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Old 27th Jan 2016, 6:02 pm   #1
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Default Do Schottky rectifiers need an RF suppression capacitior.

As the title says, I know 'normal' diodes need these to reduce RF hash, but do Schottkys?
 
Old 27th Jan 2016, 6:57 pm   #2
BottleMan
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

Normal power rectifier diodes benefit from having suppression capacitors across them to absorb the energy generated as they exhibit reverse recovery (continue to conduct momentarily). A typical 1N4001 will take about 5us. Schottky rectifiers are generally unspecified for reverse recovery. However, I have seen damping circuits across them in SMPS's, the function of which is normally to reduce ringing of the transformer windings and reduce the risk of over-volting the diode. If you want to use Schottky's in place of conventional rectifiers, then go ahead without suppression capactors. The ONLY benefit will be reduced losses.
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Old 27th Jan 2016, 7:05 pm   #3
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

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The ONLY benefit will be reduced losses
Which I want, 12V transformer, marginal with 'normal' diodes, OK with Schottky ones. I can't see a figure for the reverse recovery on the datasheet, so I will put caps across them (won't do any harm), it is powering a SW (short wave) radio setup. Still wondering though.
 
Old 27th Jan 2016, 8:19 pm   #4
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

My understanding of it would be that they're unspecified for reverse recovery because they do not exhibit this phenomenon in the same way as a conventional pn junction diode. The recovery of a pn junction is due to the finite the time it takes for the space charge within the semiconductor to return to the state it was in before the field was applied, during this time, as the charge redistributes itself, the diode appears to momentarily conduct in the 'wrong' direction.Since Schottky diodes do not work in this way there's essentially nothing to recover from.
Apologies if this explanation is a little thin on hard facts but it's been a long (long!!) time since I did any solid state physics Could have it embarrassingly wrong too!

Bottom line is you should be fine without and capacitors.

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Old 27th Jan 2016, 8:52 pm   #5
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

Even a perfect indestructible diode will be rectifying current derived from an inductive source.

I would fit capacitors to reduce RFI.
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Old 27th Jan 2016, 10:02 pm   #6
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

Thanks Steve, the old wet computer remembers now, no caps. If only for vanity, it will look neater and then never be seen again. Same reason for using lacing cord instead of ty-raps.

Quote:
current derived from an inductive source
the diode doesn't know or care where the current comes from it merrily 'diodes' away. Anyway a transformer isn't an inductive source (apart from leakage and 'transformed' source inductances, which should be minimal) it's all, well most in the real world, shorted out.
 
Old 28th Jan 2016, 1:11 am   #7
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

I'd fit suppression capacitors for Schottky diodes working from a mains transformer!

True, they don't exhibit reverse conduction and recovery. But, they are fragile and can't absorb much breakdown energy. A mains spike could kill them - a parallelled capacitor should absorb the spike and keep it below the breakdown threshold.

Other reasons - prevent modulation hum if in an AM radio.
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Old 28th Jan 2016, 2:46 am   #8
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

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Originally Posted by merlinmaxwell View Post
wet computer remembers now
Wet computer! I love it
Also very much agree with your comment re Ty wraps vs lacing cord. Hmmm there's something really nice about that waxed string stuff...

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Old 28th Jan 2016, 3:13 am   #9
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

A schottky exhibits a much higher junction capacitance than a pn. That capacitance can let through dV/dt steps that occur on the transformer secondary winding when the diodes 'commutate'. The dV/dt steps occur due to the winding leakage inductance and the high dI/dt occurring when diode current reduces and then abruptly stops.

PN diode reverse recovery can interplay with leakage inductance and poor layout and lead to MHz ringing in bad cases. Good layout and diodes like UF4007 alleviate that.

I'd suggest using schottky, but no parallel capacitance, and ensuring the wiring between transformer and diodes and first filter cap is short and twisted to minimise loop area for any higher frequency currents.

Schottky at 40V PIV was about it many decades ago, but 55-100V should provide sufficient margin.

Last edited by trobbins; 28th Jan 2016 at 3:20 am.
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Old 28th Jan 2016, 11:32 am   #10
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

The images below show 125kHz ringing in a 12V power supply using 1N4007 diodes (a), and UF4007 diodes (b). Both can be improved further by adding a snubbing network. Presumably the same applies to Schottky diodes?
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Old 28th Jan 2016, 1:43 pm   #11
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

Quote:
Originally Posted by trobbins View Post
A schottky exhibits a much higher junction capacitance than a pn. That capacitance can let through dV/dt steps that occur on the transformer secondary winding when the diodes 'commutate'. The dV/dt steps occur due to the winding leakage inductance and the high dI/dt occurring when diode current reduces and then abruptly stops.
I'd agree - in a square wave switch-mode power supply circuit - but not in a sine wave mains transformer. Here, the current will gently reduce due to transformer winding resistance and inductance, reaching zero gradually. So dI/dt will be non-zero, but limited. Inductance helps because you just can't have infinite dI/dt in an inductor!

At the instant of switch-off, the slope changes from something to a steady zero, so the second derivative is quite high, but not the first derivative.

Any reverse recovery will of course result in reverse current and an abrupt snap-off, but this is almost absent in a Schottky diode.
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Old 29th Jan 2016, 2:58 am   #12
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Default Re: Do Schottky rectifiers need an RF suppression capacitior.

Yes sorry I was a bit quick in typing that description and should have not used 'high dI/dt', but rather 'change in dI/dt'.

I agree that most people look at smps diode performance results and perceive that a mains frequency rectifier diode operates at a similar dI/dt ramp level as current falls from some high level to below zero.

In a mains supply the current waveform is somewhat similar to a half-sine and the point where current dI/dt changes from a negative value to zero has an influence on any resulting transient effect. The dI/dt level at the time current hits zero is alleviated by reducing the pulse crest factor (more series resistance, less capacitance) and using a soft diode like a valve diode where internal resistance increases significantly as diode current falls to zero.

That level of dI/dt (albeit low) still generates some voltage across transformer secondary leakage inductance, and then the voltage across the leakage inductance steps to zero when dI/dt = 0. The winding voltage at the diode anode terminal can be seen with a step in it. That dV/dt step then causes a current loop through the falling off-state capacitance of the diode. Using a diode with less capacitance reduces the level of transient current passed through the filter cap, and hence seen by the amp circuitry.
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