SPICE-Simulation using LTspice IV
SPICE-Simulation using LTspice IV
SPICE-Simulation using LTspice IV
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11.2. The Step Up Converter<br />
The schematic shows a +12V power supply , followed by an inductance with L = 100mH and the obligatory series<br />
resistor of 1.<br />
The Power-MOSFET acts as an electronic switch between R1 and D1 to connect this node to ground for a<br />
definite time.<br />
When switched ON, current flows from the power supply through the inductor and its series resistor to ground and<br />
rises linearly (...this is true, when the voltage across an inductor is constant) .<br />
So more and more magnetic energy is stored in the inductor and this can be calculated from:<br />
1<br />
E = • L I<br />
2<br />
2<br />
•<br />
When the FET is switched OFF, this energy cannot dissipate suddenly and the inductor’s current cannot switch<br />
rapidly (….this would cause a nearly infinite induced voltage...) and so stays at the same value.<br />
That means that the inductor current now flows to the right hand side, switches ON the schottky diode, charges<br />
the output capacitor and flows through the output resistor R2. When the gate of the MOSFET is fed with a square<br />
wave voltage, the output voltage rises until a “balance” is reached between picked-up-energy (by the Inductor)<br />
and the energy delivered to the output resistor R2.<br />
Important: the amplitude of the output voltage is the sum of supply voltage +12V and the inductors<br />
voltage!<br />
This means that the output voltage of this circuit will<br />
always be higher than the supply voltage!<br />
Now draw the schematic and feed the gate with a symmetric square wave voltage (frequency = 1 kHz / (Vmax) =<br />
+15V / Vmin = -15V). Choose a simulation time from 0.....50ms. Show the gate voltage and the output voltage.<br />
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