Power supply filtering
Can anybody suggest me how to design power supply filtering? I have to convert +12 volts to +5 volts. and +5 volts to -3.3 volts. I need to understand terminology first. I mean to say, when you put capacitors to ground what do you call it as? is it dc decoupling capacitor or noise filtering capacitor or power line filtering capacitor? What kind of capacitor we can use 10 uF, 0.1 uF, 10 nF and why? When you put series inductor what do you call it as ferrite beads, common mode choke or inductor? How the impedance of the inductor can matter at higher frequency and lower frequency? How to choose inductor value and capacitor value? Any particular topology?
I will appreciate all the replies. Thank you in advance.
That is a long list of questions!
Ferrite beads are used primarily to reduce RF leakage from one side of a unit to the other side. If you had a receiver/transmitter, for instance, there might be power leaking from the transmitter into the receiver along the power supply lines. You would not want to use high Q inductors dropped along the power supply line, because the line length might resonate and the inductor would then not suppress the RF leakage. If you use a ferrite bead, that acts both inductively and lossy, it will have a very low Q and will not resonate, and the inductive part will keep the RF from travelling.
As far as why us use three different capacitors, it is usually because you might need to suppress noise at different frequencies. The 10 uF big cap might reduce AC line hum. The smaller capacitor might reduce oscilations in the VHF/UHF region. And the smallest capacitor might be there to reduce oscillations or suppress noise in the microwave region. (Big capacitors have lots of metal plates, and therefore too much inductance to be effective at microwave frequencies).
Hi biff44,
When you design a filter, do you use high Q inductor or low Q inductor? Also, what I understand that when you have smaller capacitor like 10 nF, it can reduce high frequency noise because it gives low impedance path to high frequency signals from the formula of Z = 1/2*pi*f*C. Is that correct?
Capacitors to ground can be all 3, amount of capacitance is dependent on the ripple, you want to size your capacitor to the frequency of ripple. Also what you call the inductor is dependent on its function, if its a choke its a rf choke and is there to block EMI, if its and output inductor, its an output inductor and its value is typically dependent on the topology and leakage spikes (think forward converter)