An interesting trade-off arises when you choose input filter capacitors for a low-power, offline power supply. You have to choose the capacitor’s ripple current as a compromise” title=”ripple current”>ripple current rating to suit the voltage range required for the power supply to operate. By increasing the input Capacitor, you can get more ripple current At the same time, the working input Voltage range of the power supply can be reduced by reducing the voltage drop of the input capacitor. This will affect the transformer turns ratio of the power supply and various voltage and current stresses. The larger the capacitor ripple current rating, the smaller the stress, The power efficiency is also higher.

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

Figure 1 Full-wave bridge used in many offline designs

Figure 2 depicts a voltage doubler” title=”voltage doubler”>voltage doubler rectification configuration used in many 115/230 VAC applications. If you have a 230 VAC application, your input The 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 will boost the rectified voltage to near the 230 VAC input level. We can design a power supply specifically for 230 VAC line voltage to reduce the rectified voltage range over which the power supply operates. We usually use a jumper or switch to switch between different rectifier configurations. This approach The only downside is the occasional case of artificially multiplying the 230 VAC input, wreaking havoc ON the power supply. Figure 2 shows some waveforms for the voltage doubler circuit. There is no charge between the Capacitors. Two rectifiers alternately apply the input voltage to each capacitor .In one cycle, each capacitor is charged to the peak line voltage so that they each have a portion of the line frequency ripple. Since the capacitors are charged out of phase, their sum has a ripple frequency twice the line frequency.

Figure 2 Voltage doubler reduces power line voltage range

Figure 3 shows the normalized voltage drop in uF/W for the four rectifier/input voltage methods. There are three full-wave bridging methods for Low Line USA (108 VAC/60 Hz), Low Line Japan (85 VAC/50 Hz) and Low Line Europe (216 VAC/50 Hz). In addition, there is a low line voltage Japanese voltage doubler. In the case of a full-wave bridge, the normalization process simply divides the capacitance by the power. In a voltage doubler, the normalization method is to divide 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 just need to read the uF/W of the input capacitance. Finally, by multiplying by your power, you can denormalize.

Figure 3. Large Capacitors Reduce Input Line Range and Improve Efficiency

After that, you can use Figure 4 to calculate the capacitor’s ripple current rating. Figure 4 shows the normalized ripple current versus normalized input capacitance. Interestingly, the ripple current is not closely related to capacitance. This is because during discharge, the current is determined by a nearly constant current from the load. It is only during the charge cycle that the current is very different. This occurs when the capacitance (uF/W) decreases and the progressive ripple current increases. With larger capacitance and smaller conduction angle, the peak current is higher. Note that this graph includes line frequency ripple current only and does not include high frequency power supply ripple current effects.

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

In conclusion, it is important for the designer to make some tradeoffs when choosing the input capacitor and rectifier configuration. If a full-wave bridge is selected for wide-range applications, 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. Selecting the correct input capacitor based on the graphs provided in this article can limit the operating voltage range to some extent. Next time, we’ll discuss an inexpensive power protection latch circuit, so stay tuned.

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