Bench Talk

(Source: Ionatan- stock.adobe.com)

Transformers do a good job stepping up AC voltages using winding ratios. The secondary to primary winding ratios multiply the AC voltage while dividing the AC current capacity. Still, low-voltage transformers can be bulky and costly, while high-voltage transformers pose problems as well.

Modern enameled magnet wire used to insulate the wires wrapped around transformers can have an impressive kV rating, depending on insulation type and thickness, but this can degrade quickly if overheated by over-current faults. With high voltages, this can lead to arcing and potential fire issues.

Another technique of stepping up low or high voltages uses only diodes and capacitors. Both are available with high-current and/or high-voltage ratings. Both low-voltage analog and high-voltage lasers can take advantage of this technique, as can other cost- and/or space-sensitive designs. Voltage doublers, quadruplers, and multipliers are good techniques to have in an engineer's bag of tricks.

Multiplying an AC waveform can be performed using only diodes and capacitors. The simplest form is a discrete half-wave voltage doubler (Figure 1). During the negative half cycle, diode D2 is forward biased, so current will flow through it. This current will flow to capacitor C2 and charge it to peak value of input voltage (Vm). This can be looked at as a voltage clamp. C2 charges to Vpeak – VD1 (diodes forward voltage drop), The D2 Anode presents a ground with the positive side of the AC waveform during its negative half cycle.

Figure 1: The half-wave voltage doubler uses C2 and D2 as a voltage clamp, allowing positive cycle voltage ride on the charged voltage of C2. (Source: Vishay; redrawn by author)

During the positive half cycle, D2 drops out since it is negatively biased. You can visualize it better by removing D2 since it is reverse-biased. C2 is in series with the voltage source, and D1 presents as Vpeak X2 to capacitor C1 since it is ‘riding on the’ Vp voltage level from the negative cycle. In other words, the Vout = 2xVpeak. Depending on the size of the capacitor, it may take a few cycles to charge fully (Figure 2).

Figure 2: The rectifier capacitor charges every half wave until it reaches twice the peak voltage. (Source: Vishay)

The same principle can be used to create a full wave voltage doubler (Figure 3). The same waveform applies, except the output capacitor is charged by both positive and negative AC cycles. The full wave version can deliver more power, but a higher current is drawn from the AC supply. In both cases, the output voltage will drop depending on the magnitude of the load, which is determined by the sizing of the capacitors.

Figure 3: The AC waveform is full-wave rectified to double the peak voltage and will deliver more current than the half-wave rectified voltage doubler. Note the virtual ground. (Source: Vishay; redrawn by author)

A voltage quadrupler stacks two voltage doublers, each providing 2xVpeak of the input AC waveform. Here, the peak-to-peak voltages are doubled (Figure 4). Cascading voltages create two output voltages, one at 2xVm and the other at -2xVm.

Figure 4: A voltage quadrupler can create a positive and negative supply voltage; each doubles the peak voltage of the input AC waveform. (Source: Vishay; redrawn by author)

A nice feature of passive voltage multipliers is that they can be used in low-voltage and high-voltage applications. Even digital waveforms can be multiplied to create a regulated higher voltage of a dual rail voltage.

Passive voltage multipliers, for example, can be handy for creating a negative voltage supply for a dual rail Op-Am circuit. Both sides can be regulated to provide a stable positive and negative supply voltage for linear applications. Note, though, that you will create an isolating or floating ground that will not be system ground.

A Marx bank, also referred to as a generator or “stack”, is an electrical circuit used to generate brief high-energy pulses for experiments or simulations. The circuit consists of several capacitors that are charged in parallel and then quickly connected in series using spark gaps as switches. With the capacitors connected in series, a high-voltage pulse is generated, and the final gap connects the series “stack” of capacitors to the load. The circuit's high-voltage output is limited by the number of capacitors and their charge, so the output is in the form of a short high-energy pulse.

Similarly, high-voltage ceramic disc capacitors are used in applications that require operating voltage capability up to 50kV, capacitance of over 5000pF, low inductance, and a dissipation factor (DF) well under 0.5 percent. High voltage diodes with 20kV through hole and 15kV surface mount are also readily available with 30nsec recovery times.

As a design example, an off-the-shelf cap charger rated at 1500J/s can convert 500V to 4kV (Figure 5). A capacitor bank can be configured using the Vishay 715C series, as an example. These are high voltage—50kV (AC) or 10kV (DC)—disc capacitors with a low 0.2 percent dissipation factor at 1kHz. They can easily be paralleled to provide pulse currents that fit the design application, and they are ideally suited to use voltage multiplier circuits. 

Figure 5: High-power laser driver capacitor banks can take advantage of the Vishay 715C series high-voltage ceramic disc capacitors.  (Source: Vishay; redrawn by Mouser Electronics)

The Vishay 715C series uses strontium-based ceramic dielectrics, which feature negligible piezoelectric effects and low inductance. The epoxy coating provides high insulation resistance and environmental protection. These ceramic single-layer capacitors are ideal for high-voltage X-ray, CO₂, welding, industrial, and virtually any high-voltage power supplies. Capacitance values from 560pF to 8,000pF are available with rated voltages from 10kV_DC to 50kV_DC. SCRs, or thyratrons, for higher voltage operations can be used as a high-voltage switch for either full or partial discharge applications.

Engineers can use standard formulas to determine the power supply size and calculate the charge time. Using these formulas is the simplest way to estimate the energy needed for an application.

Using a typical high-voltage capacitor charging system (1500J/s), 500V to 4000V capacitor charger, we choose the Vishay 8000pf 715C10KTD80 as part of a 20-capacitor bank yielding 0.1µF that will be charged to 2.5kV.

The energy per pulse will be:

Ep = ½ CV²

Using plugins of 20Hz firing rate, with capacitance in Farads:

The charge rate—energy per pulse times the repetition rate—will be:

(50J) times 20 (Hz) = 1000 Joules/second.

We are using a 1500J/s supply to ensure we overcome any dead time—or settling time—which is usually required.

For partial discharge, the amount of time the capacitor is allowed to discharge will determine how much energy is needed to recharge the bank of capacitors. Also, pulse widths can vary from several hundred microseconds to tens of milliseconds. The maximum voltage and the droop voltage are used here:

E recharge = ½ times capacitance times (V² max – V²droop)

Discrete voltage multipliers are useful in space- and cost-sensitive designs. They offer advantages over other high-voltage step-up approaches, such as transformers. Discrete voltage multipliers not only eliminate the need for transformers in many system designs, but they can also mitigate risks such as arcing and potential fire issues in high-voltage designs.

While voltage multipliers exhibit impressive versatility in low- and high-voltage applications, capacitor chargers underscore their capacity to generate brief, high-energy pulses by leveraging capacitor parallel charging and series connection. Furthermore, the adept use of high-voltage capacitors, such as the Vishay 715C series, in tandem with voltage multipliers, provides an effective approach to amplifying operating voltage, highlighting their essential role in crafting high-voltage, short-duration energy pulses in varied electrical circuits.

After completing his studies in electrical engineering, Jon Gabay has worked with defense, commercial, industrial, consumer, energy, and medical companies as a design engineer, firmware coder, system designer, research scientist, and product developer. As an alternative energy researcher and inventor, he has been involved with automation technology since he founded and ran Dedicated Devices Corp. up until 2004. Since then, he has been doing research and development, writing articles, and developing technologies for next-generation engineers and students.