Re: battery chargers, again

How many MHz is "many"? And how much power does the biggest of the switchers have to deliver?

It almost sounds like linear is the best option if you can stomach the dissipation. Or triple banding but that doesn't work if you must keep a full spectrum view at all times at full bandwidth. If that's the case and you want switchers you'd have to run them above the upper end of your spectral range. Some ISM frequency like 13.56MHz or 27.12MHz so you don't get into trouble with the federales.

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line,

No revenue product then, I assume.

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On a sunny day (Wed, 30 Sep 2009 14:29:06 -0700) it happened John Larkin wrote in : Yeah, I suppose so. Spread-spectrum will have to do. It has its own

Big is a relative thing, with your metal housing maybe not even needed. Linear will be quieter. You do not care about wasting Watts, as you previous project proved.

On a sunny day (Wed, 30 Sep 2009 14:41:34 -0700) it happened Joerg wrote in :

supply line,

You are weaseling away from the subject of 10 mV ripple on supply lines.

supply line,

No. I said 10mV ripple on supply lines do matter in ultrasound, it's way too much when you must listen to the wee rumbles in the aortoa of an incredibly obese person. I've got well over 20 years in that business.

Anyhow, if I bought a lab bench the supply that had 10mV ripple on the output it would go back same day. This is not to diss you PIC solution, that can very well have its place and certainly for experiments with digital stuff. Just not with what I usually do. FYI, one of my recent switchers (30W) has 600uV of ripple under full load.

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Something like this

?

Best regards, Spehro Pefhany

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20-ish. And just a couple of watts. I need 5, 3.3, 2.5, 1.8, 1.2, and

-5. All from +12.

John

It will take a big spectral spike and make it into wide and low hump. If the psd gets below my 1 nv/rtHz noise floor, it's as good as gone.

No, we've got to digitize in one burst and see what's there. The next shot may be different chemicals, and we need all the data.

John

On a sunny day (Wed, 30 Sep 2009 15:40:50 -0700) it happened Joerg wrote in :

Indeed. It matters *how* you use the lab supply. Normally chips, preamps, things that need ultra low ripple have the filtering right next to it, RC for example, and you are not going to connect you bench power supply directly to those, but to the normal power input of the board.

As far as radiation from the supply leads goes, well apart from the usual ferrites and small value caps to ground, I am sure there are many applications like yours that could pick it up, especially when you open the box when you are measuring something. So that would require some extra filtering in the power supply. When we talk *industrial*, you see supplies specified at any ripple value, from

5 % to some milli volts. There are zillions of different models. Often you see 24V, if it is for relays then the ripple value is insignificant. In an industrial environment there usually are very high electrical noise levels, from relays, motors, machines, what not.

So it all depends what it is for, and how you use it. Some very advanced companies with top of the line ultra high speed products, their executives use the power supplies to charge their car battery. In such a case the ripple is not very important either.

I dreamt up a simple circuit to add to the PIC switcher to make it a true current mode controller. It only uses 2 transistors and 3 resistors, so ultra cheap :-)

+12V U out | | [ ] R0 [ ] R1 | | ------------- | | | | e e | +5V ref -- b b-------------------------| c c | | | | | [ ] R4 | | | | | | to current | | [ ] comparator [ ] R2 | | | /// /// ///

The way this works is that it lowers the voltage of the current limiter setpoint when the output voltage rises above it's setpoint. With 0V on the pot the current limit is at it's lowest value. The output voltage will be regulated at 5 x R2 / (R1 + R2), or you need 5V over R2 for equilibrium.

R0 can be a current source too of course, or you could use an opamp. You can make it as sophisticated as you want. Have not tried this yet, maybe in a next one :-) It also frees up one more PIC pin, that input can then be used for other goodies.

So, when are we going to see some circuits or code from you Joerg?

A. Spice? B. Breadboard? C. Take a chance? D. Go linear with big heat sinks?

D. Good ol' American Brute Force; sorta kill two birdies with one stone? ;-P

Cheers! Rich

Theoretically you could run it at 27.12MHz but that's nasty. If you can't stomach the dissipation of linears then you may be stuck with some spectral dodging scheme like the one I described. We do that in ultrasound sometimes but there I could always get a few clock control lines from some a DSP or FPGA. If I bought the digital and SW guys some beers that night ...

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Nasty situation ...

If you can confine them all to a corner and the loads don't change too rapidly you could LC-filter the dickens out of every rail.

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right next to it,

directly to those,

ferrites and small value caps to ground,

especially when you open the box when you

from 5 % to some milli volts.

levels, from relays, motors, machines, what not.

their executives use the power supplies

Ok, you mentioned it was a lab bench supply, not industrial. But you should be ok if you normally don't do too sensitive analog stuff or have another supply for that. A serious LC filter will also help but only if no fast load changes.

current mode controller.

setpoint when the output voltage rises above it's setpoint.

over R2 for equilibrium.

goodies.

Ok, other than freeing up a pin I can't see much advantage in tapping into the current sense here. But if the numer of free pins is close to zilch right now that'll be different. Pretty soon you've arrive at the usual discrete solution. Hint: My PIC was often a CD40106, with current mode control :-)

When I retire. Obviously I cannot share schematics of designs I do for customers here. So for now I'll have to limit myself to trying to help folks who got stuck, or ask when I get stuck.

But even when I retire I am not so sure about doing much in electronics. A friend and I have mulled over reviving an old passion of ours, brewing beer :-)

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I've got a couple of switcher eval boards around. Maybe I should fire some up and experiment with FMing the switcher freqs and see what that does to the spectrum and to the output voltage.

Or better yet have somebody do it for me.

John

Yes. And maybe slit the ground plane in artistic patterns to confine most of the circulating currents to the "power" corner of the board.

This is it so far:

ftp://jjlarkin.lmi.net/Rev30.jpg

The switchers are upper-right, and the differential-input RF stuff is the two SMBs south-east of center.

The layout is tedious, because we're optimizing the BGA FPGA routing as we go along... can't just draw the schematic and go forward to the board.

This *is* rev 30.

John

But first check the limit values. Or at least try them out with a potmeter for the frequency set resistor. Easiest would be to have PWM chips with a sync input.

Can't you get a student who is eager to learn and make a buck from the university? Maybe throw in free parking :-)

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One of my guys wants to have the FPGA synthesize a bunch of FMd clocks and dump that into the reg sync inputs. I was planning to make an analog triangle and squirt a little of it into the pins where the freq set resistors connect.

We usually have an intern or two for the summer, but we seem to have run out.

John

Reference everything to a virtual ground at +6 from the supply negative, use LDOs for the 5's, and ordinary regulators for the rest? If there's no COTS LDO at +5 or -5, it might be fun to build one with a chip and low Rdson MOSFET. ;-)

Cheers! Rich

saunter

Slits can cause grief once it is connected to the outside world. Residual noise flowing on the power input side and the input cable shields (which will meet somewhere inside the city limits) eventually makes a loop. So far I've never used slits, only removed them.

Hard to see, the jpeg isn't very hi-res. I think the inductors are crucial here. Even the ones that are called shielded by the marketeers have epoxy gaps of 20mils or more. If you can get toroids on there I'd really do that. Toroids don't talk.

Ouch.

Double-Ouch. I am firing up rev 1 of a really unorthodox cicuit this afternoon. Wish me luck, and if you hear a muffled boom north-east from you guys ...

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