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Texas Instruments Slope Compensation Design for UCC2897A Application notes
Application Report
SLUA837 – October 2017
Slope Compensation Design for UCC2897A
Frank Xiao ................................................................................................................. Field Application
ABSTRACT
The UCC2897A is a peak-current mode fixed frequency high performance pulse-width modulator, which is
ideal for active-clamp forward for the capability of driving the P-channel auxiliary switch. The chip also has
internal programmable slope compensation circuit which is important for peak-current mode control
method. In addition, this IC can set precise Dmax limit. In order to avoid sub-harmonic oscillation, slope
compensation is essential for peak-current mode when duty cycle is above 50%. Even though the duty
cycle is below 50%, it is desirable to add a slope compensation to decrease the influence of noise. This
application report will show why the sub-harmonic oscillation happens and how to design the slope
compensation parameters. Finally, experiment results are given which verify the correctness of the
theoretical analysis.
1
2
3
4
5
6
Contents
Introduction ...................................................................................................................
Slope Compensation Theory ...............................................................................................
UCC2897A Slope Compensation Design.................................................................................
Experiment ....................................................................................................................
Summary ......................................................................................................................
References ...................................................................................................................
1
2
5
7
7
7
List of Figures
1
Inductor Current Waveform With Disturbance ........................................................................... 2
2
Inductor Current and Slope Compensation Waveform
3
3
Inductor Current and Slope Compensation Waveform
4
5
................................................................
................................................................
Figure 4. UCC2897A Function Block Diagram ..........................................................................
UCC2897A 18-V Input (Blue is the Vcs, Green is the Inductor Current) ..............................................
1
Bench Results ................................................................................................................ 7
4
5
7
List of Tables
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
The UCC2897A is a peak-current mode controller which is suitable for active clamp forward topology. For
the peak-current mode control, it is usually necessary to adopt slope compensation to cancel the subharmonic oscillation phenomenon. Too large or too small slope compensation may influence the cycle-bycycle current limit accuracy. Therefore, the report gives a method for parameter design based on the slope
compensation theory.
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1
Slope Compensation Theory
2
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Slope Compensation Theory
Because the forward converter is derived from buck topology by incorporating transformer isolation, buck
converter can be used in the slope compensation analysis.
2.1
Sub-harmonic Oscillation
Figure 1 is inductor current waveform. It can be seen from the waveform that when a disturbance
happens, it still exists after a period. In order to stabilize the system operation, the disturbance should
become smaller and finally disappear after a few switching cycles.
i
E
lC
l2
T
l1
l1'
D1Ts D2Ts
l2'
T
t
Figure 1. Inductor Current Waveform With Disturbance
Calculation can be performed by using trigonometric formula as follow.
l 1 = D 1×Ts × tan
l 2 = D 2 ×Ts × tan
l 2 l 1 = ( D 2 D 1 )×Ts ×tan
'
(1)
(2)
(3)
'
l 2 - l 1 = ( D 2 - D 1 ) × Ts × tan
(4)
If the disturbance becomes smaller after one cycle, then:
ûO (D 2 - D 1 )×Ts × tan
=
>1
ûO'
' 2 ' 1 ×Ts × tan
(1- D )Vin >DVin
D < 0.5
(5)
(6)
(7)
It can be seen from the formula above that if the duty cycle is below 50%, the disturbance is reduced after
one cycle. Similarly, if the duty cycle is above 50%, the disturbance is magnified, which leads to system
instability. Therefore, proper slope compensation should be used to eliminate the problem.
2
Slope Compensation Design for UCC2897A
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Slope Compensation Theory
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2.2
Slope Compensation Function
If the inductor current rising, falling and the compensation slopes are set to be m1, m2 and m’, respectively
then the following formulas could be used to design system parameters when we want to cancel the subharmonic oscillation phenomenon.
I
m'
I L0
m1
m2
DTs
I L (T )
T
t
Figure 2. Inductor Current and Slope Compensation Waveform
m 2 - m'
m 1 + m'
<1
(8)
'
1-
m
m2
1- D m '
+
D m2
<1
(9)
'
1-
m
m2
1- D m '
1- D
+ 2
D
m id higher.
If D increases, then D decreased, which means that
Therefore, Dmax is the worst case.
If D = 1, the turning point is at m’ = 0.5 m2.
From the waveform and formulas above, if m’ = 0.5 m2, the disturbance decreases cycle-by-cycle and
finally disappear. Especially, if m’ = m2, the disturbance will vanish after one cycle (deadbeat control).
If Dmax is smaller, m’ can also set to be lower. If the Dmax = 0.66, m’ should be higher than 0.24 m2,
correspondingly.
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Slope Compensation Theory
2.3
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Accuracy of the Output Current
For buck topology, sometimes enough slope compensation is helpful to achieve current accuracy.
However, if the slope compensation is too large, it also influences the accuracy of output current.
I
m'
I L0
m1
m2
DTs
I L (Ts )
T
t
Figure 3. Inductor Current and Slope Compensation Waveform
When consider Ic0 as the peak current without slope compensation (the dotted line), Iavg is the average
current, m’ is the slope compensation, so:
i L ( DTs ) = I c0 -m 'DTs
'
(10)
'
i L ( Ts ) = I c0 -m DTs -m 2 D Ts = i L ( 0)
m 2 (1- D ) Ts
I avg = I c0 -m ' DTs 2
m 2 Ts § m 2 - 2m ' ·
I avg = I c0 +¨
¸¸ DTs
¨
2
2
©
¹
m2
m' >
When set
2 , if D↑, the Iavg will ↓.
m
2
m' <
When set
2 , if D↑, the Iavg will ↑.
m2
m' =
When set
2 .
m 2 Ts
I avg = I c0 2
(11)
(12)
(13)
(14)
From the waveforms and formulas above, it can be seen that when m’ = 0.5 m2, the output current does
not change with input voltage. If m’ < 0.5 m2, the output inductor current will be larger for low input voltage
condition, and if m’ > 0.5 m2, the output inductor current will be larger for high input voltage condition.
From all the analysis above, the m’ can be chosen to be 0.5 m2 in the slope compensation design
4
Slope Compensation Design for UCC2897A
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3
UCC2897A Slope Compensation Design
From the analysis and the function block diagram of the UCC2897A above, the following formulas can be
derived.
VIN
1
N/C
2
2.5 V
VREF
20 N/C
IRDEL
0.05 x
IRDEL
VREF
RDEL 3
19 LINEOV
0.05 x
IRDEL
CLOCK
2.5 V
ICHG
+
Start
LineOV
RTON 4
+
LineUV
1 -DMAX
2.5 V
1.27
V
±
End
±
1.27
V
+
13 V / 8 V
IDSCHG
18 LINEUV
VDD
±
CT
RTOFF 5
OUT
SYNC
VDD
VREF
PWM
OFF
17 VDD
16 PVDD
IRDEL
OUT
REF
GEN
VREF 6
SYNC
Q
RD
Q
8
15 OUT
Turn-on Delay
VREF
IRDEL
+
7
0.5 V
GND
S
14 AUX
±
+
Turn-on Delay
VREF
13 PGND
VREF
0.43 x ICHG
5 X ISLOPE
12 SS/SD
+
CS 9
CT
1 - DMAX
ISLOPE
4xR
±
R
+
RSLOPE 10
VDD
VREF
LineUV
LineOV
0.43 x ICHG
UVLO & SS
Enable
11 FB
±
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Figure 4. Figure 4. UCC2897A Function Block Diagram
The slope compensation insertion current Islope:
I slope =
2
Tonmax R slope
(15)
So the slope compensation insertion voltage Vslope:
Vslope = 5I slope R 2
m sec =
mp =
(16)
Vout
L
m sec
(17)
Np Ns
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(18)
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UCC2897A Slope Compensation Design
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Vcs = m p R cs
(19)
The m’ = 0.5 m2 can derive the following formula:
Vslope =
Vcs
2
Vout
5×2×2
R2=
N s R cs
Tonmax R slope
LN p
(20)
(21)
So the Rslope should be:
R slope =
20R 2 LN p
Tonmax Vout N s R cs
(22)
In the formulas, Tonmax is the max primary side mosfet turn on time, Rcs is the primary side current sample
resistor, R2 is the VRcs filter resistor, L is the output inductor, Np is the primary side turns of transformer,
and Ns is the second side turns of transformer.
If Rslope is a standard value, it can be selected based on the Tonmax to cancel the sub-harmonic oscillation.
V ''slope + Vcs1 + Vcs2 = 0.43v
(23)
Vslope is the slope compensation voltage added, Vcs1 is the primary side current resistor voltage derived by
second side load current, and Vcs2 is the primary side current resistor voltage derived by the magnetic
inductance field current.
Vout (1- D ) T
û,
2L
2
û, · N s
§
Vcs1 = ¨ I o +
R cs
¸
2 ¹ Np
©
Vin
Vcs2 =
R cs DT
2L m
2×5R 2
V ''slope = Vslope Ton =
D
R slope D max
10R 2
R slope D max
6
=
D+
ª Vout (1- D ) T
º Ns
V in
R cs DT + «
+Io»
R cs = 0.43
2L m
2L
¬
¼ Np
Slope Compensation Design for UCC2897A
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(24)
(25)
(26)
(27)
(28)
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Experiment
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4
Experiment
The application report uses the PMP6547 to verify the theory, and does the changes below. Change the
Rcs to 50 mΩ, and change the Rslope to 68 kΩ.
Figure 5. UCC2897A 18-V Input (Blue is the Vcs, Green is the Inductor Current)
The bench results are similar to the calculation results, which verifies the analysis above. In this case, the
document sets the slope compensation with margin. If the customers want to get better accuracy
performance, a larger value for Rslope can be chosen.
Table 1. Bench Results
INPUT VOLTAGE
5
CALCULATION RESULT IO
TEST RESULT IO
18 V
5.2 A
5A
48 V
5.37 A
5.78 A
60 V
5.39 A
6.09 A
Summary
UCC2897A is an ideal peak current mode controller for active-clamp forward. For peak current mode
controller, slope compensation is needed to cancel the sub-harmonic oscillation phenomenon. The
application report shows why the sub-harmonic oscillation happens and how to design the slope
compensation parameters. Finally, the report does experiments with PMP6547. Experimental results show
the accuracy of the theoretical analysis.
6
References
•
•
Datasheet: UCC2897A Current-Mode Active Clamp PWM Controller, SLUS829.
Application Report: Understanding and Designing an Active Clamp Current Mode Controlled Converter,
SLUA535.
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