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Variable power supply

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DavidDLC:
I want to connect a LM317 with an input of a 24 Volts SMPS.

Has anyone tried this ?

Is there any implications ?

I want to use this as a general variable voltage power supply, the reason for the SMPS is to have the unit more compact.

Please comment.

Zero999:
The output voltage will be limited to between 21V and 22.5V depending on the current draw, temperature and voltage regulation requirements.

When the output voltage is set very low the output current will be reduced due to the LM317's safe area protection (the current limit typically reduces when the voltage across it is >15V) and the fact that it might be overheating anyway.

Have you considered hacking the SMPs to give a variable output voltage? That's what I would do. Mose SMPSes use the TL431 to regulate the output voltage, which can be tweaked by replacing a fixed resistor with a pot. It should be possible to reduce the voltage down to 4V and up to over 24V, although be careful not to increase the voltage too much because some of the components on the secondary side won't be able to handle higher voltages, you should be fine to 30V though.

The reason why you probably won't be able to go much below 4V is because the TL431 has a minimum voltage setting of 2.5V and feedback is typically taken via an opto-coupler such as the 4N25 which will have a photo diode with forward voltage of 1.5V.
http://www.fairchildsemi.com/ds/TL%2FTL431.pdf
http://www.micropik.com/PDF/4N25.pdf

alm:

--- Quote from: Hero999 on July 22, 2010, 10:08:06 pm ---The output voltage will be limited to between 21V and 22.5V depending on the current draw, temperature and voltage regulation requirements.

--- End quote ---
And the lower limit will be something like 2V.


--- Quote from: Hero999 on July 22, 2010, 10:08:06 pm ---When the output voltage is set very low the output current will be reduced due to the LM317's safe area protection (the current limit typically reduces when the voltage across it is >15V) and the fact that it might be overheating anyway.

--- End quote ---
You need plenty of cooling, and might consider a high power cousin.


--- Quote from: Hero999 on July 22, 2010, 10:08:06 pm ---Have you considered hacking the SMPs to give a variable output voltage? That's what I would do. Mose SMPSes use the TL431 to regulate the output voltage, which can be tweaked by replacing a fixed resistor with a pot. It should be possible to reduce the voltage down to 4V and up to over 24V, although be careful not to increase the voltage too much because some of the components on the secondary side won't be able to handle higher voltages, you should be fine to 30V though.

--- End quote ---
I would be worried about the design (controller, magnetics) being able to cope with the very different duty cycle.

Something else to consider is that many SMPS's have a specified minimum load for proper regulation, depending on power, this might be something like 100mA (that's one huge power LED). What will the SMPS do if the output of the LM317 is shot circuited? The initial short circuit current is something like 2.5A (should be a graph in the datasheet), will the SMPS survive that? Not all SMPS designs are that stable in all circumstances, since they were not developed as lab supply. There is at least one documented case of someone who build a lab supply with an LM317/78xx and an SMPS, and the SMPS committed suicide, blew up the linear regulator (lots of voltage, since these are basically fool-proof) and whatever was attached to that. Not saying all SMPS's are unstable, but it's not as easy as a linear supply. The SMPS feedback loop is not inherently stable. Computer power supplies are even more fun, since these can make a decent welder (tens of amps continuous, even more peak).

DavidDLC:
Looks like is not a good idea at all.

Zero999:

--- Quote from: alm on July 23, 2010, 09:49:41 am ---I would be worried about the design (controller, magnetics) being able to cope with the very different duty cycle.
--- End quote ---
Yes, you're right, although lowering the voltage should be lower risk than increasing it, there's still the possibility that reducing the gain could destabilise the feedback loop, although that might be mitigated by adding an extra compensation capacitor.



--- Quote ---Something else to consider is that many SMPS's have a specified minimum load for proper regulation, depending on power, this might be something like 100mA (that's one huge power LED). What will the SMPS do if the output of the LM317 is shot circuited? The initial short circuit current is something like 2.5A (should be a graph in the datasheet), will the SMPS survive that? Not all SMPS designs are that stable in all circumstances, since they were not developed as lab supply. There is at least one documented case of someone who build a lab supply with an LM317/78xx and an SMPS, and the SMPS committed suicide, blew up the linear regulator (lots of voltage, since these are basically fool-proof) and whatever was attached to that. Not saying all SMPS's are unstable, but it's not as easy as a linear supply. The SMPS feedback loop is not inherently stable. Computer power supplies are even more fun, since these can make a decent welder (tens of amps continuous, even more peak).

--- End quote ---
Shouldn't you be able to minimise all that by choosing a high quality SMPS with over current protection and no minimum load requirement?

The minimum load requirement can be fulfilled by choosing the value of R1 in the LM317 circuit carefully, for example if 12R is used the quiescent current drawn by the LM317 will be 104mA. The trouble then becomes the lower resistor (good luck finding a reasonably priced 2W pot.) so you'll need to use a rotary switch with power resistors. If the minimum load current is too high for R2 then a more practical solution is to use a separate LM317 current regulator to fulfil the minimum load requirements.

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