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In Reply to: RE: diode upgrades posted by kkcinc on June 01, 2008 at 22:08:10
Guys,
A mini tutorial on power diodes for your possible "edification".
I copied this here from a post of mine to another forum.Skip to the last 3 paragraphs if you don't want to wade thru' any of the physics.
Whenever you have a P and N type semiconductor material the P type has an affinity for electrons (called holes - somewhere for electrons to go) and the N type wants to give electrons up for each of them to attain their most stable states. This has to do with the properties of the doping elements.
When you bung em together to form a PN Junction in a diode (Anode and Cathode respectively) or a transistor junction electrons flow from the N to the P creating a "depletion region" around the junction - that is its depleted of "holes" and electrons. The electron flow is terminated when the electric potential created by this flow of electrons (charge) is sufficient to resist any more flow. The depletion region will have a certain width
The junction (depletion region) will therefore has an electric field across it and hence has capacitance.
This junction capacitance has a Reverse Recovery charge (Qrr). That is, before a diode can turn off this stored charge has to be removed (by recombination of majority carriers). Current has to flow to remove this charge. Therefore as the AC voltage reverses on a rectifier diode there will be a pulse of current in the wrong direction before the diode actually switches off. This puts a noise spike on the power supply rail which has very high current rise time (di/dt). Filter capacitors are not very good at removing (shunting to ground) this noise spike because they have a small amount of inductance and inductors resist current change. Therefore you end up with a noise "SPLAT" on the power supply rail.
The differences in Silicon, Ultrafast Silicon, Schottky and Silicon Carbide Shottky of interest when using them as rectifiers is how fast they can turn off and how large this Current "SPLAT" is before turning off. This is obviously directly related to their Reverse Recovery Charge (Qrr). Small capacitors across each diode can help to absorb this splat and for some time they were in fact required by Electromagnetic Interference (EMI) standards for all commercial products (at least in Europe - I don't know if this is still true).
SKIP To HERE:
So here is the useful (representative ONLY) Qrr data:
Standard Silicon Diodes Qrr approx 500 nC (nano-Coulombs)
Ultrafast Silicon Diodes Qrr can be down to 100 nC
Schottky Diodes Qrr 50 to 70 nC
Silicon Carbide Schottky Diodes Qrr <20nCOh - the other thing you need to know is that the noise spike associated with the current "SLAT" makes your amp sound seriously crap and that its edge speed is often fast enough to radiate into anything that looks like remotely like an arial (resistor leads, connecting wire etc). It can make the sound very "harsh" and is typically noticed mostly in the smoothness and detail of the high frequency "top end".
So what do I use?
The Ultralfast Silicon Diodes with 10nF 2kV or 3kV ceramic caps across them.
Cheers,
Ian
Edits: 06/02/08Follow Ups:
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