Dear Mr. Rich Nute:
Thanks for your e-mail
we have made measurements of primary voltages with
respect to secondary circuits as follows::
1. using a mains-operated scope,
2. supply the SMPS from an isolating transformer where
both supply conductors are isolated from ground.
3. connect secondary's ground and primary 's ground (neutral and ground are
connected together)
4. we adjust trigger level and time base to obtain the full waveform we are
interested in
What I mean originally is what waveform of primary and secondary for SMPS
should be :
Switching waveform , sin waveform moduated by high-frenquency switching
waveform or other .
Best Regards
Xing Weibing
----- Original Message -----
From: "Rich Nute" <ri...@sdd.hp.com>
To: <xin...@cesi.ac.cn>
Cc: <emc-p...@majordomo.ieee.ogo
Sent: Saturday, December 22, 2001 1:33 AM
Subject: Re: Fw: working voltage maeasurement
>
>
> y d
>
> Hi Xing Weibing:
>
>
> One of the problems of making oscilloscope measurementsg
> 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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