative} with respect to GND, so a differential amplifier, U2, is used to sense the negative voltage and adjust it to a 0.8 V reference voltage. In this approach, both U1 and U2 are referenced to the system GND, which greatly simplifies the control and functionality of the power supply. If other output voltages are needed, the following expressions can help calculate the resistor values ​​for RF2 and RF3.

The V _{OU T2} power train uses a Cuk topology, which is widely described in the technical literature. To understand the voltages across the power train components, the following basic formulas are used.

The V _{OUT T2} efficiency curve is shown in Figure 2. In this example, the input to the LTC3892 converter is 10 V to 20 V. The output voltages are 10 A, +5 V and 5 A, -5 V.

Figure 2. Efficiency curve for negative output at 14 V input voltage.

The electrical schematic of the converter shown in Figure 3 supports two outputs: V _OUT1 at 10 A and 3.3 V, and V _OUT2 at 3 A and 12 V. The input voltage range is 6 V to 40 V.

Figure 3. Electrical schematic of the LTC3892 in a SEPIC configuration for a step-down application.

V _OUT1 is created in a similar manner as shown in Figure 1. The second output is a SEPIC converter. Like the Cuk above, this SEPIC converter is based on an uncoupled dual discrete inductor solution. The use of discrete chokes significantly expands the range of available magnetic materials, which is very important for cost-sensitive devices.

Figures 4 and 5 show the converter’s functionality during voltage drops and spikes, such as during a cold start or power cut. The rail voltage, VIN, drops or rises around the nominal 12 V. However, both VOUT1 _and VOUT2 _remain in regulation, providing stable power to the critical load. The dual-inductor SEPIC converter can be easily rewired as a single-inductor boost converter.

Figure 4. The rail voltage drops from 14 V to 7 V, and both V _OUT1 and V _OUT2 remain in regulation.

Figure 5. The rail voltage increases from 14 V to 24 V, but both V _OUT1 and V _OUT2 remain in regulation.

The above article describes a method for building bipolar and dual output power supplies based on a buck controller. This method allows the same controller to be used in buck, boost, SEPIC, and Cuk topologies, which is very important for automotive and industrial electronics suppliers because once approved, they can design power supplies with various output voltages based on the same controller.