Does anyone have any experience regarding analysing a Switch Mode Power supply with an Oscilloscope?? In particular, the DO's and DONT's and safety aspects.

Happy to discuss off-line if not relevant to others.

Cheers,
Ian

I think the first thing to remember with switchmode power supplies is that because of the small transformers in them they produce lots of volts on parts of the printed circuit board and the secondary supply voltage often 'floats' at mains potential whilst on no load. This often applies to all these tiny mains plugpacks that litter our powerboards these days and whilst they are unlikely to kill they certainly will bite wet or greasy fingers.

On the non-safety side of things, these power supplies can and often do leak their voltage back through the earth wire and when there are enough of them on one circuit they can trip RCDs with the greatest of ease.

Clarification: It is quite okay to discuss these issues here and in fact I encourage it.

Okay, let me see if I can explain my thought processes .......

Whilst reading about Switch mode power supplies, I found several dire warnings about connecting an SMPS to an oscilloscope ...... I think the wording was "catastrophic results are certain" etc etc.. No further explanation was provided.

I think the issue revolves around connecting the Negative probe to a floating primary or secondary side of an SMPS.

The SMPS that I am 'playing' with has NO Earth connection to Mains. Although, the SMPS has two caps (filters) connected to the appliance chassis. The chassis is NOT Earthed. (I do however, have another SMPS where these caps are connected to Mains Earth via the chassis).

On the secondary side, all components are isolated from the primary side by either transformers or opto-isolators ..... thus my scope should read a floating voltage ..... albeit referenced to Earth via the negative scope probe. ????????

On the primary side, Neutral (input) is connected to Mains Earth, so provided BOTH my scope probes are connected to the primary side, I should be okay. ????????

So ........assuming my scope can handle the voltages generated and I do nothing stupid like touching anything ....... provided that BOTH scope probes are connected to EITHER the primary OR secondary side of the SMPS, there should be no problem ....????????

Is it necessary to remove the Earth connection of my scope ..... safety issue here .....????????

Am I on the right track???
Or is there other issues that I have completely missed??? ... like some feedback causing over voltages????

Cheers,
Ian

One of the things that is of significance is Modern apparatus.

Much of the equipment currently being made will not withstand the voltages of valve radio's, yet alone the mains.

Do check before proceeding. I ended up putting 2KV caps on the input of my Heathkit CRO.

I have had issues with regulated PSU's and mains ground. If you are going to use the earth on the CRO I would suggest a HV cap in series with the earth.

My other question is as to why you want to get in there with a CRO?

Marc

Hi Marc,

Usually the easiest way to check the switching frequency and ripple on output voltages .... and because there could be a valuable lesson in understanding the reason for the 'dire' warnings.

Cheers,
Ian

It seems to be an issue revolving around the fact that we use SWER systems that causes the solid state regulators to go feral, if you ground their output to mains earth.

Others have noticed this as there is one commercial battery eliminator that I have used (and I was using the same method prior to this); where the earth for the battery set (especially if it is not an isolated aerial coil primary) is taken to earth via a 0.01 (103) High voltage capacitor.

The mains side of those Switch modes can be trecherous. They actually rectify the mains before feeding it to a high frequency oscillator.

DC is something that will grab you, which is why it is often more dangerous than AC. The same dangers as AC/DC Valve sets applies.

The switchmode often feeds in via series capacitors of dubious integrity (seen lots fail) which is suposed to stop them reflecting DC back into the mains.
There are a myriad of designs of thes things

My other concern with both apparatus is that once the power has passed the rectifier, there will be no protection from an RCD.

If the earthing of the CRO has any commonality with mains ground I would tend to leave it float.

Marc

Hi Marc,

........."It seems to be an issue revolving around the fact that we use SWER systems that causes the solid state regulators to go feral, if you ground their output to mains earth." ...........

Yes, I think that this is the root cause of the 'warnings' ....... but, if I float the scope probes and connect to the SMPS secondary, all should be okay .....????

Can you expand on this statement please ..... "If the earthing of the CRO has any commonality with mains ground ...... I would tend to leave it float."

My scope probes are at Mains Earth, I thought that this was standard practice???? and 'it float' ..... it??

With this type of issue, if I am in any doubt whatsoever, I will leave it alone!

Thanks for advice,
Ian

It is the MEN system, it has the neutral tied to earth in your fuse box and every second power pole. The SWER system is only in remote country areas.

First rule of switching power supplies - Use an isolation transformer.

Use the correct earth return, switching power supplies usually have the primary and secondary completely separated, only connected by an optocoupler or separate feedback winding.

Beware, the mains capacitor charges to 340 volts and can stay at this if the p/s does not work. It drops to about 320 V while it is operating. I use a 60watt BC incandescent bulb for discharging.

Do not remove the earth connection of your scope otherwise the it can be at the potential you are measuring, the case, knobs, switches, everything.

Switching power supplies are very hard on capacitors, they are charged and discharged at high frequency and usually placed near hot temperature components. alot of manufacturers use cheap components. Just replace all capacitors with quality, low ESR variety and don't forget the two little ones in the primary side. Angle them away from the hot temperature components. This will save a lot of time with the CRO.

If it won't start, test the switching transistor/device, then the resistors. There are low resistance high wattage and high resistance low wattage resistors that go open. The rest is just fault finding.

Never switch on a switching power supply with no load connected. It will blow up. Some have protection to stop them starting and others have an inbuilt minimum load resistor. (plug packs and such)

Don't overload your test equipment. If your CRO says 10 volts input and you use a X10 probe it is 100V maximum. X1, X10, X100 probes are just like switching the ranges on your voltmeter.

Duty cycle is tested the easiest with your ears. After you work on a few you get to know what a healthy switching supply sounds like. When the capacitors start drying up the supply gets louder and slower (more on time), it sounds angry.

I hope this helps.

In case it hasn't been clear in previous posts, the mains side of switchmode power supplies are live and are not tied to earth.

All points of the circuit MUST be regarded as live! Even the common lines in these power supplies are not at ground potential due to the rectified mains. Any attempt to connect grounded test equipment to a live switchmode PSU will result in lots of sparks, bangs and blown equipment.

Most switchmodes (but NOT ALL) use a isolated secondary supply circuit meaning that the common or zero volt line on the SECONDARY side can be tied to earth without explosions. Unless you are familiar with how to identify the difference, it is safest to treat all supplies as live and cannot be grounded.

To work on swicthmode power supplies you need to use an isolating transformer. These are 240v to 240v transformers whose purpose is to break the Neutral - Earth circuit that normally occurs with the mains supply. This results in a "floating" 240v supply with no earth reference. You can now safely connect grounded test equipment without blowing fuses.

Alternatively (and a bit riskier depending on the situation) would be to use unearthed test equipment. Unless the equipment is designed to be used in this way, I do not recommend removing the earth lead from your equipment.
Battery operated CRO's are great for this situation. They are not grounded and can be safely used providing the CRO's maximum input voltage is not exceeded.

A x10 (or higher) probe is a must as long as your CRO can handle at least 10v per division. Few CRO's can handle 240v AC direct to their inputs without damage. Also the CRO probes need to be rated for use at mains voltages.

I am in a country area which was originally single wire SWER and was later converted to 2 phase feed. and that Solid state issue persists.

I did point out that the mains is rectified, that does make that side of the as deadly as AC / DC Radio's. Not all CRO's have their earth connected to ground.

The more modern Mutimeters & test equipment which is now Solid State, must be watched. As I have said frequently; A lot of it is incapable of with standing the voltages in vale radio's, yet alone the mains.

My ancient Heathkit CRO (OS-1) had 400V paper caps on its input originally. These are now 2KV Green caps. I have never put any of my CRO's onto a mains switch mode, only a battery one I experimented with.

The big issue with some of those Switchmodes is that they are intolerant of mains spikes. Some have MOV's built in. As I cop a lot of lightning there are quite a few MOV's in supply lines.

The 0.01 cap will still allow AC & RF wave forms through but blocks DC

I do have an isolation transformer as I do cop the occassional AC / DC & live chassis radio from collectors.

Marc

Thanks for the responses ..... some good advice. I will proceed carefully.
Ian