In our last article covering communication systems, we discussed some basic concepts and introduced the problem of power conversion and harmonics.
How do these harmonics develop?
Well, let’s look at one common household situation:
In your computer, 120VAC 60Hz is converted into both 12VDC and 5VDC for your motherboard and other system components to use.
For the sake of argument, let’s suppose we rectify and filter down to 12VDC.
How can we efficiently convert from 12VDC to 5VDC? What if we want to convert from 5VDC back to 120VAC 60Hz?
Some might say that we could just use a voltage divider circuit. This would be correct if efficiency was not a concern.
Voltage dividers work by tapping off of a particular resistor node in a circuit composed of multiple resistors. The desired voltage can be obtained by balancing the values of the resistors proportionally to obtain the correctly proportioned voltage output. This works for us only so long as neither efficiency nor thermal waste are a concern. Also, voltage fluctuations at the input will cause similar fluctuations at the output.
What about a similar circuit known as a linear regulator? These use diodes to achieve much the same effect. Instead of a voltage divider circuit, a current limiting resistor is placed in series with multiple diodes. Diodes, once fully conducting, tend to have a steady voltage drop across them. Typical values are around 0.7V. By placing multiple diodes in series, the desired voltage drop can be obtained. By tapping off of these diodes, a relatively constant voltage can be obtained. However, there’s still a lot of thermal losses and inefficiency associated with this.
Our final solution that we’re going to look at is the use of a circuit called the “buck converter.” This circuit uses high speed switching circuits in order to charge and discharge capacitors and inductors. This allows for a highly efficient conversion from one voltage to another. The tradeoff for this is the introduction of a slight ripple in our output signal. It may be only a few millivolts; however, it is still enough to cause concern in some applications.
Similarly, a “boost converter” can be used to convert from 5VDC to 12VDC. This works very similarly to the buck converter; however, the arrangement of components is somewhat different.
What if we are trying to instead convert our 12VDC back into 120VAC? That calls for a completely different type of circuit. In the case we’ll consider now, we’ll use pulse-width modulation. This will use a similar method to the buck converter where high speed switching allows multiple voltages to be output at a high rate of speed. By merging some aspects of the buck/boost converter with a comparator circuit, we can get something that looks very close to our desired signal. However, there will be small ripples present that will produce the higher frequency harmonics we were worried about from before.
Going back to the original problem statement, imagine that our 60Hz sine wave is actually being produced by using a 5Khz switching circuit from a pulse width modulator. If we’re using a 3 stage pulse width modulator (Waveform seen above), this will cause harmonics start to form at around 10KHz and repeating out at much lower power levels much further up in frequency. By increasing the number of PWM stages and improving the filtering, we can get much closer to our desired 60Hz sine wave.
We know that all of this exists by taking our original input signal, modulating it using the desired scheme, and performing Fourier analysis of the resulting equation. If we want a pure 60Hz sine wave coming out instead of the PWM signal, we can filter out the higher frequency harmonics by using a low-pass or band-pass filter.