Teledyne DA1855A, Operator'S Manual

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DA1855A Differential Amplifier
44 922258-00 Rev A
Gate Drive Voltage
The main reason to measure the gate drive voltage is to assure rapid turn-on and turn-off and to
assure that the transistor is fully saturated.
Figure 8, Switching Power Supply with Upper and Lower FET Drive.
Care must be taken when acquiring the switching’s device gate drive signal. In off-line switching
power supplies, the switching devices are elevated to line potential. The DA1855A is ideally suited
for this application with its high CMRR.
The following discussion demonstrates how the DA1855A Differential Amplifier and a XC100
Differential Probe are used to make measurements such as upper gate drive signal on a switching
power supply. A simplified schematic of such a flyback type power supply is shown in Figure 8. In
this circuit both Q1 and Q2 are ON at the same time. D1 and D2 limit the voltage caused by the
primary’s leakage reactance to the rail voltages.
By using the DA1855A and the XC100 probe, the signal reference point can be changed to any point
in the circuit. To select the source of Q2 as the reference point, connect the – INPUT probe to that
point. To acquire the drain to source signal of Q2 place the +INPUT probe on the drain of Q2 and
select DC coupling on both inputs. The amplifier will reject the power line portion (common mode)
of the signal and allows us to see the drain to source signal. For this measurement, the XC100 is set
to ÷100 and the DA1855A is set for ÷10 attenuation and a gain of X1. The total attenuation from
probe tip to the oscilloscope is 1000.
Selecting the differential mode and setting the OFFSET to 245 Volt, will move the trace down about
2½ divisions. This means that the drain voltage with respect to the source is 245 Volt when the top
part crosses the oscilloscope’s screen horizontal center line.