discrete voltage regulator

[bocka]

I've developed a discret voltage stabilisator without a feedback loop. The output impedance I measured is about 0.03 Ohms from DC to 100kHz, slightly rising above that frequency because of the inductivity of the caps. Noise or something else I cannot measure: belows the limits of my measurement equipment because there's much rf around me. The static performace is not worse than a LM317/LM337 but a big improvement dynmically.

The 0.47R resistors in the rectifier lines together with the 100nF capacitors forms a low pass filter witch effectivly filters the recovery noise generated by the HEXFRED diodes. Another CRC filter lowers the ripple from the unregulated DC. Cut off frequency is about 25Hz so the 100 Hz ripple and yet more the harmonics are filtered in a simple but effective way.

Output voltage is about 27V, slightly arising with temperature. Voltage range can modified quite easy, just change the ZDiode and the current limiting resistors for the LEDs. For 15V just use a 16V Zener and decrease the 4k7 resistors to 2k2. The current for the LED should be about 8ma. The 0.47R resistors are 2W types, do not use any metall-oxide resistors here wire wound or metal foil is impotant to use. Other resistors are metal film. Caps are Panasonic FC-Type (150µF) and TPSH (6800µ). 100nF caps are WIMA MKP4. LED are 3mm red ones.

Output voltage :

Va = Vz - 0.6V

Vz is the Zener Voltage

The soundstage is quite good in my preamp, warm and good colored. No harsh sound or something else. And dynamically very good...

[Bricolo]

how did you calculate or measure the output impedance?

[bocka]

Feed a signal into the output and calculate the ratio between signal (driving) level and measured level. This is a simple resistor divider. Or pulse the output signal with a dynamically load and look to the ripple. With a FFT you can calculate the impedance.

[Bricolo]

I simulated the reg with simmetrix.

I run a simulation with 0.2A load, and one with 1A load
then, I calculater R=delta(U)/delta(I)
I found +- 0.1R


Does someone know how to make a current source that vary it's drawn current during the simulation? (to calculate more rapidly the resistance, but also the ripple when the load vary)


bocka, I was looking for a tutorial about discrete voltage regulators. Yours measures very good, can you explain me how it works, why you designed it this way... ?

[bocka]

My simulation shows about 0.015 Ohms (@0.1A), but that was not the design goal...

Some years ago I experimented with shunt regs (no TL431, discrete circuit) very good sounding for a preamp but getting awfully hot. So I tried to make an improved reg, better said stabilisator because I do not feed back anything here.

First it's necessary to get a stable reference. Nothing complex, a zener will do this job very well. I tried to use a resistor to set up the correct current but with some calculations I could see, that the regulation factor is quite bad, only 1/50 refered to the input signal because the intrinsic resistance of the zener is much too high. This will give a bad rejection of the 100Hz ripple. Next i tried to filter out tis voltage with a RC-Filter in the signal path of the zener. Good at high frequencies but bad in the low frequency region. Of course I can use a large cap but I won't. Seems not a good solution.

So let's make the resistor in the series with the zener higher, much higer, because the ratio between this resistnace and the zener resistance is the regulation factor. This can be done with a CCS, a simple diode (LED in this case), transistor and a resistor from emitter to the input voltage. This forms a 5 ma CCS - quick and simple.

To get rid of the zener noise I use a simple RC-Filter, 150µF are laying around 220 Ohms I need to adjust the current through the zener, thats my filter. With some experience I expect that the cut-off frequency should quite low, low enough to filter nealy any noise out of the zener voltage.

Typically anyone would now take a multi emitter follower as current amplifier. Well, in the middle of the 90'th I made amps with CFB output stages because these have a lower output resistance compared to the standard emitter follower. Seems to be good in this stabilisator too. Another advantage is that ther is only one junction between the base and the output so the output voltage is less dependant from temperature changes. It is important to drive the first stage in a CFP - the 2SA1209 and 2SC2911 transistors - with quite a large current, this minimises the output resistance. If any change in the output current has no effect of modulating the collector current of the first transtistor of a CFP then the output voltage would be independant from the output current. In this way the output resistance is... 0!

But this is not the reality. Of course output current changes are reflected to the basis of the second Transistor Ib = 1/ß * Ic. So a transistor with a high ß is needed. The larger the ratio between the current in the first transistor and modulation of this current by output current is, the better this circuit will work.

This is why maybe the differences come from: This is not a 1A regulator, it delivers about 300 ma, above this current ß of the 2SA1930/2SC5171 goes down, at 1A it's only the half @0,1A and this causes a higher output impedance at 1A. Additonally ß is higher at high temeratures and my Transistors get quite hot @3W dissipation power. All in all this results in a higher output impedance of the simulation than that I measure. 3 times higher seems to be right in this way.

Bonne nuit

[Jocko Homo]

Bocka:

Have you been taking apart commercial products designed by me? [joke]

Sure looks that way.

For those of you who like to run simulations: look at output z vs frequency. That may answer some of the questions that you guys have.

Jocko

[Bricolo]

I understand the voltage reference (led+2sa1209+zpd27+220r+4k7r), that's a classical voltage reference.

OK for the RC filter on the zener, even if a C seems to measure better on my sims.
But why is the 2sc2911's base connected between R and C and not directly at the zener? Is this the CFB? (regulating the output voltage according to the ripple that is filtered on the voltage reference?)

I'm also trying to understand the output stage.
As said before, the 2SC2911 seems to be controlled by the reference-ripple, and outputs current according to this.
This output current creates a voltage drop across the 33R, and this voltage controls the 2SA1930's current.


Am I right?


BTW, do you have some tutorials to teach me the basics about discrete regultaors, so I could be able to design mine myself?





Jocko: do you know how to use simmetrix? I'm still trying to find how to do mathematical operations on the results (even the basic + - * / ). For the momment I can only measure I and U.
So if you know how to measure output Z vs Frequency, please tell me


Thanks
Alex

[ALW]

Jocko: do you know how to use simmetrix?

Don't mention sim's to Jocko - he'll get excited

In fact I bet his lightning fast soldering iron hand would have this circuit built faster than most of you can find the models...

The o/p Z you've simulated at present seems to be a DC spec, which is almost irrelevant for audio, try adding an AC current source in Simetrix as a load for the reg, then run an AC sim.

Set the current source to 1mA, then using the add trace function (From Graphs and Data, Add Curve) in Simetrix plot: -

abs(1000*Vout), where Vout is the vector of the voltage at the reg o/p.

You'll get a nice trace showing the output impedance, directly displayed in Ohms!.

Andy.

P.S. if you attach the file here, I'll show you how!

[janneman]

Originally posted by Bricolo
[snip]But why is the 2sc2911's base connected between R and C and not directly at the zener? Is this the CFB? (regulating the output voltage according to the ripple that is filtered on the voltage reference?)[snip]Thanks
Alex

Alex, look at a power amp with CFB output stages. This is just one half of it.

Bocka, nice circuit. Hadn't seen a CFB in this app. Looks like you really optimised it. Maybe you can get further by carefull selection of output transistors with sustained-beta.


Jan Didden

[Bricolo]

I've attached my simmetrix file. But I used the transistors I have in the evaluation version's database, the input voltage isn't also the same.


Janneman: did you recieve my email?