AC-DC Power Supply Modules: Non-Isolated Topology

[trurle]

TL;DR Summary

Reduction of line noise by swapping AC-DC converter input terminals?

In AC-DC power supply modules, having non-isolated power converter topology may result in input "Line" (hot wire) been assigned to "Vss" (lower voltage rail) at output of power supply module, and N (neutral) assigned to Vdd (upper power rail). Inside single system, this usually does not results in any issues.

Typically, "Vss" or "ground" (although it is typically not grounded) is physically large net. Having a line voltage oscillating on "Vss" may result in line noise capacitively or inductively coupled to other systems easier than in case if line voltage connects to "Vdd" (upper power rail) of power supply module output.

The questions, is the "hot wire equal to ground" noise or other phenomena cause any problems? Did anybody encountered such problems? (I.e. problems which are possible to fix by flipping AC plug polarity)?

I think it is possible to device an automatic switch which will always connect AC power "correctly", but unsure if it is necessary? Usual solution is galvanic isolation, but it have costs too.

 

[Baluncore]

It is unlikely to be a problem since there will be a LPF to reduce RFI before the bridge rectifier and reservoir capacitor. The bridge rectifier diodes will be off for most of the time which isolates the supply. The bridge rectifier will make active – neutral identification unimportant.

 

[essenmein]

Most of the AC-DC converters I've worked with treat the two line inputs the same, ie both "neutral" and "hot" are interchangeable. This would also allow a single phase input to be either line-neutral connected, or line-line connected depending on available system voltages.

From an EMI perspective, generally a 3 wire filter is employed, and these also treat the two line voltage inputs identically:

 

[essenmein]

I re read the question and now I'm not sure what you are asking...

"In AC-DC power supply modules, having non-isolated power converter topology may result in input "Line" (hot wire) been assigned to "Vss" (lower voltage rail) at output of power supply module, and N (neutral) assigned to Vdd (upper power rail). Inside single system, this usually does not results in any issues. "

If the topology is not isolated then all bets are off... If you have a rectifier in there then you will see half wave line voltage from converter "ground" to actual "ground".

 

[berkeman]

Summary:: Reduction of line noise by swapping AC-DC converter input terminals?

In AC-DC power supply modules, having non-isolated power converter topology may result in input "Line" (hot wire) been assigned to "Vss" (lower voltage rail) at output of power supply module, and N (neutral) assigned to Vdd (upper power rail).

In such systems, the electronics must be isolated from the outside world so that there is no shock hazard. If the whole system is enclosed in a plastic box with no metallic controls accessible to the user, this may be okay. In fact many old TVs with 2-prong polarized power plugs were "hot chassis" like this. We always used an isolation transformer when servicing such TVs any time when we had the plastic enclosure open.

 

[trurle]

If the topology is not isolated then all bets are off... If you have a rectifier in there then you will see half wave line voltage from converter "ground" to actual "ground".

Yes, this is the sort of situation i am concerned with.

 

[Baluncore]

Yes, this is the sort of situation i am concerned with.

Without a transformer, and with only a two wire input, you must select either a single diode half-wave rectifier, or a bridge full-wave rectifier.

Where a symmetrical bridge is used the signal outcome cannot be dependent on input polarity.

The only issue is therefore when a single diode is used.
That issue is resolved simply, and at low cost, by using a bridge rectifier.

If a single diode was used, the local electrostatic field due to the asymmetric earthed neutral and active cable, will not be significantly increased by the presence of a capacitive lump antenna at the end of the cable.

 

[dlgoff]

... many old TVs with 2-prong polarized power plugs were "hot chassis" like this.

I can attest to that. That was back when I had no idea what an isolation transformer was good for.

 

[essenmein]

Yes, this is the sort of situation i am concerned with.

The only way to evaluate that then is to review the schematic of the converter in question...

 

[berkeman]

I can attest to that. That was back when I had no idea what an isolation transformer was good for.

Glad you survived.

Luckily for me, a friend is the one who demonstrated the issue for me (and he discovered it for the first time). He had purchased a new Tek oscilloscope (fairly basic analog model back in 1990 or so), and wanted to show it off a bit to me and another EE friend. We were at his apartment, so he took the back off of his TV and proceeded to try to hook up the 'scope to the TV to see some signals.

When he connected the ground lead to the TV chassis, it vaporized (and luckily did not hurt him). As we all sat around looking at each other, we realized that the nice metal chassis of that TV was *not* at Earth ground like we expected, and the 'scope ground and its ground clips were. Duh!

 

[trurle]

Seems the consensus is what "hot chassis" is safety issue rather than noise issue.

 

[Baluncore]

Seems the consensus is what "hot chassis" is safety issue rather than noise issue.

True. It is a danger whether a half-wave diode or a full-wave bridge rectifier is used.

The danger is resolved by using an isolation transformer. Those transformers were for mains frequency, expensive and heavy, until the switching supply came along and significantly reduced the mass and cost of the isolation.

During the period 1975 to 1980 there was a change in supply design. The PC arrived following the availability of small reliable switching supplies.