| Electronics > Projects, Designs, and Technical Stuff |
| Power supply with arbitrary voltage waveform and variable series resistance |
| << < (4/5) > >> |
| David Hess:
--- Quote from: nihial on March 19, 2020, 03:45:22 pm --- --- Quote from: David Hess on March 17, 2020, 11:19:20 pm ---There is no need for me to do the math when someone else has done it better. :) Today the technique I described is called "active termination" and it does exactly what is needed. I have used it in the past for exactly the reasons described in the article here: https://www.eetimes.com/active-termination-reduces-high-speed-interface-loss/ The problem now is that a variable resistance must be synthesized to control the positive feedback but at least the resistance is grounded on one end. --- End quote --- That's an interesting circuit, I rarely play with opamps so the maths burns a little bit my eyes but the concept is interesting. I began to work with duak's circuit as a base but I'll look at that circuit when I'm done --- End quote --- It works for DC and low frequencies but operational amplifier gain-bandwidth product limits performance at higher frequencies. Of course the high power requirements of the output devices also limit high frequency performance. These types of circuits readily become oscillators. |
| duak:
I remembered something from a project I was on about 20 years ago. We were working on a laser device that had about 32 chip lasers. Each needed to be driven by a constant current of at least 500 mA and modulated at about a 1 MHz rate. I didn't design the driver electronics but I recall that there was no particular problem getting the driver to meet electrical specifications. I mention this because the total current is on the same order as what is requested here. That is, use small faster devices instead of a few large, slow power devices. The driver itselft was about the same size as a 3.5" hard disk drive and used standard surface mount components. A bigger issue was cooling but only because the overall assembly was to be a particular size. |
| David Hess:
--- Quote from: duak on March 22, 2020, 06:01:52 pm ---I remembered something from a project I was on about 20 years ago. We were working on a laser device that had about 32 chip lasers. Each needed to be driven by a constant current of at least 500 mA and modulated at about a 1 MHz rate. I didn't design the driver electronics but I recall that there was no particular problem getting the driver to meet electrical specifications. --- End quote --- I would not consider 500 milliamps at a 1 MHz rate particularly difficult either but there are differences. Nihial mentioned 30 volts and 20 amps which means either using a few very large devices or dealing with an extended layout of smaller faster devices. And an LED requires a very low compliance from the driver limiting voltage slew rate. And Nihial's requirements include linear operation while I am guessing that your LEDs could have been driven with a current switched open loop source which makes operation to 100s of MHz relatively easy. |
| duak:
David, it was a bit more difficult than you make it sound. The current sources were unswitched because they generated arbitrary waveforms. LASER diodes are not as easy to drive as LEDs as their power output is non-linear (see diagram) and we were servoing as well as modulating the optical power of each diode on the fly. We were using feed forward looking at the data pattern to compensate for the wavelength shift of the LASERs. The compliance voltage had to be higher than you may think because of ripple on the power supply with some data patterns. We had tons of OSCON and ceramic caps for bypassing but switching 20-30 Amps was a problem at some frequencies. The inductance of the power cable exacerbated the situation. BTW, I was given an old set of Monster Cable speaker cables; the ones with fine gauge interwoven wires, sort of like Litz cables but with both signal conductors woven together. These helped a lot as they reduced series inductance and, not surprisingly, added a fair bit of parallel capacitance. I could envisage these cables doing in some not so stable audio amplifiers. I mentioned this driver because except for the voltage, it's not far off what the OP was asking for. If you can build a current source there's a good chance you can build a regulator with a variable output resistance. Dealing with the power, of course, is a matter of finding power transistors and getting enough together on a big enough heat sink. |
| nihial:
Never found a good analog solution for my problem so I chose to use a digipot. I tried a spice simulation but simulation crashes while voltage sources a ramping up and I get terravolts in the circuits nodes. Looks like a ltspice noobie trap. I took the models from TI's website, I autogenerated the parts with the .lib they gave. I wonder if the mistake could come from there or not. In the .lib there's these lines : .subckt VCCS_LIM_CLAWN_OUT_INA821 VC+ VC- IOUT+ IOUT- G1 IOUT+ IOUT- TABLE {ABS(V(VC+,VC-))} = +(0, 4e-5) +(2, 8.16e-5) +(4, 1.21e-4) +(8, 1.92e-4) +(10, 2.35e-4) +(12, 2.76e-4) +(13, 3.1e-4) +(13.5, 3.39e-4) +(13.9, 3.7e-4) +(18, 7e-4) .ends Each line with a + is written in red, is it an error or is it legit ? |
| Navigation |
| Message Index |
| Next page |
| Previous page |