I would like to create as an hobbistic project a power supply circuit allowing to switch between a Constant Voltage mode and a Costant Current one. When Voltage Mode is on, Current regulation should act as an upper limit as well as voltage regulator should act as a limit in costant current mode. I know how to create two separate circuit, but no idea on how to merge them in a smart way.
Electronic – Looking for a Constant Voltage/ Constant Current power supply schematic
constant-currentpower supply
Related Solutions
If somewhat more than minimum cost is acceptable you could implement a buck converter per LED. These could be very crude and low component count and not a vast price. Each converter would be able to set a current that was proportional to a PWM or analog input. Probably under $1 per channel.
Your 3 x Power supplies plus resistors idea is acceptable BUT can be improved in performance (with increased cost) by making it 3 x power supplies plus constant current sources. The constant current sources are not much different from the buck converters above but with no inductor.
Example. Imagine that your lowest Vf LED (probably red) has a Vf range of 2.0 to 2.5V.
You will need at least 2.5V supply plus headroom. Say 2.7 V.
Minimum efficincy will be when Vf = 2V = 2/2.7 = 74% BUT this is worst case Vf and in many cases the Vf range is lower.
eg say VF = 2.2 - 2.4. Vsupply = 2.6. Zmin = 2.2/2.6 = 85%.
Boost supply to 2.8V for more headroom. Zmin = 2.2/2.8 = 78%
Again, this is worst case, so acceptable.
A buck converter will improve on the above.
One solution of zillions. Rsc not needed. Rather than sensing output voltage, LED current is sensed. This C had Vref = 1.25V = far too hight. It can be reduced to 0.6V effective using a diode but still too high. More modern ICs has a lower Vref OR a 1/4 IC section can be used per channel. PWM per channel then becomes 1 x SOIC MC34063 + 1/4 LM324 + a few resistors and a catch diode per channel. May be cheaper than more modern IC.
This circuit, from sound.westhost.com may be suitable.
I independently arrived at a similar version - works reasonably well.
This is voltage feedback. Can be made current feedback. Vsense here = Vbe of Q1. This can be reducued with care.
Why don't you just invert your top diagram so it uses a P channel FET: -
This will mean a little modification to the current demand input - it has to be positive supply referenced.
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Best Answer
Produce a constant current circuit.
Feed its output into a constant voltage circuit. Can be done with 2 x LM317 or equivalent.
As long as the voltage taken by the load of the CV circuit is less than the CC setting the system will provide CV. When the load tries to draw more than the CC setting the system automatically transitions into CC mode.
To run purely as CV set CC control to max.
To run purely as CC set CV control to max.
Added:
@Felice - A single high power driver is about "how" not "what".
ie work out what you want to achieve and THEN see what circuit elements are required to do it. Olin has effectively answered your question in general terms.
Here is a simple way of doing it with linear components and a single "pass element".
Supply power via an N-Channel MoSFET - Vin to Drain, Vout from source.
Place a current sense resistor in series either twixt drain and supply or source and load.
Voltage sense Vout at the most negative point of this combination.
Convert sensed current and voltage to ground referenced voltages.
Compare these sense voltages to adjustable references and allow output voltage to rise (eg pullup on FET gate) until either of the sensed parameters is at its limit.
Limit voltage maximum using whatever senor comparator has reached its limit at any given moment.
If CV limit is reached first then the supply is a CV supply.
IOf CC limit is reached 1st supply is a CC supply.
If you turn up a reference that is the limiting one to the extent that the other non-varying sensor reaches its limit then the supply transitions smoothly CC-CV or CV-CC.
You can use the variable comparators to operate a SMPS instead of a linear supply with the same result. The SMPS can "think" that it is a Voltage controlled voltage source and it is of no "importance to it" that it is being voltage limited so that a CC condition is met.