Texas Instruments | UCC2897A Peak Current Mode Active Clamp Forward Converter Small Signal Modeling | Application notes | Texas Instruments UCC2897A Peak Current Mode Active Clamp Forward Converter Small Signal Modeling Application notes

Texas Instruments UCC2897A Peak Current Mode Active Clamp Forward Converter Small Signal Modeling Application notes
Application Report
SLUA702 – January 2014
UCC2897A Peak Current Mode Active-Clamp Forward
Converter Small-Signal Modeling Design Consideration
Power Management/Field Applications
Tony Huang
ABSTRACT
UCC2897A is a peak current mode active-clamp controller. This paper discussed the modeling process and
loop compensation for UCC2897A. An example has been implemented with the modeling and compensation.
Contents
1
UCC2897A Introduction:..............................................................................................................1
2
Peak Current Mode Small-Signal Circuitry..................................................................................2
3
Active Clamp Peak Current Modeling Analysis: .........................................................................4
4
A Design Example: .......................................................................................................................7
5
Conclusion: .................................................................................................................................10
Reference: ...........................................................................................................................................10
FIGURES
................................................................................2
Figure 1.
UCC2897A Internal Block Diagram.
Figure 2.
Control Block for Peak Current Mode.
Figure 3.
Gain from Control to Inductor Current for Peak Current Mode
Figure 4.
Peak Current Mode Active Clamp Forward Converter Topology.
.................................................6
The Peak current active clamp control block diagram.
...................................................7
Small Signal Implementation from Control to Output.
..................................................7
100-W Isolated Power Module with UCC2897A Device
..............................................................8
Overall Small Signal Circuitry Implementation
..........................................................................................................9
Simulation Results.
..............................................................................10
Lab Test Results (Vin = 72, Io = 30 A)
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
1
............................................................................3
.....................................4
.................................4
UCC2897A Introduction:
The UCC2897A PWM controller simplifies implementation of the various active clamp/reset and synchronous
rectifier switching power topologies. The UCC2897A is peak current-mode, fixed frequency, high performance
pulse width modulator. It includes the logic and the drive capability for the P-channel auxiliary switch along with
a simple method of programming the critical delays for proper active clamp operation, as showed in Figure.1.
1
SLUA702
Figure 1.
UCC2897A Internal Block Diagram.
2 Peak Current Mode Small-Signal Circuitry.
A peak current-mode converter, with continuous current mode, introduces a sampling-hold function, as shown
in Equations 1 and 2. Figure 2 shows the control block for peak current-mode:
H e (s) =
1 − e − S ×t SW
≈
s × t SW
H vi ( S ) =
2
1
1+
I L (S )
D
≈
VI ( S ) SLm
2
s
t SW
+
s2
π2
;
(1)
2
t SW
(2)
SLUA702
Figure 2.
Control Block for Peak Current Mode.
Then, the gain from the error amplifier output to the inductor current can be got:
^
iL
^
uc
= H (S )
H (S ) ≈
1
Ri
1
T L ( S + S e ) Ts
T2
− ] + S 2 s2
1 + S[ s m n
VI Ri
2
π
(3)
(4)
Define two components as below:
2
2 Lm
t SW
; and Capacitor: C a = 2
Resistor: Ra =
2( S n + S e )
π Lm
− 1]
t SW [
(S n + S f )
(5)
Where:
•
Sn is the rising rate.
•
Sf is the falling rate.
•
Se is the slope compensation rate.
Hereby, the equivalent small signal model from control to inductor current can be obtained as shown in Figure
3.
3
SLUA702
Lm
1/Rs
+
+
Vc
Figure 3.
3
Ra
-
im
Ca
Gain from Control to Inductor Current for Peak Current Mode
Active Clamp Peak Current Modeling Analysis:
Figure 4.
Peak Current Mode Active Clamp Forward Converter Topology.
The control to current gain function can be described in Equation 6:
^
uc
Rs
^
1
S
S2
1+
+ 2
Qω n ω n
=i
^
m
+
il
n
With the active-clamping topology,
4
(6)
SLUA702
Sn = (
VI VI − nVo
) Rs
+
Lm
n 2 Ls
(7)
V
DVI
Sf = (
+ o ) Rs
(1 − D) Lm nLs
n is the transformer turn ratio between primary and secondary side windings.
Therefore, Q =
π
1
; ωn =
S + Se
Tsw
π( n
− 0.5)
Sn + S f
(8)
The clamping circuitry modeling:
^
^
^
< icm >= (1− < d >) < im >; < icm >= I cm + i ; < d >= D + d ; < im >= I m + i
m
cm
(9)
Where:
•
<> means the cycle average function.
•
^ means the perturbation element.
•
d is the duty cycle of the main switch;
Considering the magnetic operating in symmetrically, the average current Ic and Im are zero.
^
^
icm = (1 − D) im
(10)
The Vds of MOSFET modeling:
^
uds
=−
^
And:
u ds
^
^
D
Vin d + (1 − D) u
cm
(1 − D)
= − SLm i
^
As result,
Define:
^
m;
Vin d = [ S
Le =
(11)
^
ucm
=−
^
1
i
SC m cm
(12)
^
(1 − D) Lm (1 − D) 3
]i
+
D
SDC m m
D
(1 − D)
Lm ; Ce =
Cm
D
(1 − D) 3
(13)
(14)
5
SLUA702
^
^
Then:
Vin d
n
1
( SLe +
)i
SC e m
;
=
n
^
And:
il =
^
1
RL
( SLs +
)
1 + SC o RL
Figure 5.
(15)
Vin d
(16)
n
The Peak current active clamp control block diagram.
^
Gco ( S ) =
uo
^
uc
=
R L (1 + S 2 Le C e )
S
S2
Rs (1 +
+ 2 )[ S 3 ( Le + n 2 Ls )C o C e R L + S 2 ( Le + n 2 Ls )C e + S (C o + n 2 C e ) RL + 1]
Qω n ω n
(17)
With C o >> n 2 C e , this formula can be simplified as in Equation 18:
^
Gco ( S ) =
uo
^
uc
≈
RL (1 + S 2 Le C e )
S
S2
Rs (1 +
+ 2 )[ S 2 ( Le + n 2 Ls )C e + 1]( SC o RL + 1)
Qω n ω n
(18)
The above Equation (18) introduced dual zeros and dual poles.
f z=
1
2π Le C e
=
(1 − D)
2π Lm C m
; fp =
1
2π ( Le + n 2 Ls )C e
=
(1 − D)
D 2
2π ( Lm +
n Ls )C m
1− D
(19)
Generally, to avoid the instability, the closed-loop crossover frequency “fc”, should be far less than half of the
pole frequency “fp”. Here, “fp” should be the value with the maximum limited duty cycle “D”.
6
SLUA702
Figure 6 shows a small-signal circuitry implementation:
Figure 6.
Small Signal Implementation from Control to Output.
4 A Design Example:
The design schematic (see Figure 7) and electric specification follow.
Figure 7.
100-W Isolated Power Module with UCC2897A Device
T1: Current transformer specification is: turn ratio is 1:100;
7
SLUA702
T2: Transformer specification is: PA0810NL; turn ratio is 6:1; Lm is 345 µH.
L1 inductor specification is: PA0373; 2.1 µH;
Vin = 72V ;Vout = 3.3V ; I out = 30 A
Based on the schematic design in Figure.7, and the small signal analysis, we can get the overall simulation
model as Figure.8:
1/n 167m
1/Rs 21.55
Ls 2u
Lm 345u
+
+
Esr1 10m
G1
-
1/n 167m
Le 909.5u
Ra 70.1 Ca 4.75n
Ce 23.8n
+
+
-
-
Co2 100n
Co1 330u
+
+
-
-
R5 2k
CTR 2.5
RL 110m
C4 330u
C1 8.2n
R2 51
UCC2897AFB 200m
Esr2 10m
R4 499
C3 270p
EA
+
+
2
3
7
6
Figure 8.
Overall Small Signal Circuitry Implementation
Figure 9 shows the simulation results.
8
R1 28.7k
+
4
C2 82n
R3 5.11k
VG1
-
Vo
SLUA702
Figure 9.
Simulation Results.
Figure.9 showed the cross-over frequency is 4.5 kHz, and phase margin is 111 degrees.
By lab test results, we can get the results as shown in Figure.10.
The test results showed cross-over frequency is 4.5 kHz and phase margin is 80 degrees.
Comparing the simulation and test results, they matched fully.
9
SLUA702
Figure 10. Lab Test Results (Vin = 72, Io = 30 A)
5 Conclusion:
The analysis shows the modeling and compensation is effective. The analysis revealed the zero and poles of
the peak current mode control active-clamp forward converter, which is critical for the design of the activeclamp converter.
Reference:
1. Texas instruments, SLUS829D, UCC2897A datasheet.
2. Texas instruments, SLUU357, UCC2897A EVM User’s Guide.
10
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