# In a synchronous buck converter operating in CCM with 50% duty cycle, why the upper MOSFET dissipates more power than lower MOSFET. consider same MOSFET, gate voltages and gate resistors at both places.

In a synchronous buck converter operating in Continuous Conduction Mode (CCM) with a 50% duty cycle, the upper MOSFET dissipates more power than the lower MOSFET. This is because of the differences in the MOSFET's operating conditions during the ON state.

During the ON state of the upper MOSFET, the input voltage is applied across the MOSFET, and the current flows through the MOSFET and the inductor to the load. The current flowing through the MOSFET is also charging the output capacitor. Therefore, the upper MOSFET operates in both the conduction and the charging modes.

During the ON state of the lower MOSFET, the current flows through the MOSFET and the inductor to the load, but the output capacitor is discharging through the diode. Therefore, the lower MOSFET operates only in the conduction mode.

Now, the upper MOSFET has to handle both the input voltage and the charging of the output capacitor, which requires more power dissipation than the lower MOSFET. The power dissipation in the MOSFET is given by the equation:

P = Vds * Id * (1 - D)

where P is the power dissipation, Vds is the voltage across the MOSFET, Id is the current flowing through the MOSFET, and D is the duty cycle.

During the ON state of the upper MOSFET, the voltage across the MOSFET (Vds) is equal to the input voltage, and the current flowing through the MOSFET (Id) is equal to the load current plus the charging current for the output capacitor. Therefore, the power dissipation in the upper MOSFET is higher than in the lower MOSFET.

During the ON state of the lower MOSFET, the voltage across the MOSFET (Vds) is equal to the output voltage plus the voltage drop across the diode, and the current flowing through the MOSFET (Id) is equal to the load current. Therefore, the power dissipation in the lower MOSFET is lower than in the upper MOSFET.

Therefore, in a synchronous buck converter operating in CCM with a 50% duty cycle, the upper MOSFET dissipates more power than the lower MOSFET because of the additional power dissipation due to charging the output capacitor.

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