Nuts & Volts - September 2006

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 inductance:

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