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Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Hello all, Since my original discussion on making a "simple" laser PSU turned more complicated and more project-like, I thought I would turn it into a project! Originally, I wanted to make it constant current from 0-4amps or so. This is great for "low power" laser diodes. (Up to 3watts) But this will not do for 20watt laser diodes as seen on ebay. I have been working hard on a good expresspcb for the laser driver, but will now likely redo it for much higher current. (Will try for 25amps!). I can get my driver up past 7amps with two mosfets, but regulation suffers.
So what I am using is a TL-082 op-amp and some IRF9640 mosfets (two of them). Accoriding to the spec sheet I found for the op-amp, its minimum supply voltage is 7 volts! No wonder I couldnt get anything when running off a 5 volts 40amp flatpac! I have since then looked up some low voltage op-amps that work down as low as 2.7volts. These are Rail-Rail as was advised earlier. (Maxim IC MAX4167EPA+) Planned mosfets of choice: FQP17P10 by Fairchild.
You can watch a video of one of my C mount lasers drawing more than 4amps here:
So far, laser driver works well. Ive left it running for about a total of 6-10 hours now. (A few hours a day). Laser diode gets a little warm after an hour at 4amps. (Just slightly warm, its on a large heatsink). I didnt expect these laser diodes to take this kind of abuse for so long! (Same laser is still alive!)
Things to do: 1-Remove temp control circuit from pcb. At 25amps, im going to guess the fan needs to run constantly. This will free up space for two more mosfets to help share the load. 2-Use a MUCH larger heatsink! 3-Try out new, better mosfets and new op-amps. (trying to lower the operating voltage to minimise heating some). 4-Finalize the expresspcb once I have worked out the details.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
I had tried using an LM723 as the ref. but the problem is is that it cannot go from 0-supply voltage. For example, if I had 5 volts in (regulated), the LM723 would output something like 1.25volts up to say 3.7volts max. Same limitations no matter what input voltage. This caused the circuit to start out at high current and would only allow me to increase the current. I still dont fully understand all there is to know about op-amps, but this trimpot seems alright. Plus, there would be no room onboard for the LM723 voltage ref. I would love to have someone correct me as to what I am doing wrong. (I imagine its trivial!)
Those resistors are the current sense resistors. They do not warm at all. (0.1ohms each).
As for the pcb, I was wondering if someone could advise me on whether or not my idea here would work. What I want to do is use both sides of the board to create large traces for the high currents. In order to maximize current flow over the two planes, could I use grids of vias to connect both sides of the board? (Once I figure out how to resize my images, I will post a pic of what I am doing) Matt
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
It should work fine to make a 'doubled up' layer by using a ton of vias. But in the end you would probably have lower resistance if you only put a line of vias at the end of the traces, since the vias out in the open serve little purpose (the current should share across the 2 layers nicley without needing to have vias to force equal current on top/bottom) but they will reduce the amout of copper remaining on the traces. Although if it were me I would just cut back the soldermask layer and put a big blob of solder on the trace and not worry about it
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Here is a picture of what I am doing with expresspcb for the higher current idea...There is also a bottom plane that mirrors the top plane. Those via grids connect the two planes to form a much larger plane. (Higher current).
EDIT: I am now considering making this driver into two pcbs. One will now hold all the mosfets, and the other for the op-amps and perhaps some kinda voltage ref. and some other goodies (overcurrent sense and whatnot) that is a little better than the trimpot. (Any ideas about that?). The driver will now have room for SIX mosfets! yay! Should help spread the current out some! At 30amps, each mosfet will only need to pass 5amps. Still seems like quite a bit for t0-220 packages, but should suffice with a large heatsink/cooling fan?
Could I run into trouble here? Three op-amps controlling 6 mosfets for up to 30amps of current draw? Matt
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
usually the current control is a bandgap voltage ref say 1.25V, then a trim pot hooked up as a divider, now you can get voltage between 0 and 1.25(keep in mind trim pots dont' usually let you play with the full range), and you can feed that to your opamp. This will be quite stable if you design it right. namely matching the output impedance of the voltage ref to your opamp's input.
Take a look at this circuit , it's a constant current dummy load, which is practically the same thing. Vref and opamps can be sampled from ti if you are tight on budget.
Are you using 3 dual ch opamps to control 6 fets? sounds good to me. As long as you have one amp for each fet, current will share evenly or as even as your current sense resistors.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Electroholic wrote ...
usually the current control is a bandgap voltage ref say 1.25V, then a trim pot hooked up as a divider, now you can get voltage between 0 and 1.25(keep in mind trim pots dont' usually let you play with the full range), and you can feed that to your opamp. This will be quite stable if you design it right. namely matching the output impedance of the voltage ref to your opamp's input.
Take a look at this circuit , it's a constant current dummy load, which is practically the same thing. Vref and opamps can be sampled from ti if you are tight on budget.
Are you using 3 dual ch opamps to control 6 fets? sounds good to me. As long as you have one amp for each fet, current will share evenly or as even as your current sense resistors.
Good luck
Hello, thank you for that schematic! Looks pretty simple enough to modify my circuit...I will try it out in a few days. Could you go into impedance matching a little more for me? I am not sure how to match the two. (Op-amp to vref.).
I am a little confused about the operation here. It was my impression that the current was controlled by controlling the difference in voltage between the Vref and the input voltage. For example, 12 volts in, 11volts on the divider (Vref) results in a 1 volt difference and thus an output from the op-amp. The higher the difference, the higher the curren output. (I am slowly building an underatanding of the op-amp).
Yes, I will be using three dual op-amps (6 total) to control the FETs. (One op-amp per Fet). I will try to get some high precision sense resistors.
Yes, I need to find a way to enclose the laser diode itself to protect it from dust. Im still thinking up a way to cheaply and easily do this. Any ideas?
Before I go all out with 6 mosfets, I will go smaller with 2 and see how much current I can get them to reliably control. Then I will scale it up! Here is my current plan via expresspcb, (I havent worked out a schematic for it yet, if something isnt clear please let me know.) THANKS! Matt
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
This is how the constant current dummy load works. First, -in of the opamp look at the voltage feeding back from your voltage sense resistor. Now it compares it to the +in voltage coming from your Vref divider.
if +in > -in, opamp will raise its output voltage, to try and make them equal.
As far as matching goes, you need to find out how much input current your opamps want(Iib max), higher power opamps have leaky inputs. But for general opamps they are in the nA range. multiply that by 6 coz you have 6 channels. That would be your worse case loading on the voltage reference.
Now you make sure your vref divider network can source that without sagging and you will be fine.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Electroholic wrote ...
This is how the constant current dummy load works. First, -in of the opamp look at the voltage feeding back from your voltage sense resistor. Now it compares it to the +in voltage coming from your Vref divider.
if +in > -in, opamp will raise its output voltage, to try and make them equal.
As far as matching goes, you need to find out how much input current your opamps want(Iib max), higher power opamps have leaky inputs. But for general opamps they are in the nA range. multiply that by 6 coz you have 6 channels. That would be your worse case loading on the voltage reference.
Now you make sure your vref divider network can source that without sagging and you will be fine.
Ok, I see now. So Basically the voltage on the vref/current sense resistance will dictate the current through the laser? Bringing it down to 1.25volts will deffinataly make it easier to adjust current SLOWLY
As for the impedence matching; I just need to size the resistors/trimpots such that I have enough current flowing through Vref for the op-amps?
Here is my semi-final version of the "low" power pcb. Feel free to add any ideas before I have it made in a few days. The bottom IC is a 741op-amp for the TEC/FAN cooling. It senses when the heatsink is too hot and turns on some cooling mechanism. It has a trimpot to adjust the trigger temp.
The top IC is an op-amp for current control. Vref is to the left and is the trimpot/resistor combo along with the previously mentioned Vref IC. (Its surface mount). The top two transistors will be the mosfets for current control. The bottom one is for the temp. control.
Power in will hopefull be from a 5volt 40amp flatpac. I will make some updates after I power the thing back on. (Moved houses and need to install new sockets with ground wires). Thanks! Matt
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High Current Laser Diode PSU
