I have a fellow engineer that intentially uses high ESR (1 - 3 ohms) tantalum caps at the power input to his boards (5V, +/-12V). His claim is that the high ESR will help to damp ringing ans oscillation caused by all the ceramic caps used elsewhere on the board. Take for example the 5V rail the board has about 40 .1uF 805 X7R caps surving local IC bypass duty and a
47uF 3ohm ESR tantalum right at the input.Does this make since to your guys?
It does make sense for some linear regulators (some low drop out variety) and some switchers that require a minimum ESR. But I doubt it helps much with the high frequency ringing of those ceramic capacitors. Once you get a few inches from them, the inductance between them and the bog capacitor pretty well isolates their resonance from this damping.
But the stability of the ESR is not very good over temperature and time, so I usually use a good, stable, low ESR capacitor and a fixed low value resistor in series when I need this effect.
Ringing rails? I've made ringers with tank circuits.
I guess on a power rail there's the trace inductance can combine with the decoupling capacitance to form a tank circuit.?? The more lossy the tank, the quicker the ring decay. To make a tank lossy a resistance is included in the network.
But I still suspect something fishy going on..
D from BC
Not really. And tantalums connected across supply rails have a bad habit of exploding.
John
Peak current (or something like i^2 * t?) is what detonates them, so they should be OK in a signal-type circuit. I'd guess that 10's of mA would be perfectly safe, but I can't quantify it any better. They do fail on the outputs of 3t regulators that limit the max current to, say, 1 amp or so.
Polymer tantalums are apparently safe.
John
What would you think of subjecting small SMT tantalum caps (eg.
22uF/35V) to several hundred Hz (current limited) from ~0 to say 80% of voltage rating ??Best regards, Spehro Pefhany
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Not if you install them right-side-up. ;-)
Maybe you have supplier problems or something, but I've never seen a tantalum cap explode.
Cheers! Rich
Yes.
Apparently limited to 25V or less rating, so maybe not a feasible fallback in this case.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Yes it does make sense to add some series resistance in some of the decoupling caps, to reduce the Q of any resonant circuit that is formed by the wiring inductance and the capacitors. This is common practice on high frequency integrated circuits. In order to not unduly increase the power supply impedance at very high frequencies, you can split the decoupling capacitance into two parallel capacitors, and put a resistor in series with only one half of the total capacitance. Choose the resistor to have a resistance equal to the magnitude of the reactance of the capacitor at the most troublesome ringing frequency. If you know the value of your wiring inductances then you can simulate all of this in SPICE too.
As for using tantalum capacitors, some people here have had some bad experience with them failing destructively, so I would suggest perhaps using a normally-low-ESR capacitor with a resistor in series as a more predictable and reliable solution.
Chris
You've been lucky. There are military and avionics specs that require series resistance be used with a tantalum cap to limit destructive currents and reduce shrapnel.
See section 15 in this Kemet document describing this failure mode and the current limiting requirement of 0.1 ohm per volt.
Steve
It's in the application information from many manufacturers of tantalum caps.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Sounds naff to me. His design is based on a component -defect-, not a way to go. He should be fitting a 47uF capacitor with external damping resistor. Tants are horrible. I'll bet he's had 'em explode in the past and is rationalising this ESR resistance in terms of 'damping', when in fact it's simply limiting the capacitor ripple current to a safe value. Cap's across input supplies (dependant on circumstance) are in a prime location to suffer spectacular ripple currents. If they can't hack the ripple then leave 'em off. (The 470uF/63V 'lytic I had explode last week, went with a helluva bang. The confetti goes everywhere).
snipped-for-privacy@jjdesigns.fsnet.co.uk a écrit :
Tantalus torture?
-- Thanks, Fred.
The PC board material, if you just make it lossy, will also damp ringing. That'd be cheaper. Heck, just a coat of paint slopped on the board after it's all soldered will achieve as much dielectric absorption as you could want. Any point-located component, like the ESR in a capacitor, will only make ONE POINT on the circuit into a node of the (presumably troublesome) high frequency standing waves on the wiring board.
That is absurd.
John
You haven't lived life to its fullest!
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