Power Supply Design

When we talk about power supplies we are talking about the electronics required in providing the proper operating voltages for the device while attached to some power source. Since power sources can be mains or derived from mains power, care must be exercised to ensure that the attached power supply can handle the input voltage. Power supplies can be internal or external to the device. Devices require low-voltage DC power while mains power exists at a much higher AC voltage. Therefore it is necessary to convert AC power to DC.

Power supplies can be considered as having three stages. The first stage converts the incoming AC to DC. The result is a DC voltage but with a high content of AC called ripple that needs to be filtered. The second stage is called an input filter that reduces the ripple to an acceptable level. The output of this stage is unregulated DC voltage. The final stage provides the necessary DC regulation for powering the low-voltage electronics used in the device. In the past, linear regulators were used but they were less efficient and dissipated excessive heat compared to the modern switching regulator. However, their function was the same—providing a regulated DC voltage from an unregulated DC source. The unregulated DC voltage must be sufficiently high so that the regulator can regulate over varying loads. Insufficient unregulated voltage leads to poor regulation and performance irregularities.

Full- and Half-wave Rectification

The device used to convert AC power to DC power is called a rectifier, and there are basically two types of rectifiers that can generate DC power from low voltage single phase AC source. The simplest approach is to use a half-wave rectifier as shown in Figure 1. The input capacitor charges through the diode during every positive portion of the input wave and then discharges through the regulator. Since there are both equal positive and negative portions, charging only occurs half the time which is not very efficient. The ripple content is also very high.

The second approach (Figure 2) adds another diode thereby allowing the capacitor to charge on both the positive and negative portions of the input wave. One diode conducts on the positive portion while the other conducts on the negative portion. This is called full-wave rectification but requires a center-tapped transformer.

By using four diodes (Figure 3), the center-tap can be eliminated. Two diodes conduct on the positive portion while the other two conduct on the negative portion. This is a variation of the full-wave approach. With full-wave rectification, the ripple content is reduced and the ripple frequency is doubled making filtering easier. The average DC content of a full-wave rectifier is twice that of a half-wave rectifier so there are indeed advantages to full-wave rectification at a modest cost in additional diodes.

Power Supply Example

Figure 4 shows a highly flexible power supply that accommodates low-voltage half-wave and full-wave AC inputs as well as low-voltage DC inputs. By limiting the input voltage to less than 30 V, a Class 2 power source can be used since the power supply requires only 10 VA of power. The two AC input connections are indicated with a ~ symbol instructing the installer that the two wires from the secondary of a Class 2 transformer are to be connected here. There is no distinction between the two inputs so there is no restriction as to what secondary wire goes where. These pins are the full-wave bridge inputs. The remaining two pins are for DC connections with the most positive DC voltage being applied at the pin with the bar symbol. The return of the DC supply is indicated by the "0vdc” symbol. You will notice that the DC input circuit is actually a half-wave AC circuit so the possibility exists for AC or DC operation. We will review the possible input configurations.