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Old 16th February 2014, 12:38 AM   #11
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In the previously shown circuit, the voltage source element consists of a current mirror loaded differential amplifier (Q3-Q6) driving an N-Channel mosfet sink/pass element (M1). Q3 and Q4 are high gain, low noise transistors - there are plenty of Euro and Japanese transistors that could serve as well. I suspect that a pair of 2N4403s selected for high gain would work as well, especially s I've already used them.... I have no trouble using the HFE function on my DVM finding 2N4403s with gains of 300 or greater. The function of the high gain in this circuit is to reduce the amount of base current, to reduce offset. The actual gain of the diff pair is determined by the tail current. I use a current source (J4, R4, R5) to set the diff pair tail current and to provide supply immunity, so that any noise on the incoming supply is not passed to the output.

I've also spun a couple of instances of this regulator using a pair of matched 2SJ74s for the diff pair, set for a tail current of 10 ma (5 per side). This works quite well and gives one the option of additional RC filtering for the reference side of the error amplifier without introducing significant bias current error. However, given the scarcity of 2SJ74s, there are probably better uses for them elsewhere.

Tail current is set for the bipolar circuit shown at 2 mA (1 ma per side), a compromise between gain/drive capability and offset/bias current.

The simulation circuit shown uses a voltage source and resistor (V2 and R15) to model the characteristics of five series-connected GaAsP deep red LEDs, each with a voltage drop of 1.6V and an incremental impedance of ~1.5 ohms at 10mA bias current. Using a fairly large number of series LEDs has a dual purpose - first, it provides sufficient bias voltage for the differential pair/current mirror stage as well as sufficient voltage to drive the mosfet pass element. Secondly, it reduces the noise contribution of the voltage reference to the output voltage by reducing the multiplication factor required to attain the desired output voltage. The noise contribution of the Series LEDs is scaled as sqrt (n), where n is the number of LEDs, while the required multiplication factor scales as n. This means you get an advantage in stacking LEDs for a higher reference voltage. You might also use a buried zener reference like the REF01 for low noise, but I already have a bunch of LEDs....

The LED reference is driven by a current source (J5, R11-12) for supply immunity. A shunt capacitor (C2, R16) provides voltage soft-start to reduce/eliminate turn-on thump.

One common feature of the shunt regulator circuits shown is that they thrive when loaded with lot of low impedance capacitors, something that would drive a lot of series-pass circuits crazy. The output capacitance (C1 and R14) is set for the equivalent capacitance/ESR of four parallel-connected Panasonic FM-series caps, 330uF, 50V (22 miliohms ESR per cap). The low ESR keeps the output excursion small for a fast current transient, while the regulation loop plays catch-up.

Output voltage is set by the reference voltage and R9-10. The vales of R9-10 are kept relatively low to reduce interaction with the bias current from Q3 (3-4 ua). The values shown program an output voltage of ~30V.

C3 and R17 are a speed-up network that passes fast output perturbations directly to the base of Q3 for correction.

I1 is a pulsed current source used to test the transient response of the regulator in simulation. The rise and fall times (100ns) are much faster than what would normally be encountered for an audio frequency load in order to determine the HF stability of the regulator and smoke out any sneaky instabilities.

The voltage source section of the regulator is driven from the unregulated supply (40V) by a current source (M2, R1-2, Q1). I would normally use a depletion mode mosfet and a pair of resistors for this purpose, but I don't have a model for the device I'm using that is easily incorporated into the Cadence/ORCAD version of PSpice.

The modified ring-of-two current source shown can be used by those who can't/wont source the depletion mode mosfets needed for a more simple current source. The output current is set by R2 and the Vbe drop of Q1. One thing that is not necessarily recognized is that the Vbe drop of Q1 is a function of its collector current. This should be set to 1-2 ma to get the B-E junction of Q1 past its "Knee". This determines the value of R1. A better solution is to replace R1 by a current source so that the collector current of Q1 is no longer a function of the unregulated supply voltage. This is shown in the collection of current source circuits a few posts ago.

The response to a fast (100 ns edge) current step is shown in the attached picture. The fast AC response to the step is determined entirely by the ESR of the output capacitors, and works out to a fast AC output impedance of 5 milliohms (no surprise, as that's the impedance of the output caps used in the simulation). The DC response to the 50mA step shows a DC output impedance of 1.8 milliohms - not bad for a small pinch of discrete components.
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File Type: gif firstshunt_bip_res.gif (75.0 KB, 437 views)

Last edited by wrenchone; 16th February 2014 at 01:01 AM.
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Old 17th February 2014, 12:17 AM   #12
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Here's a reference to the next regulator in the lineup. I'll post a newer schematic with simulation results when I get back to work on Tuesday.

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Old 18th February 2014, 02:12 AM   #13
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Here's the latest interpretation of the zener-based regulator presented in the previous post. Per Sala's comments. I've included a largish capacitor across the voltage-determining zener diode to suppress noise. It's scaled per the data sheet value of the zener impedance of the regulating diode in question. The simulation shows a 1N4750 zener diode, which, for some reason, is the largest included in the Orcad models I have available. For my purposes, I would ultimately use a 1N4751A zener, a 30V, 1W device.

Simulation-wise, this regulator does not fare badly at all, with a 5 miliohm AC impedance (determined by the shunt capacitor ESR) and a 0.8 milliohm DC impedance. I tried this regulator a few years ago and got home hiss in a sensitive circuit, most likely because I didn't bypass the voltage-determining zener with an appropriate amount of capacitance. This circuit looks like it's worth another try or two.
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File Type: gif zener_shunt_shh.gif (18.6 KB, 378 views)
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Old 18th February 2014, 02:14 AM   #14
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Here's the simulated response to a fast current transient, verifying the results I cited in the previous post. Not an opamp in sight, and only a pinch of discrete components...
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File Type: gif zener_shunt_res.gif (71.5 KB, 329 views)
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Old 19th February 2014, 10:03 PM   #15
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Here's the third of the shunt regulator series, utilizing an "amplified zener" approach.
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File Type: gif Newshunt2.GIF (14.7 KB, 296 views)
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Old 19th February 2014, 10:05 PM   #16
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Here's the response, with a "fast" output impedance of 5 milliohms (entirely a function of the ESR of the output capacitors), and a static output impedance of 2 milliohms.
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File Type: gif Newshunt2_res.GIF (44.4 KB, 293 views)
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Old 20th February 2014, 06:06 PM   #17
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The shunt regulators are done with - here's a series pass regulator that just dotes on large, low ESR output capacitors. I haven't built this one yet - before I marry it with a project, I plan to breadboard it and torture it a while. You could do that as well, as it's not all that complex. And golly gee, not an opamp in sight... The only fault I see is that the regulator will probably need about 5V of headroom to function properly. There may be ways around that, if you care.
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Old 20th February 2014, 06:11 PM   #18
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Here's the simulated response to a fast load step, showng both the transient and DC output impedance. A usual, the transient output impedance is dominated by the ESR of the output capacitors, and is ~5 milliohms. The DC output inpedance looks to be around 0.2 milliohms, a value that would be difficult to measure in a real situation. At any rate, this looks like pretty good performance for a small pinch of components, so worth the trouble to build and test the real circuit.

That's all there is. I may chime in later with some test results, but if you don't ask any questions, there won't be any answers...
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Old 22nd February 2014, 10:12 PM   #19
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As an aid to comprehension, here's the three shunt regulator circuits shorn of all simulation doo-dads so that you can see what a practical circuit implementation looks like. All three designs are set up for +40V input, +30V output, my usual working situation. Of course, these designs can be adapted for other output voltages and for negative polarity.
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Old 22nd February 2014, 10:14 PM   #20
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Here's the lonely series-pass regulator circuit, again set up for +40V input, +30V output. It, too, can be adapted for other output voltages and for negative polarity.
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