Teledyne LeCroy DA1855A - Manual (Page 49)

Operator’s Manual
922258-00 Rev A 43
Figure 7
Saturation Voltage Measurement
To measure the saturation voltage of a power device in a flyback type switching power supply,
connect the +INPUT probe to the drain and the –INPUT probe to the source of the switching device.
There is no need to ‘float’ the oscilloscope. The probe connected to the –INPUT becomes the
reference lead (same as black lead on a DMM). The DA1855A will reject the power line portion
(common mode part) of the signal and allows us to see the actual signal of interest. Set the
VOLTS/DIV to 50 mV/div and adjust the OFFSET to read 00.000 on the DA1855A front panel
indicator. Press the Autozero button to autobalance the amplifier.
The start of the saturation voltage measures close to 0.0 Volt since the switching device is OFF and
ramps up due to the increase in current through the primary of the transformer. If this starting point
does not come close to the 0 Volt line, then it could indicate an incorrect low frequency adjustment
of the probes.