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Wait a couple of seconds for things to stabilize and then reduce the sim-speed to 10uS/S to see the waveforms clearly.
A Synchronous Buck Converter is different from a traditional Buck Converter (Asynchronous) in that it has a MOSFET in place of the freewheeling diode in an Asynchronous Buck Converter (Asynchronous buck converter: http://everycircuit.com/circuit/5993410963177472). This topology is called "Half-Bridge". The benefit of having a MOSFET is that it has a low on resistance and hence wastes less power. Sometimes, a diode (schottky) is placed in parallel to the MOSFET in order to keep the power loss low during the "Dead-Time". Once the "diode-replacement" MOSFET turns on, the drop across it is very small and the schottky turns off.
This circuit doesn't use the PWM with Dead-Time circuit that I designed a while back, as the simulation keeps crashing when the output reaches 1.7V, even the control signal is correct. The circuit works till that voltage is reached. It can be seen that the two MOSFETs switch on and of properly http://everycircuit.com/circuit/5840292138450944 . I've noticed that the simulator crashes whenever the circuit uses many op-amps that are connected together.
Here's how it works. When the PMOS turns on, current starts ramping up through the inductor and energy is stored in it in the form of magnetic fields. When the PMOS turns off, the current through the inductor initially flows through the body diode of the NMOS (or a parallel schottky diode). This is because the NMOS does not turn on as soon as the PMOS turns off. There is a short delay between the closing of one MOSFET and the opening of the other. This is called "Dead-Time". It is required to ensure that both MOSFETs aren't on at the same time even for the shortest amount of time, as a huge current would flow through them. After a Dead-Time of around 3-4uS (in this circuit), the NMOS turns on and the current flows through it. Again, there is a small Dead-Time between the closing of the NMOS and the opening of the PMOS, during which current flow through the body diode (or the external diode in this case).
In order to use this topology, the inductor must be in CONTINUOUS CONDUCTION MODE (CCM), i.e., the current through it must never reach zero before the closing of the NMOS. If that happens, the output capacitors start discharging through the inductor and MOSFET.
The efficiency of this circuit, assuming that the RMS value of the input current is 93.04%. The input power is 11.625W and the output power is 10.816W. the power lost in the circuit is 0.809W.
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