Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

Introduction to Power Supplies

Intended Audience: Electrical engineers with little or no power supply background An understanding of electricity (voltage and current) is assumed A simple and functional understanding of transistors is assumed Expected Time: Approximately 60 minutes

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxiliary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

What is Electrical Power? Electrical power (P) is equal to the product of electrical current (I) and a voltage (V). Power has not changed since collegiate physics: P = V * I Output power is the product of the output current and the output voltage Input power is the product of the input current and the input voltage Input power must always be greater than output power

What is a Power Supply? Electrical Definition of Power Supply Conversion of a voltage into an desired voltage Example: Car Battery (12 V) Microprocessor (5 V) Efficiency Example: P IN = (12 V) x (1 A) = 12 W P OUT = (5 V) x (1 A) = 5 W The remaining 7W (12 W – 5 W) of power is lost as heat The efficiency η is: 5 V 1 A 12 V 1 A Power supplies are not 100% efficient.

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxiliary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Types of Power Supplies AC-DC and DC-DC Converters Vin = 110Vac Vout = 12Vdc AC-to-DC Converter V t V t DC-to-DC Converter V t V t Vin = 12Vdc Vout = 5Vdc AC to DC DC to DC Power supplies can be categorized into AC-DC and DC-DC.

DC-to-DC Converter Buck V IN > V OUT V t V t Vin = 12V V OUT = 5V DC-to-DC Converter Boost Vin < Vout V t V t Vin = 5V V OUT = 12V Boost - Step Up (Switching Regulator) Buck - Step Down ( Linear or Switching Regulators) Types of Power Supplies DC-to-DC Converters Types DC-DC Converters can be categorized as Boost or Buck. Buck can be Linear or Switching regulator.

Types of Power Supplies Input Voltage of Step Down Converter Buck - Step Down Converter V IN > V OUT V OUT t 15V 10V 5V 0V V IN t 15V 10V 5V 0V The actual input voltage does not need to be a true DC value. However V IN > V OUT for step down converter.

Types of Power Supplies What is a Switching and Linear Power Supply? Switching Power Supply The pass transistor operates in a digital fashion. When in regulation, the pass transistor (power transistor between the input and output) is either completely on or completely off. An external passive component is used in the architecture for energy storage and transfer Linear Power Supply The pass transistor operates in an analog fashion. When in regulation, the pass transistor (power transistor between the input and output) is always on. No additional passive component is needed to create the desired output voltage

Types of Power Supplies What is a Linear Power Supply? A linear power supply regulates the output by operating the pass transistor in the linear/active region.

Types of Power Supplies Types of Linear Power Supplies 1.NPN or Standard 2.PNP or Low Drop Out (LDO) 3.MOS Low Quiescent Current Linear power supply can be broadly labeled: 1.Standard 2.Low Drop Out 3.Low Quiescent

V IN V OUT CONTROL V CE ~ 0.5V V BE ~ 0.7V ~ 2.0V Types of Power Supplies NPN or Standard Linear Regulators NPN or Standard linear regulators use a NPN Darlington pass transistor and ~ 2.0 V drop out

~ 1.2V V IN V OUT CONTROL V CE ~ 0.5V V BE ~ 0.7V Types of Power Supplies Quasi Low Drop Out Linear Regulator Quasi linear regulators use a single NPN pass transistor ~ 1.2 V drop out

V IN V OUT CONTROL V EC < 0.5V I QUIESCENT Types of Power Supplies PNP or Low Drop Out (LDO) Regulator PNP or Low Drop Out (LDO) linear regulators use a single PNP pass transistor and < 0.5 V drop out

V IN V OUT CONTROL Charge Pump V DS < 0.5V I QUIESCENT 0 Types of Power Supplies MOS LDO Low Quiescent Current Regulator MOS linear regulators use a MOSFET as the pass transistor offering low quiescent current and low drop out < 0.5 V.

Types of Power Supplies Summary of Linear Voltage Regulators Standard Linear Regulator Low Drop Linear Regulator Low Quiescent MOS Linear Regulator Drop Out Voltage31 (Tie) Quiescent Current 321 Features321 Cost123 Total (Lower is better) 1076

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxiliary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

V REF V IN V OUT V INT Control Block OVERTEMP V REF Pass Transistor Voltage Divider 1) Op Amp 2) Protection Bandgap Reference Linear Voltage Regulator Functional Diagram

Linear Voltage Regulator Pass (Output) Transistor Below, the output transistor is PNP bipolar junction transistor The emitter-base voltage of the transistor will be adjusted in an analog fashion to maintain the proper output voltage V IN V OUT V INT V REF

Linear Voltage Regulator Resistor Divider The resistor divider is from the output to ground Resistors are sized such that the intermediate node is equal to the bandgap reference voltage under typical conditions Voltage Regulator V OUT R6R6 R7R7 V INT V INT = (V OUT )(R 7 ) = V REF R 6 + R 7

Linear Voltage Regulator Operational Amplifier If V INT is higher (lower) than V REF, the operational amplifiers output voltage increases (decreases). This decreases (increases) the V EB voltage, and V OUT will decrease (increase). V IN V OUT V INT V REF + - V EB IBIB ICIC

Linear Voltage Regulator Bandgap Voltage Reference Internally generated with tight tolerance, traditionally ~ 1.2V V OUT will be built from reference voltage (V REF ) TARGET V REF + 2% + 1% V REF, nom - 1% - 2% Temp V REF V REF = V BE +2(R 2 /R 1 )V T ln10

Linear Voltage Regulator Current Limit and Short Circuit Detection The current through an alternate collector tap is measured. If it is too high, the regulator can limit the current from increasing further (current limit) or turn itself off (short circuit detect) V IN V OUT V INT V REF Control Block

Linear Voltage Regulator Over Temperature Detect At temperature increases, the V BE necessary to turn on a NPN decreases, so above 150C, the transistor turns on and OVERTEMP goes LO V IN V OUT V INT V REF Control Block OVERTEMP V REF + - V BE

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxiliary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Characteristics of Linear Voltage Regulators 1.Output Voltage Accuracy 2.Output Current 3.Dropout Voltage 4.Quiescent Current 5.Thermal Resistance

Characteristics of Linear Voltage Regulators Output Voltage Accuracy Characteristic Output Voltage Symbol V OUT Min Typ 5.00 Max Unit V Condition I OUT = 1mA V IN = 14V 1mA < I OUT < 50mA 6V < V IN < 30V 1 2 Output Voltage Accuracy characterizes how reliable the output voltage will be under various operating conditions. Consider the entire operating condition when viewing the accuracy.

Characteristics of Linear Voltage Regulators Output Current Characteristic Current Limit Short Circuit Current Symbol I LIM Min Typ Max Unit mA Condition V OUT = V OUT,TYP -100mV T JUNCTION = 25C V OUT = V OUT,TYP -100mV -40C < T JUNCTION < 125C I SC Output Current Limit is the maximum amount of current that can be sourced by the regulator.

Characteristics of Linear Voltage Regulators Drop Out Voltage Characteristic Dropout Voltage Symbol V DROP Min --- Typ Max Unit V Condition I OUT = 1mA V OUT = V OUT,TYP – 100mV I OUT = 100mA V OUT = V OUT,TYP – 100mV Drop Out Voltage is the minimum voltage differential between the linear regulators input and output that is required for voltage regulation. Example: Given: VDROP = 0.3 V VOUTPUT = 5.0 V Calculate Minimum Input Voltage (VINPUT = VOUTPUT + VDROP) VINPUT = 5.0 V V = 5.3 V VINPUT = 5.3 V MIN

Characteristics of Linear Voltage Regulators Quiescent (Ground) Current Characteristic Quiescent Current Quiescent Current Symbol I Q Min --- Typ Max Unit A mA Condition I OUT < 1mA V IN = 14V I OUT = 50mA T JUNCTION = 85C Quiescent Current is the current consumed by the voltage regulator.

Characteristics of Linear Voltage Regulators Thermal Resistance Characteristic Thermal Resistance Junction-Ambient Thermal Resistance Junction-Case Symbol R thja Min --- Typ --- Max Unit C/W Condition Package mounted on FR4 PCB 80x80x1.5mm 3 To lead frameR thjc Thermal resistance indicates how much heat can be conducted by the regulator. Lower thermal resistance better thermal performance

Characteristics of Linear Voltage Regulators Thermal Resistance Calculation Example Voltage Regulator V IN V OUT I IN I OUT IqIq 2 3 GIVEN: 1)V IN = 14 V 2)V OUT = 5 V 3)I OUT = 30 mA 4)I q = 0.5 mA 5)T AMBIENT = 85° C 6)T JUNCTION = 150° C 1

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxillary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Auxillary Functions of Voltage Regulators Inhibit Function Some voltage regulator outputs that can be enabled or disabled with an INHIBIT input When a voltage regulator is turned off, the quiescent current drops dramatically Characteristic Quiescent Current Quiescent Current Quiescent Current Symbol I Q Min --- Typ Max Unit A mA A Condition I OUT < 1mA V IN = 14V I OUT = 50mA T JUNCTION = 85C INHIBIT = TRUE

Auxillary Functions of Voltage Regulators Reset Function Most automotive modules are controlled by a microcontroller with a crystal oscillator stabilization time of 1 – 10 ms. – Only when a stable clock signal is available, can a microcontroller be correctly initialized A Reset signal is sent from the linear voltage regulator to the microcontroller to indicate an established and valid operating voltage. – A small (~100nF) external capacitor controls the reset delay timing

Auxillary Functions of Voltage Regulators Watchdog Function A microcontroller can be monitored through a watchdog circuit Periodically, a microcontroller is expected to strobe (pet) the watchdog to let the watchdog know it is still functioning Voltage RegulatorMicrocontroller V OUT STROBE RESET time RESET Voltage STROBE Watchdog

Auxillary Functions of Voltage Regulators Watchdog Function However, if the microcontroller forgets to pet the watchdog, a software problem may have occurred Therefore, the voltage regulator resets the microcontroller to bring it to a known state Voltage RegulatorMicrocontroller V OUT STROBE RESET time RESET Voltage Missing STROBE Watchdog

Auxillary Functions of Voltage Regulators Early Warning Function Senses an analog input and then a transmits a digital signal to a microcontroller once the analog input threshold has been triggered. Commonly used to provide an Early Warning to the microcontroller that the battery voltage has dropped and reset may occur. Voltage Regulator V BA T V IN WARN_IN R SI1 R SI2 V OUT WARN_OUT Microcontroller RESET Voltage V OUT RESET V IN WARN_OUT time

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxillary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Types of Switching Voltage Regulators Inductive and Capacitive Inductive Switching Regulators Uses inductor or transformer for passive charge control Output current may range from 1mA to many Amps PCB design is moderately complex Traditionally used in automotive applications Automotive grade parts Capacitive Switching Regulators Uses external capacitor(s) for passive charge control Relatively low output current for the price PCB design is relatively simple Not traditionally used in automotive applications Few automotive grade parts

Types of Switching Voltage Regulators Inductive Buck Regulator (V OUT < V IN ) Buck Regulator V IN V SWITCH V OUT V FEEDBACK

Types of Switching Voltage Regulators Inductive Boost Regulator (V OUT > V IN ) Boost Regulator V IN V SWITCH V OUT V FEEDBACK

Types of Switching Voltage Regulators Additional Inductive Switching Regulators Inverting Regulators V OUT = - V IN Buck-Boost Regulators V IN,MIN < V OUT < V IN,MAX Multiple Output Regulators V OUT1 = 2V IN, V OUT2 = -V IN V IN = 16V, V OUT1 = 3.3V, V OUT2 = 5V, V OUT3 = 12V

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxillary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Characteristics of Switching Voltage Regulators Linear & Switching 1.Output Voltage Accuracy 2.Output Current 3.Dropout Voltage 4.Quiescent Current 5.Thermal Resistance + Switching 1.Switching Frequency 2.External Components Size and Cost 3.Ripple Voltage 4.Efficiency

Characteristics of Switching Regulators Switching Frequency Frequency is probably the most often cited characteristic of a switching regulator Usually (but not always!), high frequency translates into: Higher efficiency Smaller external components Higher price High frequency can also mean additional design problems

Characteristics of Switching Regulators External Components, Size and Cost The design of a power supply is a true engineering challenge in the optimization of performance, price, and space Larger valued, higher quality, higher price external components usually translate into higher performance An optimal power supply design, however, will meet the required performance requirements while using acceptable external components (smaller values of inductance and capacitance, higher values of parasitic resistance…) Possible value ranges may approach two orders of magnitude

Characteristics of Switching Regulators Ripple Voltage Because the switching power supply is constantly being switched on and off, the output voltage will oscillate around a typical value V TYP V MIN V MAX Power Supply Charging C OUT Load Discharging C OUT

Characteristics of Switching Voltage Regulators Efficiency The most important characteristic of a switching regulator is efficiency because this is the primary reason for their use. Efficiency will also vary with output current load, input voltage, and temperature 95% 85% 75% 65% 0.5A0A1.0A Load Current Efficiency vs. Load Current V IN = 24V V IN = 12V 95% 85% 75% 65% Input Voltage (V) I LOAD = 100mA I LOAD = 1A Efficiency vs. Input Voltage

Outline 1)What is a Power Supply? 2)Types of Power Supplies 3)Linear Voltage Regulator 4)Characteristics of Linear Voltage Regulators 5)Auxillary Functions of Voltage Regulators 6)Types of Switching Voltage Regulators 7)Characteristics of Switching Voltage Regulators 8)Choosing Between Linear and Switching Voltage Regulators

Choosing Between Linear and Switching Regulators When possible, most designers would prefer to use a linear voltage regulator rather than a switching voltage regulator Why Linear? 1.Linear regulators are usually lower in price 2.Linear regulators are usually simpler to implement 3.Linear regulators do not have associated noise/ripple problems apparent in switching regulators

Choosing Between Linear and Switching Regulators When to use a switching regulator: 1.When the minimum input voltage is at or below the desired output voltage because linear regulators cannot provide an output voltage greater than the input voltage 2.The heat sinking of a linear regulator is prohibitive in price or space 3.The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum (150 C)

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