High current power supply

[SparkyDave]

Hi everyone, i know this isnt really PICAXE, but I am trying to make a fairly simple power supply with high output current, namely 40A. My though was to use a voltage regulator to control a transistor. I have an old tech school text book from back in the day which has a circuit diagram and calculations for determining resistor values etc. (i cant believe I used to know how to do all that!) My question is can this setup be done with a MOSFET? I have found a MOSFET on RS that is capable of 50A for only $2.68
URL[http://australia.rs-online.com/web/p/mosfet-transistors/4862335/]URL/

Thanks in advance

 

[MPep]

May be easier to re-purpose an old PC power supply.

 

[srnet]

I am trying to make a fairly simple power supply with high output current, namely 40A

What its for ?

 

[SAborn]

You didnt say what voltage, and if it needs to be variable voltage.

What transformer or switch mode are you intending to use, as 40 amp is some serious current.

 

[Dippy]

What voltage in?
What voltage out?
What quality of regulation is required?
Is it to be an accurate PSU or a charger?
Does it have to be linear or can it be a switcher?
i.e. does it have to be super smooth or can the mystery load stand a bit of noise?

I see you've done resistor calculations, but have you done heat calcs too?
A linear reg could get bleedin' hot!

There must be zillions of PSU designs on nerdynet - have you got a list of 'candidates'?

 

[SparkyDave]

Sorry guys, I posted in a bit of a rush. The power supply is to run an automotive subwoofer amp, so 12v not adjustable. I had made one years ago for the same purpose, and it was VERY basic. Simply a toroidal transformer, bridge rectifier, capacitor. That one still works great but I need another for a mate. He has already bought the transformers rectifiers and caps, but his amp seems to be a bit more picky so I need to regulate his one somewhat. I say plural because this one is double the output of mine, so we have gone with a parallel setup.

... We are using a PSU for the car amp because he wants to use the amp and subs in the house temporarily while he doesn't have a car to fit them to

 

[srnet]

And the answers to the rest of the questions in post #5 are ?

 

[bluejets]

You might be better off using a 350CCA car battery and linking up to a 10amp charger.

Those sub-woofer amps draw a LOT of instantaneous current when it goes "boom", hence those whopping great 1 farad capacitors you see shoved in the car boots next to the amps.

Just that I think your estimate of 40 amps might be a bit light on and when you build power supplies of this capacity or larger they start to cost big bucks.

 

[hippy]

All the heavy duty, high power amplifiers I've been involved with have simply used massive transformers, rectifiers and caps. My first thoughts are "More capacitors, Igor. More!" and check the transformer is actually up to the task.

 

[Paix]

@SparkyDave, I hope that your is mate is able to get a car soon, because I can see that he is going to be very popular with his family and their neighbours, unless he lives way out in the outback once his creation is up and running.

If you go the car battery route, then at least he will have a spare battery for his wheels, unless he is planning on using a 40A PSU for a PICAXE project in the near future.
.

 

[bluejets]

Just checked ebay. A 10amp charger starts about $100 and up. Battery about $100.

Those 1 farad caps are $120 plus each. I know which way I would go.

 

[SparkyDave]

Ok, it's 240vac in, and 12vdc out. We already have the transformers, rectifiers and caps, because I had based it on my existing 20amp version, just doubled the size this time. I have it built but I get about 20v when not loaded down, so the car amplifier doesn't like it. I just need a cheap and easy way to regulate it closer to 12v. I was looking at a 55A rated MOSFET which I would like to control via a voltage regulator. I know how to do it with normal transistors, but have only found example circuit diagrams that use a different IC to control the MOSFET. I wasn't sure if I HAD to use one of those such chips or whether it is possible with a standard 3 pin voltage reg such as a 7412.

 

[hippy]

I have it built but I get about 20v when not loaded down, so the car amplifier doesn't like it. I just need a cheap and easy way to regulate it closer to 12v.

I'd suggest getting a transformer that gives closer to 12V.

I vaguely recall that transformer used, half, full or bridge rectification, affects voltage and max current capabilities so perhaps a different rectifying regime ?

 

[SAborn]

I was looking at a 55A rated MOSFET

For starters you would want a mosfet with at least twice the current rating to what you want to draw in current, or several mosfets in parallel, running a mosfet at close to max current will create a huge amount of heat in the mosfet, and is why you need to over rate the mosfet, or spread the load across several to reduce the heat within the individual packages.

Not sure how you intend to drive the mosfet, as from my understanding mosfets DONT like to be run in a linear mode and will generate lots of heat, most circuits i have seen using mosfets in regulators employ PWM to drive the mosfet, as mosfets like to be switched fully hard on and fully hard off, not in some linear mode halfway in between.

What you want to do sounds simple, but its not for high current loads like 40++ amps, just have a look at high current 24v to 12v stepdown converters, and you will soon see the cost is rather high, do you think they would cost so much if some bog basic 7812 Vreg and mosfet will do the job.

From 20v to 12v at 40 amp would mean that 320 watts will need to be lost in heat through the mosfet, hookup 3 x 100 watt light globes and see how much heat they put out, my rule is if i can touch it without loosing the skin off my fingers then its acceptable in heat dissipation.

I am guessing you purchased 12VAC transformers and forgot to allow for the gain when rectified, rule of thumb is, times the VAC by 1.4 to get VDC after its rectified. (or 12VDC / 1.4 = 8.5VAC)

 

[BeanieBots]

I agree with almost everything SAborn says. However, there's nothing wrong with using a mosfet in its linear mode. High quality audio amps (class A) do it all the time. Just make sure you take on board what he says about power dissipation. (same would be true for a bipolar transistor).
To answer your question simply, you CANNOT use a FET type transistor in the same sort of circuit (I assume emitter follower) as the majority of regulator boost circuits because they work in different way.

One question you need to ask yourself is how much effort is this worth? I get the impression it's only a temporary measure?
If yes, then the battery/charger/supercap is probably the simplest, quickest and most likely cheapest option.
You will need the supercap for the car anyway, so no money lost there. Make sure the battery is the type used by the intended car and you will have a spare (for when you run the sound system for a few minutes without the engine because you couldn't hear that it had stalled!) and a decent battery charger is always handy to have if you don't already have one.

 

[fernando_g]

For Mosfets, the limiting factor is not the current, but the Power Dissipation.

For instance, I just recently built a 24V, 10 amp supply employing a IPB70N04S3-07 Mosfet. It is rated 80 amps, and 79 watts at a case temperature of 25C. This is derated linearly to 62 watt at a case temperature of 85C.

Now, my operating conditions were 30 volts unregulated voltage (meaning a maximum power dissipation of [30-24*10]= 60 watt) and a 45C maximum ambient. So this means that I had to find a heasink with a temp rise from ambient of only 40C while dissipating 60 watt. Doing the math this means a heatsink with a thermal resistance of 0.667 C/watt...even with forced air, this is a HUGE heatsink. And my calculations did not include the thermal pad (the best ones offer around 0.33C/W), which would require a heatsink of 0.667-0.33 =0.337C/w.
Clearly I wasn't going to cut it.

To make a long story short, I ended up paralleling three mosfets, so each would dissipate only about 20 watt and therefore could have a manageable heatsink of 40C/20w-0.33C/w = 1.67C/watt.
So I ended up with 3 Mosfets, each rated 80 amps, to regulate 10 amps. You need to perform a similar thermal analysis.

You may ask, why not use another larger Mosfet? Simple answer, I had plenty of the IPB70.

 

[oracacle]

i have no idea what is reguired for this but came across something a while back when i was looking for something else

http://www.zen22142.zen.co.uk/Circuits/Power/1230psu.htm

thought it may be of some help, and maybe it can be a adapted to push 40 amp, make sure you have a good read of the data, i see 100amp is mentioned on at least 1 occasion

 

[bluejets]

One other thing to check for is that some of these sub-woofer amps are actually output rated at an input of 15 volts believe it or not.
I do not know where the manufacturer thinks this will come from but there ya go.

Might pay to do a check on the actual input voltage on the one you have and see what it is rated for and hence cut down you current requirements which will be the killer.

 

[SparkyDave]

Hi SAborn, the transformer selection is definitely the problem. We have 15v sec transformers. I am going to exchange them. The strange thing is though, my other power suplly of the same design uses 12v transformers and has an ac output of 12, rectified 12.6 and with capacitor, 16. I'm thinking I should change the transformers for 9v ones but If they follow the same rule that my other one did, Ill only be getting 13v, which is fine, but doesn't follow the 1.414 calculation....

 

[inglewoodpete]

Hi SAborn, the transformer selection is definitely the problem. We have 15v sec transformers. I am going to exchange them. The strange thing is though, my other power suplly of the same design uses 12v transformers and has an ac output of 12, rectified 12.6 and with capacitor, 16. I'm thinking I should change the transformers for 9v ones but If they follow the same rule that my other one did, Ill only be getting 13v, which is fine, but doesn't follow the 1.414 calculation....

You have to understand how a transformer, rectifier, capacitor and load interact with each other.

Assume that you have a 12 v (RMS) secondary. A meter across the secondary will show about 12v, depending on your local AC supply voltage at that moment. If you follow the transformer secondary with a bridge rectifier, you will get an output voltage that is a "bumpy" 12 v less about 1.2v for the two forward biased rectifier diodes. The 'bumps' will actually peak at more than 12v (see below). With no smoothing capacitor on the output of the rectifier, the voltage will vary from 0v to about ((12 - 1.2) * 1.42)v or 15.3v, depending on what part of the sine wave cycle you are instantaneously looking at. The peak of a sinewave is about 1.42 times the RMS value.

Add the capacitor (large or small) with no load and the capacitor will quickly charge to the Vpeak figure of about 15.3 volts. As soon as you start loading up the circuit (Eg by adding an amplifier), you will start to see ripple of the DC output across the capacitor. As a result of the ripple, the average voltage across the capacitor will drop. This ripple will be fed into the power rails of your amplifier, which in itself may or may not be a problem.

With a heavy load, Ie with lots of noise coming out them speakers, the average voltage across the capacitor will drop more. So if you use a smaller AC output voltage from the transformer eg 9 or 10v RMS to get a no-load DC voltage of 12v, then when the circuit gets heavily loaded, the average DC output across the capacitor will drop as low as 8 or 9 volts DC. And that will give less sound and more distortion. Which is great for me if you live in my street but probably not what you want: lots of nice, clear sound.

 

[Dippy]

Most people don't know about a parameter often called "Regulation".
High confusing really when the next stage is AC-DC regulation and unrelated.

Transformer regulation basically defines the weediness or strength of the transformer.
i.e. how much the output AC voltage droops at full load compared to no-load.
Essentially (Vnoload - Vfullload) / Vnoload expressed as a percentage.
Bigger% = weedier under load.
In 10 minutes everyone will be a Google-expert and will tell you all the reasons

It is essential when choosing a transformer.
You can get away with poorer values if you only have short high-power transients by using a big fat cap later.

 

[John West]

If you happen to use a transformer that was intended to operate in a saturated mode to properly regulate the DC output of the rectified voltage, then things will get interesting indeed.