Transcript
14/05/2013
Feedback Control Design in Regulated DC Power Supplies
Three General Amplifier types C2 R3
• Voltage mode FB control design example • Peak-Current-Mode Control • Control in Discontinuous-Conduction Mode (DCM)
C3
R2
C1
(verr ) R1
vc
Type 1 Type 3
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Type 2
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Current Mode Control
Limitation of Current Mode
• Difference I mode vs V mode
• Sense a current using a resistor (CT going to a resistor) • Feed it into PWM IC • However as load current decreases the magnitude of the signal must naturally decrease. • If load is light the current signal will be negligible and the current feedback loop has no effect on the system. So, current mode control becomes 4 voltage mode control at light loads.
– Current mode has two feedback loops; one to control the inductor current, the other to control the capacitor (output) voltage – Control system can be same as V mode; but for practical reasons, in inner loop, controlling the inductor current acts to remove its effect in the power stage. – Hence no resonant tank to worry about. – So only a single pole of output cap at high frequencies 3
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RHP Zero – Non minimum phase systems
RHP Z
• Inductor current CANNOT instantly change • Also duty cycle slew-rate is limited by available transient average voltage across the inductor • Solution: clamping the duty cycle speed of change by providing a means to follow the output current
• Poles and zeros are normally on the left half plane of s-plane • Poles and zeros are introduced to cancel each other • RHP zero has same 20 dB/dec raising gain magnitude as normal zero but with 90deg phase lag instead of lead • Designed is forced to roll off the loop gain at low frequency.
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RHPZ effect • Consider a flyback with load current increases – The output voltage drops instantaneously – To supply more power the feedback increases the duty cycle of the transistor to store more energy in inductor – Inturn the energy is not delivered to the load on time as the switch need to be off – This cause output voltage to drop further – Keep dropping if loop is not designed properly – This is RHP Zero; increases duty cycle, decreases voltage. 7
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Solutions? • Bandwidth of the converter to be designed such that RHP Z occurs at much higher frequencies than the bandwidth • Beware that zeros moves with loads • I mode Compensator design: – First pole at origin – Zero at 1/5 the selected crossover freq – Second pole at ESR zero of cap or RHP Zero whichever is lower 9 – Wise crossover frequency
PEAK-CURRENT MODE CONTROL
Peak-Current-Mode Control, and
Average-Current-Mode Control.
ivp
Vin
iL vvp
Q
vcp
vo
S
Clock
R
Flip-flop
Comparator
Slope Compensation iL*
ic
vo* Controller
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Slope Compensation
solution
• When the duty cycle of the current mode converter exceeds 50%, the converter will oscillate at a subharmonic of the switching frequency • Similar to the effect of RHP zero effect but at subharmonics • Current mode control, with duty cycle over 50% proves it
• Adding fixed ramp to the current signal • Constant value; so effects of variation in current are dampened • The effect is more like to make it as voltage mode control
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ivp
ivp
Vin
iL
Vin
iL
vvp
Q
vvp
vcp
vo
vcp
vo
S R
Flip-flop
Slope Compensation
Clock iL*
ic
vo*
Q
Controller
S
Clock
R
Comparator
Flip-flop
Slope Compensation iL*
ic
vo* Controller
Comparator
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sLD R(1- D) 1 (1 srC ) R(1 - D)2 vo (s) sRC iL (1 D) 1 1 D
Example 4-4 In this example, we will design a peak-current-mode controller for a Buck-Boost converter that has the following parameters and operating conditions: L 100 H , C 697 F , r 0.01 , f s 100 kHz , Vin 30V .
The output power
Po 18W in CCM and the duty-ratio D is adjusted to regulate the output voltage Vo 12V . The phase margin required for the voltage loop is 600 . Assume that in the voltage feedback network, k FB 1 .
20
0
GPS ( s ) dB .
-20
29.33dB
-40 DB(V(V_out)/I(L1)) 0d
GPS ( s ) |deg -50d
900 SEL>> -100d 1.0Hz 3.0Hz P(V(V_out)/ I(L1))
10Hz
30Hz
100Hz
300Hz Frequency
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1.0KHz
3.0KHz
10KHz
f c 5 kHz
30KHz
100KHz
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PSpice Modeling:
Gc ( s ) 20
kc s
1 s / z
1 s /
fc
fp
fz f p
fz
p
tan 45o boost 2
GC ( s) f GPS ( s) f 1 c
0
c
-20
SEL>> -40 DB(V(V_out)/I(L1)) 0d
-50d
-100d 1.0Hz 3.0Hz P(V(V_out)/I(L1))
10Hz
30Hz
100Hz
300Hz Frequency
1.0KHz
3.0KHz
10KHz
30KHz
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100KHz
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C2 (vo vo* )
R2
C1
R1 vc
R1 10 k C2
z
p R1kc
30 pF 12.026V
C1 C2 p / z 1 380 pF
vo (t ) 12.000V
vo (t )
R2 1/(z C1 ) 315 k 11.960V
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20 11.931V 2.5ms V(Vo)
2.6ms 2.7ms AVGX(V(Vo),10u)
2.8ms
2.9ms
3.0ms
3.1ms
3.2ms
3.3ms
3.4ms
3.5ms
Time
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FEEDBACK CONTROLLER DESIGN IN DCM
Simulation Results
PSpice Modeling: 80
40
CCM DCM 0
SEL>> -40 DB(V(V_out)) 0d
DCM
CCM
-100d
-200d 1.0Hz
3.0Hz P(V(V_out))
10Hz
30Hz
100Hz
300Hz
1.0KHz
3.0KHz
10KHz
30KHz
100KHz
Frequency
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Summary
Concept Quiz
Feedback Control Design in Regulated DC Power Supplies • RHP Zero • Peak-Current-Mode Control • Control in Discontinuous-Conduction Mode (DCM)
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The crossover frequency in the peakcurrent-mode control can be selected to be higher than that in the voltage-mode control, because of the phase angle of the power stage in the control bock-diagram below? A. True B. False
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