2. When the MOSFET is off, the voltage at TP rises rapidly as the inductor
tries to maintain a constant current.
The diode turns on, and the inductor
dumps current into the capacitor,
increasing the output voltage to more
than the input voltage.
3. The switching of the MOSFET will be
controlled by a microprocessor using
PWM. PWM will control the duty
cycle, D, which is the fraction of the
time the output is high. The value of D
varies such that 0 < D < 1. The output
voltage is lowest when D = 0. At zero
D, the output voltage equals the source
voltage. As D approaches unity, the
output voltage tends to infinity. Usually
D is varied such that 0.1 < D < 0.9.
A boost power supply can be driven in discontinuous conduction mode
(DCM) or continuous conduction mode
(CCM), depending on whether the current in the inductor is allowed to go to
zero or not. In DCM, the semiconductor is switched on only after the inductor current has fallen to zero. In CCM,
the switch is turned on before the current through the diode reaches zero.
We will operate in DCM because we
want to have the largest range of duty
cycles. Also, the equations for DCM are
somewhat simpler. The basic waveforms for DCM are shown in Figure 4.
Reference 1 gives the following
equation for the boost power supply
output voltage, in DCM, as a function
of the input voltage, load resistance,
semiconductor ON time, period, and
Output_Voltage = Input_Voltage*
( 1 + SQRT(1+( 4*On_Time/Period))/
K)/2, [Equation 2]
A spreadsheet to calculate the
output voltage, peak current, current
fall time, and other important boost
power supply parameters is available
from Reference 2. I generated Figure 5
using the spreadsheet, but it assumes a
K = 2*Inductance/(Output_
Load_Resistance * Period)
A/D AT VOUT
DC LOAD CURRENT
 Equivalent to two nixie tubes.
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