SWITCH-MODE POWERSUPPLY 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.

POWERAstec 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.

PR = IR2R = VR

2 / R [Eq. 1] Polarity of voltage follows direction of

current

VR = IRR [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

• VL = L dIL/dt [Eq. 3] Energy is stored in magnetic field

• EL = 1/2LIL2 [Eq. 4]

POWER IN AN INDUCTOR

Average Power is zero over period T

0

-

+Emax

Imax

Pmax

Voltage (E)time

T/4 3T/4T/2

/2 3/2 2Current (I)

T

Vol

tage

and

cur

rent

VOLTAGE AND CURRENT IN AN INDUCTOR

From [Eq. 3], the current through an inductor is delivered as an integral of voltage:

• IL = 1/L∫VLdt Also from [Eq. 3], we can see that:

• dIL/dt = VL/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.

Pave = 0

A1 = A2

V

P

I

A1

A2t

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

• IC = C dVC/dt [Eq. 4] Energy is stored in electric field

• EC = 1/2CVC2 [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, VSW = 0

In an open circuit, ISW = 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 = PO / PIN

PO = PIN – PLOSSES

Eff = PO / [PO + PLOSSES]

= [PIN – PLOSSES]/PIN

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., RdsON, 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 EL = 1/2LIL

2f , EC = 1/2CVC2f [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

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 ABOOST 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 FLYBACK CIRCUIT DIAGRAMDIAGRAM

COMPARISON OF LINEAR VS SWITCHING POWER SUPPLY Specification Linear Switcher

Linear Regulation

Load Regulation

Output Ripple

Input Voltage Range

Efficiency

Power Density

Transient Recovery

Hold-up time

0.02-0.05%

0.2-0.1%

0.5-2mVRMS

± 10%

40-55%

0.5W/in3

50µsec

2msec

0.05-0.1%

0.1-1.0%

25-100mVP-P

±20%

60-80%

2.3-40W/in3

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