Electronics > Projects, Designs, and Technical Stuff
variable power supply circuit
TheMG:
The "on" resistance of a MOSFET is a completely meaningless spec when you're using it in the linear region!!! It only comes into play when you are using it for a switching application, where the transistor is either fully off or fully on.
A linear power supply dissipates the undesired voltage as heat, plain and simple. 20V in, 5V out at 0.5A, you're sending 2.5W to the load, and the other 7.5W is dissipated in the transistor. No ifs ands or buts about it, that's how linear power supplies work.
Linear power supplies are relatively inefficient and dissipate a lot of heat. Some tricks can be used to reduce the amount of heat being dissipated, usually by switching between different input voltages (taps on transformer) depending on the desired output voltage, so you don't have as huge of a voltage drop across the transistor when outputting small voltages. Even with than, you're still not going to get anywhere near the efficiency of a purely switching power supply.
I would recommend looking up some educational electronics tutorials on linear power supply design, rather than trying to copy and modify a manufacturer's switching power supply design into a linear power supply without really knowing what you're doing. Case-in-point, the optocoupler is absolutely not necessary in your schematic! It was there in the original circuit to provide safety isolation in the feedback path between the primary (mains voltage) and secondary side of the switching supply.
rdl:
I meant to put this in my previous post but forgot. Go to https://artofelectronics.net/ and download the sample chapter which is basically about power supply design. Section 9.4.1 "Power Transistors and Heatsinking" in particular applies here.
http://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf
aneesh:
hi , plz find attached sch of korad supply which work perfectly fine and even i am using multiple tap to dissipate less heat. i completely understand the working but because i want to use single tap which is cheaper and for that reason i wanted to modify the circuit using mosfet.
i brought a dps3005 which uses mosfet and dissipate less heat. i have attached the picture as well
xavier60:
The dps3005 is a Switch Mode Power Supply, SMPS.
Its MOSFET is working as a switch, turning fully on and fully off at at high frequency.
It dissipates very little power as heat. The toroidal inductor and a capacitor smooths the output voltage.
With a linear regulator, the series pass transistor functions as variable resistance. It is partly turned on, passing load current continuously.
Just like a resistor, it dissipates power as heat.
Power = Voltage x Current
The type of transistor used makes no difference. Only the amount of heat that it can safely dissipate.
aneesh:
yes thats what the circuit i attached above is similar, it also works in switching mode. its not on continuously . it switches the comparator checks the voltage and switches off till the output is stable.
dps 3005 also works the same way
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version