Bridging Theory in Practice

Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

Introduction to Power Supplies

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 What is a Power Supply? Types of Power Supplies Linear Voltage Regulator Characteristics of Linear Voltage Regulators Auxiliary Functions of Voltage Regulators Types of Switching Voltage Regulators Characteristics of Switching Voltage Regulators Choosing Between Linear and Switching Voltage Regulators

What is Electrical Power? 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 Electrical power (P) is equal to the product of electrical current (I) and a voltage (V).

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: PIN = (12 V) x (1 A) = 12 W POUT = (5 V) x (1 A) = 5 W The remaining 7W (12 W – 5 W) of power is lost as heat The efficiency η is: 12 V 1 A 5 V 1 A Power supplies are not 100% efficient.

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

Types of Power Supplies DC-to-DC Converters Types Boost - Step Up (Switching Regulator) V V DC-to-DC Converter “Boost” Vin < Vout Vin = 5V VOUT = 12V t t Buck - Step Down (Linear or Switching Regulators) V V Vin = 12V DC-to-DC Converter “Buck” VIN > VOUT VOUT = 5V t t 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 VIN > VOUT VIN VOUT 15V 15V 10V 10V 5V 5V 0V 0V t t The actual input voltage does not need to be a true DC value. However VIN > VOUT 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 “NPN” or Standard “PNP” or Low Drop Out (LDO) MOS Low Quiescent Current Linear power supply can be broadly labeled: Standard Low Drop Out Low Quiescent

Types of Power Supplies“NPN” or Standard Linear Regulators ~ 2.0V VIN VOUT VBE ~ 0.7V VBE ~ 0.7V VCE ~ 0.5V CONTROL NPN or “Standard” linear regulators use a NPN Darlington pass transistor and ~ 2.0 V drop out

Types of Power Supplies“Quasi” Low Drop Out Linear Regulator ~ 1.2V VIN VOUT VBE ~ 0.7V VCE ~ 0.5V CONTROL “Quasi” linear regulators use a single NPN pass transistor ~ 1.2 V drop out

Types of Power Supplies“PNP” or Low Drop Out (LDO) Regulator VEC < 0.5V VOUT VIN CONTROL IQUIESCENT PNP or “Low Drop Out” (LDO) linear regulators use a single PNP pass transistor and < 0.5 V drop out

Types of Power Supplies MOS LDO Low Quiescent Current Regulator VIN VOUT Charge Pump CONTROL VDS < 0.5V IQUIESCENT 0 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

Linear Voltage RegulatorFunctional Diagram VIN Pass Transistor VOUT Voltage Divider Bandgap Reference VREF 1) Op Amp 2) Protection VINT Control Block VREF OVERTEMP

Linear Voltage RegulatorPass (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 VIN VOUT VINT VREF

Linear Voltage RegulatorResistor 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 VOUT R6 VINT R7 VINT = (VOUT)(R7) = VREF R6 + R7

Linear Voltage RegulatorOperational Amplifier If VINT is higher (lower) than VREF, the operational amplifier’s output voltage increases (decreases). This decreases (increases) the VEB voltage, and VOUT will decrease (increase). VIN VOUT VINT VREF IC + VEB - IB

Linear Voltage RegulatorBandgap Voltage Reference Internally generated with tight tolerance, traditionally ~ 1.2V VOUT will be “built” from reference voltage (VREF) VREF = VBE+2(R2/R1)VTln10 VREF VREF + 2% + 1% TARGET VREF, nom - 1% - 2% Temp

Linear Voltage RegulatorCurrent 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) VIN VOUT VINT Control Block VREF

Linear Voltage RegulatorOver Temperature Detect At temperature increases, the VBE necessary to turn on a NPN decreases, so above 150C, the transistor turns on and OVERTEMP goes LO VIN VOUT VREF VINT Control Block VREF OVERTEMP + VBE -

Characteristics of Linear Voltage Regulators Output Voltage Accuracy Output Current Dropout Voltage Quiescent Current Thermal Resistance

Characteristics of Linear Voltage Regulators Output Voltage Accuracy Condition IOUT = 1mA VIN = 14V 1mA < IOUT < 50mA 6V < VIN < 30V Characteristic Output Voltage Output Voltage Symbol VOUT VOUT Min 4.90 4.80 Typ 5.00 5.00 Max 5.10 5.20 Unit V V 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 Current Limit Short Circuit Current Symbol ILIM ILIM ISC Min 100 150 200 Typ 200 300 --- Max --- 400 --- Unit mA mA mA Condition VOUT = VOUT,TYP-100mV TJUNCTION = 25C VOUT = VOUT,TYP-100mV -40C < TJUNCTION < 125C -40C < TJUNCTION < 125C 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 Dropout Voltage Symbol VDROP VDROP Min --- --- Typ 0.20 0.40 Max 0.30 0.60 Unit V V Condition IOUT = 1mA VOUT = VOUT,TYP – 100mV IOUT = 100mA VOUT = VOUT,TYP – 100mV • Example: • Given: • VDROP = 0.3 V • VOUTPUT = 5.0 V • Calculate Minimum Input Voltage (VINPUT = VOUTPUT + VDROP) • VINPUT = 5.0 V + 0.3 V = 5.3 V • VINPUT = 5.3 V MIN Drop Out Voltage is the minimum voltage differential between the linear regulator’s input and output that is required for voltage regulation.

Characteristics of Linear Voltage Regulators Quiescent (Ground) Current Characteristic Quiescent Current Quiescent Current Symbol IQ IQ Min --- --- Typ 100 4 Max 200 8 Unit A mA Condition IOUT < 1mA VIN = 14V IOUT = 50mA TJUNCTION = 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 Rthja Rthjc Min --- --- Typ --- --- Max 120 35 Unit C/W C/W Condition Package mounted on FR4 PCB 80x80x1.5mm3 To lead frame • Thermal resistance indicates how much heat can be conducted by the regulator. • Lower thermal resistance  better thermal performance

VIN VOUT Voltage Regulator IIN IOUT Iq Characteristics of Linear Voltage Regulators Thermal Resistance Calculation Example 1 • GIVEN: • VIN = 14 V • VOUT = 5 V • IOUT = 30 mA • Iq = 0.5 mA • TAMBIENT = 85° C • TJUNCTION = 150° C 3 2

Outline What is a Power Supply? Types of Power Supplies Linear Voltage Regulator Characteristics of Linear Voltage Regulators Auxillary Functions of Voltage Regulators Types of Switching Voltage Regulators Characteristics of Switching Voltage Regulators Choosing Between Linear and Switching Voltage Regulators

Auxillary Functions of Voltage RegulatorsInhibit 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 IQ IQ IQ Min --- --- --- Typ 100 4 1 Max 200 8 2 Unit A mA A Condition IOUT < 1mA VIN = 14V IOUT = 50mA TJUNCTION = 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 RESET VOUT Voltage Regulator Microcontroller RESET Watchdog STROBE Voltage STROBE time

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 RESET VOUT Voltage Regulator Microcontroller RESET Watchdog STROBE Missing STROBE Voltage time

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. VBAT Microcontroller VOUT VIN Voltage Regulator RESET WARN_OUT RESET RSI1 WARN_IN RSI2 VIN VOUT Voltage WARN_OUT time

Types of Switching Voltage RegulatorsInductive 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 (VOUT < VIN) VFEEDBACK VIN VSWITCH VOUT Buck Regulator

Types of Switching Voltage Regulators Inductive Boost Regulator (VOUT > VIN) VSWITCH VIN VOUT VFEEDBACK Boost Regulator

Types of Switching Voltage Regulators Additional Inductive Switching Regulators Inverting Regulators VOUT = - VIN Buck-Boost Regulators VIN,MIN < VOUT < VIN,MAX Multiple Output Regulators VOUT1 = 2VIN, VOUT2 = -VIN VIN = 16V, VOUT1 = 3.3V, VOUT2 = 5V, VOUT3 = 12V

Characteristics of Switching Voltage Regulators Linear & Switching Output Voltage Accuracy Output Current Dropout Voltage Quiescent Current Thermal Resistance Switching • Switching Frequency • External Components Size and Cost • Ripple Voltage • 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 Load Discharging COUT VMAX VTYP VMIN Power Supply Charging COUT

Bridging Theory in Practice