I was trying to fix a friend's computer sub-woofer and when I tore it down I looked at all the components that were the usual suspects, like fuses and voltage regulators. They looked good.
There weren't any scorching, melted parts and nothing was warm. The board had power too, so I had to turn to the almighty internet. It turns out that these Sony wireless subwoofers are notorious for having bad Buck, or down, converters in the circuitry. The part only costs about $2, but it is quite small. I don't have the means to install a new one, except over at the SpaceFab's lab and with some help. (My soldering skills aren't the greatest, especially on microscopic things).
In any case, I decided to take a look and figure out how this little circuit works. If you wanted to simply reduce the voltage of a circuit, resistors would do, but it always uses power and gets hot. This circuit allows you to lower the voltage and increase the current at the same time without waste or heat.
Above is a diagram of a basic Buck circuit. It has a voltage source at the left, a switch, IRF820, in this case, a MOSFET, which is a type of high power transistor, which is just a type of switch. Then we have the inductor L1, diode D1, capacitor C1 and load (the thing you want to power, in our case the speaker) R1.
When the switch is closed, the battery supplies a current which grows (the growth is slowed by the back EMF of the inductor, L1,(a magnetic field is generated by the current through the coil that slows the current increase).
When the switch opens and closes, the capacitor, C1, keeps supplying voltage to the load R1, but the problem is that as you are opening and closing the circuit, the current will be huge to recharge the capacitor, since there isn't any resistance, and you're going to fry something, so you need a way to slow the current rise. That is where the inductor, L1, comes in. It slows the current rise and drop. The issue with the inductor is that when you open the switch, the current will still want to continue until the magnetic field in the inductor dissipates. This can have the undesired effect of electricity sparking or jumping the switch, so we put in a diode, D1, (a diode is a one way street for electricity) to give a second path for the current to flow.
The overall voltage that your load sees is controlled by how much of the time we leave the switch closed, compared to how much we leave it open. For example, if we have the switch in the closed position for 1/4 of the time, we say that we are running at 25% duty cycle and the voltage at the load R1 is 1/4 of the original input voltage. The PWM is pulse width modulation. These pulses control how long the switch stays closed and can be set by a timing circuit.
Pretty cool little circuit...the only issue I have with this one is that it is too tiny for my home soldering equipment.
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