40
Curtis PMC 1204/1205 Manual
controllers for use with PM motors only, the plug diode and A2 terminal are omitted; however, the
Functional Description is written in terms of series motors.
LOGIC SECTION
B- is the ground return for all of the logic circuitry. For systems over 12 volts, the battery supply
is regulated down to 14 volts to power the logic circuitry. The output of the 14 volt regulator is
switched on and off (switch) by the keyswitch input (KSI) to power up the control circuitry when
the vehicle is in use and to power it down (shutdown) when the vehicle is not in use.
The speed control input (throttle input) is usually a 5000 ohm, two-wire pot, but other types can
be accommodated, so a flexible throttle input scaling circuit conditions the control input to a
standard level. This standardized throttle input goes to the acceleration circuit which limits the rate
at which the controller output can increase. The acceleration rate is set by a resistance, and is
adjustable via a user accessible trimpot (acceleration ramp adjust).
The output of the throttle input scaling also goes to a pot fault circuit which turns the controller
output off in the event of inputs (e.g., broken wires) which would otherwise cause a runaway. An
optional protective feature, high pedal disable (HPD), inhibits controller output if the controller
is turned on with the throttle applied. After an interval measured (start-up timer) from the moment
the KSI input is turned on, the HPD circuit checks the throttle position. If an applied throttle
condition is detected, controller output is held off until the throttle input is returned to zero and
then normal operation is allowed.
The control signal then goes to the limit integrator which reduces the controller output in response
to undervoltage or overcurrent. The time-averaged response of this circuit gives a stable limiting
action. The undervoltage detector gives an output when the battery voltage is too low. The
reduction in output allows the battery voltage to recover and an equilibrium to be established at a
voltage high enough to allow the controller to function properly. The current limit function is
explained in more detail below.
The heart of the logic circuitry is the pulse width modulator in which the control input derived
from the previous stages is compared in magnitude to a 15 kHz sawtooth wave from the oscillator.
The resulting pulse output can be smoothly varied between full off and full on. These pulses
become the input to the controller’s main power MOSFET switch via a gate drive circuit that
provides the high pulse currents needed to turn the power MOSFETs on and off (see Fig. B-1). The
shape of the sawtooth wave may 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
A-2
APPENDIX A
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