It's all in the filtering...
What separates a typical, industrial power supply (and most computer-type supplies) from one that is intended to be "quiet" (RF-wise) is largely filtering, as what is contained within the box comprising the switching supply is essentially a high-power transmitter! It's pretty easy for low-level harmonics of, say, a 300 watt switching power supply (oscillator!) to leak out, and since even a few billionths of a watt at the input of a receiver make a signal that is annoyingly strong, one can appreciate the need for proper containment!
Fortunately, most switching power supplies used in this application operate in the 30-60 kHz range which means that, by virtue of the large frequency difference, the harmonics - those buzzy, raspy things that often appear every 30-60 kHz across the HF bands - are already weakened considerably, but one needs to do more to submerge them below the noise floor!
It should be no surprise that it's largely the AC input and DC output leads that conduct this energy out of the box so some fairly good filters are required.
AC Line filtering:
Take as an example the circuit depicted in Figure 1, below:
|Figure 1: 2-stage "brute-force" line filter using bifilar inductors. The AC power comes in on the left and is delivered to the "guts" of the switching supply ("load") on the right.|
This is a typical filter found on the AC power line of better-quality power supplies that are designed to be "RF quiet" and what does the most work are the two bifilar inductors. How this works is that at capacitor Cd, where there are strong RF components of the switching energy, the two sides of the AC power line are "shorted" at RF frequencies (e.g. made to be "common-mode") so that when equal amounts of RF pass through bifilar inductor, they get canceled out and "choked" by the inductance. The first of these inductors (the one on the right) isn't able to do all of the work, so another stage of filtering consisting of capacitor Cc and another bifilar inductor is applied.
For an article describing what is meant by a "common-mode" signal, read here: http://en.wikipedia.org/wiki/Common-mode_signal
Finally, at the power line we have capacitors Ca and Cb and these not only help reinforce the "common mode-ness" and the effects of the bifilar inductor, but shunt the remaining small amount of RF from the switcher to the metal box containing the switching power supply so that it does not escape on the power line. Typically the values of Cc and Cd are in the range of 0.01uF to 1 uF and the higher the capacitance, the better - but at the cost of the component itself (larger capacitors are more expensive) and the fact that as you increase the capacitance, it will draw more and more current from the AC power line on its own due to capacitive reactance. (This leakage current will not generate heat since it is very reactive - but that's another discussion altogether!)
The values of Ca and Cb can vary, but it's common to find anything from 0.001uF to 0.47uF, but some safety laws limit the values owing to the fact that at this midpoint (ground) there will be approximately 1/2 the AC mains voltage (with respect to either side of the AC mains) should the ground be disconnected: The value of these capacitors and their reactance will dictate how much of a shock hazard this might present should accidental contact occur. These capacitors must also be appropriately safety-rated since failure could put the full mains voltage on the safety ground and pose a lethal shock or fire hazard. Typically, these capacitors are blue and have imprinted on them their specific AC rated voltages as well as symbols indicating the various safety and regulatory bodies by which their use is approved.
The inductors are the most expensive components in this filter since they use fairly heavy copper wire for to handle the multi-hundred watt load as well as fairly pricey ferrite material. They are fairly large and heavy so it is not too surprising to find an off-brand or counterfeit power supply where all of these filtering components (inductors and capacitors alike!) are omitted to cut costs: Such power supplies radiate lots of noise and do not meet regulatory (or even safety) requirements in most countries!
DC output filtering:
The other place where RFI might escape is the DC output. Take the example of Figure 2, below:
|Figure 2: Simplified diagram of the DC output of a switching power supply.|
If the circuit board has been laid out properly properly and good-quality components have been used, the "V+ Out" line will be pretty clean - but notice something else: The "ground" to which the transformer center-tap, Ca and Cb are connected is different from that of the chassis (case) ground in that that they aren't even connected directly to each other!
There are several reasons for this. First of all, it is often desired that the case ground - which is usually connected to the AC mains safety ground, as well, be isolated (DC-wise) from the DC output of the power supply, this being done to prevent "ground loops" - that is, power finding its way along more than one lead and back to the same place. In extreme cases this can cause hum or, in the case of faulty mains wiring, put a shock hazard on the metal case of the gear being powered. The use of a capacitor such as Cc "connects" the two at RF, but not at DC or at mains frequencies.
In the cases depicted below, plastic capacitors rated for at least 250 volts are used which is adequate for 120 volt mains. This seemed to provide adequate bypassing - even at fairly low radio frequencies - and the value used still presents reasonably high reactance at AC mains frequencies (>800 ohms at 60 Hz, >960 ohms at 50 Hz for 3.3uF) to afford a reasonable degree of safety, minimize circulating currents (hum) at those frequencies while eliminating the possibility of any DC ground loops.
Common problems with "noisy" power supplies:
First, some warnings:
- Do not perform any modification described here unless you are familiar with the techniques involved in high voltage and high current circuits. Accidental contact with mains voltages can be lethal!
- You must make absolutely certain that all components that you use are rated for the voltage/current involved. In particular, any capacitors that bypass from the AC (mains) input to the chassis must have the appropriate voltage and safety ratings to prevent the accidental imposition of potentially lethal mains voltages on the chassis/ground of the power supply and connected equipment!
- In this article, some of the filtering components are depicted as being added prior to line fusing. In all cases, such components must be appropriately safety-rated for the voltage and current. In some areas (such as the EU) it may be permitted that such components are connected only after line fusing - and that line fusing is required on both leads of the AC power connection. In any case, take sensible safety precautions!
- Be certain that any inductors used are rated for the current involved and that their insulation is capable of withstanding the voltage applied. For the bifilar chokes on the AC input, these must be rated for at least 4-5 amps while the output choke ("L" in Figure 2) should be heavy enough to handle 23-25 amps with minimal voltage drop.
- Some of the techniques described in this article may not meet safety regulations in certain countries. Examples might include: The placement of RFI/EMI components before the fuse, the types of capacitors, the values of capacitors and the amount of leakage current that they would consume and/or place on the chassis ground, etc. Please be aware of these issues and address them in a manner appropriate. I thought that I'd mention that twice...
- There are likely other things not mentioned here. You have been warned!
For this discussion we are assuming several things about our power supply:
- It is contained in metal case. The case doesn't provide "shielding" as much as it provides a common, low-impedance point to which all filtering that helps remove switching energy can be connected. Doing this prevents the formation of differential currents between the AC input and DC output leads which could impose low-level switching supply energy onto those leads!
- It includes at least some of the above features to filter out switching components.
Case study #1: An older style model Samlex 1223
For the first example I will take an older Samlex model 1223 power supply, a 23 amp, 13.8 volt unit that I bought in the late 1990's. It was intended for use in, among other places, amateur radio stations and is an inexpensive, yet fairly well-designed power supply. Despite this, I noticed that it produced some low-level - yet annoying - spurious emissions on the lower HF amateur bands (160-40 meters) which were also weakly audible at even higher frequencies.
In disassembling the unit I noticed immediately that it had just one AC input line filter. Fortunately, there was enough room to wedge into it another bifilar choke (scavenged from a junked power supply) and the necessary bypass capacitors.
I also put an oscilloscope across the DC output terminals and noticed that even though the DC output itself was quite clean - just a few millivolts of residual switcher energy - I saw few hundred millivolts of switcher energy when I measured between the chassis of the power supply and either of the DC outputs: See Figure 4, below.
The waveform present between either DC output and the chassis of the unmodified power supply. The frequency/time noted in the box in the lower left is measured between the two purple vertical lines.
Click on the image for a larger version.
As can be seen, the magnitude of the "square" portions of the waveform are on the order of 130 millivolts with the extents of the high-frequency spikes going out to at least 268 millivolts: It is this energy that is going to cause us the most grief! This waveform looked the same whether I measured between the V- terminal and ground or the V+ terminal and ground, but this was not surprising since I already knew that from measuring across V- and V+, the waveform was quite clean.
At this point I had a choice: Should I simply short the V- to the chassis ground and risk a ground loop, or install a capacitor? Preferring to NOT subject myself to the possibility of a ground loop and the possibility of induced AC hum in the future, I rummaged around in my capacitor collection and found a large, 3.3uF plastic capacitor with a 200+ volt rating - probably something scrapped from an old switching supply or computer monitor. When I connected this between the V- lead and the chassis of the power supply, I got the waveform in Figure 5, below:
The output of the power supply after adding the capacitors to the output with the same vertical/horizontal scale as the plot in Figure 4. Notice that only a fraction of the original "grunge" remains!
Click on the image for a larger version.
As you can see, there is a significant improvement! The narrow spikes are much lower in amplitude (about 66 millivolts peak-peak rather than 268 millivolts) and, although it is a bit difficult to see in the above trace, the pulses are also much slower in their rise/fall time. This last point (pun intended!) is important since it is the rate of change (dV/dT) of these pulses that dictate how much harmonic content they have, so between their reduction in amplitude and their being "slowed" considerably, this power supply was now VERY much "quieter."
- I found a small piece of glass-epoxy circuit board material and cut it to fit the empty space above the DC output terminals.
- Flipping the power supply upside-down, I drilled a hole for a 6-32 machine screw through the case and piece of circuit board material. Flipping the case upside-down ensured that metal cuttings would not fall into the power supply.
- After de-burring the holes with a drill bit (also done with the power supply upside-down) I bolted the piece of circuit board material to the case using some "star" washers to ensure a solid, electrical connection.
- Between the piece of circuit board - which is now connected to the metal chassis ground of the power supply's box - I soldered a 3.3uF plastic capacitor between it and the V- lead. Any value of 0.47uF and up would be fine, but 2.2uF-4.7uF is better.
- I also soldered a 2200uF, 25 volt low-ESR (switching supply-type) electrolytic capacitor between the V- and V+ terminals using short pieces of heavy (#12 AWG or larger) wire. This wasn't really necessary, but it did knock down those small "spikes" in Figure 5 a bit more.
Putting the cover back on and testing it - even using it a few times during Field Day over the years - I have not observed that this power supply has caused any detectable interference, even when being placed next to a balanced-wire antenna tuner.
A Radio Shack model 22-510 power supply:
A couple years after getting the Samlex 1223 I noticed that the Radio Shack 22-510 power supply was on sale and grabbed one. Rated for 25 amps, it is almost identical in size and shape to the Samlex 1223 and it had a permanently-attached power cord rather than a detachable computer-type cord. Popping the cover I could tell that it was better filtered than the old Samlex in that it already had a 2-stage AC input line filter that strongly resembled that depicted in figure 1. As with the Samlex, I noted that across its DC terminals the output was fairly clean, but like the Samlex, I observed a waveform that was nearly identical to that in Figure 4 between the chassis and ground!
Since I had noticed a small amount of switching noise (not bad, but not as clean as that in Figure 5) I rummaged around and found a small choke on the 5 volt, high current output of a junked PC power supply that had been wound with #12 AWG wire and would thus be capable of handling 25 amps without much voltage drop. Clipping the red (V+) lead, I soldered this inline and it is shown in Figure 7, insulated with yellow heat-shrink tubing. Across the V- and V+ outputs I attached a 1000 uF, 25 volt capacitor and the combination of these two components made its output at least as clean as that shown in Figure 7. I probably would have been fine not doing this, but since I was already working on the power supply, anyway...
After I did all of the above I noticed that there was still some low-level noise on the power supply that was not at the switching frequency, but rather in the range of a few hundred Hz: It wasn't strong enough to be a problem, but it annoyed me that it was there at all and I was curious as to its source. What I soon realized was that this extra noise was coming from the small cooling fan on the chassis: Unlike the fan on the Samlex power supplies which are thermostatically-controlled with an electronic heat sensing circuit, the fan on this power supply always runs and is connected across the DC output.
One insidious problem with these brushless DC fans is that they seem to have the uncanny ability to put some of their electronic commutating noise onto their power supply leads despite the fact that they draw only a hundred milliamps or so and are, in this case, connected to a high-current power supply with lots of filtering! The fix for this is quite simple, however: A series 10 ohm resistor and a 220uF, 16 volt capacitor.
Figure 8, above, shows this modification with the capacitor connected across the "fan" side of the resistor on the fan's power supply leads. Because it was convenient to do so I used RTV ("Silicone") adhesive to attach these added components to the back wall of the power supply, but I could have also enclosed them in heat-shrink tubing. With this modification the fan electrical noise was completely removed from the power supply's output and the fan ran slightly slower due to the voltage drop across the 10 ohm resistor and would likely last a bit longer - but it still moved more than enough to keep it cool under full load.
When I was done this power supply, too, was now very "clean".
A newer Samlex 1223 power supply:
Earlier this year I spotted a brand new, in-the-box Samlex 1223 for a really good price at a swap meet and couldn't resist getting it. When I opened it up I could see that since my older '1223 had been built, they'd made some improvements:
- Like the Radio Shack unit, it now had a 2-stage input line filter.
- They'd changed the output connector from binding posts to screw-type compression terminals.
- Both the V- and V+ output leads were routed through one large ferrite bead.
To be sure, I had not tested the unmodified power supply against the others that I'd modified to see how "clean" they were in terms of causing interference to HF operations but reports indicate that these newer Samlex 1223's are better than the older version in that regard but were still known to cause objectionable interference in some cases.
Interestingly, when I placed the oscilloscope between the V- and the chassis of the power supply I got almost exactly the same waveform as I'd gotten with the older Samlex 1223 and the Radio Shack 22-510 power supply depicted in Figure 4 so I knew what I had to do.
I've made these modifications to these power supplies over the years as I've acquired them and was somewhat surprised to see that they all have the same issue in common: Significant switching energy between their cases and their DC output lines. Fortunately, the "fixes" outlined above seem to be very effective and add minimal safety risk to their use and these three switching supplies that no longer cause any noticeable RFI, even when placed very close to the feedpoint of an HF antenna.
In addition to keeping these power supplies clean at HF, I also wanted to make sure that they caused minimal disruption at LF and VLF frequencies (e.g. those below the AM broadcast band) where I occasionally listen. Because of these lower frequencies it is much more difficult to keep them from causing interference for several reasons:
- Rather than being several 10's of times higher than the switching supply frequency, I might actually be listening on the switcher frequency - or on one of its first few harmonics.
- At these lower frequencies the amount of inductance and capacitance in the filters may not be adequately high to effectively remove enough of the switching energy.
Unfortunately, I also have other power supplies around the ham shack and the house (for the computer/monitor, the DSL modem, in the compact fluorescent and LED lighting, etc.) that are far "dirtier" and, at some point, these will require some action to clean them up - but that's another article!
This page stolen from ka7oei.blogspot.com