Class D Audio Amplifier Design

Class D Audio Amplifier Design
• Class D Amplifier Introduction
Theory of Class D operation, topology comparison
• Gate Driver
How to drive the gate, key parameters in gate
drive stage
• MOSFET
How to choose, tradeoff relationships, loss calculation
• Package
Importance of layout and package, new packaging
technology
• Design Example
200W+200W stereo Class D amplifier
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Prepared Oct.8 2003 by Jun Honda and Jorge Cerezo
Trend in Class D Amplifiers
• Make it smaller!
- higher efficiency
- smaller package
- Half Bridge
• Make it sound better!
- THD improvement
- fully digitally processed modulator
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System Î Gate Drive Î MOSFET Î Design Example
Traditional Linear Amplifier
Feed back
• Vcc
•
•
Error amp
Bias
•
• Vcc
Class AB amplifier uses linear regulating transistors to
modulate output voltage. η = 30% at temp rise test condition.
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System Î Gate Drive Î MOSFET Î Design Example
How a Class D Amplifier Works
Feed back
Triangle
+V C C
Nch
Level Shift
•
•
Error Amp
•
COMP
Dead Time
Nch
-V C C
Class D amplifier uses MOSFETs that are either
ON or OFF.
PWM technique is used to express analog audio
signals with ON or OFF states in output devices.www.irf.com
Basic PWM Operation
The output signal of comparator
goes high when the sine wave is
higher than the sawtooth.
COMP
Using fPWM=400KHz to modulate 25KHz sinusoidal waveform
Class D
switching stage
LPF
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System Î Gate Drive Î MOSFET Î Design Example
Topology Comparison: Class AB vs Class D
η
η
Temp rise test condition
Efficiency
Output
Constant over Vbus
Good
Always from
supply to load
Output
Gain
Proportional to
Vbus
PSRR
0 dB
Direction of
energy flow
Both way
Creates Vbus pumping
phenomena
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System Î Gate Drive Î MOSFET Î Design Example
Analogy to Buck DC-DC Converter
Buck Converter
Class D Amplifier
Fc of LPF is above
20KHz
Gate Driver
Gate Driver
Q1
MOSFET
8
8
Q1
MOSFET
3
2
1
INDUCTOR
4
2
U1A
L1
+
L1
+
1
INDUCTOR
4
3
U1A
ERROR AMP
ERROR AMP
C1
CAPACITOR
Vref
C1
CAPACITOR
R1
LOAD
R1
LOAD
Q2
MOSFET
Q2
MOSFET
Audio signal input as
a reference voltage
Load Current Direction
Both current directions
Î Influence of dead time is different
Î Dead time needs to be very tight
Duty ratio is fixed
Duty varies but average is 50%
ÎIndependent optimization for HS/LS
ÎSame optimization for both MOSFETs
ÎLow RDS(ON) for longer duty, low Qg
for shorter duty
ÎSame RDS(ON) required for both sides
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System Î Gate Drive Î MOSFET Î Design Example
Loss in Power Device
Loss
2
Pc = 0.2 ⋅
Loss in class AB
VCC
8 ⋅ RL
π
Vcc
1
(1 − K sin ω ⋅ t ) Vcc K sin ω ⋅ t • dω ⋅ t
⋅∫
PC =
2 ⋅π 0 2
2 ⋅ RL
Vcc 2
=
8π ⋅ RL
 2K K 2 

⋅ 
−
2 
 π
Regardless of output
device parameters.
K=2/π
K=1
Loss
Loss in Class D
Efficiency can be
improved further!
PTOTAL = Psw + Pcond + Pgd
Pcond =
RDS (ON )
RL
⋅ Po
2
Pgd = 2 ⋅ Qg ⋅ Vgs ⋅ f PWM
Psw = COSS ⋅ VBUS ⋅ f PWM + I D ⋅ VDS ⋅ t f ⋅ f PWM
K=1
K is a ratio of Vbus and output voltage.
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System Î Gate Drive Î MOSFET Î Design Example
Half Bridge vs Full Bridge
Supply voltage
0.5 x 2ch
1
Current ratings
1
2
MOSFET
2 MOSFETs/CH
4 MOSFETs/CH
Gate Driver
1 Gate Driver/CH
2 Gate Drivers/CH
Linearity
Superior (No even order HD)
DC Offset
Adjustment is needed
Can be cancelled out
PWM pattern
2 level
3 level PWM can be implemented
Notes
Pumping effect
Need a help of feed back
Suitable for open loop design
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System Î Gate Drive Î MOSFET Î Design Example
Major Cause of Imperfection
Perturbation
Zo
Bus Pumping
Pulse width error
Quantization error
Audio source
PWM
+V C C
Non linear inductance /
Capacitance
DCR
Gate Driver
-V C C
Dead time
Delay time
Finite RDS(on)
Vth and Qg
Body diode recovery
R DS(ON)
ON delay
OFF delay
Finite dV/dt
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System Î Gate Drive Î MOSFET Î Design Example
THD and Dead Time
High Side Dead Time
Low Side Dead Time
ON
High Side
OFF
ON
Low Side
OFF
34
Dead Time 40nS
40
Dead Time 15nS
30
High Side edges
20
10
Vout( t )
Falling edges
0
10
20
THD=2.1%
THD=0.18%
Note: THD (Total Harmonic Distortion) is a means to
measure linearity with sinusoidal signal.
2
2
30
− 34 40
0
0
5 .10
4
Low
0.001Side edges
0.0015
t
0.002
0.0021
THD =
V2 + V3 + ⋅ ⋅ ⋅
V fundamental
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System Î Gate Drive Î MOSFET Î Design Example
Shoot Through and Dead Time
Q s t a s a f u n c t io n o f O v e r la p T im e & R g
V b u s = 6 0 V , Id = 2 A , V g s = 1 2 V
High side Vgs
Rg=10 ohm
( O v e r la p t im e m e a s u r e d f r o m 5 0 % V g s h ig h s id e f a ll t o 1 0 % V g s lo w s id e r is e )
120
Low side Vgs
100
80
R g=1O hm s
60
R g=5O hm s
rg=10O hm s
40
Shoot through current
2A/div
20
0
-10
-5
0
5
10
15
20
O v e r la p tim e (n s )
-Shoot through charge increases rapidly as dead time gets shorter.
-Need to consider manufacturing tolerances and temperature characteristics.
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System Î Gate Drive Î MOSFET Î Design Example
Power Supply Pumping
Load Current
Supply voltage Pumping
effect
+Vcc
Vo
Commutation
current
-Vcc
∆VBUS max =
Half Bridge
Load Current
VBUS
8 ⋅ π ⋅ f PWM ⋅ RLOAD ⋅ C BUS
Full Bridge
-Significant at low frequency output
-Significant at low load impedance
-Significant at small bus capacitors
-Largest at duty = 25%, and 75%
Commutation
current
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System Î Gate Drive Î MOSFET Î Design Example
EMI consideration: Qrr in Body Diode
1•
2
1.
Low side drains inductor current
2.
During dead time body diode of low side
conducts and keep inductor current flow
3.
At the moment high side is turned ON after
dead time, the body diode is still conducting to
wipe away minority carrier charge stored in the
duration of forward conduction.
3•
ÎThis current generates large high frequency
current waveform and causes EMI noises.
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System Î Gate Drive Î MOSFET Î Design Example
Gate Driver: Why is it Needed?
•
Gate of MOSFET is a capacitor to be charged and discharged. Typical
effective capacitance is 2nF.
•
High side needs to have a gate voltage referenced to it’s Source.
•
Gate voltage must be 10-15V higher than the drain voltage.
•
Need to control HS and LS independently to have dead time.
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System Î Gate Drive Î MOSFET Î Design Example
Functional Block Diagram Inside Gate Driver
International Rectifier's family of MOS gate drivers integrate most of the
functions required to drive one high side and one low side power
MOSFET in a compact package.
With the addition of few components, they provide very fast switching
speeds and low power dissipation.
Input Logic
High side well
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System Î Gate Drive Î MOSFET Î Design Example
Boot Strap High Side Power Supply
Charge
Discharge
ON
ON
When Vs is pulled down to ground through the low side FET, the bootstrap capacitor (CBOOT)
charges through the bootstrap diode (Dbs) from the Vcc supply, thus providing a supply to
Vbs.
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System Î Gate Drive Î MOSFET Î Design Example
Boot Strap High Side Power Supply (Cont’d)
• Boot Strap Capacitor Selection
To minimize the risk of overcharging and further reduce ripple on the Vbs voltage the
Cbs value obtained from the above equation should be should be multiplied by a factor
of 15 (rule of thumb).
• Boot Strap Diode Selection
The bootstrap diode (Dbs) needs to be able to block the full power rail voltage,
which is seen when the high side device is switched on. It must be a fast recovery
device to minimize the amount of charge fed back from the bootstrap capacitor
into the Vcc supply.
VRRM = Power rail voltage, max trr = 100ns, IF > Qbs x f
For more details on boot strap refer to DT98-2
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System Î Gate Drive Î MOSFET Î Design Example
Power Dissipation in Gate Driver
•
Whenever a capacitor is charged or discharged through a resistor, half of energy
that goes into the capacitance is dissipated in the resistor. Thus, the losses in
the gate drive resistance, internal and external to the MGD, for one complete
cycle is the following:
PG = V ⋅ f SW ⋅ QG
For two IRF540 HEXFET® MOSFETs operated at 400kHz with Vgs = 12V, we
have:
PG = 2 • 12 • 37 • 10-9 • 400 • 103 = 0.36W
R3
High Side
R3
High Side
SW1
SW1
R2
Low Side
C1
Ciss
For more details on gate driver ICs, refer to AN978
R2
Low Side
C1
Ciss
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System Î Gate Drive Î MOSFET Î Design Example
Power Dissipation in Gate Driver (Cont’d)
•These losses are not
temperature dependent.
150.00
200
125.00
100.00
Junction Tem perature (C )
•The use of gate resistors reduces
the amount of gate drive power
that is dissipated inside the MGD
by the ratio of the respective
resistances.
100
10
75.00
50.00
25.00
0.00
1.E +03
1.E +04
1.E +05
1.E +06
Frequency (H z)
Figure 32: IR 2010S Tj vs Frequency
R G ATE = 10 O hm , V cc = 15V w ith IR FP E50
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System Î Gate Drive Î MOSFET Î Design Example
Layout Considerations
•
Stray inductance LD1+LS1 contribute to undershoot of the Vs node
beyond the ground
IR2011
As with any CMOS device, driving any of
parasitic diodes into forward conduction or
reverse breakdown may cause parasitic www.irf.com
SCR latch up.
System Î Gate Drive Î MOSFET Î Design Example
Gate Driver for Class D Applications
Key Specs
IR2011(S)
• Fully operational up to +200V
• Low power dissipation at high switching frequency
• 3.3V and 5V input logic compatible
• Matched propagation delay for both channels
• Tolerant to negative transient voltage, dV/dt immune
• SO-8/DIP-8 Package
SO-8
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System Î Gate Drive Î MOSFET Î Design Example
How MOSFETs Work
• A MOSFET is a voltage-controlled power switch.
A voltage must be applied between Gate and
Source terminals to produce a flow of current in
the Drain.
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System Î Gate Drive Î MOSFET Î Design Example
MOSFET Technologies (1)
• IR is striving to continuously improve the power MOSFET
to enhance the performance, quality and reliability.
• Hexagonal Cell Technology
• Planar Stripe Technology
• Trench Technology
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System Î Gate Drive Î MOSFET Î Design Example
MOSFET Technologies (2)
• Power MOSFET FOMs (R*Qg) have significantly improved
between the released IR MOSFET technologies
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System Î Gate Drive Î MOSFET Î Design Example
Key Parameters of MOSFETs (1)
• Voltage Rating, BVDSS
This is the drain-source breakdown voltage (with
VGS = 0). BVDSS should be greater than or equal
to the rated voltage of the device, at the specified
leakage current, normally measured at Id=250uA.
This parameter is temperature-dependent and
frequently ∆BVDSS/∆Tj (V/°C) is specified on
datasheets.
BVDSS MOSFET voltages are available from tens
to thousand volts.
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System Î Gate Drive Î MOSFET Î Design Example
Key Parameters of MOSFETs (2)
• Gate Charge, Qg
This parameter is directly
related to the MOSFET speed
and is temperatureindependent. Lower Qg results
in faster switching speeds and
consequently lower switching
losses.
The total gate charge has two
main components: the gatesource charge, Qgs and, the
gate-drain charge, Qgd (often
called the Miller charge).
Basic Gate Charge Waveform
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System Î Gate Drive Î MOSFET Î Design Example
Key Parameters of MOSFETs (3)
• Static Drain-to-Source On-Resistance, RDS(ON)
This is the drain-source
resistance, typically
specified on data sheet at
25°C with VGS = 10V.
RDS(ON) parameter is
temperature-dependent, and
is directly related to the
MOSFET conduction losses.
lower RDS(ON) results in
lower conduction losses.
Normalized On-Resistance vs. Temperature
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•
Key
System Î Gate Drive Î MOSFET Î Design Example
Parameters of MOSFETs (4)
Body Diode Reverse Recovery Characteristics, Qrr, trr, Irr
and S factor.
Power MOSFETs inherently have an
integral reverse body-drain diode. This
body diode exhibits reverse recovery
characteristics. Reverse Recovery
Charge Qrr, Reverse Recovery Time trr,
Reverse Recovery Current Irr and
Softness factor (S = tb/ta), are typically
specified on data sheets at 25°C and
di/dt = 100A/us.
Reverse recovery characteristics are
temperature-dependent and lower trr, Irr
and Qrr improves THD, EMI and
Efficiency η.
Typical Voltage –Current Waveforms
for a MOSFET Body Diode
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System Î Gate Drive Î MOSFET Î Design Example
Key Parameters of MOSFETs (5)
• Package
MOSFET devices are available in several
packages as SO-8,TO-220, D-Pak, I-Pak, TO262, DirectFET™, etc.
The selection of a MOSFET package for a
specific application depends on the package
characteristics such as dimensions, power
dissipation capability, current capability, internal
inductance, internal resistance, electrical
isolation and mounting process.
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System Î Gate Drive Î MOSFET Î Design Example
Choosing the MOSFET Voltage Rating for Class D applications
• MOSFET voltage rating for a Class D amplifier is
determined by:
– Desired POUT and load impedance (i.e. 250W on 4Ω)
– Topology (Full Bridge or Half Bridge)
– Modulation Factor M (80-90%)
VBDSS min =
2 * POUT * RLOAD
* 1.5
M
Typical additional factor due to stray
resistance, power supply fluctuations and
MOSFET Turn-Off peak voltage
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(1)
System Î Gate Drive Î MOSFET Î Design Example
Choosing the MOSFET Voltage Rating for Class D Applications
• Full-Bridge Topology Class D amplifier
• Half-Bridge Configuration Class D amplifier
Note 1. Modulation Factor M = 85%
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(2)
System Î Gate Drive Î MOSFET Î Design Example
Calculation of Switching Loss (1)
• Switching Losses are the result of turn-on and
turn-off switching times
MOSFET Turn-On
MOSFET Turn-Off
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System Î Gate Drive Î MOSFET Î Design Example
Calculation of Switching Loss (2)
• Gate resistance Rg, and gate charge Qg, have a significant
influence on turn-on and turn-off switching times
↑ Rg ⇒ ↓ Ig ⇒ ↑ tSWITCHING ⇒ ↑ PSWITCHING
RG
↑ Qg ⇒ ↑ tSWITCHING ⇒ ↑ PSWITCHING
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System Î Gate Drive Î MOSFET Î Design Example
Estimation of Switching Losses (1)
• Switching losses can be obtained by calculating
the switching energy dissipated in the MOSFET
t
Esw =
∫
VDS(t) * ID(t) dt
0
Where t is the length of the switching pulse.
• Switching losses can be obtained by multiplying
switching energy with switching frequency.
PSWITCHING = ESW * FSW
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System Î Gate Drive Î MOSFET Î Design Example
Estimation of Conduction Loss (2)
• Conduction losses can be calculated using
RDS(ON) @ Tj max and ID RMS current of MOSFET
PCONDUCTION = (ID RMS)2 * RDS(ON)
ID RMS is determined using amplifier specifications:
ID RMS =
POUT
RLOAD
RDS(ON) data can be obtained from the MOSFET
data sheet.
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System Î Gate Drive Î MOSFET Î Design Example
Thermal Design
•
Maximum allowed power
dissipation for a MOSFET
mounted on a heat sink:
Pmax = ∆Tj / Rthja max
Pmax = (Tamb – Tjmax ) / (Rthjc max + Rthcs max + Rths max + Rthsa max)
Where: Tamb = Ambient Temperature
Tjmax = Max. Junction Temperature
Rthjc max = Max. Thermal Resistance Junction to Case
Rthcs max = Max. Thermal Resistance Case to Heatsink
Rths max = Max. Thermal Resistance of Heatsink
Rthsa max = Max. Thermal Resistance Heatsink to Ambient
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System Î Gate Drive Î MOSFET Î Design Example
RDS(ON) vs Qg
• There is tradeoff between Static Drain-to-Source OnResistance, RDS(ON) and Gate charge, Qg
Higher RDS(ON) ⇒ Lower Qg ⇒ Higher PCONDUCTION & Lower PSWITCHING
Lower RDS(ON) ⇒ Higher Qg ⇒ Higher PSWITCHING & Lower PCONDUCTION
Gen 7.5 100V MOSFET Platform
RDS(ON) vs. Qg
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System Î Gate Drive Î MOSFET Î Design Example
Die Size vs Power Loss (1)
• Die size has a significant influence on MOSFET power
losses
Smaller Die ⇒ Higher PCONDUCTION & Lower PSWITCHING
Bigger Die ⇒ Higher PSWITCHING & Lower PCONDUCTION
Gen 7 100V MOSFET Platform – Power Losses @ 384kHz
Total Loss
Conduction
Loss
Switching
Loss
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System Î Gate Drive Î MOSFET Î Design Example
Die Size vs Power Loss (2)
• Die size is directly related with RDS(ON) and RTHjc of the
MOSFET
Smaller Die ⇒ Higher RDS(ON) and Higher RTHjc
Bigger Die ⇒ Lower RDS(ON) and Lower RTHjc
55V Trench Technology MosFET
55V Trench Technology MosFET
Die Size vs. RDS(ON)
Die Size vs. RTHjc
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System Î Gate Drive Î MOSFET Î Design Example
Choosing the Right MOSFET for Class D Applications (1)
•
The criteria to select the right MOSFET for a Class D amplifier
application are:
– VBDSS should be selected according to amplifier operating voltage,
and it should be large enough to avoid avalanche condition during
operation
– Efficiency η is related to static drain-to-source on-resistance, RDS(ON).
smaller RDS(ON) improves efficiency η. RDS(ON) is recommended to be
smaller than 200mΩ for mid and high-end power, full-bandwidth
amplifiers
– Low gate charge, Qg, improves THD and efficiency η. Qg is
recommended to be smaller than 20nC for mid and high-end power,
full-bandwidth amplifiers
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System Î Gate Drive Î MOSFET Î Design Example
Choosing the Right MOSFET for Class D Application (2)
– Amplifier performance such as THD, EMI and efficiency η are also
related to MOSFET reverse recovery characteristics. Lower trr, Irr
and Qrr improves THD, EMI and efficiency η
– Rthjc should be small enough to dissipate MOSFET power losses
and keep Tj < limit
– Better reliability and lower cost are achieved with higher MOSFET
Tj max
– Finally, selection of device package determines the dimensions,
electrical isolation and mounting process. These factors should be
considered in package selection. Because cost, size and amplifier
performance depend on it.
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System Î Gate Drive Î MOSFET Î Design Example
Development of Class D Dedicated Devices
• Performance of the Class D amplifying stage
strongly depends on the characteristics of
MOSFETs and ICs.
• Designers of driver IC and MOSFET silicon need
to keep the special requirements of the Class D
application in mind.
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System Î Gate Drive Î MOSFET Î Design Example
Influences of Stray Inductance
• PCB layout and the MOSFET internal package
inductances contribute to the stray inductance (LS)
in the circuit.
• Stray inductances affect the MOSFET performance
and EMI of the system.
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System Î Gate Drive Î MOSFET Î Design Example
Influences of Stray Inductance
•
•
Drain and source stray inductances reduces the gate voltage during
turn-on resulting in longer switching time.
Also during turn-off, drain and source stray inductances generate a
large voltage drop due to dID/dt, producing drain to source overvoltage transients.
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System Î Gate Drive Î MOSFET Î Design Example
DirectFET™ Packaging
Use a single multiple-finned heat sink
to dissipate heat from devices
passivated die
copper ‘drain’ clip
gate connection
DirectFET devices
die attach material
source connection
copper track on board
Circuit board
4.
8m
Both Side
Cooling
Thermal interface gap
filler material or pad
• Remove wirebonds from package and
replace with large area solder contacts
• Reduced package inductance and
resistance
• Copper can enables dual sided cooling
m
~
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System Î Gate Drive Î MOSFET Î Design Example
DirectFET™ Packaging
DirectFET waveform
•
•
•
•
SO-8 waveform
30A VRM output current
500 kHz per phase
Silicon of the near identical active area, voltage and generation used in both
packages
Inductance related ringing greater in case of SO-8
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System Î Gate Drive Î MOSFET Î Design Example
Class D Amp Reference Design
• Specs
Topology: Half Bridge
IR Devices: IR2011S, IRFB23N15D
Switching frequency: 400kHz (Adjustable)
Rated Output Power: 200W+200W / 4 ohm
THD: 0.03% @1kHz, Half Power
Frequency Response: 5Hz to 40kHz (-3dB)
Power Supply: ~ ±50V
Size: 4.0” x 5.5”
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System Î Gate Drive Î MOSFET Î Design Example
Class D Amp Reference Board: Block Diagram
Feed back
+V C C
+
Integrator
Level Shifter
LT1220
2N5401
LPF
IR2011S
Gate Driver
GND
Comparator
IRFB23N15D
74HC04
-V C C
-VCC
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System Î Gate Drive Î MOSFET Î Design Example
Circuit Diagram
1
2
3
CH1
D
Feed Back Path
R1
R3
47K
1K
TP3
PAD
R4
GNDP
Q1
1K
8
7
6
5
1
2
3
4
8
7
6
5
HO
VB
6
LT1220CS8
TC7WH04FU
MMBT5401
TC7WH08FU
B
Q9
MMBT3904
5
B
C?
.01uF, 50V
R41
100K
TP4
PAD
Quantize
Hin
COM
LO
TP
R13
C26
10K
0.1uF,50V
3.3uF, 35V
GNDP
GNDP
VS
C
C42
E
GNDP
Lin
VCC
1
GNDP
1
2
3
4
R31
3
C23
2
0.33uF, 25V
2
1
D10
R39
1
IRFB23N15D
4
MURS120
L1
C
C38
GNDP
GNDP
CH1 OUT
RLY1A
J5
8
1
18uH
R49
4.7
7
MA2YD23
D7
8
Gate Driver
R50
IR2011
R64
10
9.1
C31
470uF, 50V
6
LPF
C30
Q5
0.22uF, 100V
U1
2
U6
9.1
D14
MURS120DICT
C49
Q6
1
D11
IRFB23N15D
MURS120
GNDP
R53 R54 R55 R56
dummy
dummy
dummy
dummy
5
1
2
255-1054 (1)
C51
MKDS5/2-9.5
R61
10, 1W
0.47uF, 100V
3
MA2YD23
D6
VDD_1
0.22uF, 100V
7
D2
R82
10K
U4
0.1uF, 100V
C9
1uF, 16V
U2
4
D1
1N4148
10K
-50V
GNDP
22K
R26
1K
1
8
C6
R10
dummy
C3
R8 10uF, 50V
330K
1N4148
1418-ND
MMBT5401
R28
PROTECT
TP
R11
10K
3
C18
1000pF, 100V
R21
5K
J1
GNDP
2
Input analog
MKDS5/3-9.5
R12
10K
470
1
100
1
2
3
3
0.1uF, 50V
Q2
J3
R37
47mOHM, 2W
C25
0.01uF
R35
Integrator
C17
1000pF, 100V
R23
C
Over Current
GNDP
Level Shift
GNDP
D
+50V
C1
220pF, 100V
+5V
C8
4
GNDP
Speaker output
C33
0.22uF, 100V
GNDP
B
-5V
R44
4.7K
SD1
R47
10
C44
Snubber
1uF, 16V
GNDP
A
3
GNDP
-50_1
GNDP
A
CLASS D REFERENCE BOARD --- CHANNEL 1
Sheet 2
of 4
File:
2
C39
Title
Drawn by:
1
C32
470uF, 50V
R14
10K
-50+VCC
0.22uF, 100V
dummy
C19
Number:
INTERNATIONAL RECTIFIER
www.irf.com
Revision:1.0
EL SEGUNDO, CALIFORNIA, USA
Approved by:
1. ClassD_Refbd_R2-0_CH1.~ch
Date: 23-Sep-2003
4
Time: 15:13:04
System Î Gate Drive Î MOSFET Î Design Example
Class D Amp Reference Board: Layout
Protection
Analog Input
(CH2)
(CH2)
Bus Capacitor
Modulator
HeatSink
±5V
Regulator
MOSFET
(CH1)
MOSFET
(CH1)
Gate Driver
Analog Input
Gate Driver
Modulator
LPF
(CH1)
LPF
(CH2)
Speaker
(CH1)
Speaker
(CH2)
+12V DC/DC
Power Supply
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System Î Gate Drive Î MOSFET Î Design Example
Performance
50W / 4Ω, 1KHz, THD+N=0.0078%
THD+N v.s. Output Power
9.35
10
HP8903B
CH1, f=1KHz, RL=4Ω
VCC=±50.0V
1
fPWM=426KHz
THD 0.1
342W / 4Ω, 1KHz, THD+N=10%
0.01
−3
7.5×101 .10 3
0.1
0.16
1
10
100
Output_Power
•Peak Output Power (f=1KHz)
120W / 8Ω / ch, THD=1%
180W / 8Ω / ch, THD=10%
245W / 4Ω / ch, THD=1%
344W / 4Ω / ch, THD=10%
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3
1 .10
342.3
System Î Gate Drive Î MOSFET Î Design Example
Performance (Cont’d)
Switching waveform
10
10
THD+N v.s. Frequency
HP8903B
CH1, Po=50W, RL=4Ω
1 VCC=±50.0V
fPWM=364KHz
THD 0.1
0.01
−3
7.1×101 .10 3
10
20
100
3
1 .10
Frequency
4
5
1 .10
1 .10
4
2×10
LPF
Residual Noise: 62.5µVrms, A-Weighted,
30KHz-LPF
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Conclusion
•
Highly efficient Class D amplifiers now provide similar
performance to conventional Class AB amplifiers If key components are carefully selected and the layout takes
into account the subtle, yet significant impact due to parasitic
components.
Constant innovation in semiconductor technologies helps the
growing Class D amplifiers usage due to improvements in
higher efficiency, increased power density and better audio
performance.
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