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power MOSFETs on and off (see Fig. B-1). The shape of the sawtooth wave can be altered so that most of
the pulse width change occurs in the earlier or in the latter part of the control input range, giving more
sensitive throttle response at high or at low speeds.
Current limiting is done by sensing the voltage drop across the main power MOSFET switch when it is on.
This voltage is compared (current limit comparators) with a current limit reference; when it exceeds the
reference, an overcurrent signal acts on the limit integrator to reduce the controller output and thus hold the
current at the limit. Because the voltage across the power MOSFET switch is high when it is off, the current
limit comparison is inhibited during the off interval by the current limit disable circuit. The current limit is
set as follows:
1.
During manufacture, the current limit is set to the model's nominal rating; it is also
user-adjustable by a trimpot (current limit adjust) to currents lower than the nominal
rating.
2.
During operation at extreme high or low temperatures, current limit is reduced to
protect the controller from damage. From a thermal sensor (temp sense) on the
heatsink, signals are produced to cut back the current limit at temperatures above
75°C or below -25°C (overtemp, undertemp). The controller's operating frequency
shifts to 1 kHz during overtemperature operation, producing an audible tone to alert
the operator.
3.
During plug braking operation, the current limit is reduced to give an appropriate
motor braking torque. The plug braking current is set during manufacture; it is also
user-adjustable by a trimpot (plug current adjust).
4.
The fixed plug option provides one level of plug braking current independent of
throttle position (provided it is at least minimally applied). For the fixed plugging
option, the plug current limit reference is derived from the current limit reference.
5.
The variable plug option provides variable plug braking current corresponding to the
position of the throttle. This allows much smoother braking under control of the
operator. For the variable plugging option, the plug current limit reference is derived
from the throttle input scaling.
6.
The emergency reverse plug option (enhanced plug braking current) is enabled by the
emergency reverse input (belly-button switch) from the auxiliary section. This signal
increases the plug current limit reference to a fixed value. Plug current limit will be at
this enhanced level, independent of accelerator position, even if the variable plug
option is installed. Emergency reverse plug current level is set to customer specifica-
tions during manufacture and is not user adjustable.
Curtis PMC 1204X/1205X/1209/1221 Manual
APPENDIX A
A-3
45
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