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M2-3 Buck Converter Objective is to answer the following questions: 1.How does a buck converter operate?

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Buck Converter The input voltage is always greater than the output voltage

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Buck Configuration The input voltage is always greater than the output voltage V OUT V IN VMVM V GATE L C I SW ILIL V IN time 20V 15V 10V 5V 0V V OUT time 7.5V 5V 2.5V 0V 10V

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Switching Regulator Components Switching Power Supply Block Diagram V IN V OUT Switching Power Supply Switch Error Amplifier Bandgap Reference PWM Controller Network

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External Network An external network (consisting of an inductor, capacitor, and diode) transforms the energy from the PWM controlled power switch into a desired output voltage Network Switch V IN V OUT V IN = 12 V V OUT = 5 V

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Recirculation Diode V OUT V IN V OUT V IN S IN S GND VMVM VMVM

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How a Switching Regulator Works V IN Switching Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage OK 50% Filter Network V OUT

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How a Switching Regulator Works V IN Voltage Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage OK 50% Filter Network V OUT

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How a Switching Regulator Works V IN Voltage Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage OK 50% Filter Network V OUT

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How a Switching Regulator Works V IN – 1V Voltage Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage Low 60% Filter Network V OUT

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How a Switching Regulator Works V IN – 1V Voltage Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage Low 60% Filter Network V OUT

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How a Switching Regulator Works V IN Switching Regulator Duty Cycle Controller Output Monitor V OUT time 5V Voltage Ok 50% Filter Network V OUT

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Step Down Switching Regulator Steady State Operation V OUT V IN VMVM V GATE + V L - C OUT I SW ILIL V GATE goes high V M ~ V IN V L = V M – V OUT t VMVM t V GATE t ILIL V OUT t I SW t R LOAD -V F -VF+-VF+

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Step Down Switching Regulator Steady State Operation V OUT V IN VMVM V GATE C OUT I SW ILIL V L Constant t VMVM t V GATE t ILIL V OUT t I SW I L and I SW increase t R LOAD C OUT is charged by I L and V OUT increases -V F -VF+-VF+ + V L -

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Step Down Switching Regulator Steady State Operation V OUT V IN VMVM V GATE C OUT I SW ILIL V GATE = 0V The pass transistor is turned off I SW = 0A t VMVM t V GATE t ILIL V OUT t I SW t R LOAD I L cannot go to 0A instantly: V M goes negative V L = V M – V OUT -V F -VF+-VF+ + V L -

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Step Down Switching Regulator Steady State Operation V OUT V IN V GATE C OUT I SW ILIL But, V M is clamped to -V F and I L decays through the diode t VMVM t V GATE t ILIL V OUT t I SW t R LOAD C OUT stabilizes the output voltage so V OUT will only slowly decay -V F V M = -V F -VF+-VF+ + V L -

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Step Down Switching Regulator Steady State Operation V OUT V IN V GATE C OUT I SW ILIL The MOSFET is turned on and off to repeat the sequence R LOAD t VMVM t V GATE t ILIL V OUT t I SW t -V F V M = -V F -VF+-VF+ + V L -

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Volt-Second Balance V OUT V IN VMVM V GATE C OUT I SW R LOAD ILIL t V GATE t + V L - ILIL

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Volt-Second Balance In steady state, the inductor current ripples about an average, I L,AVG : Therefore, the total area (or volt- seconds) under the inductor voltage waveform is zero. VLVL t V IN - V OUT -V OUT T DT (1-D)T

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Voltage-Second Principle and the DC Transfer Function From: we can calculate the transfer function of the step down switching voltage regulator

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V IN vs. V OUT and Duty Cycle, D V OUT V IN L C OUT I SW ILIL R LOAD S IN S GND During steady state: V L,AVG = 0V + V L - VLVL time V IN - V OUT -V OUT T DT (1-D)T

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V OUT Increases with D V OUT = DV IN V OUT V IN VMVM V GATE C OUT I SW ILIL V OUT R LOAD t VLVL t V GATE V IN - V OUT t + V L - -V OUT S IN S GND

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V OUT Decreases with D V OUT = DV IN V OUT t VLVL t V GATE V IN - V OUT t -V OUT V OUT V IN VMVM V GATE C OUT ILIL R LOAD + V L - S IN S GND I SW

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Ripple Current Recall, I L is the sum of the current flowing through S IN and S GND V OUT V IN C OUT ILIL R LOAD S IN S GND I GND time I SW I GND ILIL I L,AVG I SW

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Memo

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