Waveform capture of step-down converter

I built a step-down converter using LT1074 from Linear Technology, as shown in Figure 1. The '+' and '-' of the power supply (PSU) are floating type. I would like to capture the waveform of Vsw (as shown by the simulation result in Figure 2) of the circuit at pin 'Vsw'. Channel 1 of the scope is connected to pin 'Vsw'. Should I or can I connect the GND probe of Channel 1 to the '-' of the PSU or the circuit? Would it cause damage, e.g. short-circuit?

If I wish to measure the voltage waveform across the inductor, should I use two channels (differential measurement), or a single channel will do?

Thanks.

Since the psu is floating, you can directly connect the scope ground to the psu's negative output. This will not cause a short circuit or any damage.

Note that the source will then no longer be floating, so this might be a problem if your switchmode converter's load depends upon having an isolated power source.

lambtron said:

Since the psu is floating, you can directly connect the scope ground to the psu's negative output. This will not cause a short circuit or any damage.

Note that the source will then no longer be floating, so this might be a problem if your switchmode converter's load depends upon having an isolated power source.

Click to expand...

Why a psu with floating outputs, when either one connected to ground, will not cause a short-circuit, or any damage?

What is the difference between the design of a floatng power supply, and a non-floating power supply? Pls advise.

Hi,

lambtron is correct. If the PSU is floating (Very high resistance between PSU ground and scope ground - a different ground then the scope) then you could measure directly across the inductor with one probe. The differential will always work though. One of the ways to think about floating power sources might be to think of each power source as a seperate battery with no connections between them acept at one point. You can stack batteries in series plus to minus and they do not short or damage because you only have them connected at one point. However if you measure across them you measure their combined potential.

good luck

dfullmer

dfullmer said:

... If the PSU is floating (very high resistance between PSU ground and scope ground - a different ground then the scope) then you could measure directly across the inductor with one probe...

Click to expand...

(1) What does 'very high resistance between PSU ground and scope ground - a different ground then the scope' mean?

(2) '... could measure directly across the inductor with one probe...' - does it mean using one channel, one probe at one end of inductor, and the ground probe of the same channel at the another end of inductor?

BTW, would you pls give some comments for this post:

Thanks

Hi powersys,

The resistance is basically infinite or at least several meg ohms. Yes, at that point you could use the scope probe as you stated with the probe on one side of the inductor and the ground on the other. Keep in mind that as you do this you now have a reference offset of 5 volts if the ground side of the probe is on the inductor output.


Hope this helps

dfullmer

ps I will look at the other post as well.....

dfullmer said:

The resistance is basically infinite or at least several meg ohms.

Click to expand...

Did u refer the 'psu ground' to 'psu(-) output', or the 'earth point where psu casing is tied to'?

Thanks.

Hi,

The isolation resitance I am refering to is between the gnd of the scope and the gnd of the PSU. Before you connect the scope take a DVM and measure between the scope gnd and the PSU grn and you should see a very high resistance before you connect the probe or it gnd to the circuit. Some systems will tie their gns to chassi gnd and the plug that goes into the wall will also be its gnd point. If your PSU has its gnd tied to its own chassi and back to the wall plug then the two systems will have their gnd's tied in the wall socket gnd. This has caught me once or twice. Sorry if the explanation was a little confusing.

good luck


dfullmer

dfullmer said:

The isolation resitance I am refering to is between the gnd of the scope and the gnd of the PSU. Before you connect the scope take a DVM and measure between the scope gnd and the PSU grn and you should see a very high resistance before you connect the probe or it gnd to the circuit. Some systems will tie their gns to chassi gnd and the plug that goes into the wall will also be its gnd point. If your PSU has its gnd tied to its own chassi and back to the wall plug then the two systems will have their gnd's tied in the wall socket gnd. This has caught me once or twice. Sorry if the explanation was a little confusing.

Click to expand...

We have two types of psu in our lab: (a) PSU A, (b) PSU B. PSU A has two outputs, i.e. '+' & '-'.

PSU B has three outputs, i.e. '+', '-' & 'GND'. I used a DVM and measured the continuity between the 'GND' of PSU B and the 'GND' probe of scope, and the DVM produced 'beep' showing that the impedance between them was low.

PSU A has no output terminal for 'GND'. If the 'GND' of PSU A is NOT tied to chasis, meaning we have to open the chasis and access the 'GND' point somewhere on the circuit board? Pls correct me if I'm wrong. Thanks.

Hi,
Sounds like you may be able to use either supply. PSU B would be hooked to your circuit with its "+" & "-" and the gnd terminal would be connected to nothing. These PSU that have three terminals usual indicats that the "-" and gnd are seperate from gnd and the outputs are floating. If you get a low impeadance between the gnd terminal and the scope gnd this is ok as long as the "-" terminal and the gnd of the scope are high impeadance. The same can be said for the PSU A. If "-" of this unit has a high impeadance to the scope gnd it will work. Meg ohm or more.......


Regards


dfullmer