ATLAS
Buck Converter Operation
David. Nelson
SLAC
Nov-12-2006
djn@slac.stanford.edu
(650) 926-4652
DN-ATLAS-TILCAL
V2
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Basic Operation, CCM
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Figure 1 illustrates the basic buck
converter.
There are two operational phases.
Q1 turns on in phase 1 and stores
energy in L. Q1 turns off and D1
turns on in phase 2.
Figure 2 & 3 represent Continuous
Conduction Mode, (CCM).
Figure 2 illustrates wave form in L.
Time 0 to DTs is when Q1 conducts.
Ts-DTs is where D1 conducts.
Figure 3 illustrates the diode
current. The diode and inductor
current are identical during the
second phase.
The inductor current dc component
is equal to the load current.
Equation 1: I = V/R
L
+
Q1
D1
Vg
i D(t)
-
Figure 1
i L(t)
I
di
0
Conducting
devices:
DTs
Q1
V2
t
Ts
D1
Q1
Figure 2
iD(t)
I
di
DTs
Figure 3
DN-ATLAS-TILCAL
R V
C
L
t
Ts
2
L
Boundary between CCM & DCM
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Figure 4 illustrates the inductor current at
the boundary between CCM and
Discontinuous Current Mode, (DCM) of
operation. Note that I and !iL are equal
i L(t)
I
di
0
DTs
Conducting
devices:
t
Ts
Q1
D1
Q1
Figure 4
iD(t)
I
0
DTs
Figure 5
DN-ATLAS-TILCAL
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Ts
t
3
L
Discontinuous Current Mode, (DCM)
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Figure 6 illustrates the inductor
current in DCM. Note that the
inductor stops conducting
current before the cycle ends.
Figure 7 illustrates the diode
current in DCM.
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Equation 2:
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Equation 3:
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Equation 4:
(
)
(
(
DN-ATLAS-TILCAL
Q1
D3 Ts
D1
X
t
Ts
Q1
i D(t)
D ! D Ts
<
2! L
2! L
< D
1
% and T (D
crit
R ! Ts &
'
2! L
V2
D2 Ts
Figure 6
0
"K
#
$
DTs
1
D ! Vg
Equation 5:
Equation 6:
D1T s
Conducting
devices:
)
)
R ! Ts
•
0
Vg # D # D Ts
2# L
I > !iL for CCM
I < !iL for DCM
R
•
I
1
Vg " V
2# L
!iL
iL(t)
DTs
D 2 Ts
Ts
t
D1
Figure 7
D
1
4
Bricks CCM? Or DCM?
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Table 1 calculates the K factor for the bricks running a 285KHz with nominal
inductors.
Table 2 calculates the K factor with 1.3 uH inductors for the 5/3.3 volt bricks. I
found 1.3uH in one of five volt bricks which does not show up in the material
list I received.
Table 3 calculates the K factor with the frequency changed to 220KHz.
Table 4 sets the K factor to 0.71, then calculates the needed inductance.
DN-ATLAS-TILCAL
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15V Brick CCM/DCM
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The following pictures are of one 15 volt brick running at 285KHz.
Figure 1 shows L current on top trace and voltage
on diode on lower trace. This is DCM mode.
Figure 2 shows voltage on diode. L is 15uH
Figure 3 shows L current on top trace and diode
voltage on lower trace. L is 15uH and R load is
20 Ohms. K is very close to 1.
Figure 4 shows voltage on diode. L is 65uH and
R load is 40 Ohms. This is CCM mode. Notice
Figure 1
there is no switching spikes.
Figure 3
Figure 2
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Figure 4
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