LC Filter for Power Supply Design Tips

Key Takeaways

• Understand the basics of an LC filter.

• Learn why an LC filter for a power supply  is important.

• Pick up tips for designing a power supply LC filter.

AC noise affects the precision, stability, and functionality of various components. 

I have always assumed that high-quality coffee beans and an expensive coffee machine are all it takes to make the perfect espresso. After watching a world-class barista in action, I realized I was wrong. Apart from the sublime techniques, the water filter determines the end result of the espresso. 

Witnessing this master was eye-opening in terms of how meticulous you need to be to make a perfect cup of coffee. The experience also highlights a similar and often overlooked aspect when designing a power supply. There might be no water filter involved, but you will need to place an LC filter for the power supply. 

What is an LC Filter?

If you need a basic refresher, an LC filter is a low pass filter built with a capacitor and inductor network. In power supply and electronic circuits, LC filters are used to suppress unwanted AC noise while allowing the desired dc voltage and low-frequency power components to pass to the load. Depending on the topology, LC networks can also be configured as high pass filters or a band pass filter for different frequency range requirements.

A basic LC low pass filter circuit schematic.

At higher frequencies, the inductor acts as a choke, which blocks AC components from passing through. Meanwhile, the capacitor’s impedance decreases and forms a pathway for high-frequency components to pass through. The idea of the LC filter is to prevent high-frequency AC components from flowing to the load. 

In practical filter designs, parasitic inductance and capacitor equivalent series resistance (ESR) influence attenuation performance at higher frequencies. Capacitor ESL and PCB parasitics become increasingly important in high speed switching regulators where layout inductance can significantly alter filter behavior compared to ideal calculations.

Why Do You Need an LC Filter for Power Supply?

In an ideal world, the voltage from the output of a DC power supply is a perfect horizontal line when observed on an oscilloscope. However, the world is far from ideal and the voltage is never a perfect line. 

The DC power supply is susceptible to AC ripple and noise, particularly when using a switching power regulator. The switching and transition frequency produces ripples above 500 kHz, which are often coupled to the output voltage. In a switching power supply, the LC output filter must also be designed carefully to avoid interaction with the regulator compensation network. An improperly damped filter can reduce phase margin and create instability or excessive transient ringing on the output voltage. Even if you’re sticking with a linear regulator, you’ll also have AC components from the mains frequency or digital interference. 

Just like how unfiltered water could ruin a cup of espresso, the AC ripple that bleeds into the regulator output could affect system functionality. Modules like ADC, DAC, and Op-amps are particularly sensitive to AC noise on the power supply. The accuracy of these modules can be compromised, and in some instances, the components may fail to function correctly. 

Power supply noise doesn’t only affect analog parts. In one study, it was found that power supply noise reduces the clock frequency of a high-speed microprocessor by 6.7%. Even if the circuit could tolerate the AC noise, it can still be radiated as EMI and affect other electronic products. 

This becomes increasingly important in high voltage and high power systems where conducted and radiated emissions can propagate across a wide range of frequencies.

Power Supply LC Filter Design Tips

Take my word that power supply noise is annoyingly disruptive. You can’t predict how and when it will affect the circuit. Unlike a badly-made cup of coffee, the implications of power supply noise disrupting systems in the field are much larger.

Therefore, you’ll want to ensure that the power supply is designed with an LC low pass filter. Here are some tips for proper implementation:

Positioning

Naturally, you’ll think of placing the LC filter between the output of the regulator and the load. While there’s nothing wrong with that placement, you can achieve better noise reduction by placing the LC filter at the input if you’re using a buck regulator.

For buck regulators, most of the conducted noise stems from switching transitions at the input and output nodes. Placing an LC filter at the input voltage stage can help suppress conducted EMI before it propagates through the system, while output filtering reduces ripple delivered to sensitive loads.

Low ESR and ESL Capacitors

To suppress high-frequency noise efficiently, you’ll want to opt for a capacitor with low equivalent series resistance ESR and low ESL values. Low ESR capacitors reduce ripple voltage and improve attenuation at higher frequencies, though excessively low ESR can also increase resonance Q and produce underdamped filter behavior if insufficient damping is present. If it’s hard to achieve, consider using two capacitors in parallel for a lower combined ESR and ESL value.

For ceramic capacitors used in high-frequency filters, remember that dc voltage bias can significantly reduce effective capacitance. A capacitor connected to a 12 V rail may provide substantially less capacitance than its nominal datasheet value, shifting the resonant frequency and cutoff frequency of the filter network. 

Low ESR and low ESL capacitors improve high-frequency attenuation and reduce ripple propagation.

LC Values

Choosing the value of the inductor and capacitor seems to be a simple process. After all, what could go wrong with abiding by the following formula? 

f = 1 / (2π √LC)

However, there’s much more than putting together two passive components to block high-frequency noise. The resonant frequency of the filter determines how effectively unwanted noise is attenuated across the intended frequency range. Selecting inappropriate LC values can create peaking, insufficient attenuation, or unstable transient behavior depending on the input voltage, load current, and regulator topology. For a start, you’ll want to choose an inductor with a considerably higher value than the inductance in series to the power supply.

Match it with the appropriate capacitor value to achieve the desired resonance frequency and attenuation response.

You’ll want to be wary of an underdamped response of the LC filter as it could result in peak amplitude at the frequency. To prevent peaking, a parallel damping RC element is added to the LC filter.

It helps if you’re able to simulate the LC filter with an industry-leading PCB design and analysis software. OrCAD PCB Designer provides the layout needed to create your circuits and pairs well with PSpice to ensure the LC filter’s response is critically damped and attenuates the right frequencies. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts. You can also visit our YouTube channel for videos about Simulation and System Analysis as well as check out what’s new with our suite of design and analysis tools.