Universidad de Oviedo
Lección 4
Teoría básica de los convertidores CC/CC (I)
(convertidores con un único transistor)
Diseño de Sistemas Electrónicos de Potencia
4º Curso. Grado en Ingeniería en Tecnologías y
Servicios de Telecomunicación
SEA_uniovi_CC1_001
Outline (I)
Introducing switching regulators
Basis of their analysis in steady state
Detailed study of the basic DC/DC converters in
continuous conduction mode
 Buck, Boost and
 Common and
Buck-Boost converters
different properties
 Introduction to
the synchronous rectification
 Four-order converters
SEA_uniovi_CC1_012
Outline (II)
Study of the basic DC/DC converters in discontinuous
conduction mode
DC/DC converters with galvanic isolation
 How and
 The
where to place a transformer in a DC/DC converter
Forward and Flyback converters
SEA_uniovi_CC1_023
Linear DC/DC conversion (analog circuitry)
RV
ig
iO
RL
vg
vE
Av
Feedback loop
First idea
= (vOiO)/(vgig)
vO
iO  ig
Vref
Q
ig
  vO/vg
iO
RL
vg
vE
Av
Feedback loop
vO
Vref
Actual implementation
 Only a few components
 Robust
 No EMI generation
 Only lower output voltage
 Efficiency depends on input/output voltages
 Low efficiency
 Bulky
SEA_uniovi_CC1_034
Linear versus switching DC/DC conversion
Q
ig
iO
RL
vg
vE
Av
Feedback loop
ig
vO
vg
iO
RL
vg
PWM
Vref
vE
vO
Av
Feedback loop
Linear
vO
S
Vref
Switching (provisional)
Features:
vO_avg
100% efficiency
 Undesirable output voltage
waveform
t
SEA_uniovi_CC1_045
Introducing the switching DC/DC conversion (I)
S
ig
iO
RL
vg
PWM
vE
vg
t
The AC component must be
removed!!
-
Av
Vref
S
RL
C filter
Vg
vO
vg
iO
Filter
VO
RL
vO
C filter
PWM
t
It doesn’t work!!!
S
ig
iO
vg
vO_avg
vO
Feedback loop
ig
vO
vE
Av
Feedback loop
Vref
Basic switching DC/DC
converter (provisional)
SEA_uniovi_CC1_056
Introducing the switching DC/DC conversion (II)
S
ig
iO
Filter
vg
PWM
RL
ig
iL
vg
iD
+
vD
D -
C
iO
L
C
RL
vO
Infinite voltage across L when
S1 is opened
It doesn’t work either!!!
Including a diode
S
vg
LC filter
Vref
Feedback loop
iO
L
LC filter
-
Av
vE
ig
vO
iL
S
RL
+
vO
-
Vg
vD
VO
t
LC filter
Basic switching DC/DC converter
SEA_uniovi_CC1_067
Introducing the switching DC/DC conversion (III)
iL
S
ig
iD
vg
+
vD
D -
iO
L
C
RL
+
vO
-
iL
iS
ig
S
vg
LC filter
iD
+ L
vD
D -
iO
C
RL
+
vO
-
Buck converter
Starting the analysis of the Buck converter in steady state:
 L & C designed for negligible output voltage ripple (we are designing a DC/DC
converter)
 iL never reaches zero (Continuous Conduction Mode, CCM)
 The study of the Discontinuous Conduction Mode (DCM) will done later
iL
DCM
CCM
iL
t
t
SEA_uniovi_CC1_078
First analysis of the Buck converter in CCM
(In steady-state)
Analysis based on the specific topology of the Buck converter
iL
iS
ig
S
vg
vg
iD
+ L
vD
D -
iL
iO
C
RL
+
+
vO
-
L
iO
C
vD
RL
+
vO
-
LC filter
vD
vg
vO
vD
vD_avg = vO
t
t
dT
T
d: “duty cycle”
vO = vD_avg = d·vg
The AC component is
removed by the filter
 This procedure is only valid for
converter with explicit LC filter
SEA_uniovi_CC1_089
Introducing another analysis method (I)
Could we use the aforementioned analysis in the
case of this converter (SEPIC)?
ig
L1
C1
+
iS
Vg
S
iD
D
iL2
L2
+
C2 -
R
VO
 Obviously, there is not an explicit LC filter
 Therefore, we must use another method
10
SEA_uniovi_CC1_09
Introducing another analysis method (II)
Powerful tools to analyze DC/DC converters in steady-state
Step 1- To obtain the main waveforms (with no quantity values)
using Faraday’s law and Kirchhoff’s current and voltage laws
Step 2- To take into account the average value of the voltage
across inductors and of the current through capacitors in
steady-state
Step 2 (bis)- To use the volt·second balance
Step 3- To apply Kirchhoff’s current and voltage laws in average
values
Step 4- Input-output power balance
11
SEA_uniovi_CC1_10
Introducing another analysis method (III)
Any electrical circuit that operates in steady-state satisfies:
 The average voltage across an inductor is zero. Else, the net current
through the inductor always increases and, therefore, steady-state is not
achieved
 The average current through a capacitor is zero. Else, the net voltage
across the capacitor always increases and, therefore, steady-state is not
achieved
Vg
L
Circuit in
steady-state
+
vL_avg = 0
C
iC_avg = 0
12
SEA_uniovi_CC1_11
Introducing another analysis method (IV)
Particular case of many DC/DC converters in steady-state:
 Voltage across the inductors are rectangular waveforms
 Current through the capacitors are triangular waveforms
Vg
L
Circuit in
steady-state
+
vL
+
Same areas
v1
t
-
-
dT
C
vL_avg = 0
vL
iC
-v2
Volt·second balance:
V1dT – V2(1-d)T = 0
T
iC_avg = 0
+
iC
-
t
Same areas
13
SEA_uniovi_CC1_12
Introducing another analysis method (V)
Any electrical circuit of small dimensions (compared with the wavelength
associated to the frequency variations) satisfies:
 Kirchhoff’s current law (KCL) is not only satisfied for instantaneous current values, but
also for average current values
 Kirchhoff’s voltage law (KVL) is not only satisfied for instantaneous voltage values, but
also for average voltage values
 KVL applied to Loop1 yields:
Example
iL1
Node1
L1
+ vL1 -
Vg
S
iS
vg - vL1 - vC1 - vL2 = 0
vg - vL1_avg - vC1_avg - vL2_avg = 0
iC1
Therefore: vC1_avg = vg
C1
+ vC1
 KCL applied to Node1 yields:
+
vL2
Loop1 -
iL1 - iC1 - iS = 0
L2
iL1_avg - iC1_avg - iS_avg = 0
Therefore: iS_avg = iL1_avg
14
SEA_uniovi_CC1_13
Introducing another analysis method (VI)
A switching converter is (ideally) a lossless system
ig
 Input power:
iO
vg
Switching-mode
DC/DC converter
RL
Therefore: vgig_avg = vO2/RL
Pg = vgig_avg
+
vO
-
 Output power:
PO = vOiO = vO2/RL
 Power balance:
Pg = PO
 A switching-mode DC/DC converter as an ideal DC transformer
ig_avg
iO
RL
vg
+
vO
-
being N = vO/vg
ig_avg = iOvO/vg = N·iO
1:N
DC Transformer
Important concept!!
15
SEA_uniovi_CC1_14
Steady-state analysis of the Buck converter in CCM (I)
Step 1: Main waveforms. Remember that the output voltage remains constant
during a switching cycle if the converter has been properly designed
ig
+
vS
iS
-
vg
S
iO
iL
iD
+
vD
D -
L
Driving signal
C
RL
L
C
RL
vg
t
iL
iO
iL
S on, D
off
+
vO
-
+
vO
-
t
iS
t
During dT
iO
iL
S off, D
on
L
C
RL
iD
+
vO
-
t
dT
T
During (1-d)T
16
SEA_uniovi_CC1_15
Steady-state analysis of the Buck converter in CCM (II)
Step 1: Main waveforms (cont’)
+
vS
vg
iL
S
+ vL L
+
vD
D -
C
RL
t
+
vO
-
iL
L
iO
C
RL
vg
+
vO
-
vg-vO
t
- vO
T
iL + vL L
t
vL
dT
S off, D
on,
(1-d)T
DiL
iL_avg
i L + vL -
S on, D
off,
dT
Driving signal
iO
iO
C
RL
+
vO
-
 From Faraday’s law:
DiL = vO(1-d)T/L
17
SEA_uniovi_CC1_16
Steady-state analysis of the Buck converter in CCM (III)
Step 2 and 2 (bis): Average inductor voltage and capacitor current
 Average value of iC:
ig
+ vL - iL
iS
iC_avg = 0
iD
 Volt·second balance over L:
vg
L
Node1
S
D
iO
iC
C
RL
+
vO
-
(vg - vO)dT - vO(1-d)T = 0
Therefore:
vO = d·vg (always vO < vg)
Driving signal
Step 3: Average KCL and KVL:
t
 KCL applied to Node1 yields:
iL
iL - iC - iO = 0
iL_avg
iL_avg - iC_avg - iO = 0
Therefore: iL_avg = iO = vO/RL
Step 4: Power balance:
ig_avg = iS_avg = iOvO/vg = d·iO
vg-vO
t
vL
+
-
dT
t
- vO
T
18
SEA_uniovi_CC1_17
Steady-state analysis of the Buck converter in CCM (IV)
ig
+
vg
vS
iS
S
iD
+
vD
D -
Summary
iO
iL
L
C
RL
Driving signal
+
vO
-
vO = d·vg (always vO < vg)
vSmax = vDmax = vg
t
vg
vD
t
iL
iO
iL_avg = iO = vo/RL
ig_avg = iS_avg = d·iO
t
iS
DiL
t
iD_avg = iL_avg - iS_avg = (1-d)·iO
DiL = vO(1-d)T/L
iL_peak = iL_avg + DiL/2 = iO + vO(1-d)T/(2L)
iS_peak = iD_peak = iL_peak
iD
t
dT
T
19
SEA_uniovi_CC1_18
Steady-state analysis of the Boost converter in CCM (I)
Can we obtain vO > vg?  Boost converter
Step 1: Main waveforms
ig
+ vL - i L
L
vg
Driving signal
iD
iO
D
iS
+
- RL
C
S
t
+
vO
-
iL
t
iS
iL + vL -
S on, D off,
during dT
DiL
t
L
iD
vg
t
i L + vL -
S off, D on,
during (1-d)T
L
dT
iO
C
RL
+
vO
-
T
 From Faraday’s law:
DiL = vgdT/L
20
SEA_uniovi_CC1_19
Steady-state analysis of the Boost converter in CCM (II)
Step 2 and 2 (bis): Average values
ig
 Average value of iC:
+ vL - i L
L
iC_avg = 0
 Volt·second balance over L:
iD
vg
iC
D
iS
Node1
C
S
iO
+
- RL
+
vO
-
vgdT - (vO - vg)(1-d)T = 0
Therefore:
vO = vg/(1-d) (always vO > vg)
Driving signal
Step 3: Average KCL and KVL:
t
 KCL applied to Node1 yields:
iD
iD - iC - iO = 0
t
iD_avg - iC_avg - iO = 0
Therefore: iD_avg = iL_avg(1-d) = iO = vO/RL
Step 4: Power balance:
ig_avg = iL_avg = iOvO/vg = iO/(1-d)
DiL
iD_avg
vg
vL
t
dT
-(vO-vg)
T
21
SEA_uniovi_CC1_20
Steady-state analysis of the Boost converter in CCM (III)
ig
+ v L - i L - vD +
L
vg
iS
S
D iD
+
vS
-
Summary
iO
iC
C
RL
Driving signal
+
vO
-
vO = vg/(1-d) (always vO > vg)
t
vO
vD
t
iL
vSmax = vDmax = vO
iL_avg = ig_avg = iO/(1-d) = vo/[RL(1-d)]
t
iS
DiL
t
iS_avg = d·iL_avg = d·vo/[RL(1-d)]
iD_avg = iO
DiL = vgdT/L
iL_peak = iL_avg + DiL/2 = iL_avg + vgdT/(2L)
iS_peak = iD_peak = iL_peak
iD
iO
t
dT
T
22
SEA_uniovi_CC1_21
Steady-state analysis of the Buck-Boost converter in CCM (I)
Can we obtain either vO < vg or vO > vg?  Buck-Boost converter
ig
iD
iS
iL
S
vg
L
+
vL
-
iO
D
Driving signal
vO
+
C
+ RL
t
iL
DiL
ig
S on, D off,
during dT
t
iL
vg
L
iS
+
vL
t
-
iD
Charging stage
t
iO
S off, D on,
during (1-d)T
+
C vL
+
L -
iL
RL
vO
+
Discharging stage
dT
T
 From Faraday’s law:
DiL = vgdT/L
23
SEA_uniovi_CC1_22
Steady-state analysis of the Buck-Boost converter in CCM (II)
Step 2 and 2 (bis): Average values
 Average value of iC:
ig
vgdT - vO(1-d)T = 0
iS
iL
iC_avg = 0
 Volt·second balance over L:
Node1
iD
iO
+
vL
-
S
vg
L
D iC
C
+ RL
vO
+
Therefore: vO = vgd/(1-d)
Driving signal
Step 3: Average KCL and KVL:
t
 KCL applied to Node1 yields:
iD
iD - iC - iO = 0
iD_avg - iC_avg - iO = 0
Therefore: iD_avg = iL_avg(1-d) = iO = vO/RL
Step 4: Power balance:
ig_avg = iS_avg = iOvO/vg = iOd/(1-d)
DiL
iD_avg
t
vg
vL
t
-vO
dT
T
24
SEA_uniovi_CC1_23
Steady-state analysis of the Buck-Boost converter in CCM (III)
ig
i
+ vS - S
iL
S
vg
L
i D + vD -
+
vL
-
D
-
C
Summary
iO
+ RL
Driving signal
vO
+
vO = vgd/(1-d) (both vO < vg and vO > vg)
t
vO + vg
vD
t
iL
vSmax = vDmax = vO + vg
iD_avg = iO
DiL = vgdT/L
t
iS
DiL
t
iL_avg = iD_avg/(1-d) = iO/(1-d) = vo/[RL(1-d)]
iS_avg = ig_avg = d·iL_avg = d·vo/[RL(1-d)]
iL_peak = iL_avg + DiL/2 = iL_avg + vgdT/(2L)
iS_peak = iD_peak = iL_peak
iD
iO
t
dT
T
25
SEA_uniovi_CC1_24
Common issues in basic DC/DC converters (I)
L
S
vg
+
C
D
RL
-
+
vO
-
Complementary
switches + inductor
Buck
L
S
D
vg
+
C
S
- RL
d
+
vO
-
vg
D
1-d
L
C
+
- RL
+
vO
-
Boost
D
S
vg
L
Buck-Boost
C
+ RL
+
vO
-
Voltage source
The inductor is an energy buffer to
connect two voltage sources
26
SEA_uniovi_CC1_25
Common issues in basic DC/DC converters (II)
Diode turn-off
vg
L
vg
S
D
Buck
L
vg
D
S
vO
Boost
+
C
RL
-
C +
- RL
+
vO
-
+
vO
-
 The diode reverse recovery time is
of primary concern evaluating
switching losses
 Schottky diodes are desired from
this point of view
vO + vg
S
vg
 The diode turns off when the
transistor turns on
L
D C
Buck- + RL
Boost
+
vO
-
 In the range of line voltages, SiC
diodes are very appreciated
27
SEA_uniovi_CC1_26
Comparing basic DC/DC converters (I)
Generalized study as DC transformer (I)
ig
iO
L
S
vg
C -
D
L
S
iO
iO
RL
+
+
vO
-
- RL
C
+
vO
-
1:N
DC Transformer
 Buck: N= d (only vO < vg)
Boost
ig
ig_avg
vg
D
vg
iO
D
-
S
vg
RL
Buck
ig
+
vO
-
+
L
C
+ RL
 Boost: N= 1/(1-d) (only vO > vg)
+
vO
-
 Buck-Boost: N= -d/(1-d)
(both vO < vg and vO > vg)
Buck-Boost
28
SEA_uniovi_CC1_27
Comparing basic DC/DC converters (II)
Generalized study as DC transformer (II)
ig_avg
iO
RL
vg
+
vO
-
1:N
DC Transformer
ig_avg = iON = iOd/(1-d)
 Buck: ig_avg = iON = iOd
 Boost: ig_avg = iON = iO/(1-d)
Buck-Boost: ig_avg = iON = - iOd/(1-d)
29
SEA_uniovi_CC1_28
Comparing basic DC/DC converters (III)
Electrical stress on components (I)
iS
ig
vg
+
vS
iO
-
S
iD
+
vD
D -
RL
+
vO
-
DC/DC converter
 Buck:
 Boost:
 Buck-Boost:
vSmax = vDmax = vg
vSmax = vDmax = vO
vSmax = vDmax = vO + vg
iS_avg = ig_avg
iL_avg = ig_avg
iS_avg = ig_avg
iL_avg = iO
iD_avg = iO
iD_avg = iO
iD_avg = iL_avg - iS_avg
iS_avg = iL_avg - iD_avg
iL_avg = iS_avg + iD_avg
30
SEA_uniovi_CC1_29
Comparing basic DC/DC converters (IV)
Example of electrical stress on components (I)
1 A (avg)
2A
L
S
100 V
C
D
+
+
-
RL
50 V
-
100 W Buck, 100% efficiency
2A
1 A (avg)
D
S
100 V
L
C
-
+ RL +
50 V
100 W Buck-Boost, 100% efficiency
vS_max = vD_max = 100 V
iS_avg = iD_avg = 1 A
iL_avg = 2 A
FOMVA_S = FOMVA_D = 100 VA
vS_max = vD_max = 150 V
iS_avg = 1 A
iD_avg = 2 A
iL_avg = 3 A
FOMVA_S = 150 VA
FOMVA_D = 300 VA
 Higher electrical stress in the case of BuckBoost converter
 Therefore, lower actual efficiency
31
SEA_uniovi_CC1_30
Comparing basic DC/DC converters (V)
Example of electrical stress on components (II)
4 A (avg)
2A
L
25 V
D
S
C
vS_max = vD_max = 50 V
iS_avg = iD_avg = 2 A
iL_avg = 4 A
FOMVA_S = FOMVA_D = 100 VA
+
+
- RL
-
50 V
100 W Boost, 100% efficiency
2A
4 A (avg)
D
S
25 V
L
C
-
+ RL +
50 V
100 W Buck-Boost, 100% efficiency
 Higher electrical stress in the case of BuckBoost converter
 Therefore, lower actual efficiency
vS_max = vD_max = 75 V
iS_avg = 4 A
iD_avg = 2 A
iL_avg = 6 A
FOMVA_S = 300 VA
FOMVA_D = 150 VA
32
SEA_uniovi_CC1_31
Comparing basic DC/DC converters (VI)
 Price to pay for simultaneous step-down and stepup capability:
Higher electrical stress on components and,
therefore, lower actual efficiency
 Converters with limited either step-down or step-up
capability:
Lower electrical stress on components and,
therefore, higher actual efficiency
33
SEA_uniovi_CC1_32
Comparing basic DC/DC converters (VII)
Example of power conversion between similar voltage
levels based on a Boost converter
6.12 A (avg)
50 V
5A
L
1.12 A
(avg)
D
S
C
+
- RL
300 W Boost, 98% efficiency
+
-
60 V
vS_max = vD_max = 60 V
iS_avg = 1.12 A
iD_avg = 5 A
iL_avg = 6.12 A
FOMVA_S = 67.2 VA
FOMVA_D = 300 VA
Very high efficiency can be achieved!!!
34
SEA_uniovi_CC1_33
Comparing basic DC/DC converters (VIII)
The opposite case: Example of power conversion between
very different and variable voltage levels based on a BuckBoost converter
20 - 2 A (avg)
5A
D
S
20 - 200 V
L
C
-
+ RL +
60 V
300 W Buck-Boost, 75% efficiency
Remember previous example:
FOMVA_S = 67.2 VA
FOMVA_D = 300 VA
vS_max = vD_max = 260 V
iS_avg_max = 20 A
iD_avg_max = 5 A
iL_avg = 25 A
FOMVA_S_max = 5200 VA
FOMVA_D = 1300 VA
High efficiency cannot be achieved!!!
35
SEA_uniovi_CC1_34
Comparing basic DC/DC converters (IX)
One disadvantage exhibited by the Boost converter:
The input current has a “direct path” from the input voltage source to the
load. No switch is placed in this path. As a consequence, two problems arise:
 Large peak input current in start-up
 No over current or short-circuit protection can be easily implemented
(additional switch needed)
L
vg
S
D
+
C
Boost
- RL
+
vO
-
Buck and Buck-Boost do not exhibit these problems
36
SEA_uniovi_CC1_35
Synchronous rectification (I)
 To use controlled transistors (MOSFETs) instead of diodes to achieve high
efficiency in low output-voltage applications
 This is due to the fact that the voltage drop across the device can be lower
if a transistor is used instead a diode
 The conduction takes place from source terminal to drain terminal
 In practice, the diode (Schottky) is not removed
L
S
idevice
MOSFET
D
Diode
L
S1
S2
L
S1
S2
vdevice
37
SEA_uniovi_CC1_36
Synchronous rectification (II)
 In converters without a transformer, the control circuitry must provide
proper driving signals
 In converters with a transformer, the driving signals can be obtained
from the transformer (self-driving synchronous rectification)
 Nowadays,
very common technique with low output-voltage Buck
converters
L
L
S2
S1
vO
S2
S1
D
C -
RL
+
vO
-
Synchronous Buck
Q’
PWM
Q
vg
+
Av
Vref
Feedback loop
38
SEA_uniovi_CC1_37
Input current and current injected into the output RC cell (I)
 If a DC/DC converter were an ideal DC transformer, the input and output
currents should also be DC currents
 As a consequence, no pulsating current is desired in the input and output
ports and even in the current injected into the RC output cell
ig
iS
vg
+
vS
S
iRC
iD
+
vD
D -
DC/DC converter
Desired current
ig
C
+
+
v
- RL - O
Desired current
iRC
t
t
39
SEA_uniovi_CC1_38
Input current and current injected into the output RC cell (II)
iRC
ig
ig
S
vg
t
L
+
C
D
Noisy
-
iRC
ig
L
D
vg
t
t
Low noise
Buck
ig
S
Low noise
+
- RL
C
+
vO
-
iRC
t
Noisy
Boost
ig
iRC
D
ig
S
t
Noisy
RL
+
vO
-
iRC
vg
L
Buck-Boost
C
+ RL
+
vO
-
iRC
t
Noisy
40
SEA_uniovi_CC1_39
Input current and current injected into the output RC cell (III)
Adding EMI filters
iRC
ig
LF
CF
vg
+
-
L
S
D
+
C -
+
vO
-
RL
Buck
Filter
iRC
ig
L
D
vg
S
CF
Boost
vg
CF
Filter
+
-
C
Filter
ig
LF
LF
+
-
D
S
L
Buck-Boost
+
LF
CF
+
- RL
+
vO
-
R
+ L
vO
+
iRC
C
Filter
41
SEA_uniovi_CC1_40
Four-order converters (converters with integrated filters)
L1
ig
iD
C1
+
iS
vg
 Same vO/vg as Buck-Boost
 Same stress as Buck-Boost
 vC1 = vg
 Filtered input
D
-
vC1
iL2
L2
S
+
C2 -
vO
RL
ig
SEPIC
 Same vO/vg as Buck-Boost
 Same stress as Buck-Boost
 vC1 = vg + vO
 Filtered input and output
iS
C1
vg
iS
vg
- +
L1
+
C2
iL1
iD
Zeta
-
vC1
S
iD
D
-
RL
vO
C2 +
Cuk
L2
D
L2
iL2
C1
+
iL2
vC1
S
L1
-
RL
vO
 Same vO/vg as Buck-Boost
 Same stress as Buck-Boost
 vC1 = vO
 Filtered output
42
SEA_uniovi_CC1_41
DC/DC converters operating in DCM (I)
 Only one inductor in basic DC/DC converters
 The current passing through the inductor decreases when the load
current decreases (load resistance increases)
iL
L
ig
vg
S
D
DC/DC converter
iL
iL_avg
iO
RL
t
+
vO
-
Driving signal
t
dT
T
 Buck:
 Boost:
 Buck-Boost:
iL_avg = iO
iL_avg = iO/(1-d)
iL_avg = iS_avg + iD_avg = diO/(1-d) + iO
= iO/(1-d)
43
SEA_uniovi_CC1_42
DC/DC converters operating in DCM (II)
 When the load decreases, the converter goes toward Discontinuous
Conduction Mode (DCM)
Decreasing load
iL
RL_1
iL_avg
t
iL
Operation in CCM
RL_2 > RL_1
iL_avg
t
iL
RL_crit > RL_2
iL_avg
t
Boundary between CCM
and DCM
It corresponds to RL = R L_crit
44
SEA_uniovi_CC1_43
DC/DC converters operating in DCM (III)
Decreasing load
What happens when the load decreases below the critical value?
iL
iL
 DCM
RL_crit
iL_avg
 If a synchronous rectifier (SR) is used,
iL_avg
the operation depends on the driving
signal
RL_3 > RL_crit
t
RL_3 > RL_crit
iL_avg
DCM w. diode
rectifier
t
CCM w. SR
iL
starts if a diode is used as
 CCM
operation is possible with
synchronous rectifier with a proper driving
signal (synchronous rectifier with signal
almost complementary to the main
transistor)
t
45
SEA_uniovi_CC1_44
DC/DC converters operating in DCM (IV)
Remember:
iL_avg = iO (Buck) or iL_avg = iO/(1-d) (Boost and Buck-Boost)
iL
 For a given duty cycle, lower average
RL > RL_crit
CCM w. SR
iL_avg
t
 For a given duty cycle, higher average
RL > RL_crit
iL
value (due to the negative area)  lower
output current for a given load  lower
output voltage
DCM w. diode
iL_avg
t
value (no negative area)  higher output
current for a given load  higher output
voltage
The voltage conversion ratio vO/vg is always higher in DCM
than in CCM (for a given load and duty cycle)
46
SEA_uniovi_CC1_45
DC/DC converters operating in DCM (V)
How can we get DCM (of course, with
a diode as rectifier) ?
iL
After decreasing the
inductor inductance
t
iL
iL
After decreasing the
switching frequency
t
After decreasing the
load (increasing the
load resistance)
t
47
SEA_uniovi_CC1_46
DC/DC converters operating in DCM (VI)
Three sub-circuits instead of two:
 The transistor is on. During d·T
 The diode is on. During d’·T
 Both the transistor and the diode are off. During (1-
Driving signal
t
iL
iL_avg
t
iD
vg
d-d’)T
Example: Buck-Boost converter
ig
iD_avg
S
+
vg
- -vO
d’·T
t
iD
iL
t
vL
d·T
iS
L
ig
vg
D
C
+ RL
vO
+
iO
iL
T
+
vL
-
iO
L
+
vL
-
During d·T
+
C vL
+ RL
L -
iL
During d’·T
vO
+
iL
+
vL
L
-
During (1-d-d’)T
48
SEA_uniovi_CC1_47
DC/DC converters operating in DCM (VII)
Voltage conversion ratio vO/vg for the Buck-Boost converter in DCM
ig
iL
Driving signal
iL
vg
iL_max
t
iL_avg
iD
vg
t
iL_max
iD_avg
+
d·T
- -vO
d’·T
T
vL
From Faraday’s law:
vg = LiL_max/(dT)
-
During d·T
+
C vL
+ RL
L -
iL
iO
vO
+
And also:
vO = LiL_max/(d’T)
During d’·T
t
vL
L
+
t
Also:
iD_avg = iL_maxd’/2,
iD_avg = vO/R
And finally calling M = vO/vg we obtain:
M =d/(k)1/2 where k =2L/(RT)
49
SEA_uniovi_CC1_48
DC/DC converters operating in DCM (VIII)
The Buck-Boost converter just on the boundary
between DCM and CCM
iL
RL = RL_crit
iL_avg
t
 Due to being in DCM: M = vO/vg = d/(k)1/2, where: k =
2L/(RT)
 Due to being in CCM: N = vO/vg = d/(1-d)
 Just on the boundary: M = N, R = Rcrit, k = kcrit
 Therefore: kcrit = (1-d)2
 The converter operates in CCM if: k > kcrit
 The converter operates in DCM if: k < kcrit
50
SEA_uniovi_CC1_49
DC/DC converters operating in DCM (IX)
Summary for the basic DC/DC converter
Buck
Boost
N=d
1
N=
1-d
Buck-Boost
N=
2
M=
1+
4k
1+ 2
d
kcrit = (1-d)
kcrit_max = 1
1+
M=
4d2
1+
k
2
kcrit = d(1-d)2
kcrit_max = 4/27
M=
d
1-d
d
k
kcrit = (1-d)2
kcrit_max = 1
k = 2L/(RT)
51
SEA_uniovi_CC1_50
DC/DC converters operating in DCM (X)
CCM versus DCM
Driving signal
Driving signal
t
t
vD
t
iL
iS
iD
T
- Lower conduction losses in
CCM (lower rms values)
iL_avg
- Lower losses in DCM when S
turns on and D turns off
t
- Lower losses in CCM when S
turns off
t
t
dT
vD
- Lower inductance values in
DCM (size?)
t
iL
iL_avg
iS
t
iD
t
t
dT
T
52
SEA_uniovi_CC1_51
Achieving galvanic isolation in DC/DC converters (I)
- A two-winding magnetic device is needed
- The volt·second balance in the case of magnetic devices with two
windings must be used
From Faraday’s law:
+
v1
-
+
v2
n1:n2
vg
Circuit in steadystate
vi = ni d/dt

B
D= B - A = (vi/ni)·dt
A
In steady-state:
(D)in a period = 0
And therefore:
(vi /ni)avg = 0
Volt·second balance: If all the voltages are DC voltages, then:
 CCM: dT(V1/n1) – (1-d)T(V2/n2) = 0
 DCM: dT(V1/n1) –d’T(V2/n2) = 0
53
SEA_uniovi_CC1_52
Achieving galvanic isolation in DC/DC converters (II)
Transformer models
Model 1
Lm1
n1:n2
Ll1
Model 2
Ll2
Lm1
n1:n2
n1:n2
Model 1:
Model 2:
Model 3:
Circuit Theory
element
Magnetic transformer
with perfect coupling
Magnetic transformer
with real coupling
At least the magnetizing inductance must be taken
into account analyzing DC/DC converters
54
SEA_uniovi_CC1_53
Achieving galvanic isolation in DC/DC converters (III)
Where must we place the transformer?
Lm1
n1:n2
In a place where the
average voltage is zero
ig
iO
+
vg
vS
S
-
+
vD
D -
RL
+
vO
-
DC/DC converter
55
SEA_uniovi_CC1_54
Achieving a Buck converter with galvanic isolation (I)
L
S
vg
C
Buck
D
+
- RL
No place with average
voltage equal to zero
+
vO
-
New node with possible zero average voltage
L
vg
S
D
S on
S off
C
+
vO
-
+
- RL
D2
L
vg
S
Lm1
D1
+
C
-
RL
+
vO
-
n1:n2
It does not work!!
56
SEA_uniovi_CC1_55
Achieving a Buck converter with galvanic isolation (II)
vextra
A circuit to apply a given DC voltage
across Lm1 when S is off
S off
S on
D2
n3
L
vg
D1
Lm1
+
C
-
RL
+
vO
-
n1:n2
n1:n1:n2
D2
L
Lm1
vg
S
D1
+
C
-
RL
+
vO
-
D3 Final implementation: the
Standard design:
vextra = vg
n3 = n1
Forward converter
57
SEA_uniovi_CC1_56
The Forward converter
As the Buck converter replacing vg with vgn2/n1
n1:n1:n2
L
D1
Lm1
+
C
L
+
vO
-
S & D2 on, D1
& D3 off,
during dT
S & D2 off, D1
on,
S
RL
-
D3
vg
iO
iL
D2
C
RL
vgn2/n1
+
vO
-
Inductor magnetizing
stage
im1
+
D3 on, during d’T
im1
during (1-d)T
iO
iL
L
C
RL
vg
+
vO
-
Inductor demagnetizing
stage
Lm1
vL
-
Transformer reset
stage
-
Transformer
magnetizing stage
+
vg
Lm1
vL
vO = dvgn2/n1
vSmax = 2 vg
dmax = 0.5 (reset transformer)
58
SEA_uniovi_CC1_57
Achieving a Buck-Boost converter with galvanic isolation (I)
D
S
vg
L
BuckBoost
S
vg
C
+ RL
vO
+
There is a place with
average voltage equal to
zero: the inductor
D
Inductor and transformer
integrated into only one
- magnetic device (two-winding
inductor)
vO
+
C
Lm1
L
+
RL
RL
vO
+
n1:n2
S on
vg
S off
S
D
C
L
+
n1:n2
59
SEA_uniovi_CC1_58
Achieving a Buck-Boost converter with galvanic isolation (II)
vg
D
S
C
L
+
RL
vO
+
n1:n2
Two-winding
inductor
D
n1:n2
L1
L2
+
C
-
RL
+
vO
-
S on, D off,
during dT
ig
+
vg
L1
vL
-
Charging stage
iO
vg
+
S
Final implementation: the
Flyback converter
S off, D on,
during (1-d)T
vLn2/n1
-
C L2 +
RL
vO
+
Discharging stage
60
SEA_uniovi_CC1_59
The Flyback converter
Analysis in steady-state in CCM
 Volt·second balance:
L1
vg
S
dTvg/n1 - (1-d)TvO/n2 = 0
D
n1:n2
L2
+
C
-
RL
+
vO
-
 vO = vg(n2/n1)·d/(1-d)
 Therefore, the result is the
same as Buck-Boost converter
replacing vg with vgn2/n1
 vSmax = vg + vOn1/n2
 vDmax = vgn2/n1 + vO
 Very simple topology
 Useful for low-power, low-cost converters
 Critical “false transformer” (two-winding inductor) design
61
SEA_uniovi_CC1_60
Achieving other converters with galvanic isolation (I)
L1
L
D
C
Boost
S
vg
RL
+
-
+
+
vO
-
vg
S
It is not possible with
only one transistor!!
L1
C1
+
n1:n2
-
L2
C2
+ -
Vg
D
S
Cuk
C3
C1
L2
n1:n2
D
+
C2 -
RL
vO
SEPIC
 Zeta converter is also
possible
RL
VO
+
 vO = vg(n2/n1)d/(1-d)
 vSmax = vg + vOn1/n2
 vDmax = vgn2/n1 + vO
Like the Flyback converter
62
SEA_uniovi_CC1_61