When you bypass the chips in your power-supply circuit, use the normal guidelines of putting the capacitor close to the power pins and employ ceramic capacitors, preferably surface-mount, which have low ESR and ESL. Bypassing the chips that being fed by the power supply will not reduce the noise at the supply, but it will be reduced at the power pins of the chips. Perhaps less obvious, you can also reduce noise via proper bypassing of the control chips in your power-supply design. If you’re cycling power to take a measurement and then shutting down, you have to trade off the filtering effectiveness with your startup time requirement. Adding any filter may increase the startup time and transient response of your system. Using twisted-pair wiring is a good way to reduce inductance in order to prevent ringing and overshoot spikes. Here you use shielding to protect the power-supply circuit from external influences.Īlso note that your circuit board traces have inductance, and you might need to tailor that with power planes and trace widths. Another output noise source might be electromagnetic coupling from the outside world. The magnetic coupling with the output trace or wire and the bead will attenuate the noise. One filter useful for switching spikes and other high-frequency output noise are ferrite beads. That will create a dc loss term, but the resistor also adds damping to your output filter. If the supply is providing low currents, you may be able to use a resistor instead of an inductor. Thus, it may make your supply unstable or produce unacceptable ringing after transient load changes. The problem with adding LC circuits is that they have a natural resonant frequency. In essence, you’re increasing the high-frequency output impedance of the supply so that you can more effectively filter it with smaller capacitors. The inductor passes dc current with negligible loss, while providing a high-frequency impedance that the capacitor can react against to filter out the noise. In addition to the natural output capacitance of the power supply, you might add a series inductor and another filter capacitor to further reduce output noise (Fig. If you reduce it radically, say, by replacing electrolytic capacitors with ceramic ones, you may make your power supply unstable. Selecting capacitors with lower ESR and ESL will lower noise, but be careful, some power-supply circuits use the ESR to provide the error signal for feedback. Increasing the value of the output capacitance will reduce noise.īe aware that capacitors have both an equivalent series resistance (ESR) and an equivalent series inductance (ESL) (Fig. Indeed, you can consider the output capacitors part of a filter that reacts against the output impedance of the power-supply circuit. You can use a filter to remove noise from a power supply just like you use filters to remove noise from a signal. There are three common ways to deal with this noise, that often help with ripple as well: Filtering On top of any inherent ripple in the output will be random noise generated by the control chip voltage reference and all other sources of thermal, shot, and flicker noise. You can also use larger input capacitors, which will reduce the ripple on your dc input bus, so the PSRR of the control loop will apply to a smaller deviation. The higher the gain of the control loop, the smaller the error at the output input ripple is just another error that must be dealt with by the loop. A primary way to improve line regulation is to increase the gain of the control circuit. It’s not solely a function of the control chip as much as the workings of the entire circuit.Ī PSRR of 60 dB means any deviation at the input will be attenuated by 1000 at the output. This is a similar concept to power-supply rejection ratio (PSRR)-how much of the input signal a linear regulator lets pass to the output. The amount of input-referred ripple will be governed by the line regulation of your design. No matter how good the switching chip you use, a little of this frequency will bleed though the switching circuit. An ac-dc supply will have a 50-, 60-, or perhaps 400-Hz input frequency. The source of ripple is the periodic input frequency, as well as the switching frequency of the control chip. Both phenomena are an unwanted signal superimposed on the pure perfect dc output you want (Fig. Some engineers make a distinction between output ripple and output noise. While there are FCC limits on the electromagnetic interference (EMI) radiating out into the air as well as the conducted noise that your design injects back into its input, your first noise problem is getting the noise low enough in your outputs. Noise is a constant problem in power-supply design. This article is part of the TechXchange: Delving into EMI, EMC and Noise
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