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The rated forward current of the diode is 1.8V or <2V.
The goals you stated pretty much cover all of what I am looking for in the system. I am curious however as to how current is varied in the system? Where should I start with the design? I have a high current 3.3V power supply that is designed for the laser.
Ben
[QUOTE=Laser Ben;97449]The rated forward current of the diode is 1.8V or <2V.
The goals you stated pretty much cover all of what I am looking for in the system. I am curious however as to how current is varied in the system? Where should I start with the design? I have a high current 3.3V power supply that is designed for the laser.
DESCRIBE THE 3.3v SUPPLY... pICS? Was it the current regulator as well?
2.2 worse case Vdiode + .79 worse case Vce for a NPN = 2.99, ok, that means you can go either FET or NPN, but perhaps not darlington.
Steve
The current is controlled via a MOSFET.
--DDL
[QUOTE=mixedgas;97453]It's a 3.3V 50A Condor power supply if I remember correctly. The system used it as a source for the on board current control.Quote:
Originally Posted by Laser Ben
We're gonna start with the classic constant current source, however the NPN gives way to a MOSFET, actually a bunch of them in parallel.
See;
http://www.repairfaq.org/sam/glpdrv3.gif
this is a very simple thing, and needs some scaling and addons to do what you want it to do.
Steve
So I want to use an array of Q1 that are all in parallel with each other correct, AKA, wire several of them with the same pins connected together.
What is U1? Do I need to scale up the wattage of the resistors as well? It doesn't look like anything is dropping a major amount of current.
I am going to draw up a schematic and you can take a look at what I came up with.
Alright, so Q1 is a high power NPN transistor. You mentioned I would want to use a darlington transistor. Woud something like http://www.st.com/stonline/products/...261/tip102.pdf work?
certainly not for 30 amperes ;)
mosfets in parallel are a good alternative, something high current like IRFD450 perhaps? I don't have the necessary knowledge to choose the correct mosfet
it could also be doable with a darlington like these:
http://tinyurl.com/q4byge
or
http://tinyurl.com/p78hhu
you can contact djAlex1664 on laserfreak french section who managed to make such a driver with these, but he's really busy these days so it might be good to buy one (very cheap) and test some designs (they won't burn if you stress them, they're really strong parts)
Quote:
Originally Posted by Laser Ben
You need to do a lot of scaling and you need emitter ballast resistors in each parallel transistor.
Steve
Any powerful mosfet , right now I'd go with 5 or 10 in parallel, the trick is the degeneration resistors in the source or emitter leads, to force current sharing, you need a little negative feedback between each device and ground. The neat thing about that is it gives you a means to sense current.
If you do not do the degen resistors, one device will hog current and start the self destruction process, which will short out the drive to all the other devices and it shoots up to full current in a microsecond.
Lets do 10 devices. Darlington may be out because you only have 3.3 V of supply and each silicon junction adds between .6 and .8V of drop, so two junctions = 1.2V of drop, this limits you to NPNs and Mosfets.
Electronic goldmine has A11060 The FQP55N06 60V 55A N-Channel MOSFET (Fairchild) on sale, for 79 cents each. At 2.2V diode and 30 amps, you'd need P = IxE = 60 watts. Your regulating device has to dissapate 30 or more watts as you have a 3.3V supply. The spec for the FQP55N06 is
for when it is driven full on in digital mode, not analog, and with a nearly infinate heatsink. which you cannot, by definition, use.
So lets spread that out a bit. 10 devices, means 3.3 watts each device, which is a little easier to get out of a TO220 package, We will use a .1 ohm degeneration resistor in the source of each device, so we know we have 3.3 watts going down each resistor, so P = I^2 x r = 3 * 3 amps * .1 ohm = .9 watts per resistor, so you need a 2 watt .1 ohm resistor to be safe.
So right now we have 10 79 cent fets, and ten .1 ohm 2 watt resistors.
If we take a bunch of 1 K ohm resistors and tie them to the high side of the degen resistors, and then tie the other sides all together, we get a voltage proportionate to the average current of all the resistors.
Now we need a test load to run before we hook a diode to this beast, and we have not yet addressed modulation or the heatsink size.
All the mosfets need their junctions to temperature track to prevent runaway, this means they are mounted right next to each other if not touching each other in a line on the heatsink. This is also a negitive feedback technique, if one junction starts to get too hot it backs off on its conductivity, and by keeping the junctions tracking, if one backs off, the cooler one takes up the slack, and by keeping them close together they average their temperatures and thus tend to share the current.
Now this feeback loop is not the one you need for mosfets, but if you look at the lexel 88 power supply in Sam's Laser Faq, all three drawings, you can see where this is going.
DO NOTE< that the mosfets have one lead connected to the case, and thus you may need a nonconductive sil pad between the mosfets and the heatsink, to prevent fireworks.
More to come.
Steve