Power supply for voltage references

[Kleinstein]

There can be short current spikes from diode reverse recovery. This can be reduced by using fast revery diodes and limiting the voltage slew rate, e.g. with capacitance on the transformer output. Usually the diode turn on part is not an issue, and diode turn on pretty fast and not much difference (e.g. a 1N4001 is about as fast as a 1N4148 in that respect).
The other spikes come from the current pulse to the filter capacitor. Here some added resistance (could be just a fuse) or inductance can help to reduce the current pulsed and get some improvemen on the power factor. The fast rising current can otherwise be a problem and cause inductive coupling. The current puse may also be visible in the ripple voltage at the capacitors. 2 stage fitlering can reduce the shap pusle - it can also help to reduce the chance for a ground loop.
20-50 µs sounds about like the rising edge of the current pulse to charge the main filter capacitor.

[Echo88]

Heres an article that goes through the diode reverse recovery issue (if thats your issue in this case) in detail and shows how to solve it:

https://www.desmith.net/NMdS/Data/Linear%20Audio%20-%20Soft%20Recovery%20Diodes%20Lower%20Transformer%20Ringing%20by%2010-20X.pdf

[maxwell3e10]

Heres an article that goes through the diode reverse recovery issue (if thats your issue in this case) in detail and shows how to solve it:

https://www.desmith.net/NMdS/Data/Linear%20Audio%20-%20Soft%20Recovery%20Diodes%20Lower%20Transformer%20Ringing%20by%2010-20X.pdf

This looks promising! Adding resistor-capacitor network across the transformer already helped reducing the transient, just need to find optimal values. I will also get some SBYV27-200 diodes. Does anyone have other favorite diodes for general purpose linear rectifier?

[iMo]

In past while building ham radios I put 4n7-10n ceramics in parallel with each diode in the bridge rectifier..

[Kleinstein]

The difference between the different fast diodes is rather small - it looks like hard to measure that accurate (e.g. mains can vary over time). For low voltages ( keep in mind the diodes see up to 2 times the AC peak voltage) schottley diodes are attractive.  The choice depends on the current and availability. With only mains frequency there is no need to pick special or extra fast (e.g. SiC) didde - that is a point in the SMPS. The slow diodes have a Trr in the 2 µs range and even that is nearly fast enough (if mains is a clean sine) at mains frequency. Also series inductance (similar to PFC correction) can smoothen the current spike and thus make the reverse recovery less imortant.

[David Hess]

I am wondering if anyone has suggestions for reducing small sharp spikes in a simple linear power supply that are associated with rectifier diode bridge. The spikes are synched to AC line and last 20-50 microsec. I am not sure how they propagate to the rest of the circuit, perhaps magnetic or capacitive coupling. They occur when the diodes turn on, so the exact phase depends also on the power supply load. Are there perhaps better diodes to use or putting some sort of filter to smooth out the turn-on transitions?

I thought this was caused by reverse recovery "snap off", which some slow diodes display.  For 1N4004 series diodes, a couple hundred picofarads of capacitance directly across each diodes suppresses it.  Some designs include a ferrite bead in series with each diode.  Some standard recovery diodes have this problem and some do not.  Fast recovery diodes seem less likely to have this problem, or maybe they never do.

I first ran across this problem with a couple of simple fixed voltage power supplies that I had on my workbench for powering circuits during development.  Later I noticed that many linear power supplies in audio equipment include suppression capacitors across the rectifiers.  Tektronix included them in some but not all of their linear power supplies, implying that the problem was intermittent for them.

[Andreas]

The spikes are synched to AC line and last 20-50 microsec.

Hello,

It has already been said that radios and analog TVs used (up to 47nF) capacitors across each of the 4 diodes of the bridge rectifier.

some questions:
- how did you measure the spikes and with what amplitude?
  (on the AC-Side it would be very difficult so I assume that you measured across the smoothing capacitor on the DC-side)
- And what diode did you use for the bridge rectifier?

For my "ageing" cirquits I am using Schottky diodes (MBR20100) together with a dual winding transformer (2*12V).
Mainly to have not too much diode losses for the low drop regulator (15V).
But up to now I never had the Idea to measure for rectifier spikes.

with best regards

Andreas

[maxwell3e10]

Thanks for the suggestions. The spikes do in fact appear at the time of the turn-off of the diodes. The circuit in question is an old HP power supply, it has  a 50 nF capacitor across the transformer winding. The diodes used are 1N5059 which have maximum recovery time of 4 usec. I can probably do better with faster recovery diodes. The spikes are hard to see directly on rectified output, but they appear as ~0.5 mV spikes elsewhere in the circuit through some electromagnetic coupling.

[kleiner Rainer]

Everything you want to know about snubber design:

http://www.hagtech.com/pdf/snubber.pdf

Greetings,

Rainer

[David Hess]

Thanks for the suggestions. The spikes do in fact appear at the time of the turn-off of the diodes. The circuit in question is an old HP power supply, it has  a 50 nF capacitor across the transformer winding. The diodes used are 1N5059 which have maximum recovery time of 4 usec. I can probably do better with faster recovery diodes. The spikes are hard to see directly on rectified output, but they appear as ~0.5 mV spikes elsewhere in the circuit through some electromagnetic coupling.

When I had the same problem with my home built fixed output power supplies, I was working on audio circuits and the 120 Hz "buzz" showed up everywhere no matter what I did for shielding or filtering until I tracked the problem down to the standard recovery diodes and suppressed it at the source.

I added capacitance across each diode until the noise stopped, and then doubled the amount of capacitance.  The amount of capacitance needed seems to be proportional to the diode's capacitance, but it would not surprise me if it depends on the amount of stored charge.

Not all diodes of the same nominal type produced this problem.  It varied between batches of diodes from the same manufacturer, and different manufacturers.

[dietert1]

It also depends on the stray capacitance inductance of the transformer. I know that two chamber transformers with less perfect coupling between primary and secondary are much worse in this respect, independent of the type of diode.
A good/complete snubber has two capacitors and one resistor, as described in the document linked above by "kleiner Rainer". And it sits across each diode, not across the transformer secondary.

Regards, Dieter

[miro123]

Does somebody try gate driver dc-dc power supplies?
All major manufacture offer then. I have not seen any problems when I use then in intended for use application - high side SiC or GaN bridge? The only limitation is that you can use then only as single output dc-dc converter.
I don't have the equipment to measure 3pF parasitic capacitance.
https://www.murata.com/en-eu/products/power/isolated-gate-drive-power

[EC8010]

I see there's been lots of discussion about mains transformers and coupling. The key factor is C_PS (capacitance between primary and secondary). A standard foil electrostatic screen between primary and secondary on an EI transformer will reduce C_PS from 250pF on a 50VA transformer to around 5pF, perhaps 2pF. Adding capacitance to chassis from each end of the secondary creates a potential divider to common mode interference. Entirely covering the secondary with copper tape (without creating a shorted turn) reduces C_PS to about 0.1pF. It's fiddly work and requires a split bobbin transformer. Topaz Ultra-Isolator transformers fitted individual foil electrostatic screens around primary and secondary to achieve <10fF. They also have a safety screening plate between the two windings capable of sinking enough earth current to blow a fuse quickly. That's what it's there for; safety, not screening. Adding a safety screen to an EI transformer kit plus totally enclosing foil E/S screen on secondary, I achieved C_PS = 8fF +/-60% uncertainty. But what I really learned was that it's terribly easy to squander the improvement of a good transformer by sloppy external wiring. If you do something to prevent mains common mode interference getting in (low C_PS transformer, mains filter) you have to totally screen incoming mains. Folded tin plate screens work well over incoming mains wiring, with self-adhesive copper tape (gardener's slug tape) over all joints. RF crawls through the tiniest gap.

As has been mentioned, split bobbin transformers have high leakage inductance, needing an RC snubber across the secondary to damp their resonance when the diodes switch. Capacitors across diodes in the rectifier simply couple interference from the mains to your sensitive electronics. I know that's how it used to be done, but it was wrong. The Mark Johnson Quasimodo circuit works very well for determining the optimum snubber.

[alligatorblues]

If you want quiet, just get rid of the transformer. Take a metal box. Run some 120AC into it. Connect that to a 30W 500Ohm resistor with a thermostat in the circuit. Put the temp sensor in the metal box, set the thermostat for 60C, and put TEC chips in with the resistor. Put a cooler on the TEC chips, and you've got the cleanest power there is coming off the TEC chips: a thermoelectric generator. You just design the TEC pile to produce the voltage you need, and add the same configuration in parallel until you've got the current you need.

TEC power is cleaner than batteries.

[maxwell3e10]

Interesting idea, never heard of it being practically used though. Why do you say it would be cleaner than batteries? There will be some low-frequency noise due to temperature fluctuations and convection. Also one can still have some capacitive coupling to the power line coming in.

[Kleinstein]

Peltier elements / termolectric converters  have pretty poor efficiency. Even if they would run rather hot (e.g. 100 K temperature difference) the Carnot limit is only some 30% and the TE converts are something like 10% of this. So one can expect an efficientcy of less than some 3%.  A 30 W heater would thus hardly be good for some 1 W and one would need an hefty fan.

Chances are the a combination of IR LEDs and photovoltaic cells could be more efficient. At some 800-900 nm both the LEDs (laser diodes) and solar cells are relatively effcient (e.g. some 40% for the solar cell and lasers diodes).  There are systems that use power via optical fiber.

Another not much know option for isolation is an acoustic transformer: piezo elements connected with a glass rod.

For a voltage reference where size is not such an issue, there is also the option to power from batteries for the time of critical exleriments - normal standby over night could than use a conventional transformer. For those who run 24/7 the efficienty does matter a little.

[maxwell3e10]

I remember seeing a home-built HV power supply that was powered by a rotating plastic rod connected to a brushless motor. One can have a large distance between two parts and a clean AC signal at a few kHz to rectify for power. And the efficiency of motor/generator can be over 50%.

[miro123]

I've just waking up old thread instead of starting new one.
I'm considering to start journey in something different. Instead of following the  traditional approach-  tweaked push-pull, I am going with ZVS LLC converter. Back in the days such solution was quite complex.
There COTS solutions nowadays. - https://www.ti.com/product/UCC25800-Q1 

At first glance solutions seems to the perfect match . Low capacitance transformers have big leakage inductance. Leakage inductance is desirable for LLC topology.
I have few questions, just before start the PCB design and order components
1. Does somebody see any drawback of such approach
2. Why AD still propose push-pull configuration for modern design?

My toughs are - careful push pull  guarantee low capacitance and almost symmetrical noise spikes caused by hard switching. Therefore the integration ADC will make 0 of such noise.

[David Hess]

1. Does somebody see any drawback of such approach

I suspect it is more complex than needed.

2. Why AD still propose push-pull configuration for modern design?

All of the old designs that I am aware of also use a push-pull configuration.  Maybe the center-tapped primary provides less common mode swing because the center-tap is grounded?

My thoughs are - careful push pull  guarantee low capacitance and almost symmetrical noise spikes caused by hard switching. Therefore the integration ADC will make 0 of such noise.

I would consider two other possibilities before a ZVC (ZVS?) LLC converter.

High frequency linear analog sine wave drive has reasonable efficiency.  The transformer turns ratio is selected to operate the linear power amplifier close to saturation.  I might do this with an output LC notch filter and then tune the oscillator to match the notch.

Alternatively a resonate Royer converter, like that used to drive a high voltage discharge lamp, can be driven from a constant current output linear or switching regulator.  The secondary is now wound for low voltage instead of high voltage.  Tektronix sometimes did this in various forms.

[exe]

High frequency linear analog sine wave drive has reasonable efficiency.  The transformer turns ratio is selected to operate the linear power amplifier close to saturation.  I might do this with an output LC notch filter and then tune the oscillator to match the notch.

I'm not an expert, but I think, for the best result, the driving frequency should also match transformer's self-resonant frequency.

[miro123]

Many thanks David for giving me alternative options.
It seems that there are many solutions. The only way to know which is the best is to evaluate all of them.
I will start with LLC. The reasons are :
  1. I want to start with easiest for me.
  2. Open loop LLC resonant converter sounds quite complex but thanks to advancement in power electronics we have single chip solutions. COTS transformers
  3. I know the fundamentals of this topology
  4. I think that this topology fits the my requirements the best.
       - Low intertwining capacitance comes at the cost of high leakage inductance.
       - leakage inductance in undesirable for most application. It crates many problems like EMI , high current/voltages loops. etc.
       - LLC operation relies on leakage inductance to achieve ZVS.
  5. I have the equipment to
UCC 25800 circuit explanation.
Capactiors at primary winding are used to settle the middle point. Uin/2
Capacitors at the secondary defines the resonant frequency.
Resonant frequemcy is defined by leakge inducant on secondary winding. The most popular LLC use ZVC capacitors at primary.
I have to experiment between two solution - two diode doubler of full bridge rectifier. There are many shotcy diodes in that voltage range.
As start point, I will go with Wurth COTS transformers - https://www.we-online.com/en/components/icref/texas-instruments/UCC25800-LLC-Resonant

[Kleinstein]

With an open loop LLC one has to expect a relatively soft output voltage. So the output voltage would depend on the load. This may be OK, but could also need some care.

A sine voltage drive would have short current spikes with a simple rectifier. One could also consider a more overdriven sine, with more haminics in the voltage but often less hamonics in the current.

[Gerhard_dk4xp]

If you want quiet, just get rid of the transformer. Take a metal box. Run some 120AC into it. Connect that to a 30W 500Ohm resistor with a thermostat in the circuit. Put the temp sensor in the metal box, set the thermostat for 60C, and put TEC chips in with the resistor. Put a cooler on the TEC chips, and you've got the cleanest power there is coming off the TEC chips: a thermoelectric generator. You just design the TEC pile to produce the voltage you need, and add the same configuration in parallel until you've got the current you need.

The 30 Watts for the heater are not the biggest friends to nV offsets,
esp. with a thermostat.

> TEC power is cleaner than batteries.

Strong opinion. It would have to beat these Panasonic/Sanyo NiCd cells ( 4 pcs. in series).
18650 Lithium was about the same, I took no photos. Size matters.
The strong 1/f is caused by the too small input coupling capacitor.
(only 100 uF foil for 20 ADA4898 in par)
0 dB = 1nV / rtHZ.

more measurements of  battery noise in
<     http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf    >
and the cited NIST publication there in.

Cheers, Gerhard

[EC8010]

Interesting stuff. Did you measure lead-acid batteries? I haven't made a pre-amplifier as quiet as yours (mine's about 0.7nV/root Hz), but a 12V 7Ah battery does not appear to noticeably increase its noise spectrum down to 0.1Hz, suggesting it's pretty quiet.

[miro123]

With an open loop LLC one has to expect a relatively soft output voltage. So the output voltage would depend on the load. This may be OK, but could also need some care.

No worry here I am familiar with this topology. In the mean time I have measured the parasitic capacitance of proto PCB with and made toroidal transformator. Parasitic capacitance in 3,5pF range. See attached. HF bump up above 10MHz is due to the probing. I think with beter layout I can make results better. Initial results are good but it is still work in progress. The next step is output pi filter design. If ready then DC-DC buck and CUK converters.