The PSPICE circuit analysis tool is an easy method to analyze performance of various power MOSFETs in step down switching regulators. A designer can try various combinations of parts quickly using a good simulation model to see how well they work in a particular circuit.
SPICE is a Simulation Program with Integrated Circuit Emphasis and has become the standard computer program for electrical and electronic simulation. The increased utilization of PCs has led to the production of PSPICE and HSPICE.
Block Diagram
The PSPICE model operates in an open loop configuration and the switching characteristics of main switching transistor, synchronous rectifier and flyback diode are the key concerns. An L-C input filter is added for simulation purposes to smooth the input current to the regulator.
PSPICE schematic diagram of n-channel MOSFET model
All the downloaded SPICE models must be checked to the component data sheets for the accuracy. To ensure that the model matches the data sheet for diodes, a DC sweep of current is run through the diode vs. voltage drop. For MOSFETs, the switching time and on resistance matches are measured through the simulation run. It is important to run any MOSFET model in a simple switching circuit before putting them into a more complex one.
The model runs in the time domain for many cycles. This is to allow the transients to settle out. Options were set up in the transient analysis statement to bypass an initial DC operating point which PSPICE defaults to and instead uses the specified initial conditions. The initial condition on the output voltage puts an initial charge on the output capacitors. This greatly decreases the simulation time and several iterations are required during the switching transitions of the power devices.
The MOSFET models are set up as sub circuits containing numerous components. In SPICE, voltage sources have zero resistance and thus do not affect circuit operation. This model was also run using HSPICE on a UNIX work station with the simulation time on the order of 5 minutes. PSPICE and HSPICE give similar results. However, the graphics of advanced version of PSPICE models are good as compare to PSPICE models.
The power inductor was composed of a series resistor with the inductor to account for winding resistance. A capacitor was placed in parallel with the series L-C to account for the inductor self resonance. The output capacitors included series resistors to simulate equivalent series resistance. The load current is set to 1A for the analyses showing the waveforms.
The MOSFET used in the application for which the simulation run is to be conducted are P-channel MOSFET and N-channel MOSFET including A Schottky Barrier Diode.
Conclusion
The Schottky improves the circuit performance which is affected by the low RDS (on). The interaction between the body diode of the synchronous rectifier transistor and the parallel Schottky is important. The reverse recovery current of the body diode causes heating of the device and results in increased RDS (on). All these factors results in a more efficient design. Moreover, the key is to design a system that uses the lower forward drop and the fast recovery of the Schottky.
Hence, the PSPICE and HSPICE circuit simulators are powerful tools which enable the designer to asses the effects the different devices in a power converter circuit using time domain analysis.
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