Creating higher voltages, part 1: Voltage boosters

There are several ways to boost a relatively low AC or DC voltage by a factor of two or three, each with tradeoffs and constraints.

Lower-voltage circuitry dominates much of design and associated discussions, with ICs and systems operating from five, three, and one, and even sub-one volt rails. There’s good reason for this: in general, such circuits require less power and have lower dissipation than higher-voltage circuits. Further, these lower-voltage circuits also can operate at higher speeds since the voltage/current swings are smaller, and so the slewing demands (dV/dt, dI/dt) are also reduced.

However, there are many cases when a higher voltage is either preferred or mandatory.  It’s interesting to see the creative ways that have been devised to develop a high voltage from a low-voltage source, often with techniques that are a hundred years old and still in use.

Why would you want to use a higher voltage, since lower operating voltages offer advantages of lower power consumption and higher speeds, among other factors? Among the reasons:

  • First, a circuit may operate at a lower voltage, but need a higher voltage in one section to boost signal/noise ratio (SNR); this is common for sensitive front ends in RF and sensor applications.
  • Second, when a circuit must deliver power – not voltage – to a load, it’s always more efficient to do so at higher voltages due to reduced losses (internal, switching, IR, and I2R). In these cases, it may be beneficial, even if not mandatory, to use a somewhat higher voltage. In a typical situation, a battery and regulator providing 3 V for the main circuitry may also need to provide a 12-V rail for a sensor.
  • But the biggest reason is that requirement is simply unavoidable. There are many real-world applications where the voltage needed is determined by the physics of the situation, and there is no way to “get around” those requirements. For example, many scientific, medical, and test systems require higher voltages (>100 V to >1000 V) to set up specialized components such as vacuum electron devices (VEDs—the now-preferred designation for vacuum tubes) or create electric fields.

There are two very different ways to do increase a low voltage to a higher one: via a transformer, or via some type of switched-capacitor arrangement.

The transformer’s principle is simple and (hopefully) known to everyone reading this blog. An AC voltage on the primary (input) side is stepped up (or down) in proportion to the turns ratio between primary side and secondary (output) side.

For example, if the primary has 10 turns and the secondary has 100 turns – a 1:10 ratio – the voltage on the primary will be stepped up by a factor of ten (Figure 1). Of course, if you need a DC output, that ×10 AC output would need to be rectified and filtered. Use of a transformer to increase or decrease an AC voltage been known for about 150 years and is widely used to step up/down voltages in power-line installations, of course. However, it is often not the best choice for small circuits.


Figure 1 The relationship between primary (left side) and secondary (right side) turns ratio and voltage (and current) step up/step down is simple and a fundamental principle of magnetics. (Image source: Allelco)

However, while the transformer is very good at voltage step-up (or step-down) for larger systems, it is relatively large, costly, and heavy relative to a modest PC board. Further, it is not compatible with IC processes and packaging, and so would have to be mounted as a separate unit. Despite these drawbacks, it is sometimes still the right solution with respect to various tradeoffs.

The alternative is usually a circuit which uses some arrangement of switched capacitors. These clever schemes that have been in use for many years. They are often more practical and IC-compatible, because ICs can provide fast switching of the capacitors. Depending on their size, these capacitors can be in-chip or external; either way, a capacitor is more PC-board “friendly” in many cases than a transformer. These step-up approaches most commonly use a charge pump or a “flying capacitor” topology (where the capacitor is alternately switched or “flys” between input and output sides).

Charge pumps use an electronic switch to control the connection of a supply voltage across a load through a capacitor in a two-step process (Figure 2). in the first step, a capacitor is connected across the DC input supply, charging it to that same voltage. In the second step, the switches are used to reconfigure the circuit so that the capacitor is in series with the supply and the load. Now, the voltage across the load is doubled, as it is the sum of the original supply and the capacitor voltages. The switching action is repeated. Additional regulation is needed to smooth out the pulsed voltage at the output.


Figure 2 In the basic charge pump, the switching capacitor is charged from the input voltage to ground in the first phase, and then connected between the input voltage and output voltage; this “stacking” creates an output voltage which is twice the input voltage. (Image source: Texas Instruments)

An external or secondary clock circuit drives the switching, typically at tens of kilohertz up to several megahertz. A higher frequency minimizes the amount of capacitance required, as less charge needs to be stored and replenished in a shorter cycle. However, higher frequencies can also have higher losses, so there’s a tradeoff.

By adjusting the switching duty cycle, charge pumps can deliver double, triple, half, and scaled (such as ×3/2, ×4/3) ratios. With some rearrangement of the topology, they can also invert or reduce the output voltage (often called buck mode).

Charge pumps can be efficient (80-90%) but only when the components are sized for a specific load current. If the load current changes, the efficiency drops. Also, there are losses in the switching circuity which increase as the switching frequency increases; on the other hand, the output ripple is far less at higher frequencies, so the output filtering is simplified. 

These pumps are widely available and used as standalone voltage-boost ICs, or as part of buck-boost regulator ICs. They are also often embedded within an IC to provide the higher voltages needed by some (not all) internal functions or external I/O (such as enabling a 3-V RS-232/423 interface IC to provide a 5-V or even 12-V drive. The capacitors are usually external to the IC.

The switched capacitor is just one of several related capacitor-based variations which use electronic switches to transfer charge between an input-side capacitor and an output-side capacitor. The charge pump is not the only option: there’s also the “flying capacitor” (Figure 3).


Figure 3 A capacitor can be switched from a voltage input to output capacitor and load, and the resulting charge transfer can provide voltage boosting as well. (Image source: Analog Devices)

The flying-capacitor principle is this: as the charge q in a circuit is unchanged (let’s assume that there is no load, for now) then the input-side charge q = C1 × V1. Then, if this charge is switched to another capacitor on the output side, q flows to that capacitor but is unchanged, and thus the voltage on the second capacitor changes, as q now equals C2 × V2. If the output-side capacitor is smaller than the input-side one, the output-side voltage will be higher than the input-side voltage.

This almost sounds like something for nothing, but it is not. As charge is “drained” from the output side by the load, the capacitor’s voltage will decrease. To correct this, the input-to-output action must be repeated at a high-enough rate to replenish the lost charge.

Incidentally, the flying-capacitor scheme was used many years ago to provide galvanic isolation between a sensor and a circuit. The sensor for be on the “input” side, while the circuit would be on the output side. The voltage across the sensor would be captured and then transferred to the system circuitry without any ohmic path between the two sides. This scheme has largely been made obsolete by modern isolation schemes using transformer, capacitive, optical, and even RF coupling.

In theory, the transformer or switched-capacitor schemes can be used for transforming, say, 10 or 100 V to thousands of volts. However,  in practice, they cannot be used without major adjustments, as the high-voltage world has some unique issues.

However, as voltages go beyond about 60 V, issues of safety and regulatory mandates become a concern. As these voltages reach into the hundreds and thousands of volts, there are additional unavoidable and non-intuitive issues of material characteristics and electrical phenomena that show that design and construction for these voltage levels is a very different world.

Part 2 of this blog will look at how clever schemes based primarily on diodes and capacitors are used to develop those much-higher voltages using voltage multipliers.

References:

—Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors and signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing, and he also developed significant mechanical-engineering insight while designing control electronics for large materials-testing systems.

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