Introduction

Desktop power supplies (PS) tend to have an even number of ports (ignoring chassis ports): one positive and one negative. Using a bench power supply to generate a positive output is easy: set the negative output to GND and the positive output voltage to the positive output. Generating a negative supply is just as easy, just reverse the above settings. But how to produce a bipolar power supply where the load can use both positive and negative voltages? It's also relatively simple - just connect the positive port of one lab channel to the negative port of the other and call it GND. The other two ports (positive and negative) are the positive and negative power supplies, respectively. The result is a three-port bipolar power supply that provides GND, positive and negative voltage levels. Since three ports are used, there must be some sort of switching between positive and negative power supplies downstream of the power supply.

What if the application requires the same power port to be positive or negative (settings for only two ports are provided to the load)? This is not a purely academic question. In automotive and industrial environments, some applications require bipolar, adjustable dual-port power supplies. For example, applications ranging from exotic glass foils to test and measurement equipment use dual-port bipolar power supplies.

As mentioned earlier, traditional bipolar power supplies use three output ports to generate two outputs: positive, negative, and GND. In contrast, a single output power supply should have only two output ports: one GND and another that can be positive or negative. In such applications, the output voltage can be adjusted with respect to GND by a single control signal over the full range from a minimum negative value to a maximum positive value.

Some controllers are designed to implement bipolar power supply functions, such as the bipolar output synchronous controller LT8714. However, for many automotive and industrial manufacturers, testing and certifying specialized ICs requires some investment of time and money. In contrast, many manufacturers already have pre-qualified buck converters and controllers, as these devices are used in countless automotive and industrial applications. This article describes how to use a buck converter to generate bipolar power when a dedicated bipolar power IC is not an option.

Circuit description and function

Figure 1 shows a buck converter-based bipolar (two-quadrant) adjustable power supply solution. The input voltage range is 12 V to 15 V; the output is any voltage in the ±10 V range, regulated by the control block, and supports loads up to 6 A. The dual output buck controller IC is the core device of this design. One output connected to each buck-boost topology produces a regulated -12 V voltage (that is, the -12 V negative rail in Figure 1, whose power chain includes L2, Q2, Q3, and output filter CO2).

The -12 V rail is used as the ground for the second channel and the ground pin of the controller is also connected to the -12 V rail. Overall, this is a buck converter with an input voltage that is the difference between -12 V and VIN. The output is adjustable and can be positive or negative with respect to GND. Note that the output is always positive with respect to the -12 V rail, and its power chain includes L1, Q1, Q4, and CO1. The feedback resistor divider RB-RA sets the maximum output voltage. The value of this divider is adjusted by the output voltage control circuit, which regulates the output to the minimum output voltage (negative output) by injecting current into RA. The application startup characteristics are set by terminating resistors on the RUN and TRACK/SS pins.

Both outputs operate in forced continuous conduction mode. In the output control circuit, the 0 μA to 200 μA current source ICTRL is connected to the negative rail for laboratory testing, but can also be referenced to GND. Low-pass filter RF1-CF reduces fast output transients. To reduce the cost and size of the converter, relatively inexpensive polarized capacitors are used to form the output filter. Optional diodes D1 and D2 are used to prevent reverse voltages on these capacitors, especially during startup. Diodes are not required if only ceramic capacitors are used.

Converter Testing and Evaluation

This solution was tested and evaluated based on the LTC3892 and evaluation kits DC1998A and DC2493A. The converter performed well in numerous tests, including voltage and load regulation, transient response, and output short circuit. Figure 2 shows startup into a 6 A load with an output of +10 V. The linearity of the function between control current and output voltage is shown in Figure 3. The output voltage drops from +10 V to -10 V as the control current increases from 0 μA to 200 μA. Figure 4 shows the efficiency curve.

Figure 1. Electrical Schematic of a Two-Terminal, Bipolar, Adjustable Power Supply

Figure 2. Startup waveform into resistive load

Figure 3. VOUT versus control current ICTRL. The output voltage drops from +10 V to -10 V when ICTRL is increased from 0 A to 200 μA.

Figure 4. Efficiency Curves for Positive and Negative Outputs

We developed an LTspice® model of this bipolar, two-terminal power supply to simplify adoption of this method, allowing designers to analyze and simulate the above circuit, introduce variations, view waveforms, and study device stress.

Basic formulas and expressions describing this topology

This method is based on the negative voltage rail VNEG generated by the buck-boost portion of the design.
VNEG = VOUT + VOUT × Km (1)
where VOUT is the absolute value of the maximum output voltage and Km is a factor of 0.1 to 0.3. Km limits the minimum duty cycle of the buck converter. VNEG also sets the minimum value of VIN:
VIN ≥ |VNEG| (2)
VBUCK = |VNEG| + VIN
where VBUCK is the input voltage of the buck section and thus represents the maximum voltage stress on the converter semiconductors:
VBUCK(MAX) = |VNEG| + VOUT(3)
VBUCK(MIN) = |VNEG| – VOUT
VBUCK(MAX) and VBUCK(MIN) are the maximum and minimum voltages for the buck portion of the topology, respectively. The maximum and minimum duty cycle and inductor current of the buck section can be described by the following expressions, where IOUT is the output current:
DBUCK(MAX) = VBUCK(MAX) /VBUCK(4)
DBUCK(MIN) = VBUCK(MIN) /VBUCK
LBB = IOUT + Δ/1
The duty cycle of the buck-boost section of the power supply:
DBB = |VNEG|/(VIN + |VNEG|) (5)
The input power of the buck section and the corresponding buck-boost output power:
POUT(BB) = (VOUT × IOUT)/η (6)
Converter power and input current:
IOUT(BB) = POUT(BB)/|VNEG| (7)
IL(BB) = IOUT(BB)/(1 – DBB) + Δ/2
The output voltage change is achieved by injecting current into the feedback resistor divider in the buck section. The Output Voltage Control Circuit section of Figure 1 shows how to set up the output voltage control.
If RB is given, then
PBB = POUT(BB)/η(8)
IBB = PBB/VIN
where VFB is the feedback pin voltage.
When the current source ICTRL injects zero current into RA, the output voltage of the buck converter is the maximum positive value with respect to the negative rail (VBUCK(MAX)) and the maximum output voltage with respect to GND (+VOUT). To generate a negative output voltage (relative to GND) to the load, ΔI must be injected into resistor RA of the buck divider, reducing the output voltage to a minimum value, VBUCK(MIN), relative to the negative output voltage (-VOUT).

Numerical example

Using the previous formulas, we can calculate the voltage stress of the bipolar power supply, the current flowing through the power link devices, and the parameters of the control circuit. For example, the following calculations are for a power supply that produces 6 A, ±10 V output from a 14 V input voltage.
If Km is 0.2, then VNEG = -12 V. Verify the condition for the minimum input voltage VIN ≥ |VNEG|. The voltage stress on the semiconductor device VBUCK is 26 V.
The maximum voltage of the buck section is VBUCK(MAX) = 22 V, relative to the negative rail; the output voltage is set to +10 V, relative to GND. The minimum voltage, VBUCK(MIN) = 2 V, corresponds to an output voltage of -10 V (relative to GND). These maximum and minimum voltages correspond to the maximum and minimum duty cycles, DBUCK(MAX) = 0.846, DBUCK (MIN) = 0.077, DBB = 0.462 .
Power can be calculated by assuming 90% efficiency, resulting in POUT(BB) = 66.67 W, IOUT(BB) = 5.56 A, IL(BB) = 10.37 A, and PBB = 74.074 W.
For an output voltage of +10 V (according to Figure 1), the control circuit current ΔI is 0 μA, while for an output voltage of -10 V, ΔI = 200 μA.

in conclusion

This article describes a bipolar, two-terminal power supply design. The approach discussed here is based on a buck converter topology, which is the workhorse technology of modern power electronics and is therefore available in a variety of forms, from simple controllers with external components to complete modules. Using a buck topology gives designers flexibility and the option to use pre-qualified devices, saving time and cost.