Circuit Designers high current voltage reg

[badbee]

I am hoping to get Pipes and some of the other induction heater circuit designers to help with a question.

I am looking into what it takes to replace 18650s with a custom voltage regulator to power a portable vape. I already have my Ghost running off a desktop power supply. The problem is these devices, at least many, draw more than 5 amps which is the limit of the LM338 and other standard regulators. Combining regulators in parallel requires a bunch of balancing circuitry that is going to burn maybe 10 - 20 watts of power that will produce too much heat in an 18650 sized enclosure.

How would you solve this high current problem? I must be missing something because every mod can do this. What are they using for power regulation? Mods do overheat fairly easily, do they just push to the limits and count on the temp regulator to prevent runaway?

Appreciate any thoughts you may have. The folks on /r/AskElectronics are suggesting I build a custom buck converter, ugh... that's a lot of work.

 

[TommyDee]

There are two sides to this for sure... the I/O and the power converter. Amazing stuff. Safety is managed b y a 10 second timer it appears so all the gloves are off.

12v 8a is what you're after. Switching supplies in this range are also fairly hefty. The most compact supplies are the in-line computer chargers.

I've settled for 18650 cells for this very reason. A 20A BMS and my heater remains portable. Still need to finish a low cost charger (yet another design challenge) but basically 18650 is enough power to bake all day and charge overnight.

 

[badbee]

Thanks @TommyDee . I left out that important detail. The design assumption is a 12 DC supply with an appropriate amperage, but that needs to be stepped down to 8.4 Volts to match the two cells in series used by the Ghost. It would be preferable if I can find an existing regulator IC with built in overdraw and reverse polarity protection.

 

[TommyDee]

Oh, I get it. You are dealing with the same problem I am. I'm working to get 12.6v with current limit so I don't drive 2 amps into the cells. If you have cells, you can charge with a lot less. But you need a current limit with the simple BMS'. Your problem is simpler; any CV/CC buck driver will take any 12V supply and make it an appropriate battery charger for 8.4V.

 

[mr_cfromcali]

I have nothing to add to @TommyDee 's comments, which are spot on and helpful as always.

What I can offer is another good electronics forum to query for this sort of advice, the forum over at allaboutcircuits.com, and specifically their "power electronics" sub-forum. The General forum on the site is very good as well. The site has some some excellent contributors and was quite tolerant of questions like this one - they had some very good problem solvers and practical thinkers, or at least that was my experience. You may need to join the site it to view the forums there, but I believe membership is free:

https://forum.allaboutcircuits.com/forums/power-electronics-forum/

Good luck, sounds like an interesting project!

 

[badbee]

Thanks @mr_cfromcali , googling led me to some threads on that site. I'll look specifically at that forum you mentioned. I'm probably being too conservative and should test pushing an LM338 to the limits which is relatively safe since they have built in protection.

Here's a pic of my ugly testing rig from a hacked together ATX based bench supply.

 

[Pipes]

Just use 2ea LM338 in parallel for a 10 amp limit. (or 3 for 15 or 4 for 20 amp limit.)
The balancing is just adding a couple of resistors.
Check out the circuit here.

 

[TommyDee]

Thanks @TommyDee . I left out that important detail. The design assumption is a 12 DC supply with an appropriate amperage, but that needs to be stepped down to 8.4 Volts to match the two cells in series used by the Ghost. It would be preferable if I can find an existing regulator IC with built in overdraw and reverse polarity protection.

What pipes linked to is what I am use to as linear supplies. We use to parallel 2N3055 power transistors to your hearts content with a single UA723C as a regulator. I still have a schematic and layout for the control circuit somewhere.

Have you looked into things like PWM control where a bank of FET's would do? Or is PWM a nono for you circuit? I'm aiming at RC speed controls.

 

[badbee]

Just use 2ea LM338 in parallel for a 10 amp limit. (or 3 for 15 or 4 for 20 amp limit.)
The balancing is just adding a couple of resistors.
Check out the circuit here.

Thanks for replying @Pipes. I've seen that circuit but was concerned about the power dissipation in those serial resistors on the output. That's a lot of heat for that skinny tube. The low efficiency bugs me on principal but I've resigned myself to doing something like this. I might cut out part of the battery casing to expose a heat sink.

Since we are only slightly above the limits of a single LM338, which has very robust internal protections, I might try it with back flow protection diodes but no balancing feedback. The (random) higher voltage IC will supply most of the current at first but if it starts to overheat it will self limit and the other IC will take over. With a decent heat sink that might be good enough for a typical <30 second draw.

Whatever, I'm having fun investigating this...

 

[badbee]

What pipes linked to is what I am use to as linear supplies. We use to parallel 2N3055 power transistors to your hearts content with a single UA723C as a regulator. I still have a schematic and layout for the control circuit somewhere.

Have you looked into things like PWM control where a bank of FET's would do? Or is PWM a nono for you circuit? I'm aiming at RC speed controls.

I would greatly prefer a switching regulator for the efficiency and lower heat but it is really hard to find one small enough. It's the size of the main power inductor that is the problem. Someone on AllAboutCircuits pointed me to the TPSM84824MOLR, somehow it packs the inductor into a tiny 7.5mm package. I don't understand how something that tiny can pump 10 amps, I guess the 95% efficiency helps.

It only comes in surface mount which I've never dealt with before so I think I will experiment with LM338's before going down that road. By the way, have you ever used Proto Advantage adapters? How do you match your IC to the correct adapter?

I designed and built boards in university but that was a looong time ago.

 

[TommyDee]

Those coils use flat wire radially like a wide spring. And I don't mess with high density circuits.

How clean does the power need to be?

 

[badbee]

Those coils use flat wire radially like a wide spring. And I don't mess with high density circuits.

Are you referring to the coil that must be in that tiny chip? I didn't follow the spring comparison. Sorry, I'm going from a background in theory to practical application and investigating everything people tell me. I would prefer not to mess with high density either

How clean does the power need to be?

That is a very good question, one I've been considering. The Ghost must have an internal PWM regulator, it has that classic whine. How sensitive to ripple do those tend to be? Is it dangerous to stack regulators in series? That might be a reason to prefer a linear regulator for my input stage. I imagine LiPo's produce a fairly clean output so the Ghost internals might not have much filtering.

 

[TommyDee]

The chip in the image is an inductor. The coils look like a stack of plates but it is indeed a coil. High circular mills of copper in the cross section of the flat-wire in order to handle the 8 amp rating.

I just looked at a HP power adapter to the laptop. Compact and portable... 65W in a case quite a bit bigger than a pair of 18650's.

The reason I asked about how clean was to just get the right transformer; fully rectify it; put some filter caps on and worst case, zener the output to clamp the voltage.

LiPo output is the epitome of clean power!

Edit: Just saw your application - you don't need well regulated voltage. A range between 7.2-8.4V is perfect. You can probably use the transformer in the ATX power supply in front of you

 

[Hesali]

Little off-topic but when I powered my Arduino nano, a good way to calculate how much power I will dissipate in a linear regulator is to take the input voltage 60v and multiply by my output current 0.24A that was my input power 14.4W. Now my output power is 12V at .24A which is 2.88W. So that means that the converter itself is eating up the difference 11.52W. Will look after at the junction to ambient thermal resistance, as the worst case possible. Should’ve added gerber files application for them to mess around because it took me too much time.

 

[TommyDee]

Don't mix apples and oranges. Wattage in has to equal wattage out minus the losses. Convert the data to power first and then calculate. Are we even talking vape related stuff here?