Compromise selection of line voltage range for input capacitor ripple current

An interesting trade-off occurs when you select input filter capacitors for a low-power, offline power supply. You are compromising the capacitor's ripple current rating with the voltage range over which the power supply operates. By increasing the input capacitor, you can get more ripple current while also reducing the supply's operating input voltage range by reducing the voltage drop across the input capacitors. Doing so affects the power supply's transformer turns ratio and various voltage and current stresses. A larger capacitor ripple current rating reduces stress and makes the power supply more efficient.

Figures 1 and 2 show two rectifier configurations used in offline power supplies. Figure 1 is a full-wave bridge where the AC input voltage is simply rectified and then fed to the capacitor. This circuit is common in wide range AC and 230 volt AC applications. The capacitor is charged to the peak of the sine wave and then discharged for most of the half cycle. The capacitor ripple current consists of two parts: first the charging period, where the current is determined by the capacitor value and the applied dV/dt, and second the capacitor discharge. The power supply acts as a constant power load, so the capacitor discharges at a nonlinear rate, which is calculated as: W = ½ * C *V^2 = P * dt.

Figure 1. Full-wave bridging used in many offline designs

Figure 2 depicts a voltage doubler rectifier configuration that is used in many 115/230 VAC applications. If you have a 230 VAC application, your input stage needs to handle the maximum input voltage (265 VAC) multiplied by the crest factor, which is close to 400 volts. When used with a 115 VAC input, the voltage doubler steps up the rectified voltage to close to the 230 VAC input level. We can reduce the rectified voltage range over which the power supply operates by designing a power supply specifically for 230 VAC line voltage. We usually use a jumper or switch to switch between different rectifier configurations. The only drawback to this approach is that occasionally the 230 VAC input is artificially doubled, wreaking havoc on the power supply. Figure 2 shows some waveforms from the voltage doubler circuit. There is no charge between the capacitors. The two rectifiers alternately apply the input voltage to each capacitor. During one cycle, each capacitor is charged to the peak line voltage, so they each have a line frequency ripple component. Since the capacitors charge out of phase, their sum has a ripple frequency that is twice the line frequency.

Figure 2 Voltage doubler reduces power line voltage range

Figure 3 shows the uF/W normalized voltage drop for four rectifier/input voltage methods. There are three full-wave bridge methods for low-line US (108 VAC/60 Hz), low-line Japan (85 VAC/50 Hz), and low-line Europe (216 VAC/50 Hz). There is also a voltage doubler for low-line Japan. For the full-wave bridge, normalization is simply dividing the capacitance by the power. In the voltage doubler, normalization is dividing the capacitance of one of the two series capacitors by the power. To use this graph, first determine your rectifier configuration and choose an acceptable voltage drop. After that, you simply read the uF/W of the input capacitance. Finally, de-normalize by multiplying by your power.

Figure 3 Large capacitors can reduce the input line voltage range and improve efficiency

You can then use Figure 4 to calculate the ripple current rating of the capacitor. Figure 4 shows the normalized ripple current versus normalized input capacitance. Interestingly, the ripple current is not closely related to the capacitance. This is because during discharge, the current is determined by a nearly constant current from the load. Only during the charging cycle is the current very different. This occurs when the asymptotic ripple current increases as the capacitance (uF/W) decreases. The peak current is higher for larger capacitance and smaller conduction angles. Note that this graph includes only the line frequency ripple current and does not include the effects of high-frequency power supply ripple current.

Figure 4 Increasing uF/W will not significantly increase the input capacitor ripple current

In summary, it is important for the designer to make some compromises when selecting the input capacitor and rectifier configuration. If a full-wave bridge is chosen for a wide range application, the power supply may need to operate over a 4:1 input range. If the designer chooses to use a voltage doubler in the design to reduce this range, there is a risk of overvoltage due to user error. By selecting the correct input capacitor based on the curves provided in this article, the operating voltage range can be limited to a certain extent. Next time, we will discuss an inexpensive power supply protection latch circuit, so stay tuned.