A switched DC-DC converter is a circuit that transforms voltage using fast switching, inductors, and capacitors to do so with much higher efficiency than a linear regulator. They are the foundation of almost all modern power supplies.
In the previous post, we saw that linear regulators have a serious problem: inefficiency. To drop from 12V to 5V, they dump the excess as heat.
But what if, instead of braking the energy, we simply turn it on and off very quickly?
Welcome to the world of switched DC-DC converters (switching regulators). Here, we don’t waste excess voltage; instead, we use switches, inductors, and capacitors to chop up the energy and reassemble it as we wish. This is the true foundation of modern efficiency.
The concept: Power PWM
Imagine you want a light bulb to shine at half intensity.
- Linear method: You add a resistor. The resistor gets hot.
- Switching method: You turn the switch on and off very quickly (50% ON time, 50% OFF time). The bulb receives energy half the time, so it shines at half brightness. Furthermore, the switch does not heat up, because when it’s open, no current flows, and when it’s closed, it has no resistance.
DC-DC converters use a transistor (MOSFET) that opens and closes thousands of times per second (kHz or MHz).
Buck converter (step-down)
The buck converter is the efficient equivalent of the 7805. It is used to step down voltage (
Its operation is based on filling a bucket (the inductor) in sips.
Switch closed (ON): Current flows from the input through the inductor and the load. The inductor stores magnetic energy and opposes a sudden rise in current.
Switch open (OFF): We cut off the input. But the inductor does not want the current to stop. So it releases its stored magnetic energy to continue pushing electrons towards the load through the diode.
Result: At the output, we have a lower average voltage, but with higher current capability.
Conservation of Power: In a linear regulator, if 1A at 12V goes in, 1A at 5V comes out (and you lose 7W). In a buck converter, if 12V goes in and 5V comes out, the output current increases!
This calculation assumes 100% efficiency. We haven’t created energy: we trade voltage for current capacity, always within the limits of the converter and the load.
The most famous module for this is the one based on the LM2596 chip.
Boost converter (step-up)
Here comes what seems like witchcraft to beginners. How can we get 12V from a 3.7V battery? Do we create energy from nothing?
No, we use the boost converter. What we do is trade voltage for current. We increase the voltage, but reduce the available current.
The trick is to use the inductive kickback from the coil:
Switch closed (ON): We connect the inductor directly to ground (a controlled short circuit). Current rises significantly, and the inductor stores a lot of magnetic energy. The diode prevents the output capacitor from discharging backward.
Switch open (OFF): We suddenly open the circuit. The inductor, whose current path has been cut, generates a high voltage spike (its stored energy plus the source voltage) forward, passing through the diode and charging the capacitor to a voltage higher than the input.
The important idea is that (of course) we are not creating energy. What we are doing is transforming it: we increase the voltage at the cost of having less usable current at the output and incurring some small losses in the process.
The typical modules for this are the XL6009 or MT3608.
Buck-Boost (step up and step down)
There are hybrid converters that can maintain, for example, a stable 12V output whether the input is 9V (they step up) or 14V (they step down). They are ideal for car batteries, whose voltage fluctuates widely.
Efficiency vs Noise
If they are so wonderful, why don’t we use them all the time?
- High efficiency: Typically between 80% and 95%. They barely heat up compared to linear regulators.
- Versatility: They can increase voltage (boost), which linear regulators cannot do.
- Size: For high currents, they are much smaller because they don’t need giant heatsinks.
- Electrical noise (ripple and EMI): Because they switch at high frequency, they generate noise on the power line.
- A motor or an LED can tolerate some ripple, depending on the application.
- For audio or precision measurement, you must check the noise spectrum and add filtering or a post-regulator when needed.
- Complexity: Designing them from scratch is difficult (calculations for inductors, low-ESR capacitors, etc.). Fortunately, we can buy ready-made modules.
Which one to choose?
| Need | Recommended Component | Why? |
|---|---|---|
| Step down voltage with low power dissipation | Linear (7805 / LDO) | Simple and low switching noise. |
| Step down voltage with high linear losses | Buck | Improves efficiency and reduces heat. |
| Step up voltage (from 3.7V to 5V or 12V) | Boost (MT3608) | Raises a DC voltage above the input. |
| Power audio, radio, or precision circuits | Depends on noise requirements | May combine a converter, filtering, and a linear regulator. |
