MicroController control for switching regulator

What did Jan Pabntelje's uC switcher do? I remember it but not the details.

Power supply design is an art. First, you have to decide EXACTLY what you want. Not, kinda-sorta, EXACTLY. The electrons have no concept of battery charger. All they know is voltage and current and fault conditions and how to explode stuff. The devil is in the details. The rest is 99% boiler plate copy the datasheet example and 1% "why did the board catch fire?" And fixing that

I'll rattle off the top few things I can remember on short notice.

Safety first. Wear those safety glasses. Somewhere in Wilsonville, Oregon, there's a ceiling containing many chunks of epoxy and silicon from switching power supplies I exploded during evaluation. I also expect the floor still smells like pee.

The datasheet gives you clear instructions on component selection given a fixed in/out voltage and current. Extrapolating to a wide range supply is not so simple. The Onsemi datasheet mentions core saturation, but not in a way that emphasizes its importance...AKA 1% Exactly where you connect the wires is CRITICAL. They drew the circuits funny with all the connections to a single ground node for a reason. And many of the other connections can be just as critical...I can tell you stories...

If your battery charging experiments are limited to charging one technology and voltage, your requirements just got simpler... And more complicated. Using the example circuits, it's easy to come up with a design that goes up in smoke if you connect the battery before the input power. If the supply doesn't explode, the battery might.

A lot depends on exactly what you're trying to accomplish. I wouldn't attempt to build any power supply without access to an oscilloscope, but that's just me. I've redesigned many a power supply designed by engineers who didn't take the project seriously. That 1% can look mighty big when you're surrounded by smoke and flying debris...and you smell like pee.

If you want to build a power supply, go for it. If you want to experiment with batteries, buy a power supply. When your power supply fails shorted and the battery explodes, things get ugly. YMMV

Overkill.

Pabulum Boy!

The new Super Non-Hero!

Use a fast microcontroller (IOW: ARM) and use the PWM + ADC from the controller to do what you want.

The ADC in the Arduino is not too flash, as in not being stable. You will see the value "jitter" up and down. Previous advice about using purpose designed devices would seem a better idea to me :-)

Cheers .......... Rheilly P

Use this pair and make your own switcher and controller.

UC1526J & UC1832J That, with a couple op amps and you can make it a voltage and current controlled and monitored supply. Drive a pair of FETs or the like into your transformer, and then rectify or multiply and store for the final voltage. on the output. Shunt and tap that node for the feedback, monitoring and control. Voila!

On a sunny day (Tue, 25 Dec 2012 20:46:09 +0800) it happened Dennis wrote in :

It is my lab suply, on 12 hours a day: http://

I dunno about arduino, but I use a Microchip PIC PWM output, and monitor current in real time (with an internal PIC comparator), called cycle by cycle current limiting. The real time thing avoids the need for a specialized chip. Can arduino do that? Looks like overkill to me even if it can.

The LM2576 could be controlled by a multiplying DAC over some range, seems a bit dangerous...

(reference DAC on output, output DAC on feedback of LM2576). Output voltage limited to whatever the DAC can stand, add resistor divider for more, could even work, but DAC to zero would create zero feedback and push the output way up and saturate that inductor I am sure.

------- R -------- Vout |

And you cannot go to zero that way.

Cheers TallyWhacker - I'm just in the process or rebuilding my filters..........

Generate control DC voltage by microcontroller PWM. Add this voltage to the feedback input of the switcher. Thus higher PWM voltage results in proportionally lower switcher output. Two resistors, capacitor and little math is all it takes. Done that many times. Works very well.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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You might want to look at Microchip's new line of PICs with on-board switchmode controllers:

They are only about $2, and include other advanced peripherals:

For a battery charger, you may do better with one of the dedicated battery management ICs, especially if you are using LiFePO4 or NiMH or other more critical chemistry:

Paul

On a sunny day (Tue, 25 Dec 2012 09:29:20 -0600) it happened "Vladimir Vassilevsky" wrote in :

Right, way to go! That is what I did here:

Software lowpass filter the ADC samples. You'd want to do that even if the ADC was perfect. Any signal near a power switcher is going to be noisy.

The most elegant way to do this job is to use the uP PWM to drive the switcher stage, and use the ADC to measure input voltage, output voltage, and current.

Or, the OP could use a DAC (or a PWM made into a DAC) to push the feedback node of the Simple Switcher.

There are battery charger chips designed for universal applications, i.e. chemistry independent. I would go that route.

When it comes to safety, the verbiage is often repeated in the datasheet just in the event somebody doesn't read the whole thing. Sometimes repeated three times.

Got any tips on how to do this? At 20 kHz. you have 50microseconds to do two measurements, update the displays, and decide whether and how to tweak the PWM.

Even if you dedicate the whole processor to the task, you still gotta get data in and out. A 50 millisecond key debounce timer is an eternity when the output is shorted. So you make everything interrupt driven...and it gets messy.

For a 20kHz. PWM frequency, can you really get enough resolution? Or do you dither the clock and the duty factor or????

Got some sage advice on the tricks of the trade?

You don't need to do the measurement at the same rate as the PWM PRF. Even if you did, a low end PIC can cope easily.

Actually it gets remarkably elegant using interrupts. YMMV I guess. High priority interrupts A to D that controls the PWM driving the switching FET and low priority interrupts for the keyboard, serial port or whatever.

I've used a bit higher, 28Khz or so. Depending what you want (battery charger?)

you did, a low end PIC can cope easily.

FET and low priority interrupts for the keyboard, serial port or whatever.

charger?) 6+ bits is fine -

it will dither due to the measurement on the A to D correcting it.

Ok, if you're gonna tolerate the noise of a cycle/cycle regulator, I guess 2% resolution is fine. I don't have a problem with dither as long as it's filtered and doesn't show up in the output. I've never tried to use a switcher as a bench supply. I think I'd want to start with a high-res PWM and cycle skip to handle load dumps.

in

The easy way is exponential smoothing, the equivalent of a 1st order RC lowpass filter.

Filt_Val = Filt_Val + (ADC_Val - Filt_Val) / K

where, if you work in intgers, you can do the divide-by-K as a right shift.

Filt_Val = Filt_Val + ( (ADC_Val - Filt_Val) >> N )

It's all signed. N might be, say, 16 or 256 or something.

The data has to be aligned such that you don't roll bits off the end when you do the right shift. Using a 32-bit Filt_Val simplifies life; the ADC sample should be up-shifted maybe 16 bits before the math is done.

If you do this in floats, it's simpler but slower; don't worry about alignment at all.

Filt_Val = Filt_Val + J * (ADC_Val - Filt_Val)

since multiplies are faster than divs. J = 0.01 would give the filter a time constant 100x the sample rate.

If you do all the math as ints, that should be possible. We've done more complex stuff, filtering and PID control and error checks at 100 KHz on a cheap ARM.

If you don't have a real DAC, run the PWM as fast as it will go. That's not the same rate at which you might run the filtering and control algorithm.

One trick is to filter the ADC data twice; a fast filter for the control path and a slow one for display cosmetics. If you filter too much in the control path, the loop can go unstable. A P+I controller with low P may not need filtering at all.

Thanks for the replies.

So around 30 years ago in school we made an oscillator using an Op Amp, IIRC, this was a comparator circuit with hysteresis and the output caused a capacitor to charge/discharge. What if the same type of circuit was used in a buck regulator so when the voltage reached a certain level, the output would go low, when the output dropped below a lower level, the output would go back high? Then an analog output could determine the comparator voltage. That may not be perfect SMPS design but I would think it would be good enough to charge batteries, probably need to switch off to take measurements or at least filter the measurement.

RogerN