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Booster lecture
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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