Today in Edworking News we want to talk about lcamtuf’s thing The magic of DC-DC voltage conversion Resistor-based dividers, linear regulators, buck converters, and other ways to adjust voltages.
Every other consumer electronic device on the market performs some form of DC voltage conversion. The reason is simple: the voltage you get out of a typical battery or a cheap wall wart is seldom right for every part of the circuit — from motors, to LCD backlights, to latest-generation digital chips. Heck, your circuits might be doing voltage conversions without you knowing it. For example, a typical microcontroller will have a tiny internal charge pump to produce a higher voltage for EEPROM and flash memory. It might also have another regulator to produce a lower voltage for the CPU core. Despite this, voltage conversions remain black magic to most enthusiasts. Antiquated linear ICs, such as LM7805 or LM317, crop up next to ultra-modern 32-bit MCUs. And if a switching regulator makes an appearance in a hobby project, the design is usually copied from elsewhere, with no regard to its suitability for the task at hand. Today, let’s have a closer look at the task of converting DC voltages. These circuits are usually explained using complex terminology and dizzying math, but their operation is not hard to grasp. Let’s start from the beginning.
Resistor-Based Voltage Dividers
The simplest way of deriving an intermediate voltage from a known, stable supply is to use a resistor-based voltage divider. The fundamental circuit, which should be familiar to most hobbyists, is shown below:
Image: Basic resistor-based voltage divider circuit.
With no substantial load connected in the middle, the currents through R1 and R2 are necessarily equal, and each resistor develops a voltage drop proportional to the ratio of their resistances. In particular, if R1 = R2, the voltage at midpoint will be Vin/2. Easy enough. The most severe limitation of this circuit is that it works correctly only if the current sourced through the “load” leg is negligible in proportion to the current flowing via R1 and R2; as soon as this condition is violated, the voltage in the middle begins to depend on what the load is doing too. It follows that impractically low resistances would be needed to supply stable voltages to power-hungry loads, resulting in high waste currents through the R1-R2 path. In practice, it’s most common to encounter resistor-based dividers as bias networks for biasing op-amp inputs or FET gates. In such applications, the loading is practically nil, and the resistances can hover around 10 kΩ to 100 kΩ, resulting in microamp-range waste.
Linear Regulators
Linear regulators are a step up from resistor-based dividers. They use a transistor-based voltage follower to maintain a consistent voltage across the load. The basic idea is to use a MOSFET and a Zener diode to create a stable voltage reference. This setup can be further improved by using an operational amplifier as a voltage comparator to adjust the gate voltage of the transistor, thereby stabilizing the output voltage.
Image: Simplified linear regulator circuit.
Despite their simplicity, linear regulators are not very efficient. They waste energy in the form of heat, which can be problematic for battery-powered devices. Therefore, they are best suited for applications involving tiny currents or very small voltage drops.
Switched-Mode Regulators
Switched-mode regulators, such as buck converters and boost converters, offer a more efficient way to convert DC voltages. These regulators use inductors and capacitors to store and transfer energy, minimizing energy loss.
Buck Converters
A buck converter steps down the voltage from the supply to a lower, tightly controlled output voltage. It uses an inductor to limit the inrush current and a diode to provide a path for the current when the switch is open.
Image: Basic buck converter circuit.
Buck converters are highly efficient but can be complex to design. They are ideal for applications requiring high currents and excellent voltage regulation.
Boost Converters
A boost converter, on the other hand, steps up the voltage from the supply to a higher level. It uses an inductor to store energy when the switch is closed and releases it to the capacitor when the switch is open.
Image: Basic boost converter circuit.
Boost converters are commonly used to power higher-voltage devices from single alkaline cells or to generate higher voltages for specific applications like LCD backlights.
Charge Pumps
Charge pumps use capacitors to transfer charge and create higher or lower voltages. They are simple and efficient for low-current applications but lack voltage regulation.
Image: Basic charge pump circuit.
Charge pumps are ideal for applications where efficiency is crucial, and the current requirements are low.
Remember these 3 key ideas for your startup:
Efficiency Matters: When designing electronic circuits, always consider the efficiency of your voltage conversion methods. Switched-mode regulators like buck and boost converters offer higher efficiency compared to linear regulators, which can save energy and reduce heat dissipation.
Choose the Right Components: For startups and SMEs, using pre-configured modules for voltage conversion can save time and reduce complexity. Companies like CUI and MPS offer ready-made solutions that simplify circuit design.
Optimize for Your Application: Different applications require different voltage conversion methods. For low-current applications, charge pumps can be highly efficient, while buck and boost converters are better suited for high-current applications.
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