Computers are digital devices. Digital devices are distinguished from analog devices by their use of digital logic. This is the binary 1s and 0s you’ll so often hear about when discussing the nuts of bolts of a computer. More generally, digital circuits operate in discrete electrical states whereas analog circuits operate over a continuous range of electrical “states” (they aren’t really states at that point though). That is, they use discrete electrical conditions (1s and 0s or voltage signal and no voltage signal) to represent states and decisions. Everything computers can do is based on this single principle.
However without the wide array of analog circuit counterparts, digital circuitry would be fairly mundane, if not wholly useless. With that in mind, I’ll be starting a series here on interesting and useful analog electrical circuits and their very important uses in the digital devices that are so tightly integrated with our every day lives.
The first analog circuit I’d like to talk about is one of my favorites. It’s called a boost converter. One common problem with any electrical device is requiring one level of voltage, but being supplied with another. For the purposes of this discussion, we’ll assume the signals involved are DC-DC or AC-AC and no AC/DC conversion is required.
A perfect example of a situation requiring a boost converter is a cell phone. Most modern cell phone batteries are lithium-ion, which have a nominal cell voltage of 3.7 volts. If everything inside the cell phone can run off 3.7 volts or less, we’re good. Unfortunately, that isn’t the case. Some things like the cellular radio transmitter require more than 3.7 volts to operate. For the purposes of this example, let’s say that this radio circuit requires 5VDC to operate. Thanks to the boost converter, this is very possible.
So how does a boost converter work? The simplest boost converter requires only three components: a switch, an inductor, and a diode. The whole operation depends on the fact that an inductor does not like rapid changes in electrical current. An inductor is an electrical storage device that stores energy in the form of a magnetic field. The idea in a boost converter is to charge up the inductor using the power source, then switch off the power source thereby allowing the inductor’s magnetic field to collapse and create a voltage. The load being driven, which is connected to the inductor, now sees some voltage which is being supplied by the inductor itself and not the power source. The specific configuration of the circuit will determine how much of a boost in voltage our load gets.
You may or may not have guessed that the switch being used here is not like a light switch. In order for this circuit to work, the switch needs to be opened and closed very rapidly. The Maxim MAX757 for example switches at 500kHz. This rapid switching capability is made possible by semiconductors and fancy components like MOSFETs that are typically packed into a tiny integrated circuit like the MAX757. These ICs allow you to provide your own inductors, capacitors and resistors to build the appropriate type of boost converter for your specific application.
One final thing to note is that a boost converter is not a miracle device. We’re not creating more power from nothing here. If the voltage in is 3.7VDC at 400mA (1.48 watts), and we boost to 5VDC, the load receiving the 5VDC will only see 296mA of current. In reality it will likely be less, since the conversion process is not 100% efficient.