Introduction to Voltage Doublers

A voltage doubler is an electronic circuit that converts an AC input voltage to a DC output voltage that is twice the peak input voltage. It is a type of voltage multiplier circuit. Voltage doublers are commonly used in electronic devices that require higher DC voltages than the AC voltage that is readily available from wall outlets or batteries.

Compared to traditional transformer-Rectifier Circuits that step up voltage, voltage doublers offer several advantages:

1. They are cheaper since they don’t require an expensive step-up transformer
2. They are smaller and lighter weight due to the lack of a bulky transformer
3. They have good voltage regulation under varying load conditions

However, voltage doublers also have some limitations. The maximum current output is limited compared to a transformer-based supply. Also, the output voltage has more ripple than a well-filtered transformer-rectifier circuit. Despite these drawbacks, voltage doublers are a good solution for many low-current, high-voltage applications.

How a Voltage Doubler Works

A basic voltage doubler circuit consists of two capacitors and two diodes connected as shown in the schematic diagram below:

[Schematic diagram of a basic voltage doubler circuit]

Here’s how it works:

1. During the positive half-cycle of the AC input, diode D1 conducts and charges capacitor C1 to the peak voltage Vpk.
2. During the negative half-cycle, diode D2 conducts and charges capacitor C2. The voltage across C2 is the sum of the AC peak voltage and the voltage on C1. So C2 charges to 2*Vpk.
3. On subsequent cycles, C1 and C2 repeatedly charge and discharge but maintain an average voltage of 2*Vpk.
4. The output voltage Vout is taken across C2 and is equal to 2*Vpk minus the small forward voltage drops of the diodes.

Assuming ideal components, the output voltage is given by:

Vout = 2 * Vpk

Where Vpk is the peak value of the AC input voltage. For example, with a 120V RMS AC input (in the US), the peak voltage is:

Vpk = 120V * √2 = 170V

So the ideal no-load output of the voltage doubler would be:

Vout = 2 * 170V = 340V DC

In practice, the output voltage will be somewhat less due to non-ideal components and loading of the output. But this demonstrates the voltage doubling action.

Voltage Doubler Design Considerations

When designing a practical voltage doubler circuit, there are several important factors to consider:

Diode Selection

The diodes should be rated for a reverse voltage of at least 2*Vpk. They should also have a forward current rating sufficient for the load current. Schottky diodes are often used for their low forward voltage drop. For high voltage circuits, series strings of lower voltage diodes can be used.

Capacitor Selection

The capacitors should be rated for a voltage of at least 2*Vpk. A voltage safety margin of 20-50% is prudent. Their capacitance value depends on the amount of output current and the acceptable level of output voltage ripple. Larger capacitances give lower ripple but slower response to load changes. Film or ceramic capacitors are typically used.

Input and Output Filtering

While not shown in the basic schematic, input and output filters are usually added in practice:
– An input Filter capacitor reduces harmonics injected back into the AC line.
– An output filter capacitor reduces the output ripple and improves load regulation.
– For low noise applications, an LC output filter may be used to further reduce high frequency ripple.

Transformer Coupling

For applications requiring galvanic isolation, a transformer can be added before the input of the voltage doubler. This allows the output to be fully isolated from the AC line and ground. The transformer also provides some additional noise filtering.

Advantages of Voltage Doublers vs. Transformer-Rectifiers

Voltage doublers offer several advantages compared to using a step-up transformer with a rectifier for generating a high DC voltage:

Lower Cost

The main cost savings is elimination of the transformer, which is often the most expensive single component. Diodes and capacitors are relatively inexpensive. The cost advantage is greatest for low-current applications where a physically small transformer would be used anyway.

Smaller Size and Weight

Eliminating the transformer yields a large reduction in physical volume and weight. This is a major advantage for portable, battery-powered, or hand-held devices. The thin, flat form factor of surface-mount diodes and capacitors helps minimize the overall circuit footprint.

Improved Regulation

With a classical transformer-rectifier design, the output voltage drops as more load current is drawn, due to winding resistance and leakage inductance in the transformer. A voltage doubler has lower dynamic output impedance, giving better regulation under varying load conditions. The output voltage is stiffer.

Wider AC Input Range

Transformer-rectifier circuits are designed for one specified AC input voltage and frequency. Voltage doublers can often be used over a wider range of inputs without modification. This is because the doubling action depends on the peak AC voltage, not the RMS value. So a voltage doubler can accommodate power line voltage variations of ±10% or more without significant change in the DC output.

Limitations of Voltage Doublers

Along with their advantages, voltage doublers have some limitations and drawbacks compared to transformer-based designs:

Limited Maximum Current

The absence of a step-up transformer means that all of the load current must come through the diodes and capacitors. At high currents, the I2R losses in the ESR (equivalent series resistance) of the capacitors becomes substantial. And the diode forward voltage drop wastes an increasing percentage of the input power. As a result, voltage doublers are usually limited to lower power applications, typically under 10-20W.

Higher Output Ripple

The output of a voltage doubler has considerably more voltage ripple than a well-filtered classical rectifier circuit. This is because the fundamental ripple frequency is the same as the AC line frequency, not double as in a full-wave rectifier. Larger output filter capacitors are required to get low ripple.

No Overload Protection

Transformer-rectifier power supplies have some inherent overload protection due to the limited current capability of the transformer windings. Voltage doublers have no such natural current limiting (apart from the I2R losses and diode current limits mentioned above). Separate overload protection circuits must be added if deemed necessary for the application.

Potential for Higher Common-Mode Noise

Without a transformer to provide galvanic isolation, the output of a voltage doubler is directly connected to the AC line. High frequency noise on the AC line can couple directly to the output. Mitigating this requires good AC line filtering before the doubler input.

Applications for Voltage Doublers

Voltage doublers are used in a wide range of electronic devices and equipment. Some common applications include:

Cathode Ray Tubes (CRTs)

The high voltage anode supply for CRTs is often generated with a voltage multiplier circuit, with a voltage doubler being the simplest case. TV sets, computer monitors, and oscilloscopes have used voltage doublers for this. Although CRTs have largely been replaced by newer display technologies, they are still used in some specialized applications.

Photocopiers and Laser Printers

The high voltage supply for the corona wires and toner transfer in photocopiers and laser printers is often a voltage multiplier, again with a doubler as the simplest case. Voltages of 5-10kV or more are common. The low current demand of the corona wires is well-suited to the voltage doubler topology.

Strobe Lights and Camera Flashes

The high voltage needed to trigger xenon flash tubes is often generated by a voltage doubler. The fast risetime of the flash is provided by the rapid discharge of the doubler’s output capacitor through the low impedance of the flash tube once triggered. Compact battery-powered devices benefit from the small size of the doubler circuit.

Bug Zappers and Air Purifiers

Many devices for electrocuting flying insects use a voltage doubler to generate the high voltage needed for the sparking grid. Electronic air purifiers that rely on electrostatic precipitation cells also use voltage doublers. The limited current capacity is acceptable for these applications.

Charged Particle Accelerators and Detectors

Small-scale particle accelerators for research or industrial applications sometimes use a voltage doubler to generate the high voltage for the particle beam. The current is inherently low in these applications. Similarly, some charged particle detectors such as Geiger counters use a voltage doubler for the detector bias voltage.

Frequently Asked Questions (FAQ)

What is the difference between a voltage doubler and a full-wave rectifier?

A voltage doubler and a full-wave rectifier are both types of rectifier circuits, but they have some key differences:

1. Voltage multiplication: A doubler provides an output voltage that is twice the peak input voltage, while a full-wave rectifier output is approximately equal to the peak input voltage.

2. Diode count: A basic doubler uses two diodes, while a full-wave rectifier requires four (or a single package bridge rectifier).

3. Ripple frequency: The fundamental ripple frequency in a doubler is the same as the AC line frequency, while in a full-wave rectifier it is twice the line frequency. So a doubler has more output ripple for a given filter capacitance.

4. Efficiency: For low output voltages, a full-wave rectifier is usually more efficient than a doubler due to lower diode losses. The efficiency crosses over in favor of the doubler for higher output voltages.

How can I reduce the output ripple of my voltage doubler?

There are a few ways to reduce the ripple on a doubler’s output:

1. Use larger capacitance values for C1 and C2. This reduces the ripple by providing more energy storage and smoothing between AC line cycles.

2. Add a larger output filter capacitor after the doubler circuit. This will reduce high frequency ripple more than increasing C1 and C2.

3. Use an LC filter on the output. The inductor will block high frequencies while passing DC. Choose the LC values for the desired cutoff frequency.

4. Run the doubler at a higher AC input frequency. Many voltage doublers in consumer electronics run on 100-400V AC at 10-20kHz from a switching pre-regulator, rather than on 50/60Hz line voltage. The higher frequency inherently gives lower ripple for a given capacitance.

Why would I use a voltage doubler instead of a Boost Converter?

Voltage doublers and boost converters can both generate an output voltage higher than the input, but there are some differences that may favor one or the other:

Reasons to prefer a voltage doubler:
1. Lower cost and part count: A doubler uses only two diodes and two capacitors, while a boost converter requires an IC, inductor, diode, and several capacitors.

1. Simpler control: Doublers are open-loop and self-oscillating, while boost converters require closed-loop feedback control for voltage regulation.

2. Easier to work with high voltages: The diodes and capacitors in a doubler are easier to source for high voltages (>500V) than the controller ICs used in most boost converters.

Reasons to prefer a boost converter:
1. Arbitrary voltage gain: A doubler is fixed at a 2x multiplication factor, while a boost ratio can be set arbitrarily high by adjusting the duty cycle.

1. Higher efficiency: Practical boost converters can exceed 90-95% efficiency, while voltage doublers are typically 70-90% efficient at low-medium current.

2. Smaller output ripple: The high switching frequency (up to several MHz) of a boost gives inherently low ripple. Lower value filter capacitors can be used.

Can I stack multiple voltage doubler stages to get even higher voltages?

Yes, this is commonly done. A cascade of N doubler stages will ideally provide an output of 2^N times the peak input voltage. So two cascaded doublers would give a 4x multiplication, three stages give 8x, and so on.

However, there are practical limitations. Each doubler stage adds some incremental loss due to the diode drops and capacitor ESR. And the overall capacitance required for low ripple grows quickly with the number of stages. So most practical designs use only 2-4 multiplication stages.

Cascaded doublers are often called Cockcroft-Walton multipliers after the scientists who first used them for high voltage particle accelerators in the 1930s.

What are some common failure modes for voltage doublers?

Voltage doublers are relatively robust circuits, but there are a few common ways they can fail:

1. Diode reverse breakdown: If the reverse voltage across a diode exceeds its PIV rating, it can short out and fail. This is usually caused by an input overvoltage surge.

2. Capacitor dielectric failure: If the voltage on a capacitor exceeds its working voltage rating for too long, the dielectric can break down and the capacitor will short. Using capacitors with a 2-3x voltage derating reduces the risk.

3. Overheating: If the doubler is operated beyond its maximum output current for too long, the I2R losses in the capacitor ESR and diode forward drop can cause overheating. This can degrade the capacitors over time. Adequate cooling and over-current limiting prevents this.

4. Electrolytic capacitor aging: If electrolytic capacitors are used, they can dry out and lose capacitance over several years, especially at elevated temperatures. This causes the output voltage and ripple to slowly degrade. Using higher temperature rated capacitors or replacing them on a preventive maintenance schedule avoids this.

In summary, conservative component deratings, input transient suppression, and avoiding overcurrent conditions can prevent most voltage doubler failures. Proper component selection is key for long term reliability.

[TABLE]
| Component | Ideal Value | Derated Value |
|————-|—————————-|—————————-|
| Diodes | Vreverse > 2Vpk | Vreverse > 3Vpk |
| Capacitors | Vmax > 2Vpk | Vmax > 2.5Vpk |
| Output Cap | Iripple < 0.2Iload | Iripple < 0.1Iload |

This table shows some typical guidelines for choosing voltage doubler component ratings for reliable operation. Using ideal values that just meet the minimum requirements may work, but using derated values with margin allows for unexpected surges, long lifetime, and harsh operating conditions.

Conclusion

Voltage doubler circuits offer a simple, low cost, and lightweight alternative to transformer-rectifier power supplies for generating DC voltages higher than the readily available AC input voltage. They are especially well-suited for applications requiring high voltage at low-moderate current, such as CRT supplies, photocopiers, laser printers, bug zappers, air purifiers, flash lamps, and particle accelerators.

When considering a voltage doubler, it’s important to weigh the benefits of lower cost, size, and weight against the limitations of efficiency, output ripple, and maximum power throughput. With proper component selection and design margins, voltage doublers can provide reliable high voltage DC for many applications. As with any power electronic circuit, conservative design practices and accounting for real-world limitations is key to a robust and long-lived product.