Hi Xing Weibing:
One of the problems of making oscilloscope measurements
within switching-mode power supply circuits is that the
common side of the oscilloscope *MAY* change the waveform.
There are two ways to minimize the effect of the common
side of the scope from affecting the waveform:
1) If you are using a mains-operated scope, supply
the SMPS from an isolating transformer where
both supply conductors are isolated from ground.
In this way, you can connect the scope common
lead to any point in the SMPS with minimum effect
on the waveform.
2) Use a battery-operated scope such as the Fluke
Scopemeter. You can connect the scope common to
any point in the SMPS with minimum effect on the
wavefrom
When a scope measures the RMS value of a waveform, it
does so over the time displayed on the screen. If the
time base is changed, then the RMS value *WILL* change
because the displayed waveform is changed.
Likewise, when the trigger level is changed, the display
changes the starting point for RMS calculation, and the
RMS value may change if the waveform is not repetitive
for the duration of the time base.
An accurate RMS value is obtained when the scope measures
complete, full cycles of the waveform. If the number of
cycles is not full, then the RMS value is the value for
the number of full cycles plus the partial cycle, and will
not represent the RMS value of interest.
To get a useful RMS value of a complex waveform, use a
very long time base, such as 0.1 second/division or
longer. You can verify that this is a useful RMS value
if the RMS value does not change significantly with
trigger level or time base setting.
If you are making measurements of primary voltages with
respect to secondary circuits, then you must ground the
secondary circuits, and operate the SMPS from a grounded-
neutral supply. (If you use an isolating transformer for
primary-secondary voltage measurements, there is no
reference for the primary circuit, and all measured
voltages are incorrect.)
Good luck!
Best wishes for the holiday season,
Rich
> Hi Group
>
> I have a question regarding working voltage measurement of IEC 60950
>
> we have a E.U.T.(switching power supply)
> I want to clarify the measurement of working voltage
>
> 1. Using an oscilloscope having an adequate bandwith and using a high
> impedance probe (100Mohm), and adequate integration time to measure working
> voltage.
> The load on the secondary circuits is to be varied in order to find highest
> voltage across the insulation. Floating secondary outputs (capacitively
> connected to
> earth)
> are earthed.
>
> 2. don't make connection between primary winding and secondary winding.
>
> 3. we will get a stable waveform on the oscilloscope.
>
> working voltage we measure are as follows:
> The waveform we get by the above method is a kind of waveform modulated by
> high-frequency
> switching waveform.
> MEASURED voltage: 246V(rms),576V(peak)
> When we change trigger level and time base to obtain stable switching
> waveform, we get
> different rms voltage
> with different trigger level. the highest rms voltage we get is 380V.
>
> My question
> 1. the above steps are correct or not?
>
> 2. For switching power supply, what waveform of working voltage is correct
> for primary
> and secondary
> How to obtain?
>
> 3. Which one(246V, 380V) is correct for working voltage measurement? or
> other methods?
>
>
> Thank you for any comments in advance
>
> Best Regards
>
> Xing weibing
>
> 2001-12-17
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