Author Topic: MC34063 high voltage dc-dc boost converter  (Read 21425 times)

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Offline T3sl4co1l

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Re: MC34063 high voltage dc-dc boost converter
« Reply #75 on: January 23, 2020, 05:25:56 pm »
Yes, start with core type, roughly.

Typically 20 < μ_r < 60 gives best compromise between core size and copper losses, with values outside of this range being useful in less common conditions (typically, lower mu at higher frequency; or higher mu where number of turns or wire length is weighted more heavily than energy density, HV flyback transformers being a typical example).

Typical powdered iron materials are #18 and #52 (Micrometals mix numbers), MPP, or branded formulations like Hi-Flux, Sendust/Kool-Mu, etc.

Typical ferrites are gapped shapes.  Ferrite toroids are either hi-μ or hi-freq materials, and you can't easily gap a toroid, so if we're talking just toroids, yeah.

Core size: we can use an energy argument here to find minimum.  We may however still need a larger core, or lower ripple fraction or lower frequency in the design (higher inductance, slower Fpwm, slower control loop) to deal with losses, in which case that needs to be fed back into the design, and the inductor design/selection process starts over.

Energy storage is given by the Maxwell stress: σ = B^2 / (2 μ)  This has units of pressure, which is also units of energy density.  Set B = Bsat, then divide your energy storage requirement E = 0.5 L I^2, by this, to get core volume v_e.  Select parts with equal or greater volume.

Note this is inversely proportional to μ, so a lower μ core stores more energy -- but it also requires more turns for the same inductance, which increases copper losses.  The preferred range of μ is driven by typical frequencies and the resistivity of copper, of all things.  (After all, if we had casual room-temperature superconductors, we wouldn't need cores at all, just pile on thousands of turns of hair-fine wire and not care. ;D )

Core loss: determine Bpk for your application (i.e., the ripple fraction).  Find the material's core loss curves, and match up Bpk and F.  The power loss density will typically be 100-1000 mW/cm^3 (or kW/m^3 or however they label it).  Higher losses are acceptable in smaller cores (say 500 for T50 size cores?), because the surface area/volume ratio is more favorable, and lower for larger (say 100-200 for T300 size?).  You can calculate or find the core surface area, and use a typical convection rate, to solve for temperature rise, and in turn, loss density.

(Again, as a density, multiply by v_e to get watts.)

Finally, check that your wire fits the core.  Typically, Bsat will require 200-2000 At, and with passive convection, over 500-1000 At is where things start getting impractical (in terms of winding area and temp rise).

If any of these conditions is not met, try the next larger size core, etc.

Should be able to find candidates in a handful of iterations, and this method can also be applied to a table of parts to automatically select or rank them.  There are unfortunately too many variables to write out a closed-form solution -- although this is a bit easier when manufacturers provide core loss formulae.


Going back to ferrite: choose μ the same way, then calculate the air gap based on μ_avg or μ_eff (which, I don't remember the formula offhand but it's easily found; or use the figure from the datasheet, if pre-gapped cores are used).

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Online Siwastaja

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Re: MC34063 high voltage dc-dc boost converter
« Reply #76 on: January 23, 2020, 05:42:22 pm »
Also note this good source of practical information:
http://pigeonsnest.co.uk/stuff/core-saturation.html
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #77 on: January 23, 2020, 05:45:06 pm »
Thanks guys, apparently this is even more complex than I thought. I'll take my time to educate myself on the topic

ETA:

Also note this good source of practical information:
http://pigeonsnest.co.uk/stuff/core-saturation.html

Loving the rant and the colorful language there, LMFAO.
Great resource, bookmarked!
« Last Edit: January 23, 2020, 05:52:04 pm by dazz »
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #78 on: January 25, 2020, 11:45:58 pm »
OK, so I finally got around to building the prototype of the boost converter, schematic attached.

So far it's a massive failure. I feed it 12V and I get 11.7V at the output, and the mosfet get's rather hot at some 100ºC after a minute or so. That's with no load at the output. It's also drawing 1.6A from the power supply, I'm guessing most of that is being dissipated by the mosfet. I have this heatsink for the mosfet: https://www.ebay.com/itm/2PCS-TO-247-TO-220-34x38x12mm-Aluminum-Heatsink-Cool-Heat-Sink-Sound-Transistors/223287755102.
The inductor is a 100uH 0.4 ohms toroidal
Any off-the-top-of-the-head ideas on what I should check, please? My equipment is very limited unfortunately. No oscilloscope available for now
« Last Edit: January 25, 2020, 11:47:32 pm by dazz »
 

Online Siwastaja

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Re: MC34063 high voltage dc-dc boost converter
« Reply #79 on: January 26, 2020, 03:59:30 pm »
Get an oscillosscope, sorry but you just can't do switch mode converters without one. A cheap Rigol will do fine.
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #80 on: January 27, 2020, 03:47:56 pm »
Get an oscillosscope, sorry but you just can't do switch mode converters without one. A cheap Rigol will do fine.

Yeah, I know, I'll see what I can find. Thanks Siwastaja

BTW, guys, I sort of got it working. Sort of.
I replaced de 1N5817 diode with a proper rectifier diode and it's now outputting hundreds of volts. How many? Not sure because the hysteresis is insane! With a 100uH toroidal inductor, and the pot set to the minimum output voltage that should produce some 100V, it jumps to 320V, Vref to 3.5V, then the IC apparently turns off and the voltage drops to 100V, Vref to 1.2V, and back again to the massive spike.

I then replaced the inductor with a 470uH one, and as a result, I get 460V at the output and 5V at Vref. It stays like that, as if the IC never stops switching. Had to stop the test because the output cap is rated at only 450V. Also the inductor gets scorching hot, which I believe is to be expected if it's conducting all the time.
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #81 on: January 27, 2020, 05:14:15 pm »
Put a load on the output. Running a boost (or flyback) converter without a load is just asking for trouble.

And yeah, get a scope. Heck, even one of those janky little DSO Nano's is better than nothing.
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #82 on: January 27, 2020, 05:19:54 pm »
Put a load on the output. Running a boost (or flyback) converter without a load is just asking for trouble.

And yeah, get a scope. Heck, even one of those janky little DSO Nano's is better than nothing.

Oh, I thought it was only flybacks that needed a load to be stable. Right now I have a tiny 680K load, that's less than 1mA, will try that, thanks.

And I'm already shopping around for a scope  :-+
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #83 on: January 27, 2020, 06:22:33 pm »
Oh, I thought it was only flybacks that needed a load to be stable. ...

Basically, any switchmode converter in which power is supplied to the load during the switch off time is going to be unstable without a load, and the boost converter does just that, same as the flyback.

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #84 on: January 27, 2020, 06:30:14 pm »
Oh, I thought it was only flybacks that needed a load to be stable. ...

Basically, any switchmode converter in which power is supplied to the load during the switch off time is going to be unstable without a load, and the boost converter does just that, same as the flyback.

It's the sudden current spike at the inductor when the magnetic field collapses that does this, right? I noticed the effect got progressively less noticeable as I decreased the inductance of the coil. I'm at 33uH right now.

Building a constant current load with a mosfet now to test it under load. What kind of load current should I expect to make it stable? Because at 300V even a few mA will mean a few watts of wasted power and a pretty large dummy load resistor. I guess I'll find out soon enough  ;D
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #85 on: January 27, 2020, 08:36:15 pm »
Basically, any switchmode converter in which power is supplied to the load during the switch off time is going to be unstable without a load, and the boost converter does just that, same as the flyback.

It's the sudden current spike at the inductor when the magnetic field collapses that does this, right? I noticed the effect got progressively less noticeable as I decreased the inductance of the coil. I'm at 33uH right now.

Kinda/sorta/almost. The nature of an inductor is to resist a change in current, right? When the switch turns on in a boost (or flyback) converter current builds up (relatively) slowly and linearly, and when the switch turns off the current that was flowing in the inductor wants to keep flowing, creating whatever voltage is necessary to maintain said flow. Basically, when the switch turns on the boost inductor acts like a current sink and when the switch turns off it acts like a current source.

Building a constant current load with a mosfet now to test it under load. What kind of load current should I expect to make it stable?


Usually somewhere between 1% and 10% of full load power is necessary to maintain regulation. I don't know how your ratio-extender kludge with the '34063 is going to behave in real life, however; it certainly didn't behave well in the LTspice simulation...

I would also recommend against a beginner using a MOSFET current sink (load) to test switchmode converters because that pits two control loops against each other (the current regulation loop on the constant current load and the voltage regulation loop of the switcher), however given the '34063 is a hysteretic controller there is less risk of inciting full blown oscillations here (with the inevitable release of magic smoke soon thereafter).

 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #86 on: January 27, 2020, 09:52:07 pm »
Usually somewhere between 1% and 10% of full load power is necessary to maintain regulation. I don't know how your ratio-extender kludge with the '34063 is going to behave in real life, however; it certainly didn't behave well in the LTspice simulation...

What's wrong with it, please? I thought it was performing pretty well in the sim with a very respectable (I think) 85% efficiency

I would also recommend against a beginner using a MOSFET current sink (load) to test switchmode converters because that pits two control loops against each other (the current regulation loop on the constant current load and the voltage regulation loop of the switcher), however given the '34063 is a hysteretic controller there is less risk of inciting full blown oscillations here (with the inevitable release of magic smoke soon thereafter).

That might explain why I can't seem to get it to work with the boost converter. This is the one I'm trying:

« Last Edit: January 27, 2020, 10:12:44 pm by dazz »
 

Offline MagicSmoker

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Re: MC34063 high voltage dc-dc boost converter
« Reply #87 on: January 27, 2020, 11:24:32 pm »
...I don't know how your ratio-extender kludge with the '34063 is going to behave in real life, however; it certainly didn't behave well in the LTspice simulation...

What's wrong with it, please? I thought it was performing pretty well in the sim with a very respectable (I think) 85% efficiency

It made the '34063 model run in bursts with wildly varying pulse widths from cycle to cycle, some of which were way too long (ie - allowing current to build up way too high in the boost inductor). That's a good way to blow up FETs. That said, this could be the result of a dodgy '34063 model. I didn't investigate the issue too deeply (not at all, actually...).

That might explain why I can't seem to get it to work with the boost converter. This is the one I'm trying:
~snip

That circuit is almost guaranteed to oscillate because the op-amp is saddled with driving a capacitive load directly (the gate of a MOSFET is, essentially, a capacitor). At the very minimum there needs to be a resistor in between the output of the '358 and the MOSFET gate (take feedback directly from the op-amp output, however); 100R should work fine here. Also use a FET with an appropriate voltage rating for the task (the IRFZ44 is rated for 55-60V max).

That said, it's hard to argue with the cheap-n-cheerful approach of just tacking on a bunch of 330k/0.5W metal film resistors in parallel as needed to provide about 1mA of load each at 300V.
 

Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #88 on: January 28, 2020, 03:25:54 pm »
Thanks again, MagicSmoker. I'll try that resistor in series with the gate. It's the same principle as a grid stopper, right? I have a proper mosfet for the job there, an IRF740 that can handle up to 400V.
I removed the 100uF input cap of the converter and that further improved the stability of the circuit. Under a 11mA load, with 111V at the output through a 20W 10K resistor, it is stable, but it's not working properly: with 12V at the input, the current draw there is 730mA. So that's 1.2W at the output and almost 9W drawn at the input. Horrible. Vref is stuck at 1.34V which makes no sense to me. Looks like the MC34063 never switches off. I believe most of the wasted power is being dissipated at the 50uH inductor I have in there right now, because it gets very hot while the mosfet runs cool as a cucumber.

The inductor is a toroidal one, about .77 inches of diameter and AWG 20 wire (or something like that)

This thing is starting to look like a lost cause.
 

Offline VEGETA

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Re: MC34063 high voltage dc-dc boost converter
« Reply #89 on: February 01, 2020, 04:40:35 pm »
I was thinking about making an isolated DC-DC converter to get 6-8.4v from li-ion pack and convert them to 12v DC at say 200mA max. I found this IC MC34063 to be cheaper than LT8300 and I found this little SMD transformer: https://lcsc.com/product-detail/Power-Transformer_TDK-ATB322515-0110-T000_C415291.html

I wonder if this minimal circuit works with these parts?

I don't need fancy stuff, just continuous 12v isolated supply and I want it to be less than read-to-use modules like the 0505 or 1212 ones. These ones will require more circuitry to fit my application such as boost or buck to make the signal suitable for each module.

Looking forward to your feedback!

Note: I couldn't find an ltspice model for MC34063, where and how can I make it work?

thanks!


Offline dazzTopic starter

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Re: MC34063 high voltage dc-dc boost converter
« Reply #90 on: February 01, 2020, 04:50:33 pm »
 

Offline VEGETA

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Re: MC34063 high voltage dc-dc boost converter
« Reply #91 on: February 01, 2020, 04:55:50 pm »
Note: I couldn't find an ltspice model for MC34063, where and how can I make it work?

thanks!

https://www.eevblog.com/forum/beginners/simulate-mc34063-on-ltspice/?action=dlattach;attach=38508

thanks

i downloaded a .zip file and now trying to make it do the output voltage i want. I am not successful yet.

Offline T3sl4co1l

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Re: MC34063 high voltage dc-dc boost converter
« Reply #92 on: February 01, 2020, 05:52:22 pm »
On a related subject, hah, just now realizing RECOM makes Nixie driver supplies:
https://www.digikey.com/product-detail/en/recom-power/R12-100B/945-2050-5-ND/3775934
Affordably priced too, despite RECOM usually being the more expensive brand.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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