Texas Instruments TPA005D02 - Manual (Page 32)

The TPA005D02 Audio Power Amplifier Evaluation Module
3-18 Details
3.2.4.2 Losses in a Real-World Class D Amplifier
Losses make class D amplifiers nonideal, and reduce the efficiency below
100%. These losses are due to the output transistors having a nonzero
R
DS(on)
, and rise and fall times that are greater than zero.
The loss due to a nonzero R
DS(on)
is called conduction loss, and is the power
lost in the output transistors at nonswitching times, when the transistor is ON
(saturated). Any R
DS(on)
above 0
Ω
causes conduction loss. Figure 3–13
shows an H-bridge output circuit simplified for conduction loss analysis and
can be used to determine new efficiencies with conduction losses included.
Figure 3–13. Output Transistor Simplification for Conduction Loss Calculation
VDD = 5 V
RL
4
Ω
0.31
Ω
0.31 Ω RDS(on)
RDS(off)
RDS(off)
RDS(on) 5 M
Ω
5 M Ω
The power supplied, P
SUP
, is determined to be the power output to the load
plus the power lost in the transistors, assuming that there are always two
transistors on.
Efficiency + h +
I
2
R
L
I 2 2R
DS(on)
) I 2 R
L
Efficiency + h +
P
L
P
SUP
Efficiency + h +
R
L
2R
DS(on)
) R
L
Efficiency + h + 95%
ǒ
at all output levels R
DS(on)
+ 0.1, R
L
+ 4
Ǔ
Efficiency
+ h +
87%
ǒ
at all output levels R
DS(on)
+
0.31, R
L
+
4
Ǔ