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Boost converter from 12 to 19,5v

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LAN_403

Reaching

Wed Sept 05 2007, 06:23AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

IÂ´d like to build a boost converter for my car, so that i can plug in my notebook to run it from the car battery. the problem is, that the notebook needs 19,5v smoothed dc with 4,7A.

I thought about a simple boost converter where the oscillator is controlled by pwm by a comparator on the output which controls the voltage. Now i need some thoughts and tips, cause i dont want to destroy my notebook this way. the car battery supplies 12v and some 10s A (40 or so).
what inductor is suited best for this sort of application? can i use a sinmple ferrite toroid or should i go with a gapped core or something?

Dr. Slack

Wed Sept 05 2007, 07:12AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Gapped or toroid? The ideal thing would be to use the right one for the application.

If you are making a boost convertor, then you need gapped. The convertor works by storing energy in the magnetic field while it "charges" from the battery, then discharing it into the higher voltage load. The amount of energy you can store in the field (Joules), times the switching frequency (Hz), gives the max number of watts it can shift. Paradoxically, it is the air in the magnetic circuit that stores most of the energy, the core material only serves to couple the windings to the air gap efficiently, allowing a huge decrease in the amount of copper otherwise required, reducing copper losses and cost. If you try to store energy in an ungapped core, then you need a lot of costly core material.

If you are making a transformer convertor, then you would use a transfromer on an ungapped core. One nice way to go would be a 2:1 step-up, followed by a linear requlator. Quiet, bomb-proof, easy to design, but tnot power efficient.

Tips - I would recommend you look out some apps notes for boost regulator design, and follow the guidelines on inductor selection. IRF and LT certainly have extensive technical help on line. How much of the circuit do you want to design and build yourself? If you want to mess with comparators and FETs at this power level with an expensive load, then you do really need to understand what's going on, at a level far higher than the question "do I use a gapped core?" would suggest you're operating now. OTOH, if you go for a single-chip Boost regulator controller from IRF/LT or other, and follow their published design method to get to the right frequency and inductor for your load, you shouldn't go far wrong.

Steve Conner

Wed Sept 05 2007, 08:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The SEPIC or Cuk converter may be better for this kind of application, as it can go below the input voltage as well as above. Car electrical systems can see all sorts of nasty transients such as the so-called "Load Dump" which would blast straight through a boost converter.

Here is Bob Blick's SEPIC converter:

Personally, I'd have gone for an iron powder toroid, such as the output filter choke in an ATX, instead of the gapped RM pot core he used.

Marko

Wed Sept 05 2007, 09:41AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi

Definitely go for an #26 iron powder yellow toroid, it has much higher saturation flux than ferrite. It has low permeability, which means like a sort of distributed airgap, so it's not usable for transformers but perfect for inductors.

I actually have a schematic of a circuit right here which I tested out, using SG3525 but only to 50% duty cycle. It's still actually enough for the application, but ghetto as you might want current mode control, synchronous rectifier or something better.

I'd say boost converter is completely fine for 19V, and sepic is needed if laptop needs like 16V or lower where trouble would begin.
It's unlikely that car will give 19V in any case.

Steve Conner

Wed Sept 05 2007, 09:53AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, the problem is that you never quite know where the cigarette lighter socket on your car is connected to. For instance, it can be isolated from the battery when you turn the ignition off, and there can be inductive loads on the circuit with it. Voila, a big inductive-back-EMF-y spike when the ignition is shut off.

The worst case scenario would be if a battery connection came loose while you're bopping down the motorway: then you get an alternator "load dump transient" that can exceed 100V. I see on many datasheets for ICs and MOSFETs used in car electronics that they are rated to withstand a load dump.

The odds of any of these things happening might be small, sure, but laptops are expensive :(

Reaching

Wed Sept 05 2007, 09:17PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

sure. i dont understand much about smps and boost converters, . im not into all electronics stuff which can come up in the way of needs, so i hope youÂ´ll understand that im not a "pro" in all applications. thats why i asked for a gapped core or toroid, cause i have around 100 toroids but only a few gapped ones.

i think for myself and for the life of my notebook, that it would be better to buy a cheap 12 to 230v inverter to use the smps powersupply of the laptop instead
- such an inverter, for around 100watts would cost me no more than around 50$. and i doubt that i would be able to build a converter myself with less costs. my notebook is worth 1000$ or so, and i only have it since 3 months so iÂ´ll better pay 50$ instead, than 1000 for a new notebook

And yes, steve is right. The cigarette lighter is shut off when the ignition is off, so there can be voltage spikes or so. surely its a new car with eco mode after an half hour and some things i dont like for myself. and i dont want to measure or look at the cables and where are they come from etc. its a peugeot 206cc, some of the european members should have seen this car somewhere. its nice but the wiring is terrible and its a pain to look on. a simpled chinch connector from the radio to the back is a pain in the ass with all the roof electronics, cylinders, hydraulic systems etc. the worst of all would be to dissasemble half of the car to know where these wires are going :P

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