Input voltage ripple

We must confirm that the (low-frequency) input voltage ripple is less than 1% (i.e., 0.5% ). 

Once you have a stable clock, you might like to check the jitter. So how much jitter is acceptable In an AC-DC power supply with no PFC correction, especially at low line, the input voltage ripple is quite high. As this instantaneous input voltage moves up and down, a... 

The rule-of-thumb is to pick an output capacitor with a ripple current rating equal to or greater than the worst-case RMS capacitor current calculated above. Its voltage rating is usually picked to be at least 20 to 50% higher than what it will see in the application (i.e. Vin.max for all topologies). The input voltage ripple of the converter is also usually a concern because a small part of it does get transmitted to the output. There can also be EMI considerations involved. In addition, every control IC has a certain (usually unspecified) amount of input noise and ripple rejection, and it may misbehave if the ripple is too much. Typically, the input ripple needs to be kept down to less than 5% to 10% of the input... 

In general, we always need to put a scope probe on the input pin of a switcher IC and confirm that the ripple is within 1% of the input voltage. Otherwise chances are high the control sections will exhibit weird behavior. If not on one prototype, on another Without an actual measurement, you will never know if the problem is just waiting to happen. 

Click the OK button to select all of the runs. First we will plot the input voltage waveform. Add the trace V(VIN). Zoom in on the traces to see the ripple in more detail ... 

Three traces are shown, one for each temperature. The result shows that temperature does not have too much of an effect on the input voltage. Remove the trace and plot the output voltage. Zoom in on the traces and use the cursors to display the magnitude of the ripple. 

Rectification — is defined as the conversion of -> alternating current, AC, into pulsating -> direct current, DC, by any means other than the use of a motor-generator. That is, the AC is primarily converted by use of a -> rectifier into (unfiltered) unidirectional half-cycles. The percentage ratio of the DC output voltage to the peak AC input voltage of a certain rectifier device is called rectification efficiency. A circuit containing parallel -> capacitance, sometimes in combination with series inductance, is afterwards frequently applied as rectification filter in order to smooth the ripple in the DC current output. A combined device of rectifier and filter is called rectifier-filter system. 

We are ignoring the voltage ripple on the input terminals of the converter, and therefore we will take this as the dc input to the converter stage. So the duty cycle at minimum input voltage is... 

The most basic question in design invariably is — what input voltage represents the worst-case point at which we need to start the design of the magnetics (from the viewpoint of core saturation) For the forward converter choke, this should be obvious — as for any buck converter, we need to set its current ripple ratio at around 0.4 at Vinmax- But coming to the transformer, we need some analysis before we can make a proper conclusion. 

Consider a diode half-wave rectifier with a filter capacitor C and a resistor R as shown in Fig. 10.21(a). When the input voltage V) is positive, the diode conducts and charges the capacitor C. When the diode moves into cutoff, the capacitor discharges through the rest of the cycle untQ the time at which V) exceeds the voltage at the capacitor. The output waveform for the rectifier with a capacitor is shown in Fig. 10.21 (b). The ripple volts e W is given by... 

Ripple rejection in decibels. This specihcation is for the line regulation, indicating the regulator s attenuation of the line voltage ripple. It is predicated on particular AC input signal levels and frequencies with a specified value of bypass capacitor. 

The DC filter choke is used for ripple voltage and current reduction on the output of the switching power supply immediately following the existing filters. It is also used as an EMI filter on switching power supplies that have a single power line on the input, such as battery and distributed power systems. 

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