ElapE1
26/11/2012
Ingegneria dell’Informazione
Sistemi elettronici
ELETTRONICA APPLICATA
E MISURE
SENSORI
Dante DEL CORSO
AA 2012-13
CONDIZIONAMENTO IN.
CONV A/D
ELABORAZIONE
CONV D/A
CONDIZIONAMENTO OUT
Dispositivi di potenza
Limiti operativi
Analisi termica
Circuiti di comando
Problemi:
- gestire alte correnti (tensioni)
- dissipazione/temperatura
- ottenere buona efficienza
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Gruppo E: Gestione dell’energia
Lezione E1: circuiti di potenza
• Come fornire energia a un sistema elettronico
• Parti “di potenza” in un sistema elettronico
– Alimentatori
– Batterie primarie/secondarie
• Dispositivi: modelli e parametri (da Sist e Tecn. ELN)
– Diodi raddrizzatori e zener
– Transistori: MOS, BJT, altri
• Caratteristiche di componenti “di potenza”
– Limiti di correnti/tensioni
– Problemi termici, SOA
• Limiti operativi
– Safe Operating Area
– Analisi termica
• Esempi di circuiti di potenza
–
–
–
–
Alimentatori e regolatori lineari
Regolatori a parzializzazione (commutazione)
Stadi di uscita
Protezioni
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• Dispositivi BJT e MOS usati come interruttori
• Riferimenti:
– M. Zamboni: Elettronica dei sistemi di interc. e acq.: cap. 6
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Dove occorre gestire potenza
Altri moduli funzionali “di potenza”
• Alimentatore (fornisce energia quasi costante)
• Conversione dell’energia
– Fornire energia ai vari moduli, partendo da
–
–
–
–
» Tensione di rete (220V, 50/60 Hz)
» Batterie, celle solari, …
– Tensione di uscita ben definita, al variare di
» Energia di ingresso (rete, stato batterie, …)
» Energia richiesta in uscita (corrente al carico)
» Temperatura e altri parametri ambientali
ACDC: “alimentatore” classico
DCDC: alimentazioni a batteria, alim. isolate, regolatori, …
DCAC: inverter (generare 220V da batterie)
ACAC: trasformatori
– Chimica ↔ DC: batterie, accumulatori
– Meccanica ↔ AC/DC: generatori e motori
• Amplificatori/circuiti di potenza (energia variabile)
– Altro: Celle solari , …
– Visti come “alimentatori variabili”
• Per tutti: alto rendimento/basse perdite
• Entrambi: alto rendimento/basse perdite
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ATTUATORI
SISTEMA DI
ALIMENTAZIONE
E1 – CIRCUITI DI POTENZA
»
»
»
»
CIRCUITI
DI
POTENZA
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Esempio 1: Conversione AC  DC
Zener diodes
• All junctions have breakdown
• Raddrizzatore a una semionda
– Vi è AC
– Condensatore
per avere tensione
di uscita costante
(DC o quasi)
– Vo è DC
con ondulazione
• Breakdowns usually must be avoided (damage!)
• Some devices are designed to operate in breakdown
without degradation:
ZENER diodes
• Used for
• Circuito base per
la conversione da
AC a DC
– Protection circuits
– Low power voltage regulators
– Cheap reference voltage sources
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Zener diode i(v) characteristic
Zener diode equivalent circuit
• Iz and Vz have inverse sign from Id, Vd
• Vzo:
Vz for I = 0 (linear model)
– “Standard” diodes operates in forward/reverse bias
» Breakdown is a fault condition
I
Rz
• Rz:
ΔV/ΔI (actually differential rz)
– Zener operates in reverse bias (breakdown)
Iz
Vzo
I
Slope
ΔV/ΔI = Rz
• Pdmax (or Izmax):
limited by temperature rise
Vz
+
V
• Izmin:
minimum current to exit
knee region
Id
Vd
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Izmin
Pdmax
V
Vzo
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Zener diode applications
Zener diode operating point
• The reverse breakdown voltage can be used to get a
stable voltage reference
• Select I, V sign as for diodes
• Draw characterisitc of
left-side bipole (Vs + R)
– Breakdown voltage  Vz
– Output voltage of circuit
shown is equal to Vz, despite
variations in input voltage V
• Draw Dz I(V) characteristic
• Operating point (Va, Ia)
at intersection
• The resistor R limits the
current in the diode
• Detailed analysis:
I
V
Vsu
Dz
I
Va, Ia
Vsu/Rs
V
• Evaluate effect of Vsu and Rs
– Consider quiescent current
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Rs
+
– If Dz operates in breakdown
 Vo regulation
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Vsu
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Power BJT devices
Switch or amplifier?
• Fundamental relation for BJT:
• Use as amplifier
– Ic = β Ib
– Vce, Ic ≠ 0
– Active region
• Most relevant parameters for power applications:
–
–
–
–
Vcebr
Icmax
β
Vcesat
C-E breakdown voltage
max collector current
current gain (low for high currents)
C-E voltage drop in saturation
• Use as switch ON
– Vce ≈ 0
– Saturation
Ic
Ib
– Thermal parameters
• Use as switch OFF
» Max power, Thermal resistance
V be
• Use specific technology (vertical)
– Ic ≈ 0
– Cutoff
V ce
– More current in the same area (higher density)
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BJT as a switch
Critical saturation parameters
• Operating points are on the load line
• Operation is based on minority carriers
– Slow dynamic behavior, Temperature dependence
• High Vcesat (depends on Ic; about 0.1  1 V)
• β decreases for high currents
– Low current gain (5 … 20), evel lower for high voltage devices
RB
• Critical behavior near saturation
– High Ic, residual Vce  High power dissipation
• Design solution
– Guarantee deep saturation (high Ib drive, or Darlington conf.)
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Cutoff model for BJT
• Ib = 0  Ic = 0
Power MOS-FET
(ideal)
• Planar structure
– Low power devices
– Current and breakdown
voltage ratings function
of the channel W & L.
• BC junction leakage current: Icbo
– If base open, enters as Ib, causing Iceo = β Icbo
• Iceo causes power dissipation
– Temperature rise  higher leakage current
 further temperature rise  …  Thermal runaway
• Vertical structure
– Voltage rating function of
doping and thickness of
N-epitaxial layer (vertical)
– Current rating is a function
of the channel W & L
– A vertical structure can
sustain both high V & I
• Multiple devices, with current partition (avoid hot spot)
• Steer Icbo away from Base
– R to GND
– Reverse bias BE (but no breakdown!)
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Power MOS-FET parasitics
MOS-FET output characteristic
• The vertical structure creates a pn junction from body
(S) to substrate (D)
• Current can
always flow
from S to D
S
G
• The power MOS
is a 1-quadrant
switch
• Warning!
D
– Saturation in
MOS has a
different meaning
(called “active”
region in BJT)
– 4-quadrant requires at least two MOS
– The vertical structure creates also a parasitic BJT
(not indicated)
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MOS-FET switching models
MOS-FET vs BJT
• ON:
• MOS-FET use majority carriers
– Equivalent resistance Ron
– High switching speed
– Reduced temperature dependence
• OFF:
• MOSFET can use simpler driving circuit
– Leakage current Ioff
– No DC Gate current, only charge Gate-body capacitor
– Fast switching requires circuits able to drive a high-C load
• Dynamic
• ON state
– GS capacitance
– DS capacitance
– Parasitic towards substrate
– BJT modeled as Vcesat (+Ron)
– MOS modeled as Ron
• OFF state: both modeled as current source (leakage)
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Four-layer devices
SCR in CMOS logic circuits
• Some devices are designed for switching
applications:
• SCR structure intrinsic in CMOS ICs
– Responsible for latch up
– Specific physical structure (4-layer or more)
– Can be used only as switches (not for linear amplifiers)
• Triggered by
– Input levels out of GND-Vcc range
– High energy particles (space applications!)
• Examples:
– Silicon Controlled Rectifier (SCR), Tyristor
– TRIAC/DIAC
pMOSFET
VDD
• A parasitic SCR is “intrinsic” on CMOS inputs
n+
– When ON, high current may damage the circuit
– Requires proper circuit design
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S
G
p+
nMOSFET
D
D
p+
n+
G
S
n+
T1
p-well
VSS
p+
T2
n-substrate
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Lesson E1: power devices
Operating limits
• Dispositivi: modelli e parametri (richiami)
• Breakdown voltage
– If higher, insulating layers are broken
• Limiti operativi
• Max current
• Dispositivi BJT e MOS di potenza
– If higher, wires or conducting paths can melt
• Operating limits
• Max power
– Safe Operating Area
– Power dissipation
– Thermal model
– Power dissipation causes temperature rise (see max temp.)
• Max temperature
– Silicon/metal can melt, doping & parameters modified, …
• Using BJT switches
• Special application parameters
• Using MOS-FET switches
– Radiation in space, vibration, ….
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Safe Operating Area
Safe Operating Area (BJT)
• Any electronic devices can handle limited power,
voltage, current
Too high current
• The region of acceptable V,I is the
Safe Operating Area (SOA), defined by
Too high V x I (power)
- not uniform current flow
- high local power dissipation
– Power limit (V x I > Pdmax)
» Excess power cause temperature rise, with melting
» Secondary breakdown: local heating and thermal runaway
– Voltage (V < Vbrk)
Too high
voltage
Active & Safe
Operating Area (SOA)
» Excess voltage causes breakdown and insulator perforation
– Current (I < Imax)
» Excess current cause heating and metal evaporation
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Power dissipation
Power derating
• Manufacturers specify
• All electric devices dissipate a power Pd = V I
– Power dissipation causes temperature increase
– Any device has temperature limits  power limits
– Max power dissipation Pdmax
– Max junction operating temperature Tjmax
• Power dissip. modeled with thermal equivalent circuits
• Power dissipation causes temperature rise
– Power
 current
– Temperature
 node voltage
– Heat conduction capability  thermal resistance θr (°/W)
• Allowed power
dissipation
decreases
with Ta
• Diodes/MOS/BJT  power dissipated on the junctions
– Ta = Tjmax
 Pd = 0
– Heat must be brought outside, through a path including
» Junction-case – defined by manufacturer
» Case-ambient – controlled by designer using heat sinks
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Thermal model
From junction to ambient
• The thermal path from junctin to ambient consists of:
• “Electric network” model for thermal behaviour
– Power
Pd
– Temperature
T
– Heat conduction θ



current source
node voltage
thermal resistance θr (°/W)
– Junction-Case: θJC
» Thermal resistance
defined by the package
– Case-heatSink: θCS
• Electrical equivalent circuit
» Case and fixture
– Heatsink-ambient: θSA
• Tj – Ta = Pd θja
» Heatsink and
operating condition
(air flow)
• Each device has Tjmax
– Tj = Ta + Pd θja
• Designer can control θCS and θSA
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Example: Thermal specification
Lesson E1: Power devices
• From datasheet TIP30
• Dispositivi: modelli e parametri (richiami)
• Limiti operativi
• Dispositivi BJT e MOS di potenza
• Operating limits
• Using BJT and MOS switches
– ON/OFF state parameters
– Command circuits
• Example: junction temperature for Pd = 0,8 W?
– ΔTj = Pd RθJA = 62,5 x 0,8 = 50°C
– Tj = Ta + ΔTj = 75°C
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Switch interfaces
Power dissipation
• Goal: drive Power Switches from logic circuits
• Full ON or full OFF  power close to 0
– Logic circuit electrical parameters
• Intermediate state (active area)
– Load parameters
»
»
»
»
Type: R, L, C, I, V
Required V and I
Transition time (overvoltage/current, EMC)
Special requirements (Galvanic insulation, …)
–
–
–
–
Occurs during transients
V and I ≠ 0
Power dissipation Pd = V x I
Max Pd for Vce = Vsu/2
– Power device parameters
»
»
»
»
V, I, P max; SOA
Configuration; High/Low side, floating
Active device parameters (current gain, Vt, …)
Dynamic behavior
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• Reduce duration of transients
– Fast switching
– Tradeoff with parasitics losses
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BJT low-side drive circuit
MOS-FET switches
VS
• With NPN BJT
– Emitter to GND, load L on Collector
– Command to Base (close to GND)
• OFF:
RL L
IB
– Vgs < Vt
Id=0 (cutoff)
IC
• ON
• ON
– Icon = Vs/Rl
– Provide enough base current (Ib > Icon/β)
– Vgs > Vt
Id=Vds/Rd
(triode region)
• OFF
• Rd depends on
– Bring base current to 0: sub-threshold or reverse bias
– Avoid BE reverse breakdown (typ 5 V)
– Avoid thermal runaway for Icbo multiplication
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– Vgs, Vds,
Temperature, …..
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MOS low-side drive circuit
MOS switch drive from logic circuit
• Load to Vs, switch to GND
• Vt < Vh  Direct drive
• With n-MOS device
• Power devices have high Gate capacitance
–
–
–
–
Source to GND, load on Drain
Command to Gate (close to GND)
ON:
Vgs > Vt
OFF:
Vgs < Vt
– Fast transition to limit power dissipation
– High current drive (dynamic) for the Gate
– Special driving circuits
• Parasitic inductance of Gate wire combined with the
capacitor makes a LC with high Q,
• With p-MOS device, control referred to Vs
– Can cause high overvoltage on Gate
– Damping resistor close to gate
– May require non-standard logic levels
– Add n-MOS level translator
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Floating MOS switch drive
Floating load – H bridge
• Both D and S of the switch at undefined voltage
• Floating load, with complementary drive on each
side: H bridge
– Sample/hold circuit
– “floating” loads
• Voltage and current reverse in the load
• Use voltage drop on a floating resistor
• 2x voltage, 4x power
• Two commands
– Active/OFF
– Direction
• Zero power consumption when load OFF
• Used also for amplifiers: steady state has no DC
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Overvoltage protection
Lesson E1: final test
• Describe the I(V) behavior of Zener diodes
• Inductive loads create a significant overvoltage at
turn OFF, due to current flowing in the inductance.
• How can we guarantee that a BJT switch goes fully in ON
state?
– Provide a path for current when the switch is OFF
– Limit the voltage on Collector/Drain
• List relevant parameters for MOS transistors used as
switches.
• Catch diode
• Which are the boundaries of SOA for MOS power devices?
– low impedance path to
dissipate the energy
stored in the inductor
• Draw the circuit to drive a floating switch from a logic signal.
• Plot output V(I) characteristic of a MOS or BJT power device,
and identify the various operating regions.
» SW closed:
diode reverse-biased,
no current flow
» SW opens:
the current in L continue to flow through the diode
• Which parameters defines the boundary of SOA?
• How can we evaluate the actual temperature of a power
semiconductor junction?
– With MOS, internal diode can dissipate a limited energy
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