Chapter 5. The Discontinuous Conduction Mode
5.1. Origin of the discontinuous conduction mode, and
mode boundary
5.2. Analysis of the conversion ratio M(D,K)
5.3. Boost converter example
5.4. Summary of results and key points
Introduction to
Discontinuous Conduction Mode (DCM)
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Occurs because switching ripple in inductor current or capacitor voltage
causes polarity of applied switch current or voltage to reverse, such
that the current- or voltage-unidirectional assumptions made in realizing
the switch are violated.
Commonly occurs in dc-dc converters and rectifiers, having singlequadrant switches. May also occur in converters having two-quadrant
switches.
Typical example: dc-dc converter operating at light load (small load
current). Sometimes, dc-dc converters and rectifiers are purposely
designed to operate in DCM at all loads.
Properties of converters change radically when DCM is entered:
M becomes load-dependent
Output impedance is increased
Dynamics are altered
Control of output voltage may be lost when load is removed
5.1. Origin of the discontinuous conduction
mode, and mode boundary
Buck converter example, with single-quadrant switches
L
Q1
continuous conduction mode (CCM)
iL(t)
Vg
+
–
D1
C
R
iD(t)
Minimum diode current is (I – ∆iL)
Dc component I = V/R
Current ripple is
(Vg – V)
Vg DD'Ts
∆iL =
DTs =
2L
2L
+
iL(t)
V
I
∆iL
–
0
conducting
devices:
DTs
Ts
D1
Q1
t
Q1
iD(t)
I
∆iL
Note that I depends on load, but ∆iL
does not.
0
DTs
Ts
t
Reduction of load current
Increase R, until I = ∆iL
CCM-DCM boundary
L
Q1
iL(t)
+
iL(t)
Vg
+
–
D1
C
R
iD(t)
V
–
I
∆iL
0
Minimum diode current is (I – ∆iL)
Dc component I = V/R
Current ripple is
(Vg – V)
Vg DD'Ts
∆iL =
DTs =
2L
2L
Note that I depends on load, but ∆iL
does not.
conducting
devices:
DTs
Ts
D1
Q1
t
Q1
iD(t)
I
∆iL
0
DTs
Ts
t
Further reduce load current
Increase R some more, such that I < ∆iL
L
Q1
iL(t)
+
iL(t)
Vg
+
–
D1
Discontinuous conduction mode
C
R
iD(t)
V
–
I
Minimum diode current is (I – ∆iL)
Dc component I = V/R
Current ripple is
(Vg – V)
Vg DD'Ts
∆iL =
DTs =
2L
2L
Note that I depends on load, but ∆iL
does not.
The load current continues to be
positive and non-zero.
0
DTs
D1Ts
Q1
conducting
devices:
t
Ts
D2Ts
D1
D3Ts
Q1
X
iD(t)
0
DTs
Ts
D2Ts
t
Mode boundary
I > ∆iL for CCM
I < ∆iL for DCM
Insert buck converter expressions for I and ∆iL :
DVg DD'TsVg
<
R
2L
Simplify:
2L < D'
RTs
This expression is of the form
where
K < K crit(D)
for DCM
K = 2L and K crit(D) = D'
RTs
K and Kcrit vs. D
for K < 1:
2
K < Kcrit:
DCM
Kc (
rit D) =
1–D
1
for K > 1:
K > Kcrit:
CCM
2
K > Kcrit:
CCM
K = 2L/RTs
Kc (
rit D) =
1–D
1
K = 2L/RTs
0
0
0
1
D
0
1
D
Critical load resistance Rcrit
Solve Kcrit equation for load resistance R:
where
R < Rcrit(D)
for CCM
for DCM
R > Rcrit(D)
Rcrit(D) = 2L
D'Ts
Summary: mode boundary
K > K crit(D)
K < K crit(D)
or
or
R < Rcrit(D)
R > Rcrit(D)
for CCM
for DCM
Table 5.1. CCM-DCM mode boundaries for the buck, boost, and buck-boost converters
Converter
K crit(D)
Buck
(1 – D)
Boost
D (1 – D)2
Buck-boost
2
(1 – D)
max ( K crit )
0≤D≤1
1
4
27
1
R crit(D)
2L
(1 – D)T s
2L
D (1 – D) 2 T s
2L
(1 – D) 2 T s
min ( Rcrit )
0≤D≤1
2 L
Ts
27 L
2 Ts
2 L
Ts
5.2.
Analysis of the conversion ratio M(D,K)
Analysis techniques for the discontinuous conduction mode:
Inductor volt-second balance
vL = 1
Ts
Ts
vL(t) dt = 0
0
Capacitor charge balance
iC = 1
Ts
Ts
iC(t) dt = 0
0
Small ripple approximation sometimes applies:
v(t) ≈ V
because ∆v << V
i(t) ≈ I is a poor approximation when ∆i > I
Converter steady-state equations obtained via charge balance on
each capacitor and volt-second balance on each inductor. Use care in
applying small ripple approximation.
Example: Analysis of
DCM buck converter M(D,K)
L
iL(t)
+ vL(t) –
subinterval 1
Vg
+
–
+
iC(t)
C
R
v(t)
–
L
Q1
iL(t)
+
iL(t)
Vg
+
–
D1
C
iD(t)
R
subinterval 2
V
Vg
L
+ vL(t) –
+
–
+
iC(t)
C
R
–
–
iL(t)
L
+ vL(t) –
subinterval 3
v(t)
Vg
+
–
C
+
iC(t)
R
v(t)
–
Subinterval 1
iL(t)
vL(t) = Vg – v(t)
iC(t) = iL(t) – v(t) / R
+ vL(t) –
Vg
Small ripple approximation
for v(t) (but not for i(t)!):
vL(t) ≈ Vg – V
iC(t) ≈ iL(t) – V / R
L
+
–
C
+
iC(t)
R
v(t)
–
Subinterval 2
iL(t)
vL(t) = – v(t)
iC(t) = iL(t) – v(t) / R
+ vL(t) –
Vg
Small ripple approximation
for v(t) but not for i(t):
vL(t) ≈ – V
iC(t) ≈ iL(t) – V / R
L
+
–
C
+
iC(t)
R
v(t)
–
Subinterval 3
iL(t)
vL = 0, iL = 0
iC(t) = iL(t) – v(t) / R
+ vL(t) –
Vg
Small ripple approximation:
vL(t) = 0
iC(t) = – V / R
L
+
–
C
+
iC(t)
R
v(t)
–
Inductor volt-second balance
vL(t)
Vg – V
D1Ts
D2Ts
D3Ts
0
Ts
t
–V
Volt-second balance:
vL(t) = D1(Vg – V) + D2( – V) + D3(0) = 0
Solve for V:
D1
V = Vg
D1 + D2
note that D2 is unknown
Capacitor charge balance
L
node equation:
iL(t)
iL(t) = iC(t) + V / R
v(t)/R
+
iC(t)
capacitor charge balance:
C
R
iC = 0
v(t)
–
hence
iL = V / R
must compute dc
component of inductor
current and equate to load
current (for this buck
converter example)
iL(t)
Vg – V
L
<iL> = I
0
ipk
–V
L
DTs
D1Ts
Ts
D2Ts
D3Ts
t
Inductor current waveform
peak current:
iL(t)
Vg – V
iL(D1Ts) = i pk =
D 1T s
L
average current:
iL = 1
Ts
Vg – V
L
<iL> = I
ipk
–V
L
Ts
iL(t) dt
0
0
DTs
D1Ts
Ts
D2Ts
D3 Ts
triangle area formula:
Ts
0
iL(t) dt = 1 i pk (D1 + D2)Ts
2
D 1T s
iL = (Vg – V)
(D1 + D2)
2L
equate dc component to dc load current:
V = D1Ts (D + D ) (V – V)
1
2
g
R
2L
t
Solution for V
Two equations and two unknowns (V and D2):
D1
V = Vg
D1 + D2
(from inductor volt-second balance)
V = D1Ts (D + D ) (V – V)
1
2
g
R
2L
(from capacitor charge balance)
Eliminate D2 , solve for V :
V =
2
Vg 1 + 1 + 4K / D 21
where
K = 2L / RTs
valid for K < K crit
Buck converter M(D,K)
1.0
K = 0.01
M(D,K)
0.8
K = 0.1
0.6
D
K = 0.5
0.4
M=
K≥1
0.2
1+
0.0
0.0
0.2
0.4
0.6
D
0.8
1.0
2
1 + 4K / D 2
for K > K crit
for K < K crit
5.3. Boost converter example
D1
L
i(t)
iD(t)
+ vL(t) –
Vg
+
–
+
iC(t)
Q1
C
R
v(t)
–
Mode boundary:
I > ∆iL for CCM
I < ∆iL for DCM
Previous CCM soln:
Vg
I= 2
D' R
Vg
∆iL =
DTs
2L
Mode boundary
Vg
DTsVg
>
2
2L
D' R
2L > DD' 2
RTs
where
4
Kcrit ( 13 ) = 27
0.15
for CCM
Kcrit(D)
for CCM
K > K crit(D)
for CCM
for DCM
K < K crit(D)
K = 2L
and K crit(D) = DD' 2
RTs
0.1
0.05
0
0
0.2
0.4
0.6
D
0.8
1
Mode boundary
CCM
0.15
where
K > K crit(D)
for CCM
for DCM
K < K crit(D)
K = 2L
and K crit(D) = DD' 2
RTs
DCM
K < Kcrit
CCM
K > Kcrit
0.1
K
(D)
K crit
0.05
0
0
0.2
0.4
0.6
D
0.8
1
Conversion ratio: DCM boost
L
i(t)
+ vL(t) –
subinterval 1
Vg
+
–
+
iC(t)
C
R
v(t)
–
i(t)
D1 i (t)
D
L
+ vL(t) –
Vg
+
–
i(t)
+
iC(t)
Q1
C
+ vL(t) –
subinterval 2
R
v(t)
L
Vg
+
–
+
iC(t)
C
R
–
–
i(t)
L
+ vL(t) –
subinterval 3
v(t)
Vg
+
–
+
iC(t)
C
R
v(t)
–
Subinterval 1
i(t)
vL(t) = Vg
iC(t) = – v(t) / R
+ vL(t) –
Vg
Small ripple approximation
for v(t) (but not for i(t)!):
vL(t) ≈ Vg
iC(t) ≈ – V / R
L
+
–
+
iC(t)
C
R
v(t)
–
0 < t < D1Ts
Subinterval 2
i(t)
vL(t) = Vg – v(t)
iC(t) = i(t) – v(t) / R
L
+ vL(t) –
Vg
+
–
iC(t)
C
Small ripple approximation
for v(t) but not for i(t):
vL(t) ≈ Vg – V
iC(t) ≈ i(t) – V / R
+
R
v(t)
–
D1Ts < t < (D1 +D2)Ts
Subinterval 3
i(t)
vL = 0, i = 0
iC(t) = – v(t) / R
L
+ vL(t) –
Vg
+
–
iC(t)
C
Small ripple approximation:
vL(t) = 0
iC(t) = – V / R
+
R
v(t)
–
(D1 +D2)Ts < t < Ts
Inductor volt-second balance
vL(t)
Vg
D1Ts
D2Ts
D3Ts
0
Ts
t
Vg – V
Volt-second balance:
D1Vg + D2(Vg – V) + D3(0) = 0
Solve for V:
D1 + D2
V=
Vg
D2
note that D2 is unknown
Capacitor charge balance
node equation:
iD(t) = iC(t) + v(t) / R
D1 i (t)
D
capacitor charge balance:
iC = 0
hence
iD = V / R
must compute dc component of diode
current and equate to load current
(for this boost converter example)
+
iC(t)
C
R
v(t)
–
Inductor and diode current waveforms
i(t)
peak current:
Vg
i pk =
D 1T s
L
Vg – V
L
average diode current:
iD = 1
Ts
Ts
iD(t) dt
0
0
0
triangle area formula:
Ts
ipk
Vg
L
DTs
D 1T s
iD(t)
Ts
D2Ts
t
D3Ts
ipk
iD(t) dt = 1 i pk D2Ts
2
Vg – V
L
<iD>
0
DTs
D1Ts
Ts
D2Ts
D3Ts
t
Equate diode current to load current
average diode current:
iD
V g D 1 D 2T s
1
1
=
i pk D2Ts =
Ts 2
2L
equate to dc load current:
V g D 1 D 2T s V
=
R
2L
Solution for V
Two equations and two unknowns (V and D2):
D1 + D2
(from inductor volt-second balance)
V=
Vg
D2
V g D 1 D 2T s V
=
R
2L
(from capacitor charge balance)
Eliminate D2 , solve for V. From volt-sec balance eqn:
Vg
D2 = D1
V – Vg
Substitute into charge balance eqn, rearrange terms:
2 2
V
gD 1
2
V – VVg –
=0
K
Solution for V
2 2
V
gD 1
2
V – VVg –
=0
K
Use quadratic formula:
V = 1±
Vg
2
1
1 + 4D / K
2
Note that one root leads to positive V, while other leads to
negative V. Select positive root:
V = M(D ,K) =
1
Vg
where
valid for
1+
1 + 4D 21 / K
2
K = 2L / RTs
K < Kcrit(D)
Transistor duty cycle D = interval 1 duty cycle D1
Boost converter characteristics
5
0.0
1
M(D,K)
K=
4
3
K
2
=
5
0
.
0
K
.1
0
=
K
/2
≥4
M =
1
1–D
1+
for K > K crit
2
1 + 4D / K
2
for K < K crit
7
1
Approximate M in DCM:
0
0
0.25
0.5
D
0.75
1
M≈1+ D
2
K
Summary of DCM characteristics
Table 5.2. S ummary of CCM-DCM characteristics for the buck, boost, and buck-boost converters
K crit (D)
Converter
Buck
(1 – D)
Boost
D (1 – D)2
(1 – D)2
Buck-boost
with
DCM M(D,K)
2
1 + 1 + 4K / D 2
1 + 1 + 4D 2 / K
2
– D
K
K = 2L / RT s.
DCM D2(D,K)
K M(D,K)
D
K M(D,K)
D
DCM occurs for K < K crit .
K
CCM M(D)
D
1
1–D
– D
1–D
Summary of DCM characteristics
DCM
M(D,K)
st
o
o
• DCM buck and boost
characteristics are
asymptotic to M = 1 and to
the DCM buck-boost
characteristic
)
B
t
os
1
–
×
(
1
K
o
b
k
uc
B
• DCM buck-boost
characteristic is linear
1
• CCM and DCM
characteristics intersect at
mode boundary. Actual M
follows characteristic
having larger magnitude
Buck
0
0
0.2
0.4
0.6
D
0.8
1
• DCM boost characteristic is
nearly linear
Summary of key points
1. The discontinuous conduction mode occurs in converters
containing current- or voltage-unidirectional switches, when the
inductor current or capacitor voltage ripple is large enough to
cause the switch current or voltage to reverse polarity.
2. Conditions for operation in the discontinuous conduction mode
can be found by determining when the inductor current or
capacitor voltage ripples and dc components cause the switch
on-state current or off-state voltage to reverse polarity.
3. The dc conversion ratio M of converters operating in the
discontinuous conduction mode can be found by application of
the principles of inductor volt-second and capacitor charge
balance.
Summary of key points
4. Extra care is required when applying the small-ripple
approximation. Some waveforms, such as the output voltage,
should have small ripple which can be neglected. Other
waveforms, such as one or more inductor currents, may have
large ripple that cannot be ignored.
5. The characteristics of a converter changes significantly when
the converter enters DCM. The output voltage becomes loaddependent, resulting in an increase in the converter output
impedance.