5V Linear / Switching regulator question - Page 1
Forum section: electronics / projects

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 22, 2021, 05:53:35 pm
content: Hi,I know that the topic of voltage regulators has been discussed for years in many forums but I have a few questions myself that I thought I would ask.I have a existing design that use a linear voltage regulator, L78M05ABDT, which seems to work well in an existing product and has been for a number of years without any know issues.The supply is 12Vdc and the input caps to the regulator are a 330uF 50V cap and a 1uF. The circuit has two regulator output caps, a 10uF 16V and a 100nF ceramic.Each IC (MSP430, ADC, Temp sensor and a few other basic things) have a 100nF cap across the supply pins. Current drawn from the regulator varies but is usually no more than around 200mA max.The whole circuit sits in a airtight container that's is around 50 Degrees C.Now this is an older design, which has worked well, but now is getting old and needs to be redesigned for various reasons.What I did notice is that the 5V output has a high frequency ripple sitting on it. (Nice sine wave at many 10's of MHz, might even be around 100MHz, but I need to check this) with a peak to peak of around 50mV.I am looking at changing this linear regulator to a switching regulator of some type and just trying make a new design that hopefully will be better engineered.Any thoughts on this or suggestions? I would like a switching regulator that with associated components does not put the price right up but will also give a nice clean 5V supply.ThanksJohn

author: mon2
author_rank: Frequent Contributor
timestamp: May 22, 2021, 06:29:13 pm
content: Sure, just replace it with a RECOM drop in and start testing.https://recom-power.com/pdf/Innoline/R-78-0.5.pdf

author: Benta
author_rank: Super Contributor
timestamp: May 22, 2021, 06:31:10 pm
content: You can be pretty certain that a high-frequency ripple like does not come from your circuit. Do you have an FM radio transmitter or something like that in the vicinity?That being said, it's a good idea not to use just one value for decoupling caps, in this case 100 nF. Use a mix of 10, 22, 47 and 100 nF instead.Why? Because the normal ceramic decoupling caps have a self-resonant frequency, and by mixing values this phenomenon can be suppressed to a certain degree.Th 78M05 is not a bad part, but there are regulators on the market with waay better specifications today. Like ripple rejection, transient response etc. etc.

author: ledtester
author_rank: Super Contributor
timestamp: May 22, 2021, 06:33:46 pm
content: What exactly are your goals for the redesign? Your current design seems to have passed the test of time and works for your application. Moving to a switching supply will only introduce more sources of noise and I doubt it's going to reduce your BOM cost.QuoteThis sounds very suspect. Are you sure you are using a good probing technique?Engineer It - How to test power supplies - Measuring Noisehttps://youtu.be/pKXPqApOYfk?t=1mAlso be aware of possible interference from other lab equipment:EEVblog #441 - How To Track Down Common Mode Noisehttps://youtu.be/BFLZm4LbzQU

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 22, 2021, 06:48:55 pm
content: Quote from: mon2 on May 22, 2021, 06:29:13 pmAh, yes the RECOM parts. I think they are excellent and I use them here as my go to regulator for doing quick one of projects. In fact I only ordered a few more the other day from RS.But. The issue is cost. My requirements are really driven by price, mainly for 100's or perhaps 1000 devices at a push.The LM7805 and caps are cheap compared to the RECON. If a very tight budget had no play in this then a RECON would be on the list.

author: bdunham7
author_rank: Super Contributor
timestamp: May 22, 2021, 07:06:43 pm
content: Quote from: hobbyelectronics on May 22, 2021, 05:53:35 pmWhat reasons? Unless you need to eliminate the power dissipation from the linear regulator, I would think that any redesign of the voltage regulator part would be no better and possibly worse than what you have in terms of noise and reliability, as well as more expensive. Your many-megahertz noise is either external or from the MCU or related circuitry, not the regulator. Are other parts of the device being redesigned as well?

author: mariush
author_rank: Super Contributor
timestamp: May 22, 2021, 07:16:56 pm
content: If you want higher efficiency, less heat, go with a switching dc-dc converter.AP65211 is 30 cents if you buy 100 and only requires an inductor and a bunch of ceramic resistors and capacitors :https://www.digikey.com/en/products/detail/diodes-incorporated/AP65211AWU-7/6637778Not hard to place it on a circuit board.Here's datasheet :https://eu.mouser.com/pdfdocs/AP652113.pdf

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2021, 01:35:39 am
content: Quote from: hobbyelectronics on May 22, 2021, 05:53:35 pmLinear regulators like the 7805 do not need a ceramic decoupling capacitor at their output, and such a capacitor can cause trouble. One might be used at their input if the input bulk filtering capacitor is remotely located. Ceramic decoupling capacitors belong close to the load.Double check that the noise you are measuring is real. I assume you are connecting your oscilloscope probe between ground and the 5 volt output. As a test, make the same connection but with the probe tip connected directly to the same point as the ground lead. This will reveal if the 100 MHz signal is across the power or not.

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 24, 2021, 04:16:34 pm
content: Its a bit of a mystery really. Its not my regulator! I jumped the gun a bit posting this issue, rather than simply thinking about it and testing. Measured the 5V output, noticed the problem and wrongly assumed it was coming from regulatorWith the power to the board off and completely disconnected the signal is still there, just not sitting on top of the 5V of course. but just above zero.With the scope probe not connected to anything it 99.9% goes (Just a small amount of normal noise, around 20 uV). As soon as I probe anything the signal comes back.I powered down everything which is nearby (apart from the scope) but its still there? I also powered everything off and measured the output of a small breadboard PSU which had a similar signal sitting on the 5V at around 100MHz with a P/P of about 50mV.Its coming from somewhere? Maybe its just me and doing something wrong?Thanks

author: ledtester
author_rank: Super Contributor
timestamp: May 24, 2021, 05:34:47 pm
content: Just enable your 20 MHz BW limit... as they say, ignorance is bliss.You don't want to end up like Gene Hackman at the end of The Conversation:https://youtu.be/SEo7FGkmRx0

author: TimFox
author_rank: Super Contributor
timestamp: May 24, 2021, 06:58:48 pm
content: I had similar problems at work. I learned to tell the difference between AM and FM broadcasting (at the relevant frequencies) by closely watching the waveform on an analog CRO. The syllabic modulation during voice announcement is a signature. Oscillation or clock pickup is usually more stable over that timescale. Our first location was near an AM station, and then the company moved to near an FM station. In my home basement, I am roughly a mile from an AM transmitter.

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 25, 2021, 07:02:45 am
content: I finally managed to turn everything off in my house and the 93 to 98 MHz signal is still there. It varies up to a peak to peak of 100mV.Very annoying. Yeah I could limit the bandwidth to 20MHz, but then I could have just bought a 50MHz scope and saved a fortune!Maybe its coming from the house next door? Is this a big problem? I cant see how I can test anything in a reliable manner really? I have lived here for years with older scopes and developed, built and tested many projects. Never seen this before.Makes testing some circuits pointless if I have to put up with 100mV stray signals, maybe I can build a lead lined test shed in the gardenIt looks like a AM signal, but at 100mV that's a lot so it must be close. I doubt it but could it be coming form the scope somehow?

author: bobbydazzler
author_rank: Regular Contributor
timestamp: May 25, 2021, 10:13:30 am
content: 7805 recommends 330nf ceramic across the input pins(1uf +0.1uf ceramic is close enough). The 330uf input cap might be more useful on the output.

author: Trader
author_rank: Frequent Contributor
timestamp: May 25, 2021, 12:02:01 pm
content: Quote from: hobbyelectronics on May 25, 2021, 07:02:45 amI saw cases of radio noise related to the neighborhood and mains electrical lines around. Maybe your device has antenna behavior.

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 25, 2021, 12:13:54 pm
content: The 12V input comes from a separate PSU module which also feeds other 12V parts of the circuit (Fans, relays etc), which are turned on and off. I think (not 100%) the 12V PSU data sheets stated to use 330uF cap on the output.the regulator data sheets states a 0.33 uF or larger on the input. Not sure having a 10uF also on the output helps but that was there to help with giving a 5V reservoir for the MC and other chips along with the 0.1uF on the reg. output as stated.Anyway, the regulator circuit has worked perfectly in production so this will more than likely not change now that the signal noise ripple on the 5V (AM? noise?) was a red herring.Out of interest see the 3 attached scope plots. 2 with just a resistor leg on the probe and one with the signal sitting on the 5V line. The frequency is around 93 to 98 MHz up to 100mV p/p.Could be coming from anywhere really.....

author: hobbyelectronics
author_rank: Regular Contributor
timestamp: May 25, 2021, 12:20:10 pm
content: When in use the PCB is sealed in an IP rated aluminium 1/2" thick earthed box.Its just annoying trying to look at the 5V rail and see this signal on top. Which is there when the PCB is powered off and also there when the probe only has a 1cm piece of wire attached to the end.The signal is also present when the ground wire on the scope (or the ground spring) is shorted to the probe tip and simply placed on the regulator o/p pin or just a loose piece of wire.Seems their are other posts on here with exactly the same problem. With very similar traces at the same frequencies and amplitudes Interesting all the same.Thanks

author: rdl
author_rank: Super Contributor
timestamp: May 25, 2021, 02:54:31 pm
content: I had a similar noise problem. Probably still do just haven't really looked for it in years. I was able to determine the source was not in my apartment, but that leaves the rest of the world as possible suspects. While I have no proof, I'm guessing it's most likely coming from the local college radio station which is located about 3 miles away.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2021, 09:23:21 pm
content: From one envelope peak to the next envelope peak is about 800 ns, or 1.2 MHz, which is in the AM broadcast band.