conventional power supply

I wanted to share some insights I gained by spicing up a normal supply, in this case a 12V/4Adc, consisting of a transformer, bridge rectifier and smoothing capacitor. In another thread some of us gave their opinions about the choice of the required components, and I tried to verify.

  1. The transformer can be modeled with an ideal ac-source plus interiour resistance, as we know from any DC-supply. Almost all losses are due to resistance of the windings. The manufacturer specifies a certain obtainable output power, consisting of the product Vrms * Irms = P(VA). The input power is by some percentage higher and the difference is due to losses. It is the difference between the open circuit voltage Vo and the rated voltage Vrms . Ri = (Vo-Vrms) / Irms.

  1. With the rectifier and cap the load current is not sinusoidal. Only when the momentary voltage is higher than the remaining voltage on C plus the 2 diodes Vf, the current starts charging the cap. it reaches the max. before the voltage peak and goes symetrically down to zero. The max. current is a multiple of the average and the rms value is much higher as with a sine. Both are mainly dependent on the ratio of the interiour resistance to the load resistance, not much on the capacitor size. A higher Ri reduces the peak current a lot, since Ipeak is proportional to 1/sqrt(Rload*Ri)

  2. The required size of the transformer is mainly dependent on the Irms of the current thru its windings, which dissipates in the Ri and heats up the transformer. So a transformer with a very low Ri has not much advantage, since the Ipeak and Irms go up accordingly. The losses in the diode bridge raise too and can be estimated in the datasheet where there are curves for Ploss (Ipeak/Iav). Since this is for a single diode you have to double the ratio and multiply with 4. The rectifier losses plus the losses in the cap Irms^2*ESR plus the output power have to be lower than the transformer rating.

  1. The ripple voltage is not so much lower with a low Ri, mainly dependent on the cap size.

to be continued

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ciao Ban
Apricale, Italy
Reply to
Ban
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The output voltage looks like a sawtooth with a little nipple on top, its main AC component is the 120Hz fundamental, the 2nd harmonic and the 4th, the higher harmonics decrease continually.

  1. Setting up the simulation: ___ .-|___|--o-->|---o----o----o----. o-. | Ri | D | | | | | | | .-. | | / \ .---)-->|---' | |ESR | | ( ~ ) | | D | | |- .-. \_/ | | '-' / \ | | |Vo | | |+ ( I ) | | | | | === \_/ '-' | | '--|
Reply to
Ban

I hope this part finds your interest

  1. Filtering the ripple: I have set up 4 scenarios:

a) doubling the filter capacitor by paralleling another C. Advantage: the output voltage stays the same, the ripple current can be double. The filter action is obviously just 6dB better. With a 22mF/35V the cost is high.

b) the same as before, but inserting a small resistor (0R22) between the caps. We loose 0.9V and gain 13dB suppression. Same ripple current rating, but higher harmonics get reduced more.

c) instead of the resistor a 1mH/0.22R coil is inserted. Good suppression -20dB almost only the 120Hz fundamental left. I tried a 10mH instead, but it needed a long time to stabilize and would "motorboat" at 10 to 20Hz.

d) insertion of a "Gyrator" made from one transistor:

.-----------||->|--+ | | | 2.2R | 2.2R | 4001| | || | | | || | | '--||-----)---' '-||----)---------' 2.2m|| | 2.2mF || |2.2mF --- --- --- --- 2.2m| | | | === === GND GND (created by AACircuit v1.28 beta 10/06/04

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This was reducing the output voltage by 1.5V, but also the ripple by 37dB to a mere 15mVpp, as much as a10mH coil would have done. Disadvantage not short circuit proof, needs small heatsink. The second cap from the O/P is an idea of Bill Sloman. The second circuit got all protection added and can probably make 20A with a bit of cooling and modifying the Rs and Cs. I modulated also the O/P current from 0 to 6A with a 50Hz sinewave, and the resulting output voltage variation was just 200mV, hardly any 120Hz in there. Would make a good audio supply.

During the simulations I tried various caps and snubbers across the rectifier diodes and found another audiophool myth. These caps indeed reduce the even harmonics in the diode current, but... the odd ones which are 30dB higher stay identical. So what is the use?

finished, commentaries welcome.

--
ciao Ban
Apricale, Italy
Reply to
Ban

Thanks for the credit, but while I can't remeber where I got the idea from, it certainly isn't one that I invented

The protection on the second circuit is a bit iffy - two 1N4001 diodes don't have a very predictable voltage drop. Philips used to do a diode that did have a closely specified voltage drop (but I can't remember the number) and I think the BC184 and BC214 low powered audio transistor have reasonably closely specified base-emitter voltage drops.

A "synthetic diode"

-+---- ----+- | \\ ^ | | --- | | | | | v | | ___ | +---| |---+ ---

might be even better - you have to set the potentiometer to get exactly the voltage you want, but that lets you take out the tolerance on both the base-biasing transistor and the on the power transistor that you are biassing.

Nice one Ban.

Bill Sloman, Nijmegen

Reply to
bill.sloman

a écrit dans le message de news: snipped-for-privacy@e56g2000cwe.googlegroups.com...

the

filter

the

rating,

10
37dB

not

is an

Looking it the other way, this gyrator is just a 2nd order S-K filter, filtering its own supply.

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Thanks,
Fred.
Reply to
Fred Bartoli

That's not what they are for. Your entire analysis has a huge error, in that you ignore the transformer's leakage inductance. As I showed in multiple postings of measurements on ac-line transformers, voltage drops from leakage inductance typically exceed those from copper resistance. The s.e.d. threads also have extensive theory and formulas to calculate transformer leakage inductance, and these reinforced my observations.

While the inductance does not contribute to power dissipation, it has a *huge* effect on your rectifier and filter-capacitor calculations. This is good, because it serves to reduce the inrush current at the top of each half-cycle, spreading it out.

A complete and accurate analysis will have to include not only the transformer's leakage inductance but the diode's reverse- recovery time model. You have some more learning ahead of you.

Back to your snubber, this is to damp the inductive pulse and high-frequency ringing that occurs from the rectifier diode's fast snap-off after the reverse-recovery time is finished. In this case it's the transformer's leakage inductance that was charged during the reverse-recovery time and forms a resonant circuit with the winding and diode capacitances. You add more capacitance in parallel to lower the ringing frequency and take control of the resonant energy, plus a series resistor to lower the Q and damp the ringing to a single cycle. (We've written up a nice story about this scene, complete with scope photos and spice models for the 3rd edition of AoE.)

Anyway, instead of a very sharp high-voltage spike (too short to see with a scope unless you trigger properly) followed by RF ringing, you get a single small long smeared-out innocuous pulse. If left unattended to, the magnetic radiation from the inductive spikes can be picked up and create an audible buzzing signal (harmonics of 120Hz) in low-level phono or mic inputs.

--
 Thanks,
    - Win
Reply to
Winfield Hill

THX Win,

I have here the Wai-Kai Chen "Circuits and Filters" with the proper description pg.338 figure 10.65. How do I simulate it with an ideal transformer, and how big are these inductances in your opinion. I want to simulate a 500VA toroid with 2X45V.

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ciao Ban
Apricale, Italy
Reply to
Ban

Arghhh! ;)

Jon

Reply to
Jonathan Kirwan

From a 630VA, 230V-2x(2x65V) I've measured some months ago:

Lp=12.7H N=3.34 Lf, seen from the secondary side, measured @10kHz : Lf1a=Lf1b=16uH Lf2a=24uH, Lf2b=22uH

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Thanks,
Fred.
Reply to
Fred Bartoli

In article , Winfield Hill wrote: [....]

and ...

There are harmonics up to, and beyond, the AM band in the turn off of the diodes. As a result, you want the snubber to be near the rectifier to reduce the area enclosed by the wiring.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

In article , Ban wrote: [...]

On the second circuit:

Using a second transistor in place of the two 1N4001's produces a sharper knee in the current limit.

The two 2.2 Ohm resistors will have a huge current flowing in them if the output gets shorted. You could look at using a PTC device in series with one of them, or in place of it.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

I don't have that book, but their ideal transformer model isn't very useful if it fails to include Leakage Inductance. You can read one of the posts I was referring to by following this info:

From: snipped-for-privacy@rowland.org (Winfield Hill) Subject: Re: Leakage Inductance, measurements, etc. Date: 1998/01/03 Message-ID:

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If you examine my table of transformer measurements, covering 2.5 to 100-watt Signal Transformer "241" split-bobbin types,

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(223kB long)
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(6MB), you'll see the copper resistance dominates for the small ones (where the manufacturer thinks he can get away with using a small wire size without overly heating the transformer), and is on the same order as the leakage-inductance reactance for the larger ones. So, lacking better information, you could choose your modeling leakage-inductance values that way.

Google says the thread, "Leakage Inductance, Please Explain?" started by Chris Carlen on 31Dec1997, has 131 posts. But there were also offshoot threads at the same time. Anyway, you'd do well to read the entire 131-post thread, starting here: :-)

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A toroid transformer may have lower leakage inductance than the split-bobbin types I measured, if the secondary is wound directly over the primary. But if that's the case, it'll also have much poorer long-term high-voltage-spike insulation, an issue that most users have learned is very important. Anyway, we've learned that Leakage Inductance is a good thing, helping to reduce rectifier-to-capacitor in-rush charging currents.

--
 Thanks,
    - Win
Reply to
Winfield Hill

And the DCR? Was the N=3.34 an actual measurement?

--
 Thanks,
    - Win
Reply to
Winfield Hill

Arghhh?

--
 Thanks,
    - Win
Reply to
Winfield Hill

You are quoting some text I'd like to read, from a book I'd like to have.

Jon

Reply to
Jonathan Kirwan

[snip]

W.M Flanagan's book on transformer design has a chapter where he presents design curves similar to O.H Schade's, but which include the effects of leakage inductance. Very useful for transformers with high leakage inductance, such as the modern divided bobbin transformers.

Handbook of Transformer Design and Applications. McGraw Hill, ISBN 0-07-021291-0. Pages 18.1 to 18.15.

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Tony Williams.
Reply to
Tony Williams

"Ban"

** The leakage inductance of a toroidal transformer is *entirely negligible* as far as the behaviour of a mains supply frequency rectifier and capacitor input filter system is concerned.

Same goes for a mains frequency transformer where the secondary is wound over the primary - whether or not there are two such combinations on the same core ( ie as with "double C- ore " or R-core types).

Only with " split bobbin " construction does leakage inductance become a significant cause of voltage drop - but in most examples winding resistance is still the dominant cause.

....... Phil

Reply to
Phil Allison

Win, many thanks for the references, I have saved them. The book I'm referring to is giving this model:

leakage inductance leakage inductance ___ ___ ___ ___ o-|___|--UUU--+----. .--UUU--|___|-o | '-. ,--' C| )|( C| )|( C| .-' '----------------o | | o-------------+----'

magnetizing inductance

And the actually non-linear core losses can be approximated with a resistor in parallel with the magnetizing inductance. BTW Gordon E. Carlson who wrote the chapter about transformers gives AoE in his References. He must like your book very much, because I didn't find much about transformers and leakage inductance there.

That book is one that requires a lot of study, very theoretical. And

2991pages, so I have a lot to study, you are right. Thanks again for your constructive comment. It gave me the energy to start going also through the magnetics, something I never was interested, like electrical machines and motors etc. Another thing is to really check how the Spice models are fitting. I use Spice since 1984 and that was after my time at university. It works fabulous for the opamp circuits I usually do, but I know the shortcomings in this area.
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ciao Ban
Apricale, Italy
Reply to
Ban

"Winfield Hill"

** You bet your fat arse it is lower.

** Err - there is *another* method to wind a toroidal ???
** What a load of posturing drivel !!!

Toroidal transformers using impregnated cloth or plastic tape insulation have proved to be VERY reliable over many decades of use - long outlasting the equipment they power.

** ROTFL.

Better chuck out all those overwound and toroidal transformers now !!

Win the Expert has no time for them !!!

....... Phil

Reply to
Phil Allison

Arghhh! ;(

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 Thanks,
    - Win
Reply to
Winfield Hill

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