鈭?/div>
Hz
f
P2
100 k
1M
Figure 17. Type III Compensation Bode Plot
The high frequency gain and the break (pole and zero) frequencies are calculated using the following equations.
R
Z1
)
R
SET
V
O
+
V
FB
R
SET
(49)
R
SET
+
GAIN
+
R
SET1
R
SET2
R
SET1
)
R
SET2
R
PZ2
R
Z1
R
P1
R
Z1
)R
P1
1
R
P1
C
PZ1
C
Z2
^
2p
1
R
PZ2
C
P2
(51)
(52)
(53)
(54)
(50)
f
P1
+
f
P2
+
f
Z1
+
f
Z2
+
2p
2p
2p
2p
C
P2
)
C
Z2
R
PZ2
C
P2
1
R
Z1
C
PZ1
1
R
PZ2
)
R
P1
C
Z2
^
2p
1
R
PZ2
C
Z2
(55)
Using this PWM and L-C bode plot the following actions ensure stability.
1. Place two zero鈥檚 close to the double pole, i.e. f
Z1
= f
Z2
= 3559 Hz
2. Place a pole at one octave below the desired crossover frequency. The crossover frequency was selected as
one quarter the switching frequency, f
CO
= 100 kHz, f
P1
= 50 kHz
3. Place the second pole about an octave above f
co
. This ensures that the overall system gain falls off quickly to
give good gain margin, f
P2
= 200 kHz
4. The high-frequency gain is sufficient to ensure 0 dB at the required crossover frequency, GAIN = -1 脳 GAIN
of PWM and LC at the crossover frequency, GAIN = 17.6 dB, or 7.586
Desired frequency response and resultant overall system response can be seen in
Figure 18.
29