Class D Amplifier Peformance Relationship to MOSFET Parameters

Application Note AN-1070
Class D Audio Amplifier Performance Relationship
to MOSFET Parameters
By Jorge Cerezo, International Rectifier
Table of Contents
Page
Abstract ................................................................................................…………2
Introduction ..........................................................................................…………2
Key MOSFET Electrical Parameters in Class D Audio Amplifiers ........…………2
Drain Source Breakdown Voltage BVDSS ....................................…………2
Static Drain-to-Source On Resistance RDS(on).............................…………4
Gate Charge Qg ..........................................................................…………5
Body Diode Reverse Recovery Charge, Qrr ...............................…………8
Internal Gate Resistance RG(int)……………………………………...…….….11
MOSFET Package .......................................................................….…….11
Maximum Junction Temperature ................................................…...…….12
International Rectifier Digital Audio MOSFET ........................................…..……13
Conclusions…………………………………………………………………….…….…14
References...........................................................................................…….……14
This application note discusses key parameters to be considered in the selection of
MOSFETs for class D amplifier and their relationship with amplifier performance such as
efficiency, EMI, and THD. In addition, benefits and advantages of IR Digital Audio MOSFETs
in class D audio amplifier are presented.
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Abstract
This application note discusses key parameters to be considered in the selection of MOSFETs for class
D audio amplifier and their relationship with amplifier’s performance such as efficiency, EMI, and THD. In
addition, benefits and advantages of IR Digital Audio MOSFETs in class D audio amplifier are presented.
Introduction
Class D audio amplifier is a switching amplifier that consists in a pulse width modulator (with switching
frequency in order of several hundred kHz), a power bridge circuit and a low pass filter. This type of
amplifier has demonstrated to have a very good performance. These include power efficiencies over
90%, THD under 0.01%, and low EMI noise levels that can be achieved with a good amplifier design.
Details of class D audio amplifier theory are explained in Application Note “Class D Audio Amplifier
Basics” [4].
Key factors to achieve high performance levels in the amplifier are the switches in power bridge circuit.
Power losses, delay times, and voltage and current transient spikes should be minimized as much as
possible in these switches in order to improve amplifier performance. Therefore, switches with low
voltage drop, fast on and off switching times and low parasitic inductance are needed in this amplifier.
MOSFET have proved to be the best switch option for this amplifier because of its switching speed. It is a
majority carrier device, its switching times are faster in comparison with other devices such as IGBT or
BJT [1], resulting in better amplifier efficiency and linearity.
Key MOSFET Electrical Parameters in Class D Audio Amplifiers.
As in any other application, the MOSFET selection is done in base amplifier specifications. Thus,
information such as amplifier output power and load impedance (i.e. 100W into 8Ω), power bridge circuit
topology (i.e. Full-Bridge or Half-Bridge), and modulation factor (i. e. 80% - 90%) should be known before
the selection process.
•
Drain-Source Breakdown Voltage, BVDSS.
Amplifier operating voltage determines the selection of MOSFET voltage rating. However, other circuit
design related factors should be taken in consideration, such as stray resistance, MOSFET switching
peak voltages because of stray inductance, and power supply fluctuations. Otherwise, the MOSFET can
be in avalanche condition during operation degenerating amplifier performance.
Thus, the minimum BVDSS is chosen by the desired amplifier output power and load impedance, power
bridge circuit topology, modulation factor, and an additional factor related with circuit issues (frequently
used 10% - 50%) as follows:
2 * POUT * RLOAD
BVDSS min (full-bridge conf.) =
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+
M
AN-1070
Additional
factor
due
to
circuit
related issues
2
2*
BVDSS min (half-bridge conf.) =
2 * POUT * RLOAD
M
+
Additional
factor
due
to
circuit
related issues
The following table is an example of minimum MOSFET voltage rating selection for different class D
audio amplifier conditions:
Table 1. Minimum MOSFET voltage rating for different class D audio amplifier conditions.
It is important to choose the lowest BVDSS possible because this parameter is related to others such as
RDS(on). Higher BVDSS results in higher RDS(on) and higher MOSFET power losses. Therefore, this should
be taken into consideration for “factor due to circuit related issues” selection.
In summary, the voltage rating of the MOSFET should be selected according to amplifier operating
voltage. It should also be large enough to avoid avalanche condition during operation.
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•
Static Drain-to-Source On-Resistance, RDS(on).
Amplifier efficiency is related to MOSFET total power losses. These power losses are the result of
MOSFET conduction, switching, and Gate charge losses. Furthermore, the MOSFET’s junction
temperature TJ and heatsink size depend on this power losses amount. High power losses increase TJ,
and therefore, heatsink size.
MOSFET conduction losses are directly related to RDS(on) parameter. RDS(on) is the Drain-Source
resistance, typically specified on datasheet at 25°C with VGS = 10V for standard Gate MOSFETs [5].
RDS(on) along with Drain current define MOSFET conduction losses during amplifier operation, and can be
calculated as follows:
PCONDUCTION = (ID RMS)2 * RDS(on)
RDS(on) is temperature-dependent, increasing when TJ increases. Care must be taken during the thermal
design, regarding this, to avoid thermal runaway. In addition, it is important that the maximum MOSFET
junction temperature TJ max, should not be higher than specified in the datasheet during all amplifier
operating conditions. Then, the maximum MOSFET conduction losses calculation should be done at
maximum amplifier operating conditions, using RDS(on) at TJ max, typically specified in the datasheet, and
maximum ID RMS current.
Therefore, lower RDS(ON) results in lower MOSFET conduction losses and consequently better amplifier
efficiency. This is illustrated in Figure 1.
Lower RDS(ON) ⇒ Lower PCONDUCTION ⇒ Better efficiency
Pout vs. Efficiency
Different RDS(on) Comparison
Similar switching losses
Half-Bridge Configuration, Vbus = +/- 35V, fswitching = 395KHz, finput = 1KHz, Rload = 4Ω
100%
98%
10% THD
96%
1% THD
94%
RDS(on) = 51mOhm
Efficiency
92%
90%
RDS(on) = 95mOhm
88%
86%
84%
82%
80%
0
20
40
60
80
100
120
140
160
180
Pout (W)
Figure 1. Amplifier efficiency using different MOSFET RDS(on) and similar switching losses.
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•
Gate Charge, Qg.
The MOSFET Gate charge Qg is the charge required by the Gate to fully turn-on the MOSFET. This
parameter is temperature-independent and is directly related to the MOSFET’s speed. Lower Qg results
in faster switching speeds and lower Gate losses; consequently, lower switching losses and better
efficiency is achieved.
MOSFET switching losses are defined as:
PTOTAL SWITCHING = PSWITCHING + PGATE
Switching losses are the result of MOSFET turn-on and turn-off switching times illustrated in Figure 2.
Switching losses are calculated by multiplying switching energy Esw, with the amplifier’s PWM switching
frequency fsw.
PSWITCHING = Esw * fsw
And, switching energy Esw is obtained by:
Esw =
∫
t
VDS(t) * ID(t) dt
0
Where, t is the length of switching pulse.
ton
toff
MOSFET Turn -On
MOSFET Turn -Off
Figure 2. MOSFET turn-on and turn-off waveforms.
Acceptable switching losses estimations can be done by using the amplifier’s specifications and the
MOSFET’s datasheet parameters as follow:
PSWITCHING = [0.5 * ID * Vbus * (tr + tf) * fsw] + [0.5 * Coss * Vbus2 * fsw] + [K * 0.5 * Qrr * Vbus * fsw]
Where Vbus is amplifier’s bus voltage, tr and tf are MOSFET rise and fall times, Coss is MOSFET output
capacitance, Qrr is MOSFET body diode reverse recovery charge, and K is the factor due to MOSFET TJ
and specific amplifier’s conditions such as IF and dIF/dt.
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Gate losses can be estimated:
PGATE = 2 * Qg * Vdriver * fsw
Where, Vdriver is voltage of the Gate driver.
Also, amplifier linearity is affected by switching timing errors such as MOSFET turn-on and turn-off delay
times. This implies that the amplifier linearity is also affected by higher Qg. However, these timing errors
caused by MOSFET switching are not as significant when compared to dead time and, can be
significantly reduced by selecting the right dead time value [4]. This is illustrated in Figures 3 and 4.
Pout vs. THD + N
Comparison of different Qg and same dead time
Full Bridge Configuration, Vbus = 40V, fswitching = 800KHz, Rload = 8Ω
THD + N (%)
100.0
Dead Time = 12ns
10.0
MOSFET Qg = 13nC
1.0
MOSFET Qg = 8nC
0.1
1
10
100
Pout (W)
Figure 3. Amplifier THD+N at different MOSFET Qg and same dead time.
Therefore, MOSFET Qg is related to amplifier efficiency and linearity. However, it affects amplifier
efficiency more significantly than linearity. Since linearity can be improved with dead time optimization, Qg
should be lower mainly to achieve lower MOSFET switching losses and increase amplifier efficiency. This
is illustrated in Figure 5.
Lower Qg ⇒ Lower PSWITCHING ⇒ Better efficiency
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Pout vs. THD + N
Comparison of different dead time and same Qg
Full Bridge Configuration, Vbus = 40V, fswitching = 800KHz, Rload = 8Ω
THD + N (%)
100.0
MOSFET Qg = 13nC
10.0
Dead Time = 12ns
1.0
Dead Time = 4ns
0.1
1
10
100
Pout (W)
Figure 4. Amplifier THD+N at different dead time and same MOSFET Qg.
Pout vs. Efficiency
Different Qg comparison, similar R DS(on)
Half-Bridge configuration, Vbus = +/- 35V, fswitching = 395KHz, Rload = 4Ω
100%
98%
1% THD
96%
10% THD
Qg = 9nC
Efficiency
94%
92%
Qg = 13nC
90%
88%
86%
84%
82%
80%
0
20
40
60
80
100
120
140
160
180
Pout (W)
Figure 5. Amplifier efficiency at different MOSFET Qg and similar RDS(ON).
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•
Body Diode Reverse Recovery Charge, Qrr.
Intrinsically by design, the MOSFET structure has a built in reverse diode, and its reverse recovery
characteristics are related to amplifier performance as well. Reverse recovery charge Qrr, is defined as
the area under Irr during trr. This is illustrated in Figure 6. Qrr is mainly determined by IF and dIF/dt. It is a
temperature dependent parameter, increasing when TJ increases.
Qrr affects amplifier efficiency and EMI performance. The relationship with efficiency is due to the power
bridge circuit configuration. During operation, the reverse recovery current generated by the MOSFET’s
body diode after commutation current, flows also in the complementary MOSFET of bridge circuit,
causing an increase in turn-on switching losses due to the current increment. This is illustrated in figure
7. Details of these switching events are explained in Application Note “Using HEXFET III in PWM
Inverters for Motor Drives and UPS Systems” [2].
IF
Figure 6. Typical MOSFET body diode reverse recovery waveforms.
However, similar to amplifier linearity, the selection of right dead time can improve efficiency due to Qrr
reduction. Reduction of dead time causes the commutation current to flow most of the time into the
MOSFET channel, reducing MOSFET body diode current pulse width, and hence, minority carrier charge
and Qrr. Still, smaller dead time may cause shoot through current, as shown in Figure 8. This is a risky
condition for power bridge MOSFETs and also it degenerates the amplifier performance [4]. Therefore,
optimized dead time can help to reduce Qrr and improve efficiency and linearity as well. This is illustrated
in Figure 9.
Qrr is also related with amplifier EMI performance. High recovery current dIrec/dt (i. e. faster tb) generates
high dVrec/dt, and this results in large high-frequency current and voltage ringing transients in the
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MOSFET due to stray inductances and capacitances in the amplifier circuit, increasing radiated and
conducted EMI noise. Therefore, a smaller and soft recovery is fundamental to avoid these transients to
improve EMI performance. Details of this EMI and reverse recovery relationship are explained in White
Paper “Ultra-fast Recovery Diodes Meet Today Requirements for High Frequency Operations and Power
Ratings in SMPS Applications” [3].
Hence, smaller and soft reverse recovery improves amplifier efficiency and EMI performance, due to
reduction in MOSFET switching losses and current-voltage transient ringing.
Smaller and soft Qrr ⇒ Lower PSWITCHING ⇒ Better efficiency and EMI performance
Q
Drive current
Q
Commutation
current
a) Amplifier’s drive and commutation current
Drive current
Commutation current
b) Load current (Iload = Idrive +
c) High side Q1 MOSFET
Pon
VDS
Peak current due to Qrr
of complementary
MOSFET's Body Diode
ID
d) MOSFET turn-on during drive current
Figure 7. MOSFET turn-on waveforms during drive current.
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Qst as a function of Overlap Tim e & Rg
Vbus = 60V, Id = 2A, Vgs = 12V
(Overlap tim e m easured from 50% Vgs high side fall to 10% Vgs low side rise)
120
Low Side
Side VGS
VGS
High Side VGS
100
Qst (nC)
80
Shoot Through
Current (2A/div)
Rg=1Ohms
60
Rg=5Ohms
Rg=10Ohms
Device Qg = 37nC
40
Qst = ∫ Ist(t)dt
20
0
-10
-5
0
5
10
15
20
Overlap time (ns)
Figure 8. Example of shoot through charge [Qst = ∫ Ist(t)dt] as a function of dead time.
Pout vs. Efficiency
Comparison of different dead time
Full Bridge Configuration, Vbus = 40V, fswitching = 800KHz, Rload = 8Ω
95.0%
10% THD
Efficiency
90.0%
Dead Time
12ns
Dead time
4ns
85.0%
Icommutation
Icommutation
80.0%
Qrr
Qrr
75.0%
0
10
20
30
40
50
60
70
80
Pout (W)
Figure 9. Amplifier efficiency at different dead times.
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•
Internal Gate Resistance, RG(int).
Internal Gate Resistance RG(int) is a temperature dependent parameter, increasing when temperature
increases. This parameter affects MOSFET on and off switching times. Higher RG(int) increases total Gate
resistance, decreases Gate current (as shown in Figure 10), increases switching times, and thus,
MOSFET switching losses. Furthermore, a large variation of RG(int) affects dead time control. Therefore,
RG(int) parameter distribution should be taken in consideration for amplifier performance tolerances.
D
IG
RG(ext)
G
RG(int)
RGtotal = RG(ext) + RG(int)
S
Figure 10. Total Gate resistance, RGtotal = RG(ext) + RG(int).
•
MOSFET Package.
The selection of MOSFET package is of particular importance because it can significantly affects the
amplifier performance and cost. Packages characteristics such as dimensions, power dissipation
capability, current capability, internal inductance and resistance, electrical isolation and mounting process
can all be significant to define PCB and heatsink size, assembly process, and MOSFET electrical
parameters.
It is well known the importance of package thermal resistance RθJC in MOSFET performance. Mainly,
because lower RθJC reduces MOSFET junction temperature TJ during operation, improving the MOSFET
reliability and performance.
Pmax = ΔTJ / RθJC max
Again, stray inductances and capacitances in amplifier circuit affect EMI performance. Internal package
inductance can make a significant difference in EMI noise generation. Figure 11 shows EMI noise
comparison of two packages with same MOSFET die and different internal inductance, DirectFETTM
MOSFET (<1nH) vs. TO-220 (~ 12nH). It can be observed in this comparison that the DirectFET
MOSFET shows better EMI performance. Approximately 9dB lower noise than TO-220, even when the
DirecFET MOSFET rise and fall times are approximately three times faster than TO-220.
In summary, the right package selection improves amplifier reliability, performance and cost
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Test Conditions: Half-Bridge Configuration, No Heatsink, Vbus = +/- 35V,
fsw = 395kHz, Pout = 12.5W, Rload = 4 Ohms,
Input = 250mVrms Sinusoidal @ 1kHz
Note: Not shielded test room.
Both Packages are tested with the same MOSFET die
TO-220
DirectFET
a) DirectFET package EMI noise.
b) TO-220 package EMI noise.
Frequency (MHz)
Frequency (MHz)
c) DirectFET VDS rise and fall times
d) TO-220 VDS rise and fall times
Figure 11. Amplifier conducted EMI performance comparison. DirectFET vs. TO-220
•
Maximum Junction Temperature, TJ.
The maximum Junction Temperature TJ max is a parameter that is not directly related with the amplifier
performance. However, special highlight is done because it is significant to define heatsink size.
MOSFET devices with higher TJ max, allow handling higher power losses, and hence, allow smaller
heatsink reducing amplifier size and cost.
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International Rectifier Digital Audio MOSFET.
IR Digital Audio MOSFETs are devices specifically designed for class D audio amplifier applications. Its
parameters are optimized to improve amplifier performance and this goal is achieved by selecting the
right die size for specific amplifier specifications.
As mentioned before, RDS(on) and Qg are key parameters that determine MOSFET power losses. These
parameters are related to MOSFET die size and there is a tradeoff between them. A larger MOSFET die
size means lower RDS(on) and higher Qg, and vice versa.
Bigger Die Size ⇒ Lower RDS(ON) & Higher Qg ⇒ Lower PCONDUCTION & Higher PSWITCHING
Smaller Die Size ⇒ Higher RDS(ON) & Lower Qg ⇒ Higher PCONDUCTION & Lower PSWITCHING
Therefore, the optimal die size will results in lower MOSFET power losses. An example of this is
illustrated in Figure 12.
Optimal MOSFET die size ⇒ Optimal MOSFET Parameters
3.5
3.0
Total Losses = Conduction Losses + Switching Losses
Power Losses (W)
Total Losses
2.5
Optimal MOSFET
Die Size
2.0
1.5
1.0
Switching
Losses
Conduction
Losses
0.5
0.0
1
10
Normalized
Size
NormalizedDie
Die Size
Figure 12. Example of IR Digital Audio MOSFET die size selection.
Furthermore, IR Digital Audio MOSFETs have a guaranteed maximum RG(int), low Qrr, a 175°C maximum
TJ, and are assembled in most efficient packages such as DirectFET MOSFET. These features combine
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to make this MOSFET a highly efficient, robust, and reliable device for class D audio amplifier
applications.
Conclusions
There are key MOSFET parameters such as BVDSS, RDS(on), Qg, Qrr, RG(int), TJ max and packaging that are
related with Class D audio amplifier performance, size and/or cost. The right selection of these
parameters can improve the amplifier performance and reduce its size and/or cost. IR Digital Audio
MOSFETs are devices in which those key parameters have been optimized to achieve the best overall
amplifier performance.
References
1. Barkhordarian V. “Power Mosfet Basics”. International Rectifier Application Note.
<http://www.irf.com>.
2. Grant D. “Using HEXFET III in PWM Inverters for Motor Drives and UPS Systems”
International Rectifier Application Note. <http://www.irf.com>.
3. Guerra A., Andoh K. and Fimiani S. “Ultra-fast Recovery Diodes Meet Today Requirements
for High Frequency Operations and Power Ratings in SMPS Applications”.
International Rectifier White Paper. <http://www.irf.com>.
4. Honda J. and Adams J. “Class D Audio Amplifiers Basics”. International Rectifier Application
Note. <http://www.irf.com>.
5. Pearce R., Brown S. and Grant D. “Measuring HEXFET® Characteristics”. International
Rectifier Application Note. <http://www.irf.com>.
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