you should check to lm2676.I am working with this regulator for a few days and I strongly recommend to use this chip with a to220 package.
wow, this sounds similar to what I am doing atm.
Fyi, to get to 0V output, you are going to need a negative voltage rail, so don't forget to account for that. Added to that, some op-amps won't go rail to rail, so the negative rail is crucial if you want the output to go to zero.
What you feed your voltage reference from counts just as much as what you power your DAC from. Linear Regs are probably the safest bet to get a low input ripple, However, check the datasheet for the ripple rejection ratio for the switching frequency you are interested in. Most of the specs are listed at 120Hz, or double mains, for where they are mainly used.
for measuring current, I suggest using the MAX4081. Caveat: ensure it's ground is at your -5V, as both inputs need to be 5V above GND to work. thankfully the MAX4081 provides external pins for you to be able to reference your output voltage to.
The switching pre-regulator will dictate the filtering that you need to put in place on the output to try reduce noise on the input to the linear regulator. As I stated before, The ripple rejection can fall to as low as 40dB at higher frequencies.
As for the package types for all the regulators, It will depend on the power dissipated through the device. The datasheet will tell you how to calculate it.
Also google the LM350 ;-)
Looked it up, the difference is that it runs at 260kHz compared to the LM2596's 150kHz, also its max duty cycle is only 91% compared to 100% for the LM2596 which means the max output voltage will be lower and I'm going to need every single volt if I want 24V at the output. Note that about 1.5V is lost on the internal switching transistor (NPN bipolar) of the LM2596 according to the datasheet. The 2676 uses a N channel MOSFET with a charge pump so it can't go to 100%.
I'm going to use a negative supply from voltage inverting charge pump from the +5V rail as I mentioned in the first post. The opamps are LM324 for the regulator and probably MCP617 (low offset) to buffer the DAC outputs
Is your transformer outputting 24V? They are generally rated as RMS, so your peak will be RMS * SQRT(2) = 33.94V approx. This may rise and fall depending on the line regulation from your wall outlet. In Australia for instance, our mains voltage can go -6%/+10% out of the 230V. Obviously don't forget to include diode drops, and you should have enough overhead for your circuit.
Have a read of this Application Note from TI. With a simple change to a dual winding inductor and adding another diode and some capacitors, you get both rails, for quite cheap
I would suggest a gradual approach.
1) Minimum output voltage > 0v: so you can use a 3-terminal voltage regulator as the base;
2) Maximum output current < 1.5v: 3-terminal regulator again;
3) MCU-based: no for now;
4) Current limiter vs. current "fuse": do you want to maintain the maximum current output or just shut off the output?
5) display: get one of those ebay led displays, for simplicity.
Such a device can be fairly easily constructed. Once you get it going, you can add / refine features.
The issue with mcu-based controller is that its ability is difficult to main, and it has poor transient responses.
I'm not using the micro for anything else than setting the output voltage and output current limit
I'll avoid using PWM instead of DACs just because of the delay the RC filter at the PWM output will have.
Now the output voltage after rectifying and filtering should be 22 x 1.414 - 2.4 = 28.708V which is not too bad while ignoring the ripple which is another can of worms since a bridge rectifier with filter caps is not a linear load and the peak currents will be much higher than 3A... I could use some help here...
Might actually go that way, I've seen this in a MC34063 app note, thanks for the tip.
edit: I found I possible problem with that: the input voltage is variable so the duty cycle of the regulator will be variable and since the second output has no feedback it won't stay at a fixed value... am I right?
If I am right then I can only use the negative rail unregulated derived from the +5V supply and use a low voltage negative linear regulator. I'll still need three LM2596s anyway because the pregulator has to be separate and the +5V and +12V rails must be regulated, especially the +5V one. Or maybe I could get a 5V fan and ditch the +12V rail?! Decisions, decisions...
I'll avoid using PWM instead of DACs just because of the delay the RC filter at the PWM output will have.
Any idea on how to do the voltage adjustment and current limit on the LM350 with a bunch of opamps? The INA168 current sensor will output 1V/1A referenced to ground.
Also, how much will it matter if I supply the opamps from the tracking preregulator output (which is variable)?
Any idea on how to do the voltage adjustment and current limit on the LM350 with a bunch of opamps?
Hi,
Very similar to yours, my own design uses an LM2576 switcher for the pre-regulator. It's quite hard to completely remove all hint of HF noise getting through to the main psu outputs. You'll need everything from a good pcb layout, filtering and ferrites......albeit depends on just how far you want to go.
Will be interesting to see how you intend to control the switcher output so that it tracks. It ain't so easy without causing some extra noise issues (I found anyways).
DAC control of CV & CC (setting only) is great fun.......for info 10-bit is workable, but preferrably more. You always need more!
Ian.
You will likely face challenges combining the resistive divider with the CV loop; and getting the CC loop to not oscillate too much.
Can I make this any simpler?
I updated the schematic (and it's also bigger this time).