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参数资料
| 型号： | AN1042 |
| 厂商： | ON SEMICONDUCTOR |
| 英文描述： | High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs |
| 中文描述： | 高保真开关音频放大器使用的TMOS功率MOSFET |
| 文件页数： | 6/12页 |
| 文件大小： | 108K |
| 代理商： | AN1042 |
AN1042/D
http://onsemi.com
6
reduce the output to zero. At frequencies above 8 kHz, filter
phase shift makes the current limiting ineffective. This will
not be a problem unless the output is short circuited during
high frequency sine wave testing. R30 through R33 set the
bias currents for the operational amplifiers and
comparators.
The efficiency of a class B amplifier at the point of
clipping is
4
or 78.5%. As the output is reduced the
efficiency linearly drops to 0% with no voltage out. The
average power of music integrated over one second has
been measured by the author at one tenth of peak power.
This does not seem to vary appreciably with different music
or speech as long as they are continuous. Under these
conditions, a class B amplifier will have an effective
efficiency of 25%. Figure 9 shows a plot of heatsink power
loss for a class B amplifier and a switching amplifier as a
function of output power. Note that a class B amplifier
actually runs hottest at slightly less than half power.
Maximum heating occurs at 40% of maximum power. The
heat rise varies only 25% as the power changes from 10%
to 90% of maximum.
As a result, a switching amplifier has one–tenth of the
heatsink requirements of a class B amplifier. Its greater
efficiency allows it to use a power supply of one–fourth the
size of a class B amplifier power supply. The author used
a switching power supply operated off 120 vac line at 20 kHz.
If a switching power supply is used, proper shielding must
be provided to prevent pickup of power supply spikes by
sensitive portions of the amplifier. A discussion of power
supplies is beyond the scope of this paper.
Switching amplifiers have a little known property of
power supply buss runaway when producing dc or low
frequency ac. The origin of this problem can be understood
by referring to Figure 10. It shows the current in the positive
switch when a sine wave just short of clipping is produced
by the amp. During the first half cycle, the switch is on most
of the time and power is delivered to the load, with some
energy being stored in the output inductor. During the
second half cycle, the switch is off most of the time and
current flow is reversed through the switch. This reverse
current comes from the output inductor, which is returning
energy to the positive supply through the source drain diode
of Q3. The forward current of the first half cycle tends to
drop the positive supply voltage, and the reverse current of
the second half cycle will raise it.
It can be shown that the current averaged during the
switching cycle in the positive switch is
sinx
This function is shown by the dashed curve in Figure 10.
The current averaged during the first half cycle of the
4
4
current. The average current during the second half output
4
4
sin2x
2
.
output sine wave is
, which is 0.5683 times peak
of the cycle is
, which is 0.0683 times peak.
Figure 9. Power Loss versus Output Power
in Class B and Switching Amplifier
0
0.10
0.20
0.25
P
0
Output Power (Normalized)
0.2
0.4
0.6
0.8
1.0
0.15
0.05
Class B Amplifier
Theoretical Switching Amplifier
(0.3
Output Impedance, 8
Load)
The average current during the complete cycle is 0.250,
being half the average power of a sine wave referenced to
peak power. Average reverse current through the switch peaks
at 0.125 of average peak forward current. This will cause
the voltages in a conventional supply to build to destructive
levels in short order, unless the power source is a battery.
Figure 10 only applies to a switching amp that is operated
just short of clipping with a normal load. If the amp is
operated into a short, conditions worsen. The average
forward and reverse currents will both be 0.500. This
means that no net power will be taken from the supply when
averaged over the whole cycle. This reflects the fact that no
power can be delivered into a short circuit.
To accommodate shorts on the output of the amplifier
without generating dangerous voltages, special power
supply circuitry must be used. These circuits must be able
to handle reverse currents on each buss equal to one–half
of the peak short circuit output current. Conventional
rectifier based supplies will not tolerate reverse current for
sustained periods. Large filter capacitors help but they
merely postpone the inevitable reckoning. A better
solution is coupling between the positive and negative
power supply busses.
Figure 10. Supply Buss Runaway
–
°
1.0
0.75
0
0.5
0.25
–1.0
–0.75
–0.5
–0.25
0
°
3
°
6
°
9
°
1
°
1
°
1
°
2
°
2
°
2
°
3
°
3
°
3
°
3
°
4
°
Switching Frequency = 9X Modulation Frequency
Positive Switch Current
sinx
sin2x
2
相关PDF资料 |
PDF描述 |
|---|---|
| AN1042D | High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs |
| AN1062 | Circular Connector; No. of Contacts:6; Series:MS27508; Body Material:Aluminum; Connecting Termination:Crimp; Connector Shell Size:8; Circular Contact Gender:Socket; Circular Shell Style:Box Mount Receptacle; Insert Arrangement:8-35 RoHS Compliant: No |
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| AN1082 | Using the ISL6401 RSLIC PWM Controller Evaluation Board |
| AN1406 | DESIGNING WITH PECL (ECL AT + 5.0) |
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| AN1042D | 制造商:ONSEMI 制造商全称:ON Semiconductor 功能描述:High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs |
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