Mosfet drain-source voltage

I was looking at some Mosfets to make a 30V 1A constant current source for a 30W LED, and ended up finding the IRLML2502 for a pretty low price. http://www.irf.com/product-info/datasheets/data/irlml2502.pdf

The max drain-source voltage of these mosfets is 20V however, but there won't ever actually be 20V on the mosfets themselves, because the 30v will go over the led's, right?

You might need to tell us the forward voltage of the LED…

Well, 30V 1A for a 30W led might mean a 30V voltage drop...

What is your supply voltage?

Elektrix

I'm using a step up converter from a 19V laptop power supply, so about 31V? I'll need to adjust that so there won't be that much heat dissipated in the circuit.

Is it really a 30 W led ? That sounds awfully high.

michinyon:
Is it really a 30 W led ? That sounds awfully high.

Check this out: http://www.ebay.com/itm/111048590717

Inevitableavoidance:
I'm using a step up converter from a 19V laptop power supply, so about 31V? I'll need to adjust that so there won't be that much heat dissipated in the circuit.

In that case... no problem.

You might even want to go all the way to 32V to give it a bit of margin (try it and see).

Inevitableavoidance:
Well, 30V 1A for a 30W led might mean a 30V voltage drop...

Sure, it might...but when you've been around these forums a while you learn to never assume anything. :smiley:

Well that is a lamp, and it must have more than one LED junction in it. If the operating voltage is 32-36 volts, it probably has ten LED junctions in series. And probably, several strings of ten led junctions, in parallel.

So if you connect this to a 31 Volt supply, you might not get much conduction through the LED’s at all. And if you do, there will be a large voltage drop across the LED’s, so there won’t be much voltage across the mosfet at all. From that aspect, the forward voltage breakdown limit of the mosfet should not be an issue.

That 200 W led on ebay is not the one you are planning to use ? You are planning to use something else ?

You need to figure out what the characteristics of the led you are planning to use are. What is the specified voltage for it ?

While you don’t need the full 30V across the MOSFET while in constant-current mode,
you should be fine. Add a 100k resistor between drain and source if you like. I presume
you are sensing the current from a shunt between source and ground ?

The only caveat I can think of is that if the connector to the LED accidentally shorts you’ll
exceed the MOSFET ratings immediately (with a 40V device it would be dissipating 30W
during the short, but with a heatsink that wouldn’t cause prompt destruction.)

A typical "bright" LED has a voltage of about 3.5 volts and a current of about 300 mA, hence a power consumption of about 1 watt.

That 200 watt LED mentioned earlier is actually an array of 200 separate LED junctions, you can actually see them and count them, fabricated on a single chip.

Similarly, a typical 10 watt led has 9 actual LED junctions, you can see those on ebay too, particularly when the photo is of them on. On a lot of them, you can see the little wires connecting the actual semiconductor junctions together.

If you were fabricating a 10 watt led lamp with 9 actual led junctions on it, you could arrange them in several ways. For example, 9 leds in series taking 300 mA with a voltage of about 31 volts. Or three parallel strings, each of three leds in series, taking 300 mA in each string, total current 900 mA and a voltage requirement of around 11 volts.

Same concept for your 30 watt lamp. It probably actually has around 30 actual LED junctions in it. This could, in principle, be arranged as one string of 30 leds ( total voltage, about 110 volts, total current 300 mA ). Or 30 leds all wired in parallel ( total voltage about 3.5 volts, total current, about 10 amps ( not very practical unless you want to drive it from a lipo). Or 3x10. Or 10x3. Or 5x6. Or 6x5. These will all give different combinations of voltage and current to drive them.

I now just have the LED, a .47ohm resistor, and the step up converter to power the led. Then I just turn up the step up converter untill there is .47volt over the resistor.

This doesn't work for shit though, the led gets crazy hot and the forward voltage of the led is all over the place. But, it's about 30v when it's a little hot and running at 1A

I'm planning on using this circuit:

fungus:
You might even want to go all the way to 32V to give it a bit of margin (try it and see).

With this all the extra voltage will be dissipated in the mosfet. But i'll be using smt mosfets, so an extra watt to dissipate might be a little too much, right?

Inevitableavoidance:
This doesn't work for shit though, the led gets crazy hot

That's normal.

"10W" is only the power rating under perfect laboratory conditions. You can run it at 10W but don't expect to use the supplied heatsink for that (it does have a heatsink, right?)

I usually run eBay LEDs at 50-60% of the advertised power because of this.

Inevitableavoidance:
and the forward voltage of the led is all over the place

That could be due to the heat.

I know the forward voltage swings all over the place due to the heat, I meant to say that that way of driving the leds just doesn't work. That's why i'm going to try the mosfet driver.

Also, I'm using a processor heatsink on it, so it's cooled adequately.

michinyon:
That 200 W led on ebay is not the one you are planning to use ? You are planning to use something else ?

It's actually a 100W RGB led, which just has 3 30W leds's in it. Here's a video: Lamp - YouTube

That's why it's so annoying when the current of the leds changes, the mixed color will change aswell.

Inevitableavoidance:
I know the forward voltage swings all over the place due to the heat...

Also, I'm using a processor heatsink on it, so it's cooled adequately.

You contradict yourself.

fungus:

Inevitableavoidance:
I know the forward voltage swings all over the place due to the heat...

Also, I'm using a processor heatsink on it, so it's cooled adequately.

You contradict yourself.

Even though it's cooled the forward voltage still changes due to the heating of the LED. You can't expect a 100W LED not to heat up even though it has a hugeass heatsink..

The MOSFET LED driver you show in the pic has current swing as the temp. rises.

Well, not as bad as the led forward voltage swing itself I hope? How significant will this be?

Edit: The only thing influencing this is the temperature of the small transistor, which isn't really being warmed up by anything, right?