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Block Diagram of Typical AC Input, Regulated DC Output System
• Typically, a power supply front end has uncontrolled full-wave diode rectifier, followed by a bus (“hold-up”) capacitor, followed by a DC/DC converter with active feedback control
• In this example, the average value of the output voltage = DVin where D is the DUTY CYCLE in PWM (pulse-width modulation) control• D = ton/Ts, the fraction of the total switching cycle that the switch is ON
• The goal of the lowpass filter LC is to pass the DC component, while attenuating the switching components• As frequency increases, XL increases and XC decreases
Buck Converter in Continuous Conduction• In periodic steady state, inductor current flows continuously• Waveform here are for buck in continuous conduction mode; note that inductor current never decays to zero• In discontinuous conduction mode, there are 3 states
Buck Converter: Startup Waveforms• These waveforms are shown for a constant duty cycle of D = 0.5 during startup • Note large overshoot on output voltage and inductor current
Analysis for DC/DC Converters in Continuous Conduction and Steady State
• In steady state, the inductor current returns to the same value every switching cycle, or every T seconds• Therefore, the inductor ripple current UP equals ripple DOWN• Several assumptions to simplify analysis:
• Periodic steady state --- all startup transients have died out• Small ripple --- ripple is small compared to average values. For instance, output voltage ripple is small compared to the DC value
• In continuous conduction, buck converter has 2 states --- switch OPEN and switch CLOSED.• We can solve for output voltage by focusing on inductor Volt-second balance
• The inductor ripple current UP equals ripple DOWN
• We already knew this result by inspection, but this methodology of inductor Volt-second balance can be used to evaluate other more complicated DC/DC converters, such as the boost, buck-boost, etc.
Buck Converter: Discontinuous Conduction Mode• Steady state; inductor current discontinuous (i.e. it goes zero for a time)• Note that output voltage depends on load current
Boost Converter: Effect of Parasitics• The duty-ratio D is generally limited before the parasitic effects become significant• As D gets big, input current gets very large (think about power balance….); the voltage drop in inductor and switch cause efficiency to suffer
Boost Converter Output Ripple• ESR is assumed to be zero• Assume that all the ripple component of diode current flows through capacitor; DC component flows through resistor
Example 2: Boost Converter Example• Mohan, Example 7-1• Boost converter on the edge of discontinuous conduction• Vi = 12V, D = 0.75, Vo = 48V, Po = 120W
Cuk DC-DC Converter• The output voltage can be higher or lower than the input voltage• Capacitor C1 stores and transfers energy from input to output• When switch is ON, C1 discharges through the switch and transfers energy to the output• When switch is OFF, capacitor C1 is charged through the diode by energy from the input and L1
• Single-ended primary inductance converter (SEPIC)• Can buck or boost the voltage• Note that output is similar to buck-boost, but without a phase inversion• This circuit is useful for lithium battery powered equipment
• It varies significantly in various converters• PT = VTIT where VT and IT are peak switch voltage and current• In direct converters (buck and boost) switch utilization is good; in indirect converter (buck-boost and Cuk) switch utilization is poor