Post by thread:[EE] Snubber capacitor

There are a few considerations that I can think of: 1) Safety rating - as Dwayne pointed out, capacitors which are going to be placed between the AC line and ground or AC line and neutral require special safety ratings against fire and leakage current. This does not apply to your situation. These ratings are referred to as X1 or X2 or Y1 or Y2 depending on the exact application (voltage, surge rating, and whether line to GND or line to neutral) 2) capacitor life - when applying high current pulses to capacitors, there can be internal self-heating of the capacitor as well as possibly mechanical stress on the dielectric or plates. Some types of capacitors will not handle this well and will have shortened life. 3) voltage rating - the cap must be able to handle the peak voltage seen during the spikes 4) ESR and ESL - if the cap has too high an effective series resistance or inductance (ESR or ESL), then it will not behave like an ideal capacitor in the snubber. 5) Nonlinear dielectric - some dielectric materials, especially in ceramic capacitors, have a capacitance which changes with voltage level. This will alter the behavior as well. 6) Piezoelectricity - ceramic capacitors will microscopically expand and contract as they are charged and discharged. If the spikes are at an audible frequency, then the caps will "buzz". Probably not an issue with an engine running nearby but it could be annoying in office equipment, for example, or especially audio equipment. I have definitely used ceramic caps in snubbers and the practice of doing so is well-established. However, you need to operate the cap below its max peak current and peak voltage, take ESR and ESL into account, and probably select a dielectric type which is not very nonlinear over the voltage range involved. Unless you are using type NP0 or C0G capacitors, then this means that you will likely have to restrict the peak voltage on the snubber to less than 25% of the voltage rating of the capacitor. Sean On Sun, May 13, 2012 at 4:27 AM, Electron <electron2k4KILLspaminfinito.it> wrote: {Quote hidden}> At 19.54 2012.05.12, you wrote: >>At 12:29 AM 5/12/2012, Electron wrote: >> >>>Can I use a 0805 ceramic (rated with enough voltage of course) or >>>must I use a big MKT (polypropylene) and such? >> >>What's the application? Triac on AC line voltage or MOSFET driving a DC load? > > Capacitive discharge. It's mostly to reduce EMI and such. > > Cheers, > Mario > > >> >>dwayne >> >>-- >>Dwayne Reid <.....dwaynerKILLspam.....planet.eon.net> >>Trinity Electronics Systems Ltd Edmonton, AB, CANADA >>(780) 489-3199 voice (780) 487-6397 fax >>http://www.trinity-electronics.com >>Custom Electronics Design and Manufacturing >> >>

> 6) Piezoelectricity - ceramic capacitors will microscopically expand > and contract as they are charged and discharged. If the spikes are at > an audible frequency, then the caps will "buzz". Probably not an issue > with an engine running nearby but it could be annoying in office > equipment, for example, or especially audio equipment. A ceramic cap hit with a steep voltage waveform that causes it to ring piezoelectric can then generate a higher voltage than the input in response to it's own mechanical effects. This may not be what you want to happen in a snubber circuit :-). I've seen reports of LDO regulators with Vin max ratings not much higher than Vin_usual failing when a ceramic input capacitor was used because the ringing spike from the capacitor when input voltage was applied caused Vin to exceed the LDO Vin ratings. Russell McMaho

Thank You Sean and Russel for the precious insights. The ESR and ESL are not even rated in the datasheet of these ceramic caps I own.. I wanna be safe and will go to the polypropylene route, also because the pulses are quite steep (dv/dt wise), and polypropylene handles it well. Not worth the risk just to gain some square mm of board space and some cents. Thanks anyway for the knowledge contribution, I'm sure it has been useful to many. Cheers, Mario At 18.32 2012.05.13, you wrote: {Quote hidden}>There are a few considerations that I can think of: > >1) Safety rating - as Dwayne pointed out, capacitors which are going >to be placed between the AC line and ground or AC line and neutral >require special safety ratings against fire and leakage current. This >does not apply to your situation. These ratings are referred to as X1 >or X2 or Y1 or Y2 depending on the exact application (voltage, surge >rating, and whether line to GND or line to neutral) >2) capacitor life - when applying high current pulses to capacitors, >there can be internal self-heating of the capacitor as well as >possibly mechanical stress on the dielectric or plates. Some types of >capacitors will not handle this well and will have shortened life. >3) voltage rating - the cap must be able to handle the peak voltage >seen during the spikes >4) ESR and ESL - if the cap has too high an effective series >resistance or inductance (ESR or ESL), then it will not behave like an >ideal capacitor in the snubber. >5) Nonlinear dielectric - some dielectric materials, especially in >ceramic capacitors, have a capacitance which changes with voltage >level. This will alter the behavior as well. >6) Piezoelectricity - ceramic capacitors will microscopically expand >and contract as they are charged and discharged. If the spikes are at >an audible frequency, then the caps will "buzz". Probably not an issue >with an engine running nearby but it could be annoying in office >equipment, for example, or especially audio equipment. > >I have definitely used ceramic caps in snubbers and the practice of >doing so is well-established. However, you need to operate the cap >below its max peak current and peak voltage, take ESR and ESL into >account, and probably select a dielectric type which is not very >nonlinear over the voltage range involved. Unless you are using type >NP0 or C0G capacitors, then this means that you will likely have to >restrict the peak voltage on the snubber to less than 25% of the >voltage rating of the capacitor. > >Sean > > >On Sun, May 13, 2012 at 4:27 AM, Electron <EraseMEelectron2k4spam_OUTTakeThisOuTinfinito.it> wrote: >> At 19.54 2012.05.12, you wrote: >>>At 12:29 AM 5/12/2012, Electron wrote: >>> >>>>Can I use a 0805 ceramic (rated with enough voltage of course) or >>>>must I use a big MKT (polypropylene) and such? >>> >>>What's the application? Triac on AC line voltage or MOSFET driving >a DC load? >> >> Capacitive discharge. It's mostly to reduce EMI and such. >> >> Cheers, >> Mario >> >> >>> >>>dwayne >>> >>>-- >>>Dwayne Reid <dwaynerspam_OUTplanet.eon.net> >>>Trinity Electronics Systems Ltd Edmonton, AB, CANADA >>>(780) 489-3199 voice (780) 487-6397 fax >>>http://www.trinity-electronics.com >>>Custom Electronics Design and Manufacturing >>> >>>-