MFJ MFJ-4418, Instruction Manual

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in the boost function. Diode D2 is a transient suppressor diode and acts to protect the unit against
transient voltages greater than 15 Volts.
SMPS controller: IC1 is a packaged SMPS controller with built in voltage reference. This is a
constant frequency pulse width modulated regulator with built in driver for the MOSFET gates.
The switching frequency is set to 50Khz. The controller is enabled or disabled by the soft start
control input. This input is grounded to disable the SMPS boost supply. An open collector of Q8
and series resistor R44 provide the control function. Feedback from the output voltage is
provided by the voltage divider consisting of R9, electronic pot IC5, and R10. This feedback is
compared against the voltage reference to adjust the pulse width and resulting voltage output.
The voltage reference is supplied to the analog control section for voltage level decision making.
MOSFET drivers and transformer: A push pull configuration using two FDP56N06
MOSFETs drives the transformer T1. T1 uses two tapped windings allowing the input voltage to
be placed in series with the SMPS output voltage. The turns ratio is 1.57:1 in series with the DC
input voltage results in a boost of 1.57 over the input voltage. The tapped points on the two
windings are fed to the common cathode rectifier pair D3. Using MOSFET Q1 as an example for
both the Q1 and Q2 circuitry. The gate circuit consists of R6, R3 and FB1. R6 and FB1 minimize
ringing and parasitic oscillations while R3 is a base drain that discharges the gate to substrate
capacity while the unit is not powered. The storage of energy in the gate capacity as a result of
residual energy from previous operation or static can allow both transistors to attempt to conduct
during the next power up cycle before the control IC takes control of the gate usually destroying
one or both MOSFETs.
Output Filtering:
The output filter consists of toroid inductor L2 with capacitors C14 through
C22 that act as a low pass filter and energy storage bank. The total energy storage is 23,500 UF
distributed over 5 capacitors to minimize ESR. Capacitor C22 is located close to the output and
serves as an EMI suppressor. Diode D4 is a 15 volt transient suppressor diode and protects the
unit from transient seen on the output terminals.
Over-voltage Protection: The microprocessor monitors the output voltage and when the voltage
exceeds the programmed set voltage the input PMOS transistor Q9 is turned off turning off
power to the remainder of the MFJ-4418.
Microprocessor Control: The CPU is the brains of the MFJ-4418 and controls most aspects of
the operation.
The MFJ-4418 has direct control over the power feed and unlike the MFJ-4416 booster it can
turn off the output voltage under CPU control. This allows the MFJ-4418 to provide both
operational shutoff of the attached radio and shutoff in a failure mode to protect both the MFJ-
4418 and the radio. The operational power control mode is based on the use of the ignition lead
and power will be switched on and off by this lead. In addition a hold on timer can allow the
MFJ-4418 to provide power for a programmable period of time post the ignition turn off. This
allows the attached radio to operate with the ignition off without the concern of forgetting to turn
it off resulting in a dead battery. The failure modes detected and used for power shutoff are low
input voltage below the programmable point, high output current above the programmable limit
and high output voltage above 15Volts.
The MFJ-4418 provides soft start and this is activated on power up or when the ignition lead
12V. The software waits 500 milliseconds for the supply to stabilize and then switches