03 amplificadores de potência
TRANSCRIPT
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Amplificadores de Potência(Estágios de Saída)
Prof. Jader A. De Lima
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
η% - eficiência do amplificador
Pout – potência de saída do amplificador entregue à cargaPdc – potência DC retirada da fonte de alimentação
Ex: amplificador de audio
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Output Stage Requirements:
• deliver a specified amount of signal power to a load with acceptably low levels of signal distortion;
• high input impedance/low output impedance (why?);
• low quiescent power (when the input signal is zero
the power dissipation should be low).
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Collector current waveforms for transistors operating in (a) class A, (b) class B,
Estágios de Sáida (Estágios de Potência)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
(Continued) (c) class AB, and (d) class C amplifier stages.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Estágios de Sáida (Estágios de Potência)
Classe A - Seguidor de Fonte
oLm
0ioutin
outV
r//Rg
11
1
v
vA
x
xin
i
vr
mL
moL0vinout
g
1//R
g
1//r//Rr
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Class A amplifiers ( the transistor conducts for the entirecycle of the input signal) are highly (power) inefficient.
• Large power dissipation occurs even for no signal input (standby).
• Why save power?
• Preserve natural resources/reduce pollution
• Extend battery life
• Reduce costs, improve reliability (power wasted
is dissipated in the active devices: temperature,performance , chance of failure and larger
devices are required cost
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Vin
VL
Vbe
- RL Is
Vcc - Vcesat
Vcc - Vcesat + Vbe
- RL Is + Vbe
Vin
VL
Vcc
-Vcc
IsRL
Q1
~
Va
Vs
Rs
• Classe A (seguidor de emissor) com fonte de corrente
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• class-A efficiency:
CEsat
CCV
Vvo
2max
CEsat
CC
LLV
V
RR
voIQ
2
1maxmin
CC
CEsatCC
V
VV 2
4
1max
Ex: VCC = 3V e VCEsat = 0.3V → max = 20.5%
< 25% !!
{
CEsatCC
L
CCL
CEsatCC
VVR
VR
VV
2
22
12
max
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• class-A amplifier with inductive coupling
• small speaker of 3.2 (8) needs only 100mW (500mW) to operate
• class-A amplifier may be adequate for output power of a few hundred mW
• using the transformer impedance reflexion, speaker load apperas (Np/Ns)2 largerat the collector; Ex: if turns ratio is 10:1, a 3.2-speaker appears as 320 load.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Classe B – Push-Pull
Transfer characteristic for the class B output stage
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Distorção de cruzamento (crossover distortion)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
≈ 78.6%
• class-B efficiency:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• class-B amplifier with inductive coupling
• however, audio transformers are bulky and expensive
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Classe AB – Push-Pull (eliminar distorção de cruzamento)
Class AB output stage. A bias voltage VBB is applied between the bases of QN and QP, giving rise to a bias current IQ .Thus, for small vI, both transistors conduct and crossover distortion is almost completely eliminated.
• quiescent current
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• D1 (D2) must match VBE curves of QN (QP)in saturation current , area and temperature;
→ only good approach for integrated deisgn
• compensating biased diodes
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:Determinar o rendimento do estágio:
i) Ibias (resistores)ii) IC_pk (transistor limite saturação)iii) IC_aviv) Idcv) Pdcvi) PL_maxvii)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:
AI
ICMAX
AV 309.01416.3
97.0
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:
AAmAIdc 311.0309.038.2
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
AI
ICMAX
AV 309.01416.3
97.0
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
ex:
WAVxPdc 22.6311.020
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
AI
ICMAX
AV 309.01416.3
97.0
AAmAIdc 311.0309.038.2
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
W
R
VVVP
L
CCCEsatCCL 70.4
10
103.020
2
15.0
2
1 22
max
ex:
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
AI
ICMAX
AV 309.01416.3
97.0
AAmAIdc 311.0309.038.2
WAVxPdc 22.60311.020
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
%5.75%10022.6
70.4%100
maxmax xx
P
P
dc
L
ex:
A
R
VVVI
L
CCCEsatCCC 97.0
10
103.0205.0max
AI
ICMAX
AV 309.01416.3
97.0
AAmAIdc 311.0309.038.2
WAVxPdc 22.60311.020
W
R
VVVP
L
CCCEsatCCL 70.4
10
103.020
2
15.0
2
1 22
max
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• push-pull com multiplicador de VBE
VBB = VBE1 (1 + ( R2 / R1 ))
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
QPNPBEQQNPNBEQBB IVIVV @@ __
- curvas dos BJTs devem ser consultadas para se determinar correto valor de VBB
R2/R1 definido
Projeto Multiplicador VBE
• passo #1
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
1
1_2
max_max__max__
21_max__max_3
max_max__max_33
R
VI
R
VII
IIII
VVIRV
QBER
LNPN
o
NPN
NPNCNPNB
RQCNPNBR
onpnBERCC
• no máximo de excursão no semiciclo positivo tem-se:
R3 definido
para IB_Q1 << IR2
Obs:assume-se um valor inicial para IC_Q1 para se determinar VBE_Q1 a partir
Da curva característica IC x VBE de Q1
• passo #2
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
1
1_1
max_max__max__
11_max__max_4
max_max__max_44
R
VI
R
VII
IIII
VVIRV
QBER
LPNP
o
PNP
PNPCPNPB
RQEPNPBR
opnpBERCC
• no máximo de excursão do semiciclo negativo tem-se:
R4 definido
• passo #3
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• passo #4
• re-calcular valores de IR2 e VBE_Q1 • no caso de diferença importante, reiniciar a partir do passo #2
Homework
• Considerando npn Q2N2222 e pnp Q2N3906, projetar um estágio classe-AB para IQ = 5mA, RL = 8 e Vo_max = 2.5V. Admitir fontes simétricas, sendo VCC = 5V.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• capacitive coupling is not the preferred coupling mechanism for audio push-pull stages (bulky caps!)
• common-emitter driver: In addition to providing a higher input resistance, the buffer Q1 biases the output transistors Q2 and Q3
driver(Av ~ R3/R4)
small
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
The compound-pnp configuration.
The Darlington configuration.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• overload protection
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• overload protection• short-circuit protection occurs by sensing current threough R6
• VR6 = VBE_Q15
• When load current reaches a given limit, Q15becomes forwardly-biased and diverts any further base current of Q14
→ load current no longer increases
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• thermal shutdown
• Q2 acts as a swicth and is normally off at operating temperatures
• with temperature increase above a given threshold, positive tempco of Zener and negative tempco of VBE_Q1 increses Q1 current
→ VBE_Q2 increases and Q2 turns on
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• power opamp
low-powergain stage
current booster
• when Q5 turns on, it sources additiona load current
• when Q6 turns on, it sinks additiona load current
buffer
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Class B circuit with an op amp connected in a negative-feedback loop to reduce crossover distortion
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• bridge amplifier
critical match
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 741
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Class C (tuning amplifier)
• power devices conducts less than 180o
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
tank is driven by current pulses
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
rich in harmonics (f, 2f, 3f, ..., nf)
only ressonance frequency f(like pure sinewave)
fundamental frequency f
Very-low impedance at harmonics → no gain
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Series to Parallel Conversion for RL Circuits
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Coil Q > 50• class-C amps have Q > 10 usually
(for overall circuit)
narrowband operation
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
- for QL_coil = 100, determine:• resonance frequency: fr• bandwidth: BW
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
class-A, B, AB
class-D
Class D
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• power devices (normally MOSFETs) operate as switches (either fully ON or OFF) → reducing their power losses (efficiency 90 – 95% is possible, as swictheshave zero DC current when not switching and low VDS when conducting)
• input signal modulates a PWM carrier that drives the output switches
• commonly used in audio power amplifiers
PWM
~ lossless filter
high-side
low-side
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Despite the complexity involved, a properly designed class D amplifier offers the following benefits:
• Reduction in size and weight of the amplifier
• Reduced power waste as heat dissipation and hence smaller (or no) heat sinks
• Reduction in cost due to smaller heat sink and compact circuitry
• Very high power conversion efficiency, usually better than 90% above one quarter of the amplifier's maximum power, and around 50% at low power levels
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• The value of deadtime should be based on the device characteristics, ambient operating conditions, parasitic parameters of switching devices and load conditions.
• Reduces the RMS output to a certain extent and increases THD.
No deadtime:
deadtime:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Using Feedback to Improve Performance
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Many class D amplifiers utilize negative feedback from the PWM output back to the input of the device.
• A closed-loop approach:• improves linearity• allows better power-supply rejection.
• Open-loop amplifier inherently has minimal (if any) supply rejection.
• In closed-loop topology, as the output waveform is sensed and fed back to the input of the amplifier, deviations in the supply rail are detected at the output and corrected by the control loop.
• Drawback: control loop must be carefully designed and compensated to ensure stability under all operating conditions
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Many Class D amplifiers are implemented as full-bridge output stage.
• A full bridge uses two half-bridge stages to drive the load differentially.
• The full-bridge configuration operates by alternating the conduction path through the load. This allows bidirectional current to flow through the load without the need of a negative supply or a DC-blocking capacitor.
Half Bridge vs. Full Bridge
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Half-bridge amplifier:
• output swings between VDD and ground and idles at 50% duty cycle→ output has a DC offset equal to VDD/2
• efficiency >90% while delivering more than 14W per channel into 8Ω.
• Full-bridge amplifier:
• does not require DC-blocking capacitors on outputs when operating from a single supply→ offset appears on each side of the load, which means that zero DC current flows at the output.
• can achieve twice the output signal as the load is driven differentially. → 4x increase in maximum output power over a half-bridge amplifier operating from the same supply (at cost of twice as many MOSFET switches)
• efficiency in the range of 80% to 88% with 8Ω loads
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
deadtime:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Class E
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• current I is diverted through C1 when S1 is opened (see IC and IS)• RFC: only DC current• Theorectical zero overlap between VDS and IS → ideally 100% efficiency• LC resonator ensures single tone at output
RFC
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• C1: shunt cap to switch ( + device parasitics) – exact value for max efficiency
• L2 – C2 resonates below the operating frequency (↑Q → sinewave output current)→ excessive inductive reactance → max efficiency at center frequency (not max power)
• ↑ L1 RF choke (only DC current)
high Qhigh L
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• switch driven with 50%-duty cycle
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Rise of Vds is delayeduntil Is = 0
Vds returns to zero before Is increases
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• efficiency as function of duty-cycle
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Safe Operating Area (SOA)
• voltage and current conditions over which the device can be expected to operate without self-damage
(only BJT´s)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Transistor Power Rating
• temperature at collector junction places a limit on allowable power dissipation PD.
Ex: 2N3904 → Tj (max) = 150oC 2N3710 → Tj (max) = 200oC
• ambient temperature: heat produced in junction passes through the transistor case (metal or plastic) and radiates to the surrounding air (ambient temperature, usually around 25oC)
• Derating Factor: data sheets often specify PD (max) @25oC.Ex: 2N1936 has PD (max) @25oC = 4W.
• What happens if temperature is higher than 25oC? → power rating must bederated (reduced)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Power Derating
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Heat Sinks
• increase transistor power rating→ area of transistor case is increased
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Ex: assuming the circuit below must operate from 0oC to 50oC, what is themaximum power rating of the transistor?
• for TO-92 case, PD(max) = 625mW@25oC derating factor D = 5mW/oC
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Ex: assuming the circuit below must operate from 0oC to 50oC, what is themaximum power rating of the transistor?
• for TO-92 case, PD(max) = 625mW@25oC derating factor D = 5mW/oC
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Failure mechanisms in ICs are accentuated by increased temperatures (leakage in reverse biased diodes, electromigration, and hot-electron trapping).
• To prevent failure, the die temperature must be kept within certain ranges:
• commercial devices [0° to 70°C]• military parts [–55° to 125°C]
• 40-pin DIP has a thermal resistance of 38 °C/W (natural) and 25 °C/W (forced air convection).
→ DIP can dissipate 2 watts (natural) or 3 watts (forced), and still keep the temperature difference between the die and the environment below 75 °C
• PGA has thermal resistance from 15 ° to 30 °C/W.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Electromigration
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
REFERÊNCIAS:
• Fundamentals of Microelectronics, B. Razavi, John Wiley and Sons, 2006
• Microelectronic Circuits, A. Sedra and K. Smith, Oxford university Press, 5th Edition, 2003
• Analysis and Design of Analog Circuits, Gary, Hurst, Lewis and Meyer, 4th Edition, 2001