Lecture # 12&13 SWITCHING-MODE POWER SUPPLIES

Switching-Mode Power Supplies

Introduction to Switching Regulators Objective of topic is to answer the following questions: What is a switching power supply? What types of switchers are available? Why is a switcher needed? How does a switcher operate in general? How does a buck converter operate? How does a boost converter operate? How does a buck-boost converter operate? How many design topologies of a switching power supplies?

Introduction to Switching Regulators Intended Audience: Electrical engineers with limited power supply background A simple, functional understanding of inductors and capacitors is assumed A simple, functional understanding of transistors is assumed Expected Time: Approximately 120 minutes

Outline of the presentation Switching Regulator Overview What is a Switching Regulator? Why is a switcher needed? What are the main differences between a switching and linear regulator? Buck, Boost, Buck-Boost (Inverting) Five designing topologies.. Flyback converter. Forward converter. Push-pull converter. Half Bridge converter. Full-Bridge converter. 3. Switching Regulator Operation . How does a Switching Regulator works? Switching Regulators components. Switching mode DC power supplies

What is Switching Power Supply?

What is Switching Power Supply? The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers. This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).

What is a Switching Regulator? Converts an input voltage into desire output voltage. The power transistor operates as a switch, completely on or off. An energy storage part (inductor) is used in the architecture

Choosing Between Linear and Switching Regulators When possible, most designers would prefer to use a linear voltage regulator rather than a switching voltage regulator Linear regulators are usually lower in price Linear regulators are usually simpler to implement Linear regulators do not have associated noise/ripple problems apparent in switching regulators So, when should you use a switching voltage regulator instead of a linear voltage regulator? Well, in the simples terms, most designers will use a linear voltage regulator wheneve it is possible for all the above reasons. The linear voltage regulators are easier and cheaper to use. It is hard to beat that kind of combination. However, there are instances when a switching regulator is the right choice.

Choosing Between Linear and Switching Regulators When to use a switching regulator : When the minimum input voltage is at or below the desired output voltage Linear regulators cannot provide an output voltage greater than the input voltage VIN < VOUT The first example of when to use a switching regulator – when you can not use a linear voltage regulator. Linear voltage regulators are not capable of step-up (boost) operation. Therefore, if the minimum input voltage to your system is at or below the desired operating voltage you will need to use a switching voltage regulator.

Choosing Between Linear and Switching Regulators When to use a switching regulator: The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum The maximum junction temperature is usually 150C The efficiency of linear regulators often prohibit their use in high voltage, high current applications As stated before, under most situations, linear regulators have a much lower efficiency than switching regulators. Low efficiency means a lot of power can be lost as heat. To make matters worse, linear regulators are lose efficieny as their input voltages (and power losses) increase. Sometimes even a giant heatsink will fail to keep the linear regulators junction temperature within the specified operating temperature range. Under these conditions, the high conversion efficiency of a switching voltage regulator may be the only option.

Why are switching regulators needed? The power dissipation is too high for a linear regulator The efficiency of a linear regulator cannot maintain the junction temperature below maximum (150 °C) The heat sinking of a linear regulator is prohibitive in price or space Output Power Switching Regulator Linear Regulator Maximum Power Dissipation Linear Regulator

Why are switching regulators needed? The desired output voltage is greater than the input voltage Linear regulators cannot provide an output voltage greater than the input voltage The desired output voltage is opposite polarity than the input voltage Linear regulators cannot invert an input voltage Power Supply 1.5 V Battery 5 V Required Power Supply 12 V Battery -12 V Required

Types of Switching Regulators AC-DC, AC-AC, DC-AC, and DC-DC Converters 110 Vac 110 Vac 12 Vdc 12 Vdc t t t t 220 Vac Power supplies can be classified into a number of different categories. Two such categories are AC/DC converters and DC/DC converters. In an AC/DC converter, the input power is delivered to the power supply as a true AC signal. The AC/DC power supply then creates a DC output voltage. We will not examine this type of power supply in this module. Rather, we will address the DC/DC converter category of power supplies. With rare exception, DC/DC converters are used as the power supplies for automotive applications. 110 Vac 12 Vdc 5 Vdc t t t t

Types of DC-DC Converters Step Down, Step Up and Inverting Vin = 12 V Step Down Buck Vout = 5 V t t V V Vout = 12 V Step Up Boost Vin = 5 V t t The DC/DC converter category of power supplies can be sub-divided by the ratio of input voltage to output voltage. If the input power is delivered to the power supply at a voltage (VIN) which is less than the output voltage of the power supply (VOUT), the DC/DC converter must “boost” or “step-up” the voltage and the power supply is called a “Boost Converter” or a “Step-Up Converter”. If the input power is delivered to the power supply at a voltage which is greater than the output voltage of the power supply (VOUT), the DC/DC converter must “step down” or “buck” the voltage and power supply is called a “Step-Down Converter” or a “Buck Converter”. We will examine both buck and boost converters in this training module. V V Inverting Buck-Boost Vin = 5V t Vout = -10 V t

Basic Circuit Configuration Buck VIN > VOUT Boost VIN < VOUT Buck-Boost VIN < -VOUT < VIN VIN VIN VIN ISW ISW VGATE IL VGATE IL L VOUT VOUT VOUT L VM VM VM C C C VGATE IL ISW L All topologies consists of the same basic components but are arranged differently

Buck Configuration VIN VIN VOUT ISW 20V 10V VGATE 15V IL 7.5V VOUT 10V 5V VM L 5V 2.5V C 0V 0V time time The input voltage is always greater than the output voltage

Boost Configuration VIN VIN VOUT 24V 20V 20V IL 15V L 15V VOUT 10V VM 10V C 5V VGATE 5V ISW 0V 0V time time The input voltage is always less than the output voltage

Buck-Boost Configuration VIN VOUT VIN time ISW 0V 20V VGATE VOUT 15V -5V VM 10V -10V C IL 5V L -15V 0V -20V time The input voltage is always not constrained by the output voltage

Switched-Mode DC Power Supplies Five Designing Topologies. Flyback converter. Forward converter. Push-pull converter. Half Bridge converter. Full-Bridge converter. Operate at high frequencies Easy to filter out harmonics

Flyback Converter

Mode 1 Operation -- Q1 ON Current builds up in the primary winding Secondary winding has the opposite polarity D1 OFF C maintains the output voltage, supplies load current

Mode 2 Operation -- Q1 turned OFF Polarity of the windings reverses Diode D1 conducts, charging C and providing current to the load RL Secondary current falls to 0 before the next cycle begins

Waveform Summary

Double-ended Flyback Converter

Forward Converter

Features Includes a “reset” winding to return energy. Secondary “dot” so that D2 forward biased when Q1 is ON – no energy stored in the primary. Operates in continuous mode.

Mode 1 Operation -- Q1 ON Current builds up in the primary winding Energy transferred to the load

Mode 2 Operation -- Q1 turned OFF Polarity of transformer voltages reverses D2 turns OFF, D1 and D3 turn ON

Waveform Summary Vo D3

Double-ended Forward Converter

Push-Pull Converter

Push-Pull Operation Q1 ON, Vs across the lower primary winding Q2 ON, Vs across the upper primary winding

Half-Bridge Converter

Mode 1 Operation Q1 ON, D1 conducting Energy transferred to the load

Mode 2 Operation Both transistors are OFF D1 continues to conduct due to current in L1

Mode 3 Operation Q2 ON, D2 conducting Energy transferred to the load

Mode 4 Operation Both transistors OFF D2 continues to conduct due to current in L1

Full-Bridge Converter

Mode 1 Operation Q1,Q4 ON, Q2,Q3 OFF D1 conducting, energy transferred to the load

Mode 2 Operation All transistors are OFF D1 continues to conduct due to current in L1

Mode 3 Operation Q2,Q3 are ON, Q1,Q4 OFF D2 conducting, energy transferred to the load

Mode 4 Operation All transistors are OFF D2 continues to conduct due to current in L1

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

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

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

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

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

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

Switching Regulator Components

Switching Power Supply Block Diagram VIN VOUT Network Switch Network PWM Controller Error Amplifier Bandgap Reference

PWM Controller In a switching voltage regulator, the pass transistor is used as a switch - it is either on or off The output voltage, however, is an analog value PWM controller senses error in VOUT via the error amplifier PWM controller updates the duty cycle of the of transistor adjusting the output voltage 0-100% Error Amplifier PWM Controller VOUT

Switching Transistor Bipolar and MOSFET Switch Speed Slow Fast Drive Method Current Voltage Drive Circuit Complex Simple ESD Robustness High Low Drain Collector Base Gate Emitter Source

Switching Power Supply Block Diagram VIN VOUT Network Switch Network PWM Controller Error Amplifier Bandgap Reference

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 Switch Network VIN VOUT VIN = 12 V VOUT = 5 V

Switching Power Supply Block Diagram VIN VOUT Network Switch Network PWM Controller Error Amplifier Bandgap Reference

Step Down Switching Regulator Steady State Operation VIN VGATE VGATE goes high VM ~ VIN VL = VM – VOUT t ISW VM VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + IL COUT t VOUT t

Step Down Switching Regulator Steady State Operation VIN VL Constant VGATE t ISW VM IL and ISW increase VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + IL COUT is charged by IL and VOUT increases COUT t VOUT t

Step Down Switching Regulator Steady State Operation VIN VGATE = 0V The pass transistor is turned off ISW = 0A VGATE t ISW VM VM goes negative VL = VM – VOUT VGATE IL t -VF VOUT ISW VM + VL - t RLOAD - VF + IL IL cannot go to 0A instantly: COUT t VOUT t

Step Down Switching Regulator Steady State Operation VIN But, VM is clamped to -VF and IL decays through the diode VGATE t ISW VM VGATE IL t -VF VOUT ISW VM = -VF + VL - t RLOAD - VF + IL COUT stabilizes the output voltage so VOUT will only slowly decay COUT t VOUT t

Step Down Switching Regulator Steady State Operation VIN The MOSFET is turned on and off to repeat the sequence VGATE t ISW VM VGATE IL t -VF VOUT ISW VM = -VF + VL - t RLOAD - VF + IL COUT t VOUT t

REFERENCES For student study : Read Chapter 21 discrete & integrated voltage regulators topics (21.5) from the book introductory electronic devices and circuits by (Robert T .paynter)conventional flow version. Wikipedia & world wide web.

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