#Pi filter circuit design how to#

A mid-range frequency transient response from the LC filter poles.High frequency ripple as seen in the red curve above.Here, we have multiple contributors to the transient characteristics observed above: What Contributes to This Transient Response? This could produce a large current spike that could damage your load. In certain cases, the overshoot can reach as high as 50% of the load current when the converter switches between two voltage states, i.e., when switching between two PWM frequencies or duty cycles. In fact, the transient overshoot depends on the rise time of the PWM signal and the parasitics in the MOSFET, as well as the poles present in the filter circuit. This transient response is very important. Power output from the DC/DC converter circuit shown above. However, there is a clear low frequency transient response as the converter switches from OFF to ON. The filter does a decent job of cleaning up the switching noise from the converter. From this result, we can compare the unfiltered output (red curve, top graph) with the filtered output (blue curve, top graph). The image below shows a transient simulation showing the voltage across the capacitors (top graph) and the current delivered to the load (bottom graph). First, we’ll look at the actual transient response with Altium Designer’s new simulation dashboard feature, then we’ll look at a range of filter component values that give the lowest noise. Rather than focusing on the allowed range of PWM or passives values that give a specific power output, we want to focus on the range of filter component values that give us the lowest noise. Buck-boost converter SMPS schematic with output filter.įinally, we have the following parameters on the PWM: 100 kHz switching frequency, 10 ns rise time, 30% duty cycle. The filter has a standard pi-filter topology to provide low-pass filtering. In this circuit, the output capacitor is part of the switching power supply output filter. I’ve highlighted two sections: the switching converter section (in green) and the output filter section (in red). The image below shows a switching buck-boost converter schematic with a power PMOS transistor (you could use NMOS and change the V1 and V2 polarities). The output filter then removes the higher frequency switching noise on the output from the filter, giving clean DC power to the load. The reason for this is that the function of a switching converter is to exchange low frequency ripple from AC-DC power conversion into higher frequency switching noise from a switching transistor. This can be as simple as a shunt capacitor, although the typical method is to place a pi-filter to shunt AC noise to ground. The output filter on a DC/DC converter (whether buck/boost or other topology) is a low-pass filter.

Starting a Switching Power Supply Output Filter Design

#Pi filter circuit design how to#

As an example, let’s look at a buck-boost converter topology to see how to implement an output filter for a switching power supply. As I’ve discussed in an earlier article on this blog, and as we’ll see from some simulation results, reducing noise depends on the values of the components in the output filter and the inductor in the circuit. In this article, I’ll show how a switching power supply output filter can be used to dampen output noise and how you can use some simulation tools to optimize your filter design for low noise. Keeping the output noise-free and stable might require using an output filter, which can be implemented using passives in your PCB layout. It’s also ideal to dampen the effects of any residual ripple from rectification or remove any noise on the input. The goal in designing these systems is to ensure stable DC power delivery to the rest of your system with minimal noise. Switching power supplies come in many forms, such as in a high-power benchtop lab power supply, or embedded onto a PCB with specialty ICs and passives.