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APPENDIX A
POWER SECTION
An array of paralleled power metal oxide semiconductor field effect transistors (MOSFETs) switches pulses
of current from the battery to the motor. During the interval when the MOSFETs are off, the motor current
continues to flow in the freewheel diode, which is actually a number of paralleled fast recovery rectifiers. An
array of filter capacitors connected directly across the battery provides the instantaneous current required by
the power switching circuitry and in this way provides battery ripple current filtering and voltage spike
suppression. The plug diode provides a path for armature current to flow during plug braking.
LOGIC SECTION
B- is the ground return for all of the logic and auxiliary 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) from the auxiliary section 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. Controller output
(PWM pulses) will be inhibited (pulse disable) by the auxiliary section if a fault is detected or until the proper
control sequencing is applied to the keyswitch input and forward and reverse inputs.
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) that 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, overvoltage, 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 overvoltage detector produces an output when battery voltage
is too high (e.g., overcharged batteries) to protect the controller from excessive voltage transients. 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
Curtis PMC 1204X/1205X/1209/1221 Manual
A-2
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