Switched mode power supply noise

[king.oslo]

Hello there,

I am making my first power-supply. I have been told to use switch mode controller to increase efficiency. However, someone told me that the noise of switch-modes are considerably worse.

What is your evaluation?

Thank you!

Kind regards,
Marius

 

[crutschow]

What is the power level of the power supply you want to build? That helps determine whether it is worthwhile to build a switcher.

Switch-mode power supplies generally do have greater output noise, which occurs at the switching frequency. If you are powering low noise analog circuits, then you would want a linear power supply, otherwise for digital and many analog circuits, a switching power supply will be fine.

If you want the lowest noise from a switching power supply you might consider one of these from Linear Technology.

Layout is also critical for proper switching operation and to minimize switching noise, so use the recommended PCB layout shown in the data sheets. Do not try to build it on a breadboard or vector board.

 

[king.oslo]

At the minute I have only experimented with microcontrollers and an amplifier, but I expect to do all sorts in the years to come. I love electronics. Perhaps a linear is more suitable after all?M

 

[crutschow]

If you don't need more than a amp or so of current, then a linear supply is probably better and a lot easier to build. A very simple way to make a linear adjustable supply is to use the very common LM317 regulator. It can go to over 1A and up to 30V output (but not necessarily at the same time), depending upon the raw supply voltage, and can be adjusted down to 1.2V minimum (which is low enough for most applications). It also has a current limit and over temperature cutoff (in case the heat sink is not sufficient to maintain the temperature below its rated value) to protect the device. They are almost bullet proof.

If you use two transformers (or one with two separate output windings) you can make two isolated supplies. That way you can connect them as a plus and minus supply, or parallel them to get the same voltage at double the output current.

 

[king.oslo]

I bought a 130VA dual winding secondary transformer, 6 heatsinks of 2.5 C*/W thermal resistance that I wanted to strap to the outside of a a medium sized aluminium case, so the transistors could stay on the outside to cool better. But I do not really want to use a fan. What are your thoughts?

Thanks.M

 

[Blueteeth]

I say, do some maths Practice is different fm theory, but theory will get you solid figures to work on. There are many resources for calcuating power dissipation of heatsinks, and with linear power supples you can work out how much power the pass element (transistor) in said supply is kicking out. If you want 12V, and you draw say 2.1A from it.... if the linear regulator is powered by 15V then the pass element is P = IV = 2.1A * (15-12) = 6.3W. But power it with 30V, and set the output to 5V @ 2A, and P = 2*25 = 50W (!!).

Switching power supplies can be very efficient, but not always more efficient than a linear supply And as you rightly pointed out, they produce noise - both radiated, and conducted (noise on the output line). A comprimise is often when both are used - a switching supply to convert a voltage from say, 30V to 15V, followed by a linear regulator from 15V to 12V. The switching supply's efficiency keeps heat production low, and the linear regulator afterwards cleans up the output voltage, as well as possibly making regulation better. So you sacrifice some efficiency for a cleaner output. Radiated noise can be reuced with good layout, sheilded inductors, and shielded enclosures.

What is the voltage output of your transformer? This will determine maximum current output, maximum voltage output, as well as the voltage difference between your transformers secondary, and your regulators output.

Edit, sorry crutschow, I see you've pretty much covered everything, apologies if I've just repeated what you've already stated.

 

[king.oslo]

Thank you to both of you!

I will try and do some maths M