RIP Jim Williams. I always enjoyed your writings and the sense of humor that showed in them. I wonder what you and Bob Pease are designing in that great analog world in the sky?
Be wary if your linear regulators are powering oscillators. The ferrite beads can resonate with their switching frequencies and you'll see a massive jump in voltage ripple going back to the power supply!
I recommend getting rid of the ripple right at the switcher's output, which means a ferrite bead and / or inductor (depending on frequencies) BEFORE the switcher's output cap. This can be tricky as it can introduce another pole in the feedback loop and turn your power supply into a big oscillator. However, in places where you have a clock somewhere on your board multiplied up into the GHz range, this is necessary. As is filtering that supply with the FB / inductor, large cap as close as possible with the shortest traces possible, and DON'T drop vias to your power plane and ground planes until AFTER this filter. If that diode ringing from the switcher makes it on to the power and ground plane, it will bounce those entire planes, and adding ferrite beads around the linear regulator won't help you. The power/ground plane makes a far better transmission line than it does a capacitor, and there is no way to stop the entire ground plane from bouncing around once that noise gets on it. So stop that ringing right at the switcher!
Ok, so lets discuss how a Surface power supply would go about eliminating switching noise. Looking at the attached drawing, I've captured a generic switching regulator on the left, and have it's output [ Vin ] feed into an EZ81 vacuum tube rectifier [ pins 1 and 7 ]. Output of the EZ81, nice clean DC, as shown [ Vout ]. How is possible ? Let me disclose what is not seen here. What is not seen here, the vacuum gap between plates and cathode of the tube. The DC voltage will forward bias the rectifier, thus plate current will flow. However, the switching noise does not jump to the cathode, due to the free-air-space between plate(s) and cathode. What does it cost ? First, you need to supply the filaments 6.3 volts, at about 1 amp. Also, in this configuration, series plate resistance on the order of 180/2 ohms. So there is some insertion loss to factor in. However, if the application does not require current sourcing up to 1 amp, other tubes could used, and you would save on filament power. Also, some care is required to set this up, with the dual inputs, or you can red-plate these tubes. I would also recommend some type of simple BITE circuits to keep an eye on tube health and performance. You set this up right, these tube last almost forever. . . . . ..... ... . . ...... Side notes. I show output filter cap at 100uF. The data sheet for the EZ-81 states 50 uF max. So, why do I run output filter cap at 100uF. It goes back to understanding how data sheets are written. You see, the EZ-81 come out in 1956. At that time, the biggest filter capacitors back then, were these wax and paper rolled jobs, max value was 47uF. So, this is just a guess, but I would say they were saying this tube could handle the biggest cap available. The higher capacity electrolytics did not come out until the mid 1970's. Secondly, there is no issue with surge current for two reasons. 1. This tube rated for 0.5 amp continuous, per plate. 2. It takes about 12 seconds for this tube to warm up, and gently ramp up to full conduction. I even ran this tube feeding a few 1000uF caps over a few days. It did not care. Why? Because a vacuum tube is not an active electronic device. It is a mechanical device, with - certain - electrical - properties. It does not generate any waste heat internal to the device, because is a vacuum between plate and cathode.
From what I saw in real life, the ferrite bead is better for 2 reasons - an LC filter made with a proper inductor has a clearly defined resonant frequency and you can get the filter to oscillate at this frequency when applying load steps, so you might filter some noise but generate other unwanted oscilations; on the other hand the ferrite bead behaves like a resistor at high frequency rather than an inductor, this is because its absorbing the magnetic field (and turning it to heat) rather than store it like a proper inductor, so the magnetic energy does not get put back into the circuit.
I love how both presenter and viewers are chocking their chicken due to excitement of this presentation. 😊
Jim Williams was an analog genius!
RIP Sir.
The man radiates intellectual honesty. RIP.
RIP Jim Williams. I always enjoyed your writings and the sense of humor that showed in them. I wonder what you and Bob Pease are designing in that great analog world in the sky?
Be wary if your linear regulators are powering oscillators. The ferrite beads can resonate with their switching frequencies and you'll see a massive jump in voltage ripple going back to the power supply!
Great man, my only regret I got to know his work very late.
The world is now a better place, thanks to you! RIP Jim.
Thanks Jim! I'm glad you taught me something!
Thanks Jim! Rest in Peace.
Top bloke, serious case of the smarts rip Jim.
Jim William's is a switching regulator god.
Amazing that his wisdom is on TH-cam!
Really sad to see from the comments he's no longer around. :(
Jim William's legacy lives on thanks to YT.
Thank you Jim
Simple and elegant solution
You absolutely understand electronics.
I recommend getting rid of the ripple right at the switcher's output, which means a ferrite bead and / or inductor (depending on frequencies) BEFORE the switcher's output cap. This can be tricky as it can introduce another pole in the feedback loop and turn your power supply into a big oscillator. However, in places where you have a clock somewhere on your board multiplied up into the GHz range, this is necessary. As is filtering that supply with the FB / inductor, large cap as close as possible with the shortest traces possible, and DON'T drop vias to your power plane and ground planes until AFTER this filter.
If that diode ringing from the switcher makes it on to the power and ground plane, it will bounce those entire planes, and adding ferrite beads around the linear regulator won't help you. The power/ground plane makes a far better transmission line than it does a capacitor, and there is no way to stop the entire ground plane from bouncing around once that noise gets on it. So stop that ringing right at the switcher!
"...parasitic internal pass basically from everywhere to everywhere" hahaha
Thanks, Jim!
Great analog magician!
Beautifully explained.
LOL at the appnote last page: Megahurts to Minihurts Converter :)
Very useful info. Thanks Jim
Great presentation. Thank you for the info!
The goat...rip
Ok, so lets discuss how a Surface power supply would go about eliminating switching noise. Looking at the attached drawing, I've captured a generic switching regulator on the left, and have it's output [ Vin ] feed into an EZ81 vacuum tube rectifier [ pins 1 and 7 ]. Output of the EZ81, nice clean DC, as shown [ Vout ]. How is possible ? Let me disclose what is not seen here.
What is not seen here, the vacuum gap between plates and cathode of the tube. The DC voltage will forward bias the rectifier, thus plate current will flow. However, the switching noise does not jump to the cathode, due to the free-air-space between plate(s) and cathode.
What does it cost ? First, you need to supply the filaments 6.3 volts, at about 1 amp. Also, in this configuration, series plate resistance on the order of 180/2 ohms. So there is some insertion loss to factor in. However, if the application does not require current sourcing up to 1 amp, other tubes could used, and you would save on filament power. Also, some care is required to set this up, with the dual inputs, or you can red-plate these tubes. I would also recommend some type of simple BITE circuits to keep an eye on tube health and performance. You set this up right, these tube last almost forever. . . . . ..... ... . . ......
Side notes. I show output filter cap at 100uF. The data sheet for the EZ-81 states 50 uF max. So, why do I run output filter cap at 100uF. It goes back to understanding how data sheets are written. You see, the EZ-81 come out in 1956. At that time, the biggest filter capacitors back then, were these wax and paper rolled jobs, max value was 47uF. So, this is just a guess, but I would say they were saying this tube could handle the biggest cap available. The higher capacity electrolytics did not come out until the mid 1970's. Secondly, there is no issue with surge current for two reasons. 1. This tube rated for 0.5 amp continuous, per plate. 2. It takes about 12 seconds for this tube to warm up, and gently ramp up to full conduction. I even ran this tube feeding a few 1000uF caps over a few days. It did not care. Why? Because a vacuum tube is not an active electronic device. It is a mechanical device, with - certain - electrical - properties. It does not generate any waste heat internal to the device, because is a vacuum between plate and cathode.
very nice and informative thanks
Would an inductor with a well-known value (LC filter) serve the same purpose or even do a better job?
Hello, for an answer by one of our engineers, please visit the Power by Linear technical community at ez.analog.com/community/power. Thanks!
From what I saw in real life, the ferrite bead is better for 2 reasons - an LC filter made with a proper inductor has a clearly defined resonant frequency and you can get the filter to oscillate at this frequency when applying load steps, so you might filter some noise but generate other unwanted oscilations; on the other hand the ferrite bead behaves like a resistor at high frequency rather than an inductor, this is because its absorbing the magnetic field (and turning it to heat) rather than store it like a proper inductor, so the magnetic energy does not get put back into the circuit.
@@FesZElectronics If you could make a video with ferrite bead experiments, it would be very cool.
Surface power supply can filter out all that switching noise . . . .
Tried it. Didn't work.
Don't use black beads