problem with data interfaces and SMPS

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problem with data interfaces and SMPS

Post by SemiconductorCat » Mon Mar 25, 2013 2:04 pm

In this post I'm going to talk about interfacing with a computer or a MCU system which operated on
SMPS power supplies.

Well the basic safety rules of thumb , and your checklist will be.
1. Check level of the voltages. (TTL or RS232)
2. Check ground and active outputs are not short circuited.
3. When doing bi-directional input to a computer, always check whether the line is in hi-impedance level and
ready to accept the incoming input.
4. Check for parasitic capacitances and switching harmonics and EMI effects. Add ferrite breads if necessary.

One more rule should be there, Where this is what I'm going to discuss now.
This is a main problem with SMPS power supplies. Where you all know that voltage expressed relative to a
reference, and when talking about computer data lines it's reference is ground PCB track on the computer motherboard.
But the problem is what if another device (your microcontroller system) ground reference is at another voltage level????

This issue is called ground looping. A firmware engineer should always check for this.Otherwise it will transfer high
AC voltages across microprocessor bus lines and damage your computer/MCU hardware or sometimes interfere to it's
purposed operation.


1. Galvanic isolation.
Where almost all new SMPS power supplies were build with this. So different ground voltages on different devices would
not affect the data communication and will not appear across the microprocessor pins.

Where it's very reasonable and wise to reject simple buck and boost converters.
On a mobil phone charger.

On a modem.

2. Tight ground.
This is a old solution, and this would not always possible. For a example now , my printer is on a iron rack
and my laptop is on a wood table. So it's completely obvious how it may possible of two different ground levels
on these two devices. Fortunately my laptop uses isolated transformer flyback converter topology. And I don't
know what's there on printer.

3. Optical Isolation.
Where this could be easily implemented by those opto-couplers. One disadvantage of it is it's response time,
so it will be limited bandwidth.

if you know more please feel free to add more.

--Thanks For Reading--
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