variable power supply circuit - Page 1
Forum section: electronics / projects

author: aneesh
author_rank: Contributor
timestamp: June 15, 2020, 08:09:59 pm
content: hi,i am designing a 30v5A variable supply. the circuit is similar to korad 3005 power supply, the difference is i am replacing the power transitor with pmos but it becomes very hot and temp goes on increasing. the circuit is attached.whats wrong? why so hot ? at 20v 500ma

author: bin_liu
author_rank: Contributor
timestamp: June 16, 2020, 04:29:54 am
content: You need to choose the heat sink according to the power loss of 150W.Also choose a MOS tube that can withstand a lot of power continuously.

author: aneesh
author_rank: Contributor
timestamp: June 16, 2020, 04:43:18 am
content: mosfet that i am using has good continuously current.my plan is to design 30v 5A supply. but i cannot go above 500mA. i am using heatsink but i think there is some other problem. As there are PS in the market like dps3005 which uses IRF5210 with the same heatsink and achieves 5A.

author: xavier60
author_rank: Super Contributor
timestamp: June 16, 2020, 05:00:53 am
content: Quote from: aneesh on June 16, 2020, 04:43:18 amWhat is your source voltage? The dps3005 will likely have tap switching to reduce dissipation and a fan to reduce temperature.The MOSFET could also be oscillating although it need not necessarily increase dissipation.

author: aneesh
author_rank: Contributor
timestamp: June 16, 2020, 05:13:19 am
content: input is 20v dc output is correctly regulated to 5v as per requirement but when i start drawing current 500mA it gets heated badly... the circuit is uploaded whats the prob with the design

author: xavier60
author_rank: Super Contributor
timestamp: June 16, 2020, 05:24:37 am
content: I did a search for "dps3005". It's an efficient Switch Mode Power Supply that doesn't produce much heat.You are experimenting with a linear regulator. The MOSFET has to dissipate the difference between input and output power.You will need a larger heatsink or use a SMPS IC.

author: aneesh
author_rank: Contributor
timestamp: June 16, 2020, 06:16:46 am
content: i am using larger heatsink and the Ic is also the same only the circuit may vary. if you can suggest me improvement in my current design to reduce heat. i will be great.might be duty cycle, frequency, etc

author: xavier60
author_rank: Super Contributor
timestamp: June 16, 2020, 06:38:05 am
content: Quote from: aneesh on June 16, 2020, 06:16:46 amThe hand drawn diagram in your first post looks like you are designing a linear regulator.Can you post pictures?

author: aneesh
author_rank: Contributor
timestamp: June 16, 2020, 06:45:26 am
content: yes its a linear bench power supply .. i am using 20v dc from transformer rectified source.

author: kallek
author_rank: Regular Contributor
timestamp: June 17, 2020, 06:03:01 am
content: As already mentioned, with linear regulator you need much bigger heatsink than with switching regulator. Take a look at some thermal calculations and you will find out why that very small heatsink get so hot. Can you post pictures?

author: xavier60
author_rank: Super Contributor
timestamp: June 17, 2020, 06:09:05 am
content: Start with about 1°C/W for the TO-247's thermal pad.EDIT: Or is it a TO-220 part? which will be much worse.

author: aneesh
author_rank: Contributor
timestamp: June 17, 2020, 09:22:37 am
content: it is t0 220 with large heatsink . plz refe the above circuit

author: xavier60
author_rank: Super Contributor
timestamp: June 17, 2020, 09:49:41 am
content: One way to calculate the power dissipated by the MOSFET, measure the voltage between Drain and Source, then multiply it by the current.

author: aneesh
author_rank: Contributor
timestamp: June 17, 2020, 10:53:50 am
content: drop is something nearly 15v and 0.5A

author: xavier60
author_rank: Super Contributor
timestamp: June 17, 2020, 10:59:20 am
content: Quote from: aneesh on June 17, 2020, 10:53:50 am15 x 0.5 = 7.5WWhat information do you have about the heatsink?

author: rdl
author_rank: Super Contributor
timestamp: June 17, 2020, 12:11:28 pm
content: You probably won't be able to get by with one of those small heat sinks made from stamped and folded metal. Some kind of extrusion will be best, something like what Dave used for his dummy load (EEVBlog #102).

author: aneesh
author_rank: Contributor
timestamp: June 17, 2020, 03:04:33 pm
content: no its not about heatsink.. if u see the circuit attached i have given relevant information about the mosfet.the internal resistance is just 3m ohm which is the best available .so the power dissipation should be low but i think its because of the region of operation.

author: kallek
author_rank: Regular Contributor
timestamp: June 17, 2020, 03:21:39 pm
content: Resistance is that only when mosfet is fully open. In your circuit mosfet is dissipating 15 volts.You can reduce heat with transformer taps, but still you need bigger heatsink if your goal is 30V/5A.

author: aneesh
author_rank: Contributor
timestamp: June 17, 2020, 05:37:15 pm
content: look at this pic its dps3005 i.e 30v 5A with small hearsinki am using bigger than this and better mosfet but i m not achieving this output

author: kallek
author_rank: Regular Contributor
timestamp: June 17, 2020, 06:52:29 pm
content: Just can't compare switching and linear regulator no matter what your pass transistor is.If you want much better efficiency you need some sort of switching pre-regulator.

author: TheMG
author_rank: Frequent Contributor
timestamp: June 17, 2020, 09:41:02 pm
content: 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.

author: rdl
author_rank: Super Contributor
timestamp: June 17, 2020, 10:12:51 pm
content: I meant to put this in my previous post but forgot. Go tohttps://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

author: aneesh
author_rank: Contributor
timestamp: June 18, 2020, 03:36:03 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: June 18, 2020, 05:54:29 am
content: 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 CurrentThe type of transistor used makes no difference. Only the amount of heat that it can safely dissipate.

author: aneesh
author_rank: Contributor
timestamp: June 18, 2020, 06:35:02 am
content: 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

author: xavier60
author_rank: Super Contributor
timestamp: June 18, 2020, 09:45:31 am
content: Quote from: aneesh on June 18, 2020, 06:35:02 amYou have forgotten to include your age in your profile.The hand drawn design in the first post will operate as a linear regulator and likely an unstable one.If you want to make a switch mode regulator, the best chance of success is with a self contained Buck IC like the LM2596.Without an oscilloscope you will be working in the dark were SMPS is involved.

author: aneesh
author_rank: Contributor
timestamp: June 18, 2020, 12:03:17 pm
content: yes i want to build linear supply which works with power transistors but why does it become very hot with mosfet.some manufacturers have achieved good results with mosfet.let me be clear is it because of high freq, duty cycle or turn on or off cycle because of gate source resistor

author: mikerj
author_rank: Super Contributor
timestamp: June 18, 2020, 12:15:20 pm
content: Quote from: aneesh on June 18, 2020, 06:35:02 amYour circuit is a linear regulator, NOT a switching regulator. Which part of your circuit do you think provides the PWM waveform for driving the MOSFET? Where is your inductor and freewheel diode?

author: aneesh
author_rank: Contributor
timestamp: June 18, 2020, 12:28:26 pm
content: yes linear regulator but if you see its the opto pulses which switches the transistor on/off but i think am i not switching it on and off properly.i have attached a ref circuit of another manufacturer.

author: kallek
author_rank: Regular Contributor
timestamp: June 18, 2020, 12:58:11 pm
content: You can't beat Ohm's law! Unregulated voltage minus output voltage is turned heat in mosfet! Mosfet is doing it's stuff to work as resistor!

author: ArthurDent
author_rank: Super Contributor
timestamp: June 18, 2020, 02:24:00 pm
content: “…i am replacing the power transitor with pmos but it becomes very hot and temp goes on increasing.”“…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.”Whether you use bipolar or mosfets for the pass elements of a power supply, with the same design and same load you will be dissipating the same amount of heat.You aren't understanding what everyone is explaining to you. Until you're willing to read and understand what they are saying, you will continue to have these overheating problems. There is a big difference between linear and SMPS topologies and once you understand that and design your power supply accordingly, it will work much better. Perhaps you should read some of the references suggested above to understand the basics before you try to build a power supply beyond your current understanding.

author: exe
author_rank: Unknown
timestamp: June 18, 2020, 04:45:29 pm
content: Peeps, I think this is a buck converter. Look at the diode, inductor and capacitor. Or at least the intent was to build one. I don't see any feedback.

author: mikerj
author_rank: Super Contributor
timestamp: June 18, 2020, 05:03:05 pm
content: Quote from: aneesh on June 18, 2020, 12:28:26 pmThat circuit appears to be part of a buck converter, it has the inductor, the diode and the output capacitor which are all required for a switching regulator. Where are these parts in your design?

author: aneesh
author_rank: Contributor
timestamp: June 18, 2020, 06:38:55 pm
content: ok i understand ...so i have to use some kind of pre regulator instead of multiple taps .. as transformer becomes expensive.any reference of pre regulator

author: kallek
author_rank: Regular Contributor
timestamp: June 18, 2020, 08:18:12 pm
content: Do you know what you are going to use your power supply for?Because many times for example 20V/3A or even 1 amp is more than enough for bench power supply. And when you have working design you can modify it for higher current.

author: aneesh
author_rank: Contributor
timestamp: June 19, 2020, 02:07:32 am
content: yes i have the design working with me using transistor but i wanted to make it more compact. I wanted to remove such big heatsinks as they are custom made and transformers that are single tap.