Reference no: EM133203325

Question 1: This simulation assignment examines the use of synchronous rectification to generate the 1 V, 20 A power supply required by a microprocessor. A 12 V bus is available, and hence the object is to design a 12V:1V buck converter. We will model the microprocessor with a 50 milliohm resistor.

Download the buck converter zip file for this assignment.

This file contains a conventional buck converter that employs a MOSFET (IRFZ44N) and a schottky diode (MBRB2545CT). There is both a 12 V power input Vg and a 12 V floating gate driver power supply. The switching frequency is 100 kHz. Do not change the element values or circuitry.

Manually adjust the commanded duty cycle Vduty so that the output voltage is 1 V +/- 25 mV. What value of Vduty is required to achieve the required output voltage?

Question 2: For the conditions of Question 1, what is the converter efficiency?

Carefully measure average output voltage and average input currents over an integral number of switching periods, with the circuit operating in steady state. Enter a numerical value that lies in the range (0,1), with an accuracy of +/- 0.005

Question 3: Now modify the buck converter to operate with a synchronous rectifier, realized using an IRFH5004 MOSFET in place of the schottky diode. Add a dead time generator (copy this from the synchronous boost simulation lecture file), with a dead time of 100 ns. Add another driver for the synchronous rectifier, and use a bootstrap power supply for the high side driver. Adjust the values of the gate driver resistors as shown. Power both drivers from Vg. A schematic is given below.

Adjust the duty cycle so that the output voltage is 1.00 V +/- 25 mV. What value of Vduty is now required?

Question 4: It is highly suggested that you examine the circuit waveforms for steady-state operation of the converter:

• Examine the switch node voltage. Can you explain the small steps in this waveform?
• Examine the gate-to-source voltages of the two MOSFETs, in relation to the switch node voltage. Can you explain the sequence of events in the switching process?
• Compare the current waveform in Vg or M1 with the switch node voltage. Can you explain the sequence of events in the switching process?
• If you have time, try changing the gate circuit resistance values or the dead time value. What is the effect on the switching transition waveforms and the converter efficiency? Be sure to set these parameters back to their original values before answering the questions below.

For your synchronous buck converter of Question 3, what is the converter efficiency?

Question 5: Although the synchronous rectifier improves efficiency by substantially reducing the forward voltage drop of the rectifier, the body diode of the synchronous rectifier induces significant switching loss through its reverse recovery. Estimate the charge contained in the current spike drawn out of Vg during the turn-off transition of the synchronous rectifier, as predicted by your LTspice simulation. Enter a numerical value in coulombs, accurate within a range of a factor of 2. You may use scientific notation such as 5e-6

Note: package and layout inductance is not included in this simulation model. As a result, the body diode di/dt and the recovered charge are substantially larger than the values listed in the data sheet for this MOSFET.

Question 6: How much average power is represented by the current spike of Question 5? Most of this power is switching loss induced by the body diode reverse recovery plus transistor output capacitances.