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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Heya Sig!
Cool to see you're fooling around with boost converters, though I do have a question; why are you using an ATTiny, vs. just using a 555 timer?
You can actually build almost exactly what you're attempting to do using that vs the ATtiny.
I think one of the biggest issues with the circuit is how complicated your gate driving is. Your gate driver IC's may be blowing up because of oscillations in their input; When the output reaches whatever you have it set at it'll drop momentarily, and then rise again repetitively. Problem with that is if that occurs too fast, it'll destroy your gate drive IC's because they're not made to run quite that fast. You need to add some hysteresis to prevent the oscillations from occurring quite that fast.
Alex's suggestion of adding a small cap (experiment with the value!) on the input to the ATTiny; that'll slow down how fast the input is and acts as the hysteresis.
You can also try series-ing some of the inductors you have. It sounds like you may have too low of an inductor value for it to work properly. Most SMPS inductors are usually low value, like 5-15 uH. You need to have around 100-200 uH for it to work more effectively, according to my experience with boost converters.
You can typically get an idea of how high inductance is of an inductor by looking at how many windings it has, and how thick the wire is. More windings and tinier wire usually means high value. USUALLY.
Another thing to deeply, deeply consider is that the FDL100N50F, while an absolutely wonderful mosfet, has an EXTREMELY high gate capacitance, which would explain why you're also having issues driving it. If you have something that's not quite so large it'll be a lot easier to drive.
I'm using the ATTiny/Arduino as I'm fresh out of comparators. I originally built the circuit using a 555 and it fared no better, and of course changing the frequency was much more of a pain as I was limited to real components vs. just changing some code. Same with the comparator aspect, it is a lot simpler to change one line of code than dig out two new resistors to raise or lower the voltage set point for the comparator, especially when parts are limited.
Yep, I tried adding several values of capacitance (300pF to 1000uF in x10 increments!) across the feedback and all it did was yield some atrocious ringing at the input to the uC. I mean it was simply just horrid. Cleanest signal was with no added capacitance.
I was thinking it may have been the lack of hysteresis to the gate drive ICs blowing them up but I'm only getting maybe 30Hz of oscillation between ON and OFF states of the uC. I have a monitor LED for when the over-voltage detection trips that turns off the output of the uC and at worst it isn't above the persistence of the eye, usually quite low at maybe 5Hz. AND they blew up identically when running with NO feedback whatsoever, just pure open loop. It was simply a 555 feeding the TC4420 which drove the gate through a 5R resistor. Blew up every single time in less than 10sec.
Yep, the Ciss is pretty gigantic, but I'm limited by what I've got on hand unfortunately. I've got some HUF75652G3s I can use too but I wanted something with at least 200Vds as I was hoping I'd be able to get about 170V @ 50mA output when going full tilt. Ideally I'd love to get 100mA out but I could make due with half that. Looking at the gate waveform though it doesn't look like the typical signs of too much gate capacitance (or insufficient drive) though.
You may be right about the inductors. I can try daisy chaining a few. I didn't think hundreds of uH would be necessary.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
It also might help to peruse through some SMPS ic datasheets, like
To get an idea of what kind of components they use, characteristics, etc.
But if your gate drive IC's are blowing up that frequently it sounds like there's something else amiss. What, I'm not 100% sure. My experience with gate drive ic's just shows not to leave their outputs open, make sure they have enough decoupling capacitance, and to make sure the signals feeding into them are high quality and free of HF oscillations.
I hear ya. I've pushed gate drive Ics to over 1MHz (both CW and with short pulse widths) in SSTC use with no issues, but a damned <100KHz smps and they die. Hell I've pushed those FDL100N50F's to 1MHz with no problems! Leaves me quite puzzled.
OOoh I like that LT1170! I love when they put application notes in datasheets!
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
do you use a breadboard? if so their parallel contact lines have an horrible inter-capacitance, and the adhesive underside tends to be a little conductive when humidity and HF meet...
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I wish I could calculate things properly for this, but I don't have a LCR meter, and these inductors are all unknown value, salvaged from various junk.
In that case, what you've made is a rig for measuring inductor value and saturation current, if you have a 'scope. Look at the voltage across R3, differentially, although it's easier to swap R3 and the inductor round so the resistor is grounded. It now becomes a current measurement resistor. The rate of rise of current gives you the inductance, the current at which the current suddenly increases is the saturation current. Now you've got the inductor values, conveniently measured at the sorts of voltage and power you will want for them, you can leave R3 out and build the SMPS properly.
If you haven't got a scope, then you're doing it the hard way
I wish I could calculate things properly for this, but I don't have a LCR meter, and these inductors are all unknown value, salvaged from various junk.
In that case, what you've made is a rig for measuring inductor value and saturation current, if you have a 'scope. Look at the voltage across R3, differentially, although it's easier to swap R3 and the inductor round so the resistor is grounded. It now becomes a current measurement resistor. The rate of rise of current gives you the inductance, the current at which the current suddenly increases is the saturation current. Now you've got the inductor values, conveniently measured at the sorts of voltage and power you will want for them, you can leave R3 out and build the SMPS properly.
If you haven't got a scope, then you're doing it the hard way
Yup I've got a scope alright! (See vid linked earlier to see it). I assume the basic V = L dI/dT is the applicable formula, right? If so I know the dI/dT by scoping the resistor, what do I use for V? If you could take a look at the video I posted and point out either a good method for getting a useful measurement out of the badly triggered mess I was getting or a means of getting a better triggering would be great. Extracting info out of the mess I saw seems daunting.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Ideally, you use the actual V that's across the inductor. To a first approximation, that's the supply voltage, as the drops across the switch and R3 will be relatively small while the inductor is at low current.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I'd always be tempted to LTSpice it, although LTSpice may not be able to handle some of the components, you can find out so much more from LTSpice particularly WRT the start up transients and so on; which could well be blowing stuff up.
You may need to redesign it for LTSpice to be able to handle it though; but even if you simplify it, you may be able to work out what's going on.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
especially with the ltSpice group on yahoo groups, you'll find heaps of models for you to play with that are not in their original database, and copy-pasting SPICE directives and models in your schematic to feed a model is not that hard if you look a bit on the net (you can even find good valve models)
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DIY Boost Converter: help me understand my failures please.
