Designing POL regulator input filter

Ericsson POL regulators are implemented through the use of non-isolated synchronous buck topology shown in figure 1. During normal operation of a buck power stage, QH and QL are alternatively switched on and off with the on and off times governed by a control circuit with a fixed frequency PWM scheme. Output current for a buck power stage is smooth as result of the inductor/capacitor combination on the output side. But input current for a buck power stage is pulsating or chopped due to the power switch QH current that pulses from zero to full load every switching cycle. Obviously, the input capacitor is critical for proper operation of the regulator and to minimise noise emissions from a switching regulator.

Figure 1: Simplified schematic and input waveforms of a buck converter.

In many applications, a fairly conventional intermediate bus architecture (IBA) is used as shown in figure 2. In an IBA, a board-level intermediate bus converter (IBC) feeds multiple POL regulators which are located in proximity to the load circuitry and supply the final operating voltages. All these switching converters generate ripple and noise on the common DC input bus which should be suppressed. If it isn't filtered, input ripple and noise of a regulator can reach levels high enough to interfere with other devices powered from the same source. In addition to the input ripple and noise generated by the POL converters, the IBC has its own output voltage ripple and noise.

Figure 2: In an IBA a board-level IBC feeds multiple POL regulators.

So the input filter on a POL regulator may play two important roles. One is to prevent electromagnetic interference, generated by the switching source from reaching the power line and affecting other equipment. The second purpose of the input filter is to protect the converter and its load from transients that appear in the input voltage thereby improving the system reliability.

So what are the sources of input ripple and noise in POL regulators and how can input filters be better designed to attenuate its occurrence?

Stability
A filter having attenuation sufficient to meet noise and ripple specifications is constructed and added to the input. If the input filter consisting of only capacitors (C), stability is not a problem. If the input filter also includes inductors (LC), the stability must be checked: the input filter changes the dynamics of the regulator. The output impedance may become large over some frequency range, possibly exhibiting resonances. The audio susceptibility may be degraded. The problem is that an LC input filter can affect the dynamics of the converter, often in a manner that degrades regulator performance.

An important yet often overlooked aspect of input filter design is meeting the Middlebrook criterion. According to the criterion, the input filter does not significantly modify the converter loop gain if the output impedance curve of the input filter is far below the input impedance curve of the converter, as reflected by the Equation 1: