crazy boost converter for LED use is too noisy!

If you *hear* anything from a capacitor, the displacement current is going to kill it. Seems your design needs improvement.

I'm just learning to make best guesses for pcb and component parasitics in spice for smps design. Throw in some Cs and Ls here and there and a sim'd smps gets really ugly. Ringing, spikes, oscillations, feedback, mistiming, heat, loss of control, EMI,.... huhhhh :(

For even more fun, make a controller from scratch too :P

D from BC British Columbia Canada.

The low sense voltage is to achieve high efficiency, i.e. you don't have too many LEDs in a string. If you are doing 90V output, dropping a bandgap worth of voltage across a sense resistor isn't that big of a deal, percentage wise. I'm trying to recall the hack. Here it is on the maxim website:

This isn't specific to any part, but rather topology.

I haven't done this first hand, so feel free to say I'm blowing it out my arse. There is always some sort of gotcha in real life.

The chips themselves often don't work the first pass, so I suggest not rolling your own. A few companies have tried this with disastrous results, i.e. recalls.

I did a self timed boost converter that needed a second pass due to not modeling enough stray on the pin where the inductor and diode intersect. Of course, if you are building a discrete design, your design mistakes are not as costly.

...

Yes, I've come to this conclusion as well :) The output capacitor is seeing voltages go quickly up to 80V, then while the circuit is not used the cap voltage leaks down to say 40V, then another PWM cycle comes and raises it to 80V again. What I'm guessing is that it is this repetitive high dV/dt behaviour that is 1) sounding and 2) destroying the cap (which in itself is a quite interesting phenomena which I'd like to know more about, but maybe more on that later :).

Apart from avoiding the dV/dt itself, are there any capacitor parameters or brand/models that are more tolerant to this ? E.g. putting multiple smaller capacitors in parallel. The current capacitor is a 1 uF/100V X7R part.

Best regards, Bjorn

...

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You mean the part in the Maxim note where there is a current sense resistor that says "7.5?" on it ? :)

Anyway, yes, thanks for the idea. The 0.5A passing through the sense resistor is PWM'ed at a quite low duty cycle so I could increase the resistor and voltage drop somewhat and take the resistor heat with no problem probably. This would probably reduce the impact of the noise on the current regulation feedback at least.

I'm still afraid of the magnetic coupling to other parts of the circuit though, and EMI in general, so I guess I have to revise the driving scheme anyway..

Thanks,

Bjorn

Why 90V and not, say, 25V * 4?

LEDs seem to parallel just fine under peak current...the "joule thief" I made runs at 5MHz and the two white LEDs light up just fine in parallel. Peak voltage is something like 3-4V.

Tim

-- Deep Fryer: A very philosophical monk. Website @

If the Vf-spread is truly random, this would probably work fine. A big problem is that if one of the strings are opened, the other will get the full current and will burn out as well (unless you also skip the current regulation and go with the old resistor-in-series current limiter).

Another problem might be if the different strings accidentally are manufactured from different batches having non-random Vf - for example, some manufacturers have 3 or more Vf bins in the production process, it is random from which bin you get your LED's but might not be random in the individual bags. Then one of your 4 parallell strings will need 18 volts and the other 25 volts. This might or might not be visible/desirable depending on the v-i characteristics of course..

/Bjorn

This sounds like a ceramic capacitor. I have known these to be piezoelectric, even microphonic. If it is not getting warm, it's probably OK - but I would be leery about putting anything in production that has capacitors producing audible sounds.

I did see mention of a current sense resistor in another post in this thread. I would try increasing it to get the current down (your duty cycle will have to increase).

I might also suggest a non-ceramic capacitor, of a kind with an AC voltage rating and noted as suitable for use in pulse forming networks, snubbers, etc.

- Don Klipstein ( snipped-for-privacy@misty.com)

I've designed a few of those but semi-discrete, the only chip in there being a Schmitt. All the way up to 12:1 step-up. Works fine, in production for >10 years now. The trick is to do an RF-style layout and _not_ split any grounds. And don't use boutique chips or the folks from purchasing will be all over you some day.

--
Regards, Joerg

http://www.analogconsultants.com/

If the circuit ran fine in LTspice, your pcb layout may be at fault. Switchers are very sensitive to pcb layout. I've seen switcher controllers go berzerk because the designer thought ground was ground. If you don't understand that wierd comment, then you need to read the section on layout in the data sheet.

When probing your circuit, you need to pay attention to oscilloscope probe ground lead length if you're using a 10x passive probe. You can't use the standard ground lead on a scope probe. Active differential probes work great for the low voltage parts on a switcher.

LEDs look like resistors in over-current conditions. I've sucessfully ran 6 strings of IR emitters in parallel running 1 amp thru each LED. The current sharing balances out fairly well. The emitters were rated for 50mA continuous. Amazingly, they worked for over 1 million pulses at 9A with a very low duty cycle.

You might want to post your pcb layout on ABSE for the switcher section to see if that could be the problem.

--
Mark

Thanks, yes, I'm going to dig out the active probes for further probing.

Interesting! I should hook up the LED's on a breadboard and see if they behave as nice..

Which of these two solutions for a low-dutycycle flash would you consider chosing then:

1) same as my original booster, but more LED's in parallell thus lower voltage and higher current 2) booster with the same lower voltage as in 1), but with a current as low as the PWM duty cycle charging a cap and then "flashing" using a standard linear current regulator for the LED's

The result would be the same, but the circuit in 2 would be more complex but would put much lower demands on the switcher.. OTOH it would perhaps put more demand on the cap, which would have to be specified for high di/dt use.

/Bjorn

Actually, I screwed up. Take a look at this chip:

This is about as simple as a boost gets. Now look at the voltage divider. Replace the resistor from "output" to "FB" with your LED string. The feedback loop will set the voltage across the lower resistor to that of a bandgap. The value of the resistor sets the current in the LEDs.

This is how some of the early LED drivers were done. Now a full 1.24V across the lower resistor wastes power, but if you have a long LED string, the waste is not significant.

You don't need to use a Maxim boost. There are a few vendors of this minimal style boost chip. I'm pretty sure National makes these as well as JRC. The advantage here is the voltage levels are higher, so there is less of a noise margin issue.

Maybe 10uF will give less change, but that will be at the expense of a much larger charging current over a brief time, (over?) stressing something else.

I would use a lower voltage, since it's easier to design a boost converter. There are a few converters with an integrated driver transistor if you keep the voltage below 36 volts. Since you don't show a schematic, hard to say what your circuit is doing.

In the past, I have used a boost converter and set the current so it will charge a capacitor bank in a timely fashion to a specified voltage. If you connect a small capacitor from the voltage feedback input to ground, you can slightly overcharge the capacitor bank which will shut down the boost converter until you fire the emitters. Use some beefy ceramic capacitors (around 10uF) near the IR emitter string to handle the fast edges. Use long life, low esr aluminum electrolytics for the bulk capacitance. Look thru Digi-Key for suitable capacitors.

This sort of scheme will give you a output pulse with decreasing light output over time. The amount of droop is controlled by bulk capacitance and pulse width.

Since you are over driving the emitters, be sure that the light output remains stable with a stiff supply for your particular scenario. If it starts to droop, you are overheating the emitter die. To look at the light output, you need to use a reverse biased PIN photo detector if you're below tens of microsecond pulse widths. Hamamatsu S5973-01 works well and is about $20. Pearson current transformers work well for measuring the pulse current.

The IR flash unit I built incorporated 14 parallel strings. Each string had 10 series connected emitters. Current was 1.0 A thru each emitter. I used a power FET to flash the emitters. Pulse duration was in the single digit microsecond region.

--
Mark

While I'm of the school that pulsing LEDs is more efficient than continuous illumination, there are some that make the opposite argument. Why not try the design I suggested in my second post, which would dump a regulated current into one string of LEDs.

If you want to pulse the LEDs with regulated current, i.e. your solution #2, consider something along the lines of "current regulated diodes." They are just jfets running at IDSS. I think it would be difficult for any feedback circuit to regulate the current over a short pulse.

The pulsing in this case is because the camera does not take images continously anyway.

The design I started with was the same as in the Maxim application you suggested - and the problem was partly the low feedback voltage, which I easily raised to 1V, but also partly (and mostly) the inherent noise- generating capability of switching a 4-5 amp current at 1 mhz through a large inductor on a PCB... that will never be silent, it seems.

This is interesting. I looked into the current regulating diodes but most off-the-shelf parts seemt o be in the < 50 mA range while I need

500 mA. Should I "roll my own" with a JFET then perhaps ? And spice simulate it..

(The jfet is a feedback circuit as well of course, albeit with a shorter roundtrip maybe than an op-amp and a BJT....)

/Bjorn

The JFET is open loop.

I didn't catch that this was for a flash. What kind of synchronization time do you require? What I"m thinking is you could just "charge" and inductor, then dump the energy into the string of LEDs. How does the camera tell the "flash" to turn off? Or do you put out the same amount of light each time? What I suggest is writing an objective first, then the circuit topology will follow.

Consider a toroid. As they say, "toroids don't talk much".

Very tolerance prone. And expensive if you want to by one at a desired current. IOW those have become boutique parts.

--
Regards, Joerg

http://www.analogconsultants.com/