Voltage doubler uses inherent features of push-pull dc/dc converter
Paralleled Darlingtons drive a wide-range voltage doubler.
Ajoy Raman, Aeronautical Development Establishment, Bangalore, India; Edited by Charles H Small and Fran Granville -- EDN, August 16, 2007
This Design Idea presents a minimal-parts-count, wide-range voltage doubler using the inherent voltage-doubling characteristics of a one-transformer push-pull dc/dc converter. The implementation uses a high-voltage Darlington-array driver, ULN2023A. The circuit exhibits a wide input-voltage range of 5 to 30V and provides a typical power output of 1 to 4W at moderate efficiency.
Figure 1 shows a simple, one-transformer dc/dc converter in which cross-coupled RC networks from the collectors of Q1 and Q2 to the corresponding bases provide regenerative feedback. In operation, the transformer alternates between positive and negative saturation, with collapse in transformer flux leading induced voltages to drive the transistors alternately off and on. The input-voltage and saturation characteristics of the transformer core determine the operating frequency based on the relationship
where VCC is the input voltage, βS is the saturated flux density in gauss, A is the cross-section area of the core in square centimeters, and N is the number of turns in half of the primary. The circuit uses the property that the collector-to-emitter voltage of each device is approximately twice the supply voltage, VCC, plus induced voltages, which occur because of leakage inductance. Rectification and filtration of the collector voltages of Q1 and Q2 through D1 and D2 directly provide an output voltage that is approximately double the input voltage, VCC.
The internal schematic of the high-voltage Darlington-array ULN2023A driver in Figure 2 exactly matches the requirements for the circuit in Figure 1 by providing rectifier diodes at the collector outputs. The voltage-breakdown specification of 95V meets the maximum requirement of twice VCC plus transients when operating at an input of 30V. The device exhibits a low collector-to-emitter saturation voltage at the desired current level of approximately 100 mA and low switching times when switching at rates as high as tens of kilohertz.
Figure 3 shows the final circuit configuration. Three drivers operate in parallel, sharing the drive current, minimizing the collector-to-emitter voltage, and maximizing the permitted power dissipation. Table 1 shows the experimental results with the voltage-doubler circuit operating over the input voltage of 5 to 30V. In that range, the input current is less than 300 mA to remain within the current values of the transformer at lower input voltages and within the power-dissipation limit of the ULN2023A at higher input voltages. Figure 4 shows the plot of the experimental results, clearly indicating the operation of a low-power, moderately efficient, wide-range voltage doubler.
