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Old 19th June 2017, 12:53 PM   #611
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Connect an LTSPICE current source to the shunt regulator's output and have it draw a square wave of current from the regulator. Something like Ilow=5mA, Ihigh=12mA, risetime=2ns, falltime=2ns, hightime=1us, period=2us. Then plot the current which flows in the 33uF capacitor across the regulator output. The capacitor current is the amount of stress removed from the regulator. Every milliamp that flows in the capacitor is a milliamp that doesn't flow in the regulator. The capacitor's action is similar to a bulletproof vest; it absorbs and spreads out much of the impact force of the bullet, so the resulting wound is survivable rather than lethal.
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Old 19th June 2017, 01:53 PM   #612
Bigun is offline Bigun  Canada
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I can't run a sim today. But I can imagine that the current will be shared between the shunt and cap in proportion to their impedance. The lower impedance will dominate and over a large range this is the shunt. Why do I care about some stress on the shunt, it's simply doing the job it was designed for ??
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Old 19th June 2017, 03:55 PM   #613
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If you do run a simulation with and without the 33uF capacitor across the output, you might see results that resemble the attached image. It comes from a different shunt regulator, not yours, which uses a different error amplifier and a different voltage reference scheme than yours. But the simulation results might be similar anyway.

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Old 19th June 2017, 06:01 PM   #614
AndrewT is offline AndrewT  Scotland
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For the same reason one fits local supply rail decoupling.
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Old 19th June 2017, 06:25 PM   #615
Bigun is offline Bigun  Canada
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I can see the blue is smoother. It says that the shunt regulator is unable to keep up with the current step demand (but it gives up very very little change in actual voltage). But if this has a rise time of ns ? the it's asking for response from the shunt regulator at extreme frequency. I can include a small rail decoupling capacitor, small enough to use something that would work well at high frequency. But isn't this academic ? where will such high frequency signals be generated ? the input to the phono amp has limited bandwidth. Why do I want to engineer the power supply to have low impedance at r.f. ?? - I know I'm being stubborn :-)
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Old 19th June 2017, 07:55 PM   #616
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You are the designer, you make the design decisions. How good is good enough? Your call. How many parts is too many parts? Your call. What simulations are worth running? Your call.
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Old 19th June 2017, 08:19 PM   #617
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Bigun View Post
I can see the blue is smoother. It says that the shunt regulator is unable to keep up with the current step demand (but it gives up very very little change in actual voltage). But if this has a rise time of ns ? the it's asking for response from the shunt regulator at extreme frequency. I can include a small rail decoupling capacitor, small enough to use something that would work well at high frequency. But isn't this academic ? where will such high frequency signals be generated ? the input to the phono amp has limited bandwidth. Why do I want to engineer the power supply to have low impedance at r.f. ?? - I know I'm being stubborn :-)
In a domestic environment since the 1990's there has been an increasing use of new technologies such as switch mode power supplies, cell phones microprocessors, remote controls, fluorescent displays lighting etc etc.

Radio detection involves a feeding diode and a capacitor to earth. You have these elements in a transistor package with the base-emitter junction and capacitance between the base and the collector and emitter junctions - so the potential is there to create a disturbance if the circumstances are ripe.

I mentioned the use of ferrite coils by Technics in series with RIAA input stages - as far back as the 1990's at least.

I suggest you look at the Ie of the RIAA LTP under dynamic conditions to see what impact these have on your supply rails. Mark Johnson commented recently about how much these vary.

I agree with Andrew T about local decoupling capacitors for the RIAA.
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Old 20th June 2017, 01:24 AM   #618
Bigun is offline Bigun  Canada
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I don't mind if the shunt has to work for a living, take the stress. But I didn't like what I saw in terms of the output impedance (plotted on dB scale) peaking when the capacitance on the rail was to small. I ran a quick simulation with 1uF on the V+ rail and 10uF on the V- rail. There was obvious peaking on the V+ rail. A transient simulation with fast pulse current source produced some awkward moments. The designer has opted to keep rail caps in place
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Last edited by Bigun; 20th June 2017 at 01:27 AM.
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Old 20th June 2017, 02:15 AM   #619
Bigun is offline Bigun  Canada
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do regular Aluminium Electrolytics work well at such high frequencies?
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Old 20th June 2017, 06:09 AM   #620
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Aluminum electrolytics have a self-resonant frequency, like all other caps, but resonance looks more like a big, broad, non-resonant trough in its Z vs. F curve. IIRC, some 470F 35V caps with cans around 7 x 15mm have self resonant frequencies around 400kHz - 600kHz, and caps with smaller cans will have SRFs around 1MHz or 2 - it's usually more of a function of the physical size of the cap, and less related to the capacitance value or voltage rating.

A small to medium valued (22F - 470F) high quality wet Al electrolytic will work very well to smooth a power supply rail without generating resonances of its own. If you look at the impedance plots, even above its resonance, it may have an impedance comparable to a large value monolithic ceramic cap, so it can work with an MLCC to help tame any MLCC resonances, and provide low impedance at frequencies that are too low for the relatively low capacitance MLCC to handle.

Lossy caps like aluminum electrolytics and X7R ceramics are useful on power supply rails. With X7R however, some modern parts lose a lot of capacitance when you run them at 10-20V DC bias, so check the specs or run a simulation on the manufacturer's web site first. Murata and Kemet have good simulators that let you specify DC bias and then show you the resulting specs and curves.

In general, higher voltage rated and physically larger chip X7R caps will 'squish' less with DC voltage (i.e. thicker ceramic dielectric layers squish less), but just check the site's simulation first. The difference is not subtle - some caps will have 80% of their faceplate capacitance at 17V, and some will have 20%, even though both are well within their voltage ratings. Again, the voltage induced capacitance loss is largely from the use of really thin dielectric layers, so in general, a tiny, high value X7R cap won't do much when shunting 15V rails, even though the cap may be rated for 50V. Check the manufacturer's simulations rather than guessing.

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