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Old 29th September 2005, 06:56 AM   #1
Pierre is offline Pierre  France
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Default SMPS control loop estability

As some of you know, I am trying to build an offline half-bridge SMPS for audio use.
My prototype is almost working, but I have an issue with regulation. I started another thread, but as the problem is now different I have decided to keep them separately for ease of search.

The turns ratio gives a peak secondary voltage around 70V, while I want a regulated output of around 60V.

If I leave the supply unregulated, going to max. duty cycle, the output is too high. If I regulate it "manually" adjusting duty-cycle, (but open loop), the voltage is good for relatively heavy loads, but goes to more than 70V with no load or light load, and load regulation is not too god.
The obvious solution is close the loop, of course.

BUT: as I have it now, it oscillates: at light load, there is a 5Hz, 2Vpp oscillation (not sine, but more like triangular) around the correct set value. As load increases, its freq. increases to, say, 30Hz at 150W.
At some load, it even oscillates badly from almost 0V to 50-60V, I suppose that duty cycle falls to 0 and the controller stops.

I have tried to compensate with no results so far.

I attach the schematics of my current feedback setup. By the moment it is using only the internal SG3525 opamp. I know that the optocoupler diode excitement is very simple and will have some temperature drift, but that's not very important for me by the moment.

Can anyone tell me what's wrong??? Thanks!
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Old 29th September 2005, 12:07 PM   #2
Eva is offline Eva  Spain
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I've said many times that this arrangement is not going to work, and I've also explained how to get it right.

Your output filter requires a zero at 280hz for compensation, but it has to be implemented before the optocoupler, and also two voltage rails can't be sensed in that way.
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Old 29th September 2005, 01:43 PM   #3
Pierre is offline Pierre  France
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Sorry, Eva. I only wanted to put it to work reasonably well without adding more complication, but if that's not possible...

What's the reason for the requirement that the zero is implemented before the optocoupler? You'll agree with me in that it adds complexity and if that can be done in the control side... much better. But I am sure you have a very good reason.

About sensing both rails, well, it is sensing both of them, but it is true that it has no GND reference, so if the load is not somewhat symmetric it can produce different voltages in each rail.

Thanks for your help
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Old 29th September 2005, 02:15 PM   #4
Eva is offline Eva  Spain
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Rather than being useless as you could think at first, the output ripple voltage waveform is very important for the control circuit, but it must be precisely attenuated and averaged by the pole/zero/pole response of the error amplifier before it goes through the optocoupler, because optocouplers suffer from limited and asymmetric slew rates, delays and storage times, thus distorting high amplitude high frequency signals badly.

Also, placing the error amplifier and the compensation after the optocoupler will allow the optocoupler to saturate and cutoff during transients without the error amplifier being able to track the average value of the output voltage waveform and compensate for it in average.

Concerning output voltage sensing, note that the frequency response of the output filter of each rail changes as load is increased. If you take feedback in such a way and apply a different load to each rail you are likely to get instability because you are summing two different transfer functions that may probably interact producing notches.

Also, even when compensation and output sensing are right, you are going to need a minium dummy load to ensure stability.
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Old 29th September 2005, 02:17 PM   #5
Pierre is offline Pierre  France
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Please have a look at this reference I have just found. It is a push-pull DC/DC, but the output voltages are very similar and in terms of control loop it is almost identical.

http://valveaudio.tripod.com/images/...bar9b1012a.jpg

They use a fb. scheme very similar to mine, with no additional opamps. Eva, what do you think of this arrangement? Perhaps it can't work properly?

The only compensation is a pole at the output of the error amplifier. I really don't know how it works. It is wired as a comparator instead of an amplifier with moderate gain.
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Old 29th September 2005, 02:20 PM   #6
Eva is offline Eva  Spain
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That link doesn't work, this site doesn't allow to access their images from anywhere except their own links.

EDIT: I've managed to reach the schematic, but, does it work? is it stable? Who knows? Note that they are using the optocoupler in a very different way.
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Old 29th September 2005, 02:57 PM   #7
Pierre is offline Pierre  France
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Well, in fact they are using the optocoupler in a very similar way to what I do. (although they put the output voltage directly to the error amp used as a comparator).

I have been reading the SG3524 datasheet (very similar to SG3525) and they recommend compensation by means of a capacitor+resistor from COMP to GND.

I still don't understand why compensation should be done before the optocoupler. Please could you explain me that, Eva.

Sorry for so many questions but I think that this issue is interesting for a lot of people too. Thanks for your time!
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Old 29th September 2005, 04:03 PM   #8
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Default Feedback Compensation

Pierre/Eva,

Some years ago, I built a DC-DC PWM converter using the SG3525 chip, with compensation on the primary (control) side of the ckt. This was from the Audio Amateur 04/88 and 01/89 issues from an article by Mr. Randy Vikan and his 75+75W all-MOSFET Car amplifier. Pt I was about the SMPS, and Pt II- the Amp itself.

In this schematic, he uses a parallel r-c network (49.9kW and 1000pF), going from the comp pin (pin 9) of the '3525 to ground. Switching frequency was just over 26kHz, so I upped this to 33kHz, further removing it from the audible spectrum. This didn't seem to make any difference in the performance of the converter from a noise perspective. Though I didn't have a ScopeMeter at the time, audible inspection and voltage measurements verified this. The optocoupler, an MCT271, is no longer made and is replaced by the garden-variety 4N36 or similar optocoupler. "Compensating" for the variations of CTRs (Current Transfer Ratios) in different optocouplers is a biasing network, using the very-familiar TL431 and a few resistors. I believe the '431's reference pin taps a 61.9kW (upper) and 6.19KW (lower) resistor divider chain that sits across the +/-37V Buses. From the +37V line comes a 2.21kW resistor, through the LED portion of the optocoupler, and to the Anode of the '431. The '431's cathode simple dumps to the -37V line. Sensing was about the +/- buses, with no reference to the output side's groung node. BTW, input and output grounds are isolated from each other, coupled only by a couple of 0.01uF caps.

Anyway, I have built this DC-DC Converter with little-to-no noise on several discrete and chip- amps. This is used as simple voltage-mode, no current-mode sensing is used. That is not to say there may be some advantage(s) in implementing current-mode sensing with the '3525 (yes, it can be done with this chip), but, since this was a pre-made board, I went with what was made space for.

The only difference between my version and the author's was the substitution of two FT-140-77 toroid cores (Amidon), stacked, in place of the single 3622 pot-core (Ferroxcube). If I can find a web-based schematic, I will post it here.

Going a step further, I transformed the '3525's control circuit into a "modularized" half-bridge off-line SMPS, and powered a pair of 40W discrete Amps, with excellent results. I kept the same feedback compensation scheme, and achieved good results. Unfortunately, I have since disassembled the circuit for use of the parts in other circuits. Boy, am I wishing that I didn't do that!

Anyway, this is getting long-winded, so I will close by saying, that, in my experience, compensation on the control (primary) side with the 3525 is indeed possible. Pierre, can you go back and check your parts layout? You'll be suprised how much this can affect stability.

All the best,

Steve
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Old 29th September 2005, 05:05 PM   #9
Pierre is offline Pierre  France
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Hi, N-Channel.
Nice to hear that.
Althouth Eva's proposed approach will undoubtfully work better, I still think it is possible to operate SG3525 without additional opamps in a estable way.

Let's talk a bit more about the circuit you talk about:

+VCC (positive rail output) goes to a resistor, then photodiode anode, then TL431 _cathode_, then -VCC (negative rail output).
The terminal control of TL431 has a 69K resistor to +VCC and a 6.9K resistor to -VSS.
Is this correct?

Thus, if voltage increases, ref. voltage will increase and so TL431 zener voltage, so current through the LED will decrease.

So... the control side should be something like that:
Phototransistor collector to +12V, emitter to GND with around 1K resistor in series. The error signal is taken from the emitter, and goes to the SG3525 error opamp, configured as a simple unity-gain noninverting buffer, right? (inverting input to COMP, + input to phototransistor emitter.

COMP pin with R||C to GND (49.9k and 1000pF)

Is all this correct or is it wired in a different way? An schematics would be great.
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Old 29th September 2005, 07:56 PM   #10
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Pierre,

A few corrections on your last comment:

Quote:
So... the control side should be something like that:
Phototransistor collector to +12V, emitter to GND with around 1K resistor in series. The error signal is taken from the emitter, and goes to the '3525 error opamp, configured as a simple unity-gain noninverting buffer, right? (inverting input to COMP, + input to phototransistor emitter.

COMP pin with R||C to GND (49.9k and 1000pF)

Is all this correct or is it wired in a different way? An schematics would be great.

The (+) error amp input of the '3525 is connected th the junction between phototransistor's emitter and a 4.99kW resistor. The resistor goes to GND, and the phototransistor's collector goes to the Vref pin (pin 16) of the '3525, which puts out 5V up to 50mA.

The (-) error amp input of the SG3525 is connected to the junction of a pair of 4.99kW resistors. One resistor goes to GND, and the other goes to Vref.

The output pin of the '3525's error amp goes to ground, via the 49.9kW resistor and 1000pF cap in parallel.

I hope this clears up your confusion. Your understanding of things on the secondary side of the opto are correct.

Putting two 4.99kW across the +5V reference voltage gives a bias current of 500mA, for the inputs of the error amp.

There is a complete design procedure for sensing the output voltage of isolated DC-DC Converters the book titled "Power Supply Cookbook", by Marty Brown, ISBN: 075067329X, EDN Series for Design Engineers, Motorola Series in Solid State Electronics, Butterworth-Heinmann, MA, 1994, 248 p. Look on p.75 of the Revised 2001 edition of the book

His book uses a cut-n-dried approach to properly biasing and compensating feedback circuits on either the primary OR secondary side, depending on the topology used (flyback -v- forward), and whether the converter is output-isolated or not.


Steve
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