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The simplistic Salas low voltage shunt regulator
The simplistic Salas low voltage shunt regulator
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Old 10th August 2012, 10:50 PM   #5231
Salas is offline Salas  Greece
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The simplistic Salas low voltage shunt regulator
Sure there are differences, 9610 picks up more Vgs, the way to get less CCS is either Vbe decreases due to the BJT or some oscillation settles in the BJT-Mosfet combination. But you had the right hunch and solved it.
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Old 11th August 2012, 09:10 AM   #5232
AndrewT is offline AndrewT  Scotland
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Using two 47k for R2 & R3 allows about 0.22mA for Q1 Ic, when Vs=25Vdc.
This seems very low.
With R4 passing ~100mA, then the base current of Q1 is ~0.25mA if hFE=400 @ Ic = 0.22mA.

I would suggest you aim to run Q1 such that Ic>>Ib preferably a ratio of 10 or more.
That seems to indicate that your first choice of 4k7 was about the maximum for these resistors.

I think you need to look at other reasons for the non constant CCS current.
Replacing R3 with a 3mA to 4mA jFET may help fix the Ic of Q1 at a more suitable value, but I suspect that instability is at the root of the problem. 431 plus 9610 for Q6 and the eight separate connections for the sense circuit could all give stability symptoms.
Forget Remote Sensing if it does not suit your needs. BUT YOU MUST properly collect the sense connections together and attach just TWO sense measurement points into the +ve & -ve rails. You have shown 5 connections spread along the top +ve rail and 3 connections spread along the bottom -ve rail.

And why did you adopt a pair of 9610s when a 510+(520 or 530) combination would almost certainly perform better and guaranteed to not perform worse.
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Last edited by AndrewT; 11th August 2012 at 09:14 AM.
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Old 11th August 2012, 10:23 AM   #5233
judby is offline judby  Denmark
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Hi Andrew, thanks for your comments

Why is 0,22mA for Ic for the BJT very low?
How do you get the value 0,25mA for Ib? Given hFE of 400 it would be more like 0,55uA?
Maybe you could explain to me how this CCS works?

It looks obvious: BJT will regulate Ic/Vce to keep Vbe about 0,6V. If Id (FET) increases, so will Vbe(=R4*Id), increasing Ic. Now V drop over R2+R3 will increase and Vgs(=Vce) will decrease and Id will decrease and we're back, constant Id (In-/decrease as numerical values, ie. |Vgs| etc).

Then what happens when Vin increases (assuming we keep Ic and Id constant)?
Vce and Vds will increase. For the BJT we assume constant Ic so only Vbe must decrease. With increase in Vds AND Vgs (=Vce) FET will see 2 factors giving increase in Id => Vbe(=Vr4) will increase. But it has to decrease to keep Ic konstant at higher Vce.
Vce then has to decrease. If Ic is given by Vin-Vce/(R2+R3) then Ic will increase...
Eventually Ic, Vce(=Vgs) and Vbe(=R4*Id) will find a new "steady state" and it is not obvious to me if it is with higher or lower Id than before Vin increase.

BTW: |Vgs| in my prototype for the CCS FET is 4,27V.

I don't see any stability issues with my prototype. All connections from D of CCS FET to S of shunt FET and output are connected to the same breadboard copper trace (shown with black marker on top).

I use IRF9610s because they were in my "stash" and are more than adequate dimensioned. One of the purposes of this prototype was exactly to test if they can be used in this circuit.

I have a number of things to try in a new version of the prototype. I do not like the "BJT controlled" CCS as I cannot easily comprehend its functionality as described above, so it has to go. Also the TL431 for a number of reasons.

But please keep comment, I'll learn some (analog-) electronics which is obviously too far away in my little head
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Old 11th August 2012, 10:25 AM   #5234
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by judby View Post
........Why is 0,22mA for Ic for the BJT very low?
How do you get the value 0,25mA for Ib? Given hFE of 400 it would be more like 0,55uA?
Maybe you could explain to me how this CCS works?
OOPS, I divided the 100mA by 400 to get 0.25mA.
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Old 11th August 2012, 09:06 PM   #5235
Salas is offline Salas  Greece
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The simplistic Salas low voltage shunt regulator
I run a simulation, no problem at say 25V in with 4k7,4k7, to the contrary Vbe naturally is a bit higher for more Ic so it gains 10mA for the Vbe/R1 CCS governing rule (R4 in yours). Only thing I saw is you better put 10R in series on your bootstrap capacitor (C4) between now nodes, especially if its low ESR it gives a PSRR curve kink.
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Old 11th August 2012, 09:14 PM   #5236
Salas is offline Salas  Greece
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The simplistic Salas low voltage shunt regulator
P.S. From 1.2 and 1.2R experience tight layout matters in such CCS. And/or an interaction with the lab PSU? Maybe a 0.1uF decoupling cap when test feeding from bench PSU long leads. A good thing in this CCS type is it gives you a steady parameter (Vbe) to choose setting resistors against, when LEDs and Vgs solution give play. Especially in hot rod settings the one Vbe solution is signifiicantly cooler for Rset too.
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Old 12th August 2012, 08:34 AM   #5237
judby is offline judby  Denmark
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Quote:
Originally Posted by Salas View Post
I run a simulation, no problem at say 25V in with 4k7,4k7, to the contrary Vbe naturally is a bit higher for more Ic so it gains 10mA for the Vbe/R1 CCS governing rule (R4 in yours). Only thing I saw is you better put 10R in series on your bootstrap capacitor (C4) between now nodes, especially if its low ESR it gives a PSRR curve kink.
Hi, being and old-school engineer, I usually try to understand the circuit before simulating

But thanks for the input - could I ask you to post the schematic + result from the simulation?

The R in series with the bootstrap C is noted
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Old 12th August 2012, 08:35 AM   #5238
judby is offline judby  Denmark
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...A good thing in this CCS type is it gives you a steady parameter (Vbe) to choose setting resistors against, when LEDs and Vgs solution give play. Especially in hot rod settings the one Vbe solution is signifiicantly cooler for Rset too.
Yes, I like(d) this CCS too because of the tight Vbe setting, it gives low loss and you can expect Id to be close to 0,6V/Rset (R4 in my version).

But as said before, I'm not sure I can understand exactly what happens when Vin changes, which gives me headache...

I might choose a 3xLED driven by a CCS FET (K170) as "Vref" for the CCS, experiment with a proper Rset value and accept the higher loss.

Regards Jesper
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Old 12th August 2012, 07:37 PM   #5239
Salas is offline Salas  Greece
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The simplistic Salas low voltage shunt regulator
Quote:
Originally Posted by judby View Post
But thanks for the input - could I ask you to post the schematic + result from the simulation?
I modified and watched a standard version file so I avoided saving it over with the modifications. Sorry.
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Old 12th August 2012, 07:51 PM   #5240
Salas is offline Salas  Greece
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The simplistic Salas low voltage shunt regulator
Quote:
Originally Posted by judby View Post
Hi, being and old-school engineer, I usually try to understand the circuit before simulating
Picture a BJT instead of a MOSFET and its nothing more than the classic ''ring of two'' CCS concept.

http://www.diyaudio.com/forums/tubes...stability.html
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