WCL488 Series Water Cooled Electronic Loads
Operation & Programming Manual
TDI-Dynaload Division
Page 34
402828, Rev. B1
Transconductance
One characteristic of power MOSFET’ S is called transconductance. Today’ s MOSFET’ S are
designed for high-speed switch mode operation where the operation is full ON or full OFF. The
Dynaload uses these FET’ s in their linear region where the transconductance effect becomes
apparent. When the gate of a FET is pulsed, the drive circuitry must overcome the inherent miller
capacitance to reach the desired gate voltage. When the pulsed gate signal is very small the
transconductance of the component will limit the rise time of the FET.
When operating the Dynaload at pulsed currents less than 10% of full scale, the rise time of the
current waveform will be much slower than expected. Since this roll off in response is dependent
on the number of FET’ s used in the power tray and the actual current pulse desired, it is very
difficult to provide exact equations to define the effect. It is suggested to select a load that will
provide the desired pulse level while operating at current levels greater than 10% of the full
current rating of the load. If the baseline current is greater than 10% of full scale current and the
pulsed waveform is added above this baseline the effects of transconductance will be eliminated.
The greatest distortion occurs when pulsing from a zero current baseline.
NOTE:
Range switching will have no effect on operation in the transconductance region.
Location, Airflow, Maintenance
The figures shown in
WCL488 Outline Drawings
are the dimensions of your Dynaload. In
addition to the rack mounting ears, the use of slides or shelf type supports is required. The WCL
chassis is equipped with mounting holes that match Jonathan 110QD-24-2 slides. The slide
mounting screws
must be #10 - 32 x 5/16 truss head.
The internal fan cools the unit by drawing in air from the front and exhausting it out the back.
Keeping the airflow inlet and outlet screens open and free of dust and other airflow inhibitors will
help keep your Dynaload unit’ s operating temperature within the intended design limits. We
suggest that the loads be cleaned and free of dust build-up at least once a year.
The load can operate without performance derating over the temperature range of 0 to 40°C and
with derated power dissipation capability up to 50°C.
