Two-quadrant power supplies can supply positive or negative voltages to the same output port, and can be easily fabricated using the LT8714 4-quadrant controller. The two-quadrant power supply shown here can be used in a variety of applications, from glass foils (changing polarity changes the arrangement of crystal molecules) to test and measurement equipment.

The Lt8714 data sheet describes how a two-quadrant power supply operates in the first quadrant (positive input, positive output) and the third quadrant (positive input, negative output). Note that in both quadrants, the power supply is sourcing current, so it is producing power rather than receiving it. The second quadrant and the fourth phenomenon generate the received power.

Circuit description and function

Figure 1 shows the circuit diagram of the two-quadrant power supply LT8714. The power system consists of NMOS QN1, NMOS QN2, PMOS QP1, PMOS QP2, inductor L1, inductor L2, coupling capacitor CC, and input and output filters. Inductors L1 and L2 are two discrete uncoupled inductors, which can reduce the cost of the converter.
Proper selection of active and passive components requires an understanding of the voltage stress and current levels present in each quadrant. To do this, look at the positive output functional topology shown in Figure 2.

Figure 1. Circuit diagram of a two-quadrant power supply based on LT8714 with VIN 12 V and VO ±5 V at 6 A

Figure 2. Two-quadrant operating topology with positive output

When the volt-second balance is in steady state, the duty cycle can be derived from:

To validate the design, we modified the demo circuit DC2240A to match the schematic shown in Figure 1. For both cases, the output voltage is ±5 V at a nominal input voltage of 12 V and a maximum current of 6 A.

The measured efficiency of this design is shown in Figure 3. The positive output exceeds the negative output, which is consistent with the theoretical calculation. In a negative output configuration, the voltage stress and current on the components are higher, which increases losses and reduces efficiency.

Figure 3. Converter Efficiency Curve: 12 V at VIN, +5 V and –5 V at VOUT, 6 A Maximum IO

Figure 4 shows a good linear relationship between the output voltage and the control voltage VCTRL. For this configuration, the circuit is loaded with 1 Ω resistors and the control voltage range is 0.1 V to 1 V

Figure 4. Graph of output voltage VOUT versus control voltage VCTRL. As VCTRL increases from 0.1 V to 1 V, VOUT gradually changes from –5 V to +5 V.

Using two LTspice® models, we can analyze the performance of the LT8714, the first model showing good power supply and the second model using an uncoupled inductor.

in conclusion

This article shows a simple two-quadrant voltage supply circuit using the LTC8714. The design has been tested and verified to demonstrate excellent linearity using the LTC8714 controller.