Datasheet

UCC28050, UCC28051
UCC38050, UCC38051
SLUS515F−SEPTEMBER 2002 − REVISED MARCH 2009
15
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APPLICATION INFORMATION
A controller that has integral control at low frequencies requires a zero near the crossover frequency in order
to be stable. The resulting g
M
amplifer configuration is shown in Figure 3.
UDG−02122
V
REF
V
OUT
+
R
V1
C
V1
C
V2
Figure 3. g
M
Amplifier Configuration
The compensator transfer function is:
A
V
+
g
M
C
V1
) C
V2
1 )
ǒ
R
V1
C
V1
s
Ǔ
s
ǒ
1 )
ǒ
R
V1
ƪ
C
V1
C
V2
ƫ
ƪ
C
V1
)C
V2
ƫ
Ǔ
s
Ǔ
where g
M
= dc transconductance gain = 100 μs
The limiting factor of the gain is usually the allowable third harmonic distortion, though other harmonics can
dominate. The crossover frequency of the control loop will be much lower than twice the ac line voltage. In order
to choose the compensator dynamics, determine the maximum allowable loop gain at twice the line frequency
and solve for capacitor C
V2
. This also determines the crossover frequency.
C
V2
+
ǒ
V
AC(max)
4p f
AC
Ǔ
2
ǒ
g
M
k
1
V
OUT(avg)
R
S1
k
(crm)
C
OUT(max loop gain @ 2 f
AC
)
Ǔ
f
CO
+
V
AC
p
g
M
k
1
C
V2
V
OUT
R
S1
k(cmr) C
OUT
Ǹ
Select C
V1
so that the low frequency zero is one-tenth of the crossover frequency.
C
V1
+ 9C
V2
Select R
V1
so that the pole is at the crossover frequency.
[
1
2 p f
CO
C
V2
(14)
(15)
(16)
(17)
(18)