DC-DC Converters

Lower or Raise DC Voltage, Drive Devices Off a Battery, and Create Smart Battery Chargers

A DC-DC Converter takes a variable input voltage and changes it to a fixed output voltage. When the converter supports constant current, it can then further reduce the voltage so that the current never exceeds a given threshold. These regulators work with DC. They cannot work with AC.

For AC circuits, transformers are used to change one voltage into another using alternating magnetic fields. Since DC does not use alternate current and thus does not produce alternating magnetic fields, transformers do not work for DC.

DC-DC Converters change the voltage electronically by using components that can temporarily store energy, such as capacitors or coils. In combination with a high frequency switch (transistor), they can add or reduce voltage as needed, producing a constant voltage (CV).

The typical switching frequency is in the range of 50-500kHz, and depending implementation, this frequency can either be fixed or variable.

Why Should I Care About Switching Frequency?

In hobby projects, you typically do not need to care much about the switching frequency of DC-DC Converters, and whether they are fixed or variable. Most cheap hobbyist DC-DC Converters are fixed frequency PWM converters.

DC-DC Converters with fixed frequency typically use the same PWM (pulse width modulation) that you may have used in your electronics projects to dim LEDs. Since the frequency is fixed and well above audible frequencies, converters will never emit annoying hissing sounds (coil whine). Their weak spot are light loads: due to the fixed frequency and PWM, the pulse width can only be shortened so much. With light loads, energy is lost and turned into heat, and efficiency decreases.

In DC-DC Converters with variable frequency, typically PFM (pulse frequency modulation) is used: the pulse width stays the same but the frequency of pulses changes. They are more efficient with light loads as the frequency can be easily lowered in a wide range. This can lead to a different problem: when the frequency is lowered so much that it enters audible ranges, these converters can produce an audible annoying high pitched hissing sound. This is also known as Coil Whine and can be produced by other parts of circuits as well when frequency drops into audible ranges.

High switching frequency also helps reducing component size (and cost) because due to the high switching frequency, only relatively small amounts of energy need to be stored in capacitors or coils.

Lower switching frequencies require larger components but may create a better conversion efficiency and less EMI (radio interference).

For example, an LM2853 (designed to lower voltage from 5V to 3.3V at a maximum current of 3A), running at a switching frequency of 550kHz, requires a total component area of 206mm2 with larger components and an inductor height of 4.6mm. The same conversion can also be done by a LM2833Z at a switching frequency of 3MHz. Now, the total component area is reduced to just 57mm2, and the inductor height reduced to 2mm.

While all of this may be crucial in microelectronics and consumer devices such as tablets and phones, for hobbyist projects it does not matter much. That’s why most DC-DC Converter breakout boards are relatively bulky and use switching frequencies in the lower range.

There are two types of regulators: Buck (to lower the input voltage) and Boost (to raise it).

With pure Buck or Boost, the output voltage must always be higher (Buck) or lower (Boost) than the input voltage.

For example, even if a Buck converter supports an input voltage range of 8-32V and an output voltage range from 1.25-28V, once you actually supply a voltage, the output voltage must always be lower than the input voltage. If you supplied 10V, you now can only produce an output voltage of 1.25-10V (practically rather 1.25-9.5V as there needs to be a certain voltage difference between input and output).

This limitation does not exist for combined Buck-Boost converters. They can always accept any allowable input voltage and produce any desired output voltage within the supported range: such converters are smart enough to decide whether the input voltage needs to raise, stay the same, or fall to produce the desired output voltage.

Buck Or Boost: Lower Or Raise Voltages

DC-DC Converters can lower or raise the voltage:

  • Buck (lower the voltage): A *buck converter decreases the input voltage. This is the most common converter which is typically used to supply microcontrollers (which require very low voltages) from powerbanks or car batteries.
  • Boost (raise the voltage): A boost converter increases the input voltage. This way, you could i.e. run a 5V microcontroller off a single 3.7V lithium-ion battery. Boost converters can be used in many places, i.e. if you want to run a series of LEDs from a battery, you could “pump up” the input voltage.

There are also combinations of both: Buck-Boost-Converters convert a DC voltage to another DC voltage, regardless of whether the input voltage is lower or higher than the output voltage.

DC-DC Converters are often used to stabilize the voltage. Lithium-Ion batteries, for example, can supply a wide range of voltage between 3.7V and 4.2V, based on their charge state. If you wanted to run a voltage-sensitive device like a microcontroller off such a battery, you could either use a Buck (to lower the voltage to precisely 3.3V for an ESP8266) or a Boost (to raise the voltage to precisely 5V for an Arduino).

Constant Current (CC)

Simple DC-DC Converters produce a constant voltage (CV): regardless of input voltage, the output voltage is constant.

More advanced DC-DC Converters can also produce a constant current (CC): the output voltage is set to a given maximum but is automatically lowered to a level where a constant current flows. A constant current can be useful for a variety of use cases, i.e.:

  • Protection: CC can protect you from accidental shortcuts: if the output is shortened, only a safe maximum current will flow, and the DC-DC Converter will automatically lower the voltage to almost 0V in this case, protecting you from burning wires and damaged components.
  • LED: LEDs are current-driven. They need a certain current to light up brightly. This is why you typically add a current-limiting resistor to LEDs. Instead, you could also supply the LEDs directly from a CC power source. This is even better than using resistors because not every LED is equal, and what really matters in the end is the exact current that is supplied. A resistor is always just an approximation.


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(content created Feb 20, 2024 - last updated Feb 27, 2024)