Laptop Power Supplies

[spsnyder]

Why are the power supplies for lap tops are so large? This is a homework question for a power electronics class I am taking. I think it mostly has to do with the required inductor for a Buck or Buck-boost converter. I also think it has to do with the frequency being so small (and therefore the Period being so relatively large) The inductor has to be sized such that the voltage ripple on the output is minimized. Since the inductor size is proportional to the duty ratio times the period a large inductor is required. Any thoughts? My other thoughts were that the input voltage could vary significantly from country to country (120 v. 240 etc.) Are lap top power supplies typically rated for different voltages. I'm a desktop user myself, so I don't know.

Thanks for your input.

 

[bphgravity]

Re: Laptop Power Supplies

Not knowing anything about this stuff at all...I would guess that it could be for the purposes of heat dissipation. There is probably a large heat sink material within the casing drawing heat of the electrical components. That would be my guess.

 

[steve66]

Re: Laptop Power Supplies

A better question might be "How are they so small?"

web page

 

[hbiss]

Re: Laptop Power Supplies

Why are the power supplies for lap tops are so large? This is a homework question for a power electronics class I am taking.

I think my answer to a dumb question like that would be large in comparison to what? Show me a supply with comparable specs that is smaller and I can tell you why.

Lots of things determine the size and weight, but wattage and design with choice of components are primary factors.

-Hal

 

[spsnyder]

Re: Laptop Power Supplies

Hal,

I don't think it's a dumb question. Computers are becoming smaller and smaller. Speeds are faster and faster. Moore predicted in the 70's that a transistor could never be smaller than the microns realm. We're approaching nano technology. Every 2 years speed doubles (A correct Moore theory). I don't think it's a dumb question to ask why a power supply has remained the same size. Obviously, it has nothing to do with the Power MOSFET in the converter since that most likely is becoming smaller as all other transistors are. Thanks.

 

[bthielen]

Re: Laptop Power Supplies

Hal, I agree with spsnyder. There is no such thing as a dumb question. I think we have to be careful not to label others as I have said before, until we learned what we now know, we knew nothing.

Bob

 

[bphgravity]

Re: Laptop Power Supplies

I don't think Hal was calling the poster dumb, he was just stating he felt the homework question is dumb. I know alot of the homework questions I give are dumb too...

 

[spsnyder]

Re: Laptop Power Supplies

I have a lot of respect for Hal. I've learned a lot from his posts. Also, I didn't think Hal's comment was directed to me, but rather the question. I think the question would have been better worded if I had asked why as computers are continually becoming smaller are the power converters not following suit. I didn't phrase it that way because the answer is simply that the power converters do not require VLSI to operate while computers do. Thanks.

 

[steve66]

Re: Laptop Power Supplies

I think computer power supplies have gotten smaller if we look across a couple of decades. Thats why I posted the photo, and asked how they are so small.

On the other hand, one major item limiting any reduction in size of the power supply is this:

Computers keep getting faster. An increase is speed usually requires an increase in the power needed to drive the computer. Think of all the tiny electronic switches in a computer. They are like small capacitors. Place an electric charge on them, and they store a zero. Place an opposite charge on them and they store a one.

Now, if we want to read or write to these capacitors, we have to draw some current from them, or supply some current to them. It is easy to realize that the faster you want to do this, the more current you need to supply.

Lots of current means lots of power and lots of heat. So we are really back to what Bryan said about having to get rid of the heat.


Of course there are other things that also contribute to the size of the power supplies:

Do laptops have fans like desktops? That adds a lot of size.

Are we counting the battery as part of the power supply? Can't get that smaller without recharging more often.

Oh, and the power supply also has to run the display. It's not easy getting a 17" display to be nice and bright from a battery.

All this adds up to make me wonder how they do make the power supplies as small as they do.

Steve

[ February 27, 2006, 04:41 PM: Message edited by: steve66 ]

 

[hbiss]

Re: Laptop Power Supplies

Bob, sorry if I confused you. I think the homework question is dumb, certainly not spsnyder for asking it.

Steve pretty much sums it up. I might add another perspective, that being there is not much incentive to making it smaller to begin with. It's not like you have to use it while you are carrying the laptop around. You use it to charge the batteries and when it's not running on them.

-Hal

 

[got_nailed]

Re: Laptop Power Supplies

It has to do with amp hours of batteries. The newer metals / bimetals combinations help. You could drop the size and still get it to start up but just think about getting a laptop and seeing that it will only run for 1 hour on the batteries. Most laptops are made to make it through a good part of a work day with out needing to be plugged in.

 

[bthielen]

Re: Laptop Power Supplies

Hal,

I appologize for my misunderstanding. Truthfully, I should have known better anyway considering your reputation on this site.

I humbly retract my comments.

Bob

 

[physis]

Re: Laptop Power Supplies

I'm just taking a guess here. I haven't taken apart any recent model laptop supplies.

It might be that they're using a switching power supply because they dissipate a lot less heat and make a cleaner output than a linear supply (the tranformer, rectifier, regulator type). Switching supplies have a lot of components and use up more space amp for amp in a lot of cases.

 

[Onitram]

Re: Laptop Power Supplies

Well, the power supply is so big is because the current needed to operate the pc Volt Amps, put a small one and it wont work,and to charge the batteries..

 

[spsnyder]

Re: Laptop Power Supplies

Sam,

You're correct about the switching power supply. I should have stated that the class is on switching power supplies. There are at minimum a power MOSFET, Inductor, Capaitor, and Diode. I tend to think of these idems the inductor is the biggest. I submitted my homework last night, should get it back Thursday or more likely Tues and will let you know what the prof thinks.

Thanks to all.

 

[spsnyder]

For those interested, a major reason the power supply is as big as it is has to do with the substatial inductor required, but more to do with an isolation transformer required for safety. Thanks to all.

 

[steve66]

We are talking laptops right? Isn't the isolation transformer usually a separate box that is part of the line cord? I guess it's been a while since I've used a laptop.

Steve

 

[al hildenbrand]

For those interested, a major reason the power supply is as big as it is has to do with the substatial inductor required, but more to do with an isolation transformer required for safety. Thanks to all.

Interesting.

I think your instructor is still giving "symptoms" of the size of the power supply. Sure the inductor and transformer take up space. The real question is why they are as big as they are.

There's a whole lot of power management built in to laptops that didn't exist a while back. . .still the supply is not shrinking in proportion to the rest of the machine.

I would point to the steady ballooning of the number of transistors, diodes, etc, in each "shrinking" laptop generation. They all need current to bias them. Even though the dye keeps getting smaller, and with it, the power needed to bias the individual component, the number of components keeps increasing offsetting some, or all, of the savings. . .and that's with all the power management that shuts down whole portions of the machine that have been idle.

The pressure to provide more computational capability with sharper, bigger, higher detail display drives the demand on the power supply.

 

[steve66]

That's a very good point about the number of transistors, Al.

I also think its interesting that the microprocessors keep getting faster (GHz now vs. the 16MHz my first Windows machine ran at), but computers don't seem to run any faster.

You know what they say: "Intel giveth and Microsoft taketh away."

Steve

 

[al hildenbrand]

It really is a kick, isn't it?

In 1985, I installed Windows 1.01 (Really!) on my IBM XT clocking at 4.77 MHz.

256K of RAM and two floppies. . .no harddrive.

Windows didn't like the disk swapping, and I was so fascinated at getting the program to run, I coughed up $440.00 for a 20 megabyte hard disk.

We're light years from there.