DC-DC Converter Isolation

[dknguyen]

For DC-DC converters, what does it mean exactly when it says the converter is isolated? I'm pretty sure it means that the input/output grounds are not connected but are still at the same potential. I am not sure what this does exactly. Noise? Voltage spikes/transients? (If you need an example to work with, a 24V system with a +5V MCU that controls a motor driver is most relevent to me).

Thanks.

 

[Nigel Goodwin]

For DC-DC converters, what does it mean exactly when it says the converter is isolated? I'm pretty sure it means that the input/output grounds are not connected but are still at the same potential. I am not sure what this does exactly. Noise? Voltage spikes/transients? (If you need an example to work with, a 24V system with a +5V MCU that controls a motor driver is most relevent to me).

It simply means it uses a transformer, so the primary and secondary aren't connected in any way - just like using a mains transformer. This allows you to connect the output as you wish - so you could have the output floating, or as positive, or negative, with respect to the input 'ground' - or even added to the input supply.

Basically it give you full versatility.

 

[justDIY]

also, it gives you some degree of safety as well ... should the circuit fail, mains current shouldn't be available on the outputs ... I suppose the primary and secondary could short but that's not too likely if you have a fuse on the primary side?

there's quite a bit of app notes and whitepapers on isolated switchers, it is regulatory requirement for most consumer grade equipment.

I noticed you specified DC-DC, and I'm mainly talking AC-DC but thats what came to mind

 

[Styx]

For DC-DC converters, what does it mean exactly when it says the converter is isolated? I'm pretty sure it means that the input/output grounds are not connected but are still at the same potential. I am not sure what this does exactly. Noise? Voltage spikes/transients? (If you need an example to work with, a 24V system with a +5V MCU that controls a motor driver is most relevent to me).

Thanks.

Isolated DCC converter are just that, there is no electrical connection from the primary to secondary side.
They do not have to be at the same potential, the output floats to a nominal potential (w.r.t. the input).

It allows you to power a cct which could be floating w.r.t. to a more convientient power source.

Take this example

a Common 28V power bus is user to power a controller-board in a 3phase inverter. The Inverter operates at a DC-link potential of 600V, the DCC converters can then be powered from this common 28V to provide a +/-15V isolated/floating suplly to control the upper IGBT's

 

[Hero999]

You can disable smilies so you can write DCC without turning into a smily.

 

[Oznog]

Some designs would never work without DC isolation. For example, say I want to make an voltmeter that reads out values back through a computer's serial port. Wow, the circuit would have to use the computer's ground so now I have to tie the "-" probe to the computer so I can read it? What if the "-" probe has to tie to a signal that already have a potential relative to the computer ground? It would short out and burn. It could damage what you're trying to measure.

In this case you want an electrically isolated power supply (DC DC or transformer) to power your circuit and it sends signals back to the serial port via an electrically isolated optocoupler. Then you can attach your probes to anything without fear of causing a new path to computer ground.

 

[dknguyen]

I see. So in a non-isolated converter both input and output have the same ground reference, but in an isolated converter the output reference, it is not the same and can be whatever you wish it to be.

 

[Nigel Goodwin]

I see. So in a non-isolated converter both input and output have the same ground reference, but in an isolated converter the output reference, it is not the same and can be whatever you wish it to be.

Pretty well, except a non-isolated one doesn't have to have the same 'ground' reference, it just needs some reference in common, but it's generally fixed and you can't change it.

 

[dabbler]

A Television set would be a good example. The input supply (mains) has to be isolated from the circuitry, otherwise you would get a hefty kick each time you connect a (RCA) cable to the AV connections

 

[Nigel Goodwin]

A Television set would be a good example. The input supply (mains) has to be isolated from the circuitry, otherwise you would get a hefty kick each time you connect a (RCA) cable to the AV connections

Not 100% correct, the external sockets have to be isolated, but the circuitry doesn't - historically just the external sockets were isolated, but since the advent of switch-mode it's more common for the PSU to do the isolation. Although there have been a few exceptions, Hitchi for example had a live-chassis set not too many years ago, with the external AV sockets isolated via opto-couplers, and the aerial (as was usual) via capacitors.

 

[dknguyen]

Question

Grrr I had a really important but subtle question to ask Nigel a couple hours ago, but I don't remember it anymore. I know it was important because it was something I always took for granted as is, but needed a clarification...

 

[Nigel Goodwin]

'Subtle!' - if you have a large enough hammer you CAN fit a square peg in a round hole