[SOLVED] Mains Input into SMPS

[Fred_B]

Hi,

I am interested in using a switch mode controller for a project I am working on. I am having some trouble understanding the generic diagram presented in the datasheet for a particular controller. Here is a schematic for reference.

My issue is with what is apparently line input to a common mode choke which then feeds into a bridge rectifier. The jumper for the AC input is labeled L on one terminal and N on the other terminal. I take this to mean line and neutral respectively. Unless I'm missing something, feeding this with a direct mains connection such as what seems to be indicated here will simply blow the bottom two diodes of the bridge rectifier. I modeled this in LT spice, and indeed the bottom two diodes flow hundreds of amps.

So, my question is am I missing something here. The data sheet seems to be implying this will work with a direct means connection. It also fails to imply that there should be an isolation transformer before the jumper where AC is shown being inputted into the circuit. How am I supposed to make this work?

Regards,

fred b

 

[betwixt]

It looks good to me. Are you confusing SMP and PFC? This schematic is a PFC, it has no earth connection, the chassis symbols are floating but common to the the next stage which normally would be the SMPS itself.

Brian.

 

[KlausST]

Hi,

The GND symbol they use ... I´ve initially learned that it is EARTH_GND. But later I´ve learned that other countires use this symbol as the APPLICATION_GND. So not connected to EARTH at all. The GNDs in this circuit are floating with respect to EARTH_GND.

It also is neither connected with N nor L.

--> no high current flow.

Klaus

 

[Fred_B]

It looks good to me. Are you confusing SMP and PFC? This schematic is a PFC, it has no earth connection, the chassis symbols are floating but common to the the next stage which normally would be the SMPS itself.

Brian.

I am aware that this is a PFC, although the output except for the isolation issue will work for my application as is. I should have used PFC instead of SMPS in the title line. There's more to this reply in the following reply to the other poster.

--- Updated Dec 19, 2023 ---

Hi,

The GND symbol they use ... I´ve initially learned that it is EARTH_GND. But later I´ve learned that other countires use this symbol as the APPLICATION_GND. So not connected to EARTH at all. The GNDs in this circuit are floating with respect to EARTH_GND.

It also is neither connected with N nor L.

--> no high current flow.

Klaus

I get that. I know that neutral in mains wiring is connected to Earth ground at the power box for each facility. I also realized the circuit would work just fine if I isolated the power input from ground.

So that morphs my question into how do I isolate the output of this PFC so that I can use it relative to Earth ground. There must be a way because 200 watt ATX power supplies lack any heavy iron low frequency isolation transformers.

Because my application is over 100 Watts I believe I'm going to need a PFC stage for legal reasons, so I suppose I could look for a PFC design that uses an isolated output, or use an isolated SMPS stage after it. It would be convenient if the data sheets mentioned and offered an example of such.

 

[betwixt]

The isolation so real 'ground' can be used is provided by the transformer in the SMPS stage. As it will typically switch at 10-100KHz, the transformer will use a Ferrite rather than iron core.

Brian.