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RF Electronics Chapter 2: Computer Simulation Page 28 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Time Domain Transient Circuit Simulation The time domain transient analysis is illustrated with the simulation of a Buck DC-DC converter, used in many power supplies. For this circuit the maximum voltages and currents in the start-up transients are important as well as the steady state operations. This example is used to show both transient and harmonic balance analysis operation. Example 2.4: Buck DC-DC Converter The specifications are for the buck converter to change V in = 48 Volt DC to V out = 12 Volt DC. A maximum output current of 5 Amp is required. MOSFETs have low ON resistance and result in a high efficiency converter design. An IRF9530 – SiHF9530 100V, 12A, P Channel Power MOSFET, with an ON resistance of 0.3 Ω is chosen. The PSPICE model for that is readily available from the Vishay web site [26]. Change the extension from .lib to .cir and import it in AWR DE. A switching frequency of 100 kHz, is chosen to keep the filter elements small while minimising switching losses. Figure 2.31. Curve tracer use and output for IRF9530, SiHF9530 PMOSFET. AWR DE allows the transistor's IV curves versus bias current to be plotted using the IVCURVE measuring device. Figure 2.31 shows that for a 6 Amp drain current, and a -48 V, drain source voltage, a > 6 Vpp gate voltage is required. Varying the gate drive voltage in the simulation shows that a 10 Vpp gate drive is required to switch the MOSFET hard ON and this minimise the losses in the converter. For a 5 A nominal output current, the nominal load resistor must be 12/5 = 2.4 ohm. A minimum current of I min = 0.2 A is required, corresponding to a maximum load resistance of 60 Ω. � � � ��� �� ����� � �� �� ����� Eqn. 2.3 � �� �� ��� � �� � ��� � Eqn. 2.4 ��� � ��� � � � ����� ��� Eqn. 2.5 If the diode voltage drop is 0.5V, then equation 2.3 shows that the duty cycle D = 0.2577. For a 100 kHz switching frequency, that results in an ON time t on = 2.577 s. Equation 2.4 then results in a minimum inductance of 232 H. For a peak-to-peak ripple voltage V ppr = 50 mV, a C min = 9.6 F is required. In the simulation of the Buck Converter L min = 240 H and C min = 10 F are used. For realistic results, a 0.1 Ω resistive loss is included for the inductor. That results in a 0.5 V drop across the inductor and a RF Electronics: Design and Simulation 28 www.cadence.com/go/awr