Voltage Converter Design
finalised design of the core converter is shown in fig 5.
5. Dual converter design.
design uses two converters 'firing' 1800 out of phase with a 50:50
M/S ratio. The operational frequency is 50 kHz. The output diode structure channels
current to all three capacitors (they all get charged to almost the same voltage)
and ensures that as a capacitor discharges it doesn't get recharged by any of
the other capacitors that have yet to be fired.
50:50 M/S ratio confines the system to operation in 'discontinuous' mode, ie the
current in the inductor returns to zero during the delivery phase. The alternate
firing sequence places a more even load on the battery pack compared with running
the converters in phase (the peak battery current would be roughly doubled).
control system which governs the charging voltage limit is show in fig 6.
6. Charge/Boost control circuit
circuit basically consists of an oscillator, an opamp voltage monitor, and a dual
MOSFET driver chip. The oscillator is based on a 7555 chip and produces a 50 kHz
pulse train with a 50 % duty cycle. The inverse of this waveform is provided by
a MOSFET inverter and both signals are fed into the MOSFET driver.
opamp voltage monitor is based on a dual LM358 chip. One of the opamps provides
a 'charged' indication to the microcontroller 'fire control' system. This is also
made visible via an LED. The second opamp stops the charging process when the
capacitor voltage is a few volts above the desired charged level. Some hysteresis
is provided by positive feedback so that the control system doesn't go into high
frequency on/off chopping - ie so it doesn't go nuts ;).
charging process can also be enabled/disabled by either a hard switch (connected
via CN1) or a voltage level (via CN12). These options allow the charging to be
enabled only when a magazine is loaded and when the fire control system is 'happy'
with the state of the system.