Gheorghe Plasoianu EDN Many applications require positive and negative supply voltages, with only one voltage requiring tight regulation. This Design Idea describes a dualoutput, hybrid SEPICĆuk converter whose positive output voltage can be lesser or greater than the input voltage (Figure 1). The unregulated negative output is a mirrored replica of the positive output.
To find out the output voltages we apply the principle of inductor voltsecond balance and capacitor charge balance. To simplify the calculus, we neglect the voltage drop over the MOSFET and diode and we consider only continuous conduction mode.
When the MOSFET is on we have this equivalent network, shown in Figure 2, and the inductor voltages and capacitor currents for this interval, using smallripple approximation, are: When the MOSFET is off we have this equivalent network, shown in Figure 3, and the inductor voltages and capacitor currents for this interval are: Equating the average inductor voltages and capacitor currents over one switching period to zero, we get: where D is the duty cycle.
Solving for V4 and V5: In practice, due to the feedback, the positive output voltage, V4, is fixed. Extracting the duty cycle from the V4 equation and inserting it into V5 results in: Therefore, this topology is most suitable when the output currents do not differ much. When the two loads are equal, then: and A variation of the topology can supply a “floating” load is shown in Figure 4.
The opamp converts the differential output voltage to singleended for regulation. The differential output voltage is: For Figure 4: where V_{FB} of the LM3488 is 1.26 V.
I measured V_{OUT} = 18 V for R_{L} = 47 Ω, and 18.02 V with a 94 Ω load. That gives a load regulation of about 0.1 V/A, or 0.55% of V_{OUT}.
Figure 5 shows the gate, source, and drain voltages of the MOSFET. The duty cycle is about 60%. ReferencesMaterials on the topic 

