Power


Catch here


Add an efficient positive rail to a unipolar negative supply


Victor Khasiev, senior applications engineer, ADI S


ometimes you need a positive power supply, and the most available rail (or only available rail) is negative. In fact, negative- to-positive voltage conversion is used in automotive electronics, and the biasing circuitry for a variety of audio amplifiers, and industrial and test equipment. Even though power is distributed in many of these systems via a negative—relative to ground—rail, the logic boards, ADCs, DACs, sensors, and similar devices found in them still require one or more positive rails. This article presents a simple, low component count, efficient circuit for the generation of a positive voltage from a negative rail.


Circuit description and power train functionality


Figure 1 shows a complete solution for efficient conversion of a negative voltage to a positive voltage. This particular solution uses a boost topology. The power train includes switching MOSFETs, bottom Q1, top Q2, inductor L1, and input/output filters. The synchronous high efficiency boost controller IC regulates output voltage by changing the state of the switching MOSFETs in the power train. For the purposes of describing this circuit, system ground (SYS_ GND) serves as the reference regarding polarity, with a negative—relative to SYS_GND—input rail (–VIN), and positive—relative to SYS_ GND—output rail (+VOUT).


The converter works as follows. If transistor Q1 is on, then current flows from the SYS_GND to the negative rail. The transistor Q2 is off and inductor L1 stores energy in its magnetic field. To complete the switching period, Q1 turns off, and Q2 turns on. Current starts to flow from the SYS_GND to the +VOUT rail, discharging L1 energy to the load.


Basic expressions for the power train components selection The topological diagrams in Figure 2 of the switching behavior illustrate the negative-to-positive converter behavior. For the first interval of a switching cycle,


over a length of time defined by the duty cycle, the bottom switch, BSW


, is shorted and the top switch, TSW , is open. The


voltage across the inductor, L, is equal to –VIN


inductor. At the same time, the output filter capacitor discharges, supplying current to the system load.


The second interval of the cycle flips both switches—BSW


is open and TSW is


shorted. The polarity across inductor L changes, and the inductor starts sourcing current (stored in the first interval of the cycle) to both the load and COUT


, the


. Throughout this interval, current in inductor L increases, generating a voltage polarity matching –VIN


across the


Figure 1. A negative-to-positive converter electrical schematic, with VIN and with VOUT


+12 V at 6 A.


–6 V to –18 V (–24 V peak)


Figure 2. Negative-to-positive converter topological diagrams


Figure 3. Efficiency curve for VIN –12 V and –18 V with natural convection cooling.


Figure 4. Output current derating curve for absolute value input voltages below –9 V.


14 June 2020


Components in Electronics


www.cieonline.co.uk


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