TDI-Dynaload Division
WCL488 Series Water Cooled Electronic Loads
Operation & Programming Manual
402828, Rev. B1
Page 31
Effects of Cable Inductance On Pulse Loading
When the Dynaload is used for high current pulse loading, the effects of cable inductance must be
considered. The critical parameters are the rise time and the minimum compliance specifications.
If the inductance of the cables from the voltage source is great enough to cause the voltage at the
Dynaload to go below the minimum compliance level, then excessive current waveform distortion
will occur. This is the result of the power devices driving into saturation. They attempt to reach
the programmed current, however they cannot because of the low drain voltage. Once in a
saturated state, the response time is much slower. The result is a significant overshoot on the
rising edge of the pulse.
In order to prevent this from occurring, it should be noted that:
1 microhenry = 2.4 feet of wire (total)
50A @ 50 microseconds rise time = 1 volt drop with 1 microhenry
The inductive drop cannot exceed the difference between the source voltage and the
minimum compliance.
EXAMPLE:
Referring to Equation 1 that follows, to test a 10V source with a 100A pulse, and
assuming a 3 Volt minimum compliance, the maximum cable length would be:
Equation 1. Establishing Maximum Cable Length
In the example above, the maximum cable length is equal to 8.4 feet total or 4.2 feet per cable
from the source to the Dynaload.
If the distance from the load to the source must be greater than this, there are several methods to
increase the maximum distance. One way is to use several insulated conductors. This cuts the
inductance in half if four are used instead of two, or by 1/3 if six are used. This double or triples
the maximum length, respectively.
Another method is to slow down the rise time of the pulse generator before applying it to the
regulation loop. Increasing the slew rate to 100 microseconds will double the maximum cable
length.
The third method is to use a large electrolytic capacitor at the Dynaload studs that can supply
current necessary to counteract the inductive drop of the cable.
E M ax drop = 7V
L = 3.5 m icrohenries m axim um
E = L
7V = L
402825-02-12.C D R
di
dt
100A
50µs