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SMPS control loop estability
SMPS control loop estability
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Old 29th September 2005, 09:42 PM   #11
Pierre is offline Pierre  France
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A lot of thanks for the clarifications, N-Channel.
I am going to test that. Let's see how it works.

A couple of things more: in your previous post, I think you have put the TL431 polarity backwards. The cathode should go to the cathode of the LED while the anode goes to -37V.

Did you test this supply with light loads or no load at all? Did the output go higher than being loaded? I am particullarly worried about that because I plan to set the voltage to +/-60V, but don't want it to go higher than that with no load as I am using 63V capacitors and the amplifier can be damaged also when no audio power is produced and hence current drawn from the supply is very low.
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Old 30th September 2005, 07:17 AM   #12
Pierre is offline Pierre  France
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After thinking a bit more about N-Channel proposal, I see one problem when output voltages are high (say, +/-60V), as TL431 only accepts 36V, unless the resistor in series with the photodiode is absorbing the excess voltage drop.

However, there is still one thing I don't understand about the secondary part of the circuit:
TL431 works as a "variable zener" whose voltage is V=Vref*1+(R1/R2), when R1 is connected to the cathode to the adjust, and R2 from adjust to the Anode. (according to the datasheet basic application).
But, in the proposed circuit, it is not wired that way, so, how can I calculate V between cathode and anode as a function of output voltage and the resistors?

If I knew that, I could dimension the series resistor accordingly so LED current is ok and voltage rating of the TL431 is not exceeded.

Sorry if that's a very basic question but I am a bit confused.
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Old 30th September 2005, 02:15 PM   #13
N-Channel is offline N-Channel  United States
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Pierre,

You're right about the TL431. I mis-described its connections backward. However you describe backwards, the anode-cathode connections of the TL431's LED.

I'm not too sure about the resistor in series with the opto's LED, as I do not have the design procedure in front of me, but I think it goes something like this:

For a 500uA biasing current at the error amp inputs with an optocoupler having a 100% CTR, the biasing current for the opto's LED ahould also be 500uA, so size the resistor for a current of 500uA.

The TL431's Vref is 1+(R2/R1), for R2 being the upper bias resistor, ands R2 being the lower bias resistor. So, for 74V (+37V-(-37V)), the Vref of the TL431 is 1/11th of this voltage difference, or (74V/11) = 6.72V above the -37V bus, or -30.27V.

So, for a current of 500[FONT=symbol]m[/FONT=symbol]A sinking into the '431 from the +37v line, we get 37-(-30.27) = 67.27V. Now take 67.27V - V(LED - opto) = 67.27-2.2V = 65.072V.

And since R = V/I, 65.072V/.0005A = 130K. For 1.0mA of bias current, make the series resistor 65.072K. (Use 66k as the next std value). Obviously, for your application of +/-60V, your values will be different- this is just an example.

Anyway, these are for optos with 100%CTRs. If yours is lower, say 50%, then a 2.0mA LED bias will yield 1.0mA at the error amp input. Also, the two 4.99k resistors tied to the other error amp input will now be 2.50k across the 5Vref of the SG3525. Taking a CTR of 50%, the biasing resistor should be 32.5k. Use 33.2k as the next standard 1% value.

This arrangement keeps things below the TL431's 36V limit.

Hope this Helps.

Steve
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Old 30th September 2005, 04:30 PM   #14
Pierre is offline Pierre  France
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Wow! Thanks, N-Channel.
I will have a look at it slowly as soon as I have time and luckily test it soon.

Best regards,
Pierre
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Old 30th September 2005, 05:42 PM   #15
N-Channel is offline N-Channel  United States
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No Problem!

After going back and reading my last post, I discovered some minor math errors, but the general theory holds. Also, I should have said thar R2 is the upper resistor, and R1 is the lower resistor. I acciidentally listed R2 as both upper and lower.

Here is a pic of the DC-DC converter I built. More to follow......

Regards,

STEVE
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File Type: jpg rkv dc-dc other side.jpg (74.4 KB, 543 views)
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Old 30th September 2005, 05:46 PM   #16
N-Channel is offline N-Channel  United States
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Default SG3525-based DC-DC Converter pics

Here's another
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File Type: jpg rkv dc-dc input side.jpg (75.2 KB, 510 views)
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Old 30th September 2005, 05:47 PM   #17
N-Channel is offline N-Channel  United States
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Default SG3525-based DC-DC Converter pics

...........and another........
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File Type: jpg rkv dc-dc output_side.jpg (75.5 KB, 474 views)
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Old 30th September 2005, 05:48 PM   #18
N-Channel is offline N-Channel  United States
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Default One More

.....one more.......
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File Type: jpg rkv dc-dc side.jpg (88.3 KB, 461 views)
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Old 30th September 2005, 05:57 PM   #19
N-Channel is offline N-Channel  United States
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Default Sg3525

...last one!

These are old parts. Note the Motorola symbol on the '3525. In fact, all the semi's are Motorolas, the PWM IC (SG3525AN), the N-Channel MOSFETs (IRF540), the output rectifiers (MBR10010), the Optoisolator (MCT-271), and the 19V 1W zener diode (1N5???), the TL431, and the reverse-polarity protection diode (1N5406).

On subsequent versions, I will substitute the MCT-271 with the CNY17-2, which has a CTR ranging from 63-125%, or the 4N35, which is steady at 100%.

Also, note the pot I put on the very edge of the board- this allows me to tweak the output voltages +/- a few volts.

Also, the biasing resistor has been lowered from 36.5k to 33.2k, yielding voltages for the amp's output section at +/-33V, and voltages for the amp's input stages at +/-42V. This tiered output arrangement is necessary for the amplifier's output MOSFETs, which require a voltage higher than the Vdd and lower than the
-Vss to operate over their entire linear region.
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Old 1st October 2005, 01:28 AM   #20
Eva is offline Eva  Spain
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I'm just curious: Where are the ouput inductors?
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