Voltage Multipliers

Voltage multiplier is a modified capacitor filter circuit that delivers a dc voltage twice or rnore times of the peak value (amplitude) of the input ac voltage. Such power supplies are used for high-voltage and low-current devices such as cathode-ray tubes (the picture tubes in TV receivers, oscilloscopes and computer display). Here we will consider half-wave voltage doubler, full-wave voltage doubler and voltage tripler and quadrupler.

Half-Wave Voltage Doubler

The circuit of a half-wave voltage doubler is given in figure shown below. During the positive half cycle of the ac input, voltage, diode D₁ being forward biased conducts (diode D₂ does not con­duct because it is reverse-biased) and charges capacitor C₁ upto peak values of secondary voltage V_smax with the polar­ity, as marked in figure shown below.

Half-wave voltage doubler

During the negative half-cycle of the input voltage diode D₂ gets forward biased and conducts charging capacitor C₂. For the negative half cycle, the lower end of the transformer secondary is positive while upper end is negative. The polarity of the capacitor C­₂ has also been marked in the figure. Now starting from the bottom of the transformer secondary and moving clockwise and applying Kirchhoffs voltage law to the outer loop we have

-V_smax – V_c1 + V_c2 = 0

Or

V_c2 = V_smax + V_c1= Vsmax + Vsmax = 2V_smax = Twice the peak value of the transformer secondary voltage. (Since V_c1 = V_smax)

During the next positive half-.cycle diode D₂ is reverse-biased and so acts as an open and capacitor C₂ discharges through the load If there is no load across the capacitor, C₂ both capacitors stay charged – C₁ to V_smax and C₂ to 2V_smax. If, as expected there is a load connected to the output terminals of the voltage doubler, the capacitor C₂ discharges a little bit and consequently the voltage across capacitor C₂ drops slightly. The capacitor C₂ gets recharged again in the next half-cycle. The ripple frequency in this case will be the signal frequency (that is, 50 Hz for supply mains.)

Full-Wave Voltage Doubler

The circuit diagram for a full-wave voltage doubler is given in the figure shown below. During the positive cycle of the ac input voltage, diode D₁ gets forward biased and so conducts charging the capacitor C₁ to a peak voltage V_smax with polarity indicated in the figure, while diode D₂ is reverse-biased and does not conduct.During the negative half-cycle, diode D₂ being forward biased conducts and charges the capacitor C₂ with polarity shown in the figure while diode D₁ does not conduct. With no load connected to the output terminals, the output voltage will be equal to sum of voltages across capacitors C₁ and C₂ that is, V_C1 + V_C2 or (V_s max + V_s max) or 2 V_s max. When the load is connected to the output terminals, the output voltage V_L will be somewhat less than 2 V_s max. The input voltage and output voltage waveforms are also shown in the figure below.

Full-wave voltage doubler

Voltage Tripler and Quadruples

The half-wave voltage doubler, shown in the earlier figure can be extended to provide any multiple of the peak input voltage (that is, 3 V_s max, 4 V_s max or 5 V_s max), as illustrated in the figure shown below. It is obvious from the pattern of the circuit connections how additional diodes and capacitors are to be connected to provide output voltage, 5,6,7 or 8 times the peak input voltage from a supply transformer of rating only V_s max, and each diode in the circuit of PIV rating 2 V_s max. If load is small and the capacitors have little leakage, extremely high dc voltages can be obtained from such a circuit using many sections to step-up the dc voltage.

In operation capacitor C₁ is charged through diode D_l to a peak value of transformer secondary voltage, V_s max during first positive half-cycle of the ac input voltage. During the negative half cycle capacitor C₂ is charged to twice the peak voltage 2 V_s developed by the sum of voltages across capacitor C₁ and the transformer secondary. During the second positive half-cycle, diode D₃ conducts and the voltage across capacitor C₂ charges the capacitor C₃ to the same 2 V_g max peak voltage. During the negative half-cycle diodes D₂ and D₄ conduct allowing capacitor C₃ to charge capacitor C₄ to peak voltage 2 V_{S max}. From the fogure shown below it is obvious that the voltage across capacitor C₂ is 2 V_{s max}, across capacitors C₁ and C₃ it is 3 V_s max and across capacitors C₂ and C₄it is 4 V_s max.

If additional diodes (each diode of PIV rating 2 V_s max) and capacitors (each capacitor of voltage rating 2 V_s max) are used, each capacitor will be charged to 2 V_{s max}. Measuring from the top of the transformer secondary winding (figure below) will give odd multiples of V_g max at the output, while measuring from the bottom of transformer secondary winding will give even multiples of the peak voltage, V_s max.

Voltage Tripler and Quadruplar

Some electronic devices, such as cathode ray tubes (in picture tubes in TV receivers, oscilloscopes and computer display) need dc power supply at high voltage with low current. This requirement can be met with either by employing a step-up transformer with a rectifier circuit or by employing voltage multiplier. Since transformers are very bulky and costly, voltage multipliers are preferred. By using voltage multipliers, the voltage level is usually raised well into the hundreds or thousands of volts.

Generally such circuits are employed when both the supply voltage and load are maintained constant.