NE555 based boost whining

[rankot]

I made this NE555 based boost converter for my tube pedal, but I hear whining when it is on. Most interesting thing is that whining is audible from PCB, although circuit's output is not connected anywhere - I didn't even come to the point where I can test is it audible through signal chain - just fired it on to check voltages and it started to sing (and voltages are correct, BTW)!!!

So it seems that one of the components decided to try a new career as a singer and trouble is that I have no idea which one is that! I have built similar (or same) supplies before, and never had a trouble like this. Any hint is welcome!

[Rob Strand]

Sound from the PCB can come from the inductor and sometimes the output caps.

It might be the inductor saturating.

Try reducing the timing cap.

You might find the situation changes with light and heavy loads.

(I would have to analyse the currents and timing to comment further).

[Rob Strand]

From what I can see it should work, at least at light loads.
Very roughly a 1A inductor will give just under 1W output.
*However* I haven't checked the output ripple.

Maybe the 1uF cap on the output is too small.  What type of cap are you using?

Also, try putting a 10u to 100u cap across the 555 power pins.

[idy]

Yes, check it under load. Many power supplies make funny noises when not under load. Audio or electronic whines. Noises that go away when the supply is actually feeding something...

[rankot]

I already have 330u capacitor on 555 power lines, it is just not shown here. I will try to use different inductor (toroidal core instead of this cylindrical), and post what happens. Frequency does change when I adust voltage with trim pot. And putting top of screwdriver near inductor change the pitch too, like I built metal detector

I have built almost the same circuit on separate PCB, but with slightly different values and it works like a charm, no whining loaded or not. That version also has voltage doubler with diodes and capacitor, so it can provide upto 350V. I removed those parts since I need only 120V for this project.

[rankot]

Using toroidal inductor solved this problem. Obviously cylindrical inductors are not capable to withstand this, although I used two in parallel, both rated 1.6A max.

And one more question - do I have to use zener diode to protect this power MOSFET (IRF740) from static discharge, as I always do with small signal MOSFETs (like BS170), or not?

[Rob Strand]

It could be the inductor.  However I'm surprised you can hear it.

What 555 are you using LMC555 or NE555 or other?

The screw driver can affect things if it is magnetized.
The other way is the core is bobbin type:
http://images.lulusoso.com/upload/20120330/inductor_bobbin.jpg
http://www.token.com.tw/smd-coils/image/tw-bobbin-inductor.jpg

Also try a load.

[Rob Strand]

Using toroidal inductor solved this problem. Obviously cylindrical inductors are not capable to withstand this, although I used two in parallel, both rated 1.6A max.

It's possible.  With no load or a light load the current naturally decreases, *provided* the 555 can switch in a short enough time.

And one more question - do I have to use zener diode to protect this power MOSFET (IRF740) from static discharge, as I always do with small signal MOSFETs (like BS170), or not?

For that circuit you could get away without it.   To some degree it's a judgment if an ESD zap outside of the circuit will make its way to that point in the circuit.

[rankot]

It could be the inductor.  However I'm surprised you can hear it.

What 555 are you using LMC555 or NE555 or other?

It's NE555P by TI.

The screw driver can affect things if it is magnetized.
The other way is the core is bobbin type:
http://images.lulusoso.com/upload/20120330/inductor_bobbin.jpg
http://www.token.com.tw/smd-coils/image/tw-bobbin-inductor.jpg

Also try a load.

It is bobbin type, I didn't know the exact English word I still can't try it with load, but will do during weekend. However, replacing this bobbin type with toroidal inductor solved the problem.

[antonis]

replacing this bobbin type with toroidal inductor solved the problem.

You might have problems at start-up..
(rare but possible with heavy loads..)

[duck_arse]

toroids are good, aren't they? the bobbin type former is also known as a drum type. [as in cable drum, not as in snare drum.]

[Rob Strand]

toroids are good, aren't they?

For off the shelf parts operating at say less than 100kHz they are probably a safe choice.
For high current inductors they are likely to be powdered cores.
They will have a lower stray field than bobbins/drums.

Unfortunately magnetics is one of those topics where the performance depends on every single factor:
Material, wire size, frequency, how it is constructed.    Very hard to compare apples to apples.

For off the shelf parts you really don't know what you are getting.  If you wanted best performance for
production it would be easier to buy samples, try them and pick the best one!

For modern stuff operating at 200kHz to 500kHz this style of shielded inductor is likely to be better. They don't radiate much crap:
http://katalog.we-online.de/media/images/v2/Family_WE-PD3.jpg
http://img.futurenowinc.com/nimg/72/00/dc74586eb77593ddb14d3892178a-600x600-0/shielded_smd_power_inductors_with_1_20_to_5_600_h_inductance_ranging.jpg

[rankot]

I have finally tested it under load - and it works just as expected! No noise, voltage is stable. Pedal itself is a little bit noisy because it is still unboxed, but the sound is great. Few more tweaks and I will put a project here.