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| Please comment: 10A 13.8V linear power supply |
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| duak:
If I wanted variable current limiting, I would use an opamp as I described comparing a variable voltage (corresponding to the current limit) to the voltage drop across the pass transistor emitter resistors. By referencing this function to VCC rather than GND, I don't have to worry about matching the resistors around U2A nor the Common Mode Rejection Ratio of the opamp. It should be possible to have a relatively stable current limit range down to 10 mA - certainly to 100 mA. |
| David Hess:
--- Quote from: duak on September 04, 2019, 05:57:35 am ---If I wanted variable current limiting, I would use an opamp as I described comparing a variable voltage (corresponding to the current limit) to the voltage drop across the pass transistor emitter resistors. By referencing this function to VCC rather than GND, I don't have to worry about matching the resistors around U2A nor the Common Mode Rejection Ratio of the opamp. It should be possible to have a relatively stable current limit range down to 10 mA - certainly to 100 mA. --- End quote --- That is how I would do it. In the 1970s, Tektronix did it that way in their discrete transistor regulators using differential pairs for the error amplifier and current limit instead of operational amplifiers and later they converted these designs to use operational amplifiers just like Duak describes as shown below. |
| thinkfat:
That's a rather interesting design. It seems their reference is a stabilized +50V supply through a 9:1 voltage divider, which U114A provides to the base of the driver transistor through a diode. U114B keeps the base positively biased when the current limit is not reached. U114A pulls the base down to the reference voltage (+0.6V). When the current limit is reached, U114B pulls the base down and the diode isolates the output of U114A. The foldback is through the voltage divider on the negative input of U114B, which the +5V rail couples into. For my own design, I tried sensing referenced to the unregulated power supply rail before. I don't think it worked very well. There's a significant 100Hz ripple on the VCC rail at higher loads which will probably play all kinds of funnies with the current limiter. |
| thinkfat:
Allright, updated the schematics. I think I'm almost happy with this version now. It's got a nice foldback mechanism that actually pulls down the short circuit current to 0 once it goes over the trip point. Drawback - it's got an interesting bootstrap problem: if you start it under full load, it will go into foldback immediately, due to the ramp-up of the different voltages. If I want to avoid that, I need to implement some kind of delay switch that would only connect the load after approx. 20ms or so. If anyone has a suggestion how to do that easily, please chime in. |
| thinkfat:
Here's some screenshots of the pcb, managed to get it all into a 100x80 mm form factor. Of course I realized after I launched the PCB that 16V as an analog supply rail, connecting to the V+ input of the UA723 might be a bit tight. The error amplifier might not have enough headroom to drive the output stage. In the simulation it works, but I have no idea how accurate the spice model is with these boundaries. On the other hand I cannot go higher than that because the TLC2272 doesn't like more than 16V difference at its supply rails. Anyway, if it doesn't work I can still cut the VCCA trace to V+ and wire it up to VCC instead. It'll still work just the load regulation will not be as nice. The simulation shows more ringing on VOUT. |
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