SWITCH-MODE POWER SUPPLY or SMPS SMPS are power supplies that operate on a switching basis.

SWITCH-MODE POWER SUPPLY Why? The principal reason for the move from linear power supply to SMPS is their much greater efficiency. Normally, SMPS’ power efficiency is ranging from 70 – 88%. This greatly reduces the cooling requirements and allows a much higher power density.

POWER Astec Custom Power  Rate of ENERGY per unit TIME  P = dW/dt [Work done per unit time]  Measured in WATTS [ 1W = 1 Joule / sec]  May be DELIVERED POWER [Energy OUT / sec]  or ABSORBED POWER [Energy IN / sec]

VOLTAGE SOURCES  May DELIVER or ABSORB power.  Voltage is ideally fixed, but current may be leaving or entering the positive terminal.  May be a DC source or an AC source

SMPS BASIC COMPONENTS SWITCH (transistor) INDUCTOR CAPACITOR DIODE LOAD PWM (controlling and monitoring)

RESISTIVE LOADS  ALWAYS ABSORB power. P R = I R 2 R = V R 2 / R [Eq. 1]  Polarity of voltage follows direction of current V R = I R R [Eq. 2]  Electric Energy is converted into Heat.

INDUCTORS  May temporarily DELIVER or ABSORB power.  The net power eventually goes zero.  Voltage is proportional to the rate of change in current V L = L dI L /dt [Eq. 3]  Energy is stored in magnetic field E L = 1 / 2 LI L 2 [Eq. 4]

POWER IN AN INDUCTOR Average Power is zero over period T E max I max P max Voltage (E) time T/43T/4T/2 /2/2  3/23/2 22 Current (I) T Voltage and current

VOLTAGE AND CURRENT IN AN INDUCTOR  From [Eq. 3], the current through an inductor is delivered as an integral of voltage: I L = 1/L∫V L dt  Also from [Eq. 3], we can see that: dI L /dt = V L /L  This means that the current slope is proportional to voltage for any given L

VOLTAGE AREA AND CURRENT SLOPE IN AN INDUCTOR  For current to return to the original value, positive volt-seconds must equal negative volt-seconds.  P ave = 0 A 1 = A 2 V P I A1A1 A2A2 t t t

INDUCTORS IN SWITCHING POWER SUPPLIES  When a voltage pulse is applied across an inductor, the current through it rises linearly until the end of the pulse.  The longer the pulse, the higher the final value of current.  If the current is fed into a capacitor, the capacitor voltage can be regulated by applying a square- wave across the inductor and varying cycle.

CAPACITORS  May temporarily DELIVER or ABSORB power.  The net power is eventually zero.  Current is proportional to the rate of change in voltage I C = C dV C /dt[Eq. 4]  Energy is stored in electric field E C = 1 / 2 CV C 2 [Eq. 5]

CAPACITORS IN SWITCHING POWER SUPPLIES  Capacitors smooth out the output voltage of a power supply.  In a switching power supply, the shunt capacitor, together with the series choke, form an LC filter which smoothens out the switching square wave input.

POWER IN A SWITCH  An ideal switch is either “ON” [closed] or “OFF” [open]  In a short circuit, V SW = 0  In an open circuit, I SW = 0  Therefore, Power in a switch is ideally 0.  An actual switch may have significant power losses during the switching interval [rise time & fall time], called “switching loss”.

EFFICIENCY [2-TERMINAL NETWORKS]  Efficiency is the ratio of output power to input power Eff = P O / P IN  P O = P IN – P LOSSES  Eff = P O / [P O + P LOSSES ] = [P IN – P LOSSES ]/P IN

INTRODUCTION TO POWER SUPPLIES  Almost all electronic devices use DC sources  DC source can be a battery or a power supply  DC source needs to be well-filtered and well- regulated

TYPES OF POWER CONVERSION  AC-DC rectifier  DC-AC inverter  DC-DC step-up or step-down converter

CHARACTERISTICS OF AN IDEAL POWER SUPPLY  Constant output voltage  Output impedance is zero at all frequencies  100% efficient [No power loss]  No ripple or noise on the output voltage

A REAL POWER SUPPLY  Losses in semiconductors and transformers. [e.g., Rds ON, switching loss, hysteresis & Cu loss]  Although well-regulated, the output does change with load. It also changes with line voltage and temperature.  Even with above changes, output must still meet specifications.

LINEAR POWER SUPPLY  Uses a 50/60Hz [low frequency] power transformer followed by a rectifier, a filter and a linear regulator.  Low efficiency of 40% to 60%

BASIC FUNCTIONS WITHIN A POWER SUPPLY  Voltage transformation  Rectification  Filtering  Regulation  Isolation

POWER SUPPLY WITH REGULATOR

SWITCHING POWER SUPPLY  Generally, of the “off-the-line” type  AC input voltage is directly rectified and filtered without using a 50/60Hz transformer.  Rectified DC is chopped by a power switch at high frequency to produce an AC signal which is then impressed across an inductor for energy storage.  The inductor current is fed to a capacitor which acts like a stable voltage source for the load.  Output voltage-regulation is accomplished by varying the switch duty cycle.

SWITCHING POWER SUPPLY  High frequency switching [20KHz to 500KHz] enables reduction in size of transformer, capacitors and inductors.  P = E/t, t = period  E L = 1 / 2 LI L 2 f, E C = 1 / 2 CV C 2 f[Eq.6]  High efficiency, normally 70% to 88%

BASIC SMPS TOPOLOGIES  Buck Converter  Boost Converter  Forward Converter  Flyback Converter

EVOLUTION OF POWER SUPPLY  Old technology: Linear power supply  New technology: Switching Power Supply [ also called Switch-Mode Power Supply or SMPS]

CHARACTERISTICS OF A BUCK CONVERTER  DC – DC switching regulator  Output voltage is always lower than the input voltage (i.e., “step-down”) -example: cell phone chargers for cars(12V battery voltage steps down to 8V)  OUTPUT is not isolated from the INPUT

BUCK CONVERTER APPLICATIONS  Small size imbedded systems  Used as post regulators

BUCK CONVERTER CIRCUIT DIAGRAM

BASIC OPERATION OF A BUCK CONVERTER  DC input voltage is chopped by SW to produce a rectangular voltage with respect to ground at the diode cathode.  LC filter smoothens out this chopped voltage to produce DC output with very low ripple.  Regulation of the output voltage is accomplished by varying the duty cycle of the SW with respect to input voltage changes.

DETAILED OPERATION: BUCK CONVERTER ”OFF” STAGE  SW is open (no current), but current continues to flow out of the inductor.  Reverse inductor-voltage forward biased diode.  Energy stored in L is now delivered to the load.  C Smoothens out the continues inductor current.

Buck Converter

PWM

ADVANTAGES AND DISADVANTAGES ADVANTAGES - high efficiency - simple - no transformer - low switch stress - small output filter - low output ripple voltage DISADVANTAGES - no isolation between input and output - potential over voltage if Q1 shorts - normally only one output possible - high-side switch drive required - high input ripple current

CHARACTERISTICS OF A BOOST REGULATOR DC-DC switching regulator OUTPUT voltage is always higher than the INPUT voltage (during normal operation) OUTPUT cannot be isolated from the INPUT

BOOST REGULATOR APPLICATIONS Low output power levels for auxiliary supply e.g., to step-up a 5V computer logic level to 15V for use with Op-Amps. Almost exclusively used to Power Factor Correction (PFC)

BOOST REGULATOR CIRCUIT DIAGRAM D = (Vo – Vin) / Vo

FORWARD CONVERTER CIRCUIT DIAGRAM D = (Vo/Vin)(Np/Ns)

FLYBACK CIRCUIT DIAGRAM

COMPARISON OF LINEAR VS SWITCHING POWER SUPPLY SpecificationLinearSwitcher Linear Regulation Load Regulation Output Ripple Input Voltage Range Efficiency Power Density Transient Recovery Hold-up time % % 0.5-2mV RMS ± 10% 40-55% 0.5W/in 3 50µsec 2msec % % mV P-P ±20% 60-80% W/in 3 300µsec 32msec

SWITCHING VS. LINEAR PSU  Advantages of Switching over Linear: Wider input range Higher efficiency Higher output density Longer hold-up time  Advantages of Linear over Switching: Better line and load regulation Lower output peak to peak ripple (lower output noise) Faster transient recovery

BASIC REQUIREMENTS OF A POWER SUPPLY  Provide required VOLTS and AMPS.  Provide basic protection such as: OVP – Over voltage protection S/CP – Short circuit protection  Provide additional protection as needed: OCP – Over current protection OTP – Over temperature protection

OTHER POWER SUPPLY CONCERNS  EMI [conducted and radiated]  Safety [UL standards, etc.]  Quality and Reliability  Manufacturability  Cost