Download Chapter 4. Switch Realization

Chapter 4. Switch Realization
4.1. Switch applications
Single-, two-, and four-quadrant switches. Synchronous rectifiers
4.2. A brief survey of power semiconductor devices
Power diodes, MOSFETs, BJTs, IGBTs, and thyristors
4.3. Switching loss
Transistor switching with clamped inductive load. Diode
recovered charge. Stray capacitances and inductances, and
ringing. Efficiency vs. switching frequency.
4.4. Summary of key points
Fundamentals of Power Electronics
1
Chapter 4: Switch realization
SPST (single-pole single-throw) switches
Buck converter
SPST switch, with
voltage and current
polarities defined
with SPDT switch:
1
L
iL(t)
+
1
+
i
Vg
2
+
–
C
R
V
–
v
–
with two SPST switches:
iA
0
L
A
iL(t)
+
+ vA –
All power semiconductor
devices function as SPST
switches.
Fundamentals of Power Electronics
Vg
+
–
–
vB
B
R
V
+
iB
2
C
–
Chapter 4: Switch realization
Realization of SPDT switch using two SPST switches
●
●
●
●
A nontrivial step: two SPST switches are not exactly equivalent to one
SPDT switch
It is possible for both SPST switches to be simultaneously ON or OFF
Behavior of converter is then significantly modified
—discontinuous conduction modes (ch. 5)
Conducting state of SPST switch may depend on applied voltage or
current —for example: diode
Fundamentals of Power Electronics
3
Chapter 4: Switch realization
Quadrants of SPST switch operation
1
+
i
switch
on-state
current
A single-quadrant
switch example:
v
ON-state: i > 0
–
OFF-state: v > 0
switch
off-state voltage
0
Fundamentals of Power Electronics
4
Chapter 4: Switch realization
Some basic switch applications
Singlequadrant
switch
switch
on-state
current
Currentbidirectional
two-quadrant
switch
switch
off-state voltage
switch
on-state
current
Voltagebidirectional
two-quadrant
switch
Fundamentals of Power Electronics
switch
on-state
current
switch
off-state
voltage
switch
on-state
current
Fourquadrant
switch
switch
off-state
voltage
5
switch
off-state
voltage
Chapter 4: Switch realization
4.1.1. Single-quadrant switches
1
+
i
v
–
0
Fundamentals of Power Electronics
Active switch: Switch state is controlled exclusively
by a third terminal (control terminal).
Passive switch: Switch state is controlled by the
applied current and/or voltage at terminals 1 and 2.
SCR: A special case — turn-on transition is active,
while turn-off transition is passive.
Single-quadrant switch: on-state i(t) and off-state v(t)
are unipolar.
6
Chapter 4: Switch realization
The diode
• A passive switch
i
1
+
i
• Single-quadrant switch:
• can conduct positive onstate current
on
v
off
v
–
0
Symbol
instantaneous i-v characteristic
Fundamentals of Power Electronics
7
• can block negative offstate voltage
• provided that the intended
on-state and off-state
operating points lie on the
diode i-v characteristic,
then switch can be
realized using a diode
Chapter 4: Switch realization
The Bipolar Junction Transistor (BJT) and the
Insulated Gate Bipolar Transistor (IGBT)
BJT
C
• An active switch, controlled
by terminal C
1
i +
i
• Single-quadrant switch:
v
–
0
IGBT
• can conduct positive onstate current
on
off
v
1
i +
C
v
–
instantaneous i-v characteristic
0
Fundamentals of Power Electronics
8
• can block positive off-state
voltage
• provided that the intended
on-state and off-state
operating points lie on the
transistor i-v characteristic,
then switch can be realized
using a BJT or IGBT
Chapter 4: Switch realization
The Metal-Oxide Semiconductor Field Effect
Transistor (MOSFET)
• An active switch, controlled by
terminal C
i
1
i +
C
• Normally operated as singlequadrant switch:
on
v
v
off
–
on
0
• can conduct positive on-state
current (can also conduct
negative current in some
circumstances)
(reverse conduction)
• can block positive off-state
voltage
Symbol
instantaneous i-v characteristic
Fundamentals of Power Electronics
9
• provided that the intended onstate and off-state operating
points lie on the MOSFET i-v
characteristic, then switch can
be realized using a MOSFET
Chapter 4: Switch realization
Realization of switch using
transistors and diodes
Buck converter example
iA
L
A
iL(t)
+
+ vA –
Vg
+
–
–
vB
B
C
R
V
+
Switch A: transistor
iB
–
Switch B: diode
iA
SPST switch
operating points
switch A
on
iB
switch B
iL
on
switch A
switch B
off
Vg
off
vA
Switch A
Fundamentals of Power Electronics
iL
–Vg
vB
Switch B
10
Chapter 4: Switch realization
Realization of buck converter
using single-quadrant switches
iA
+
vA
L
–
+
–
vB
+
+
–
Vg
iL(t)
vL(t)
–
iB
iB
iA
switch A
on
switch B
iL
on
switch A
switch B
off
off
Vg
Fundamentals of Power Electronics
vA
–Vg
11
iL
vB
Chapter 4: Switch realization
4.1.2. Current-bidirectional
two-quadrant switches
• Usually an active switch,
controlled by terminal C
i
1
i
on
(transistor conducts)
+
C
v
–
0
BJT / anti-parallel
diode realization
Fundamentals of Power Electronics
v
off
on
(diode conducts)
instantaneous i-v
characteristic
12
• Normally operated as twoquadrant switch:
• can conduct positive or
negative on-state current
• can block positive off-state
voltage
• provided that the intended onstate and off-state operating
points lie on the composite i-v
characteristic, then switch can
be realized as shown
Chapter 4: Switch realization
Two quadrant switches
switch
on-state
current
i
1
+
on
(transistor conducts)
i
v
off
v
switch
off-state
voltage
–
0
Fundamentals of Power Electronics
on
(diode conducts)
13
Chapter 4: Switch realization
MOSFET body diode
i
1
i
on
(transistor conducts)
+
off
v
C
v
–
on
(diode conducts)
0
Power MOSFET
characteristics
Fundamentals of Power Electronics
Power MOSFET,
and its integral
body diode
14
Use of external diodes
to prevent conduction
of body diode
Chapter 4: Switch realization
A simple inverter
iA
+
Vg
+
–
Q1
v0(t) = (2D – 1) Vg
D1 vA
–
L
iL
+
+
Vg
+
–
Q2
D2 v
B
–
C
R
v0
–
iB
Fundamentals of Power Electronics
15
Chapter 4: Switch realization
Inverter: sinusoidal modulation of D
v0(t) = (2D – 1) Vg
v0
Sinusoidal modulation to
produce ac output:
Vg
D(t) = 0.5 + Dm sin (ωt)
D
0
0.5
1
–Vg
The resulting inductor
current variation is also
sinusoidal:
iL(t) =
Vg
v0(t)
= (2D – 1)
R
R
Hence, current-bidirectional
two-quadrant switches are
required.
Fundamentals of Power Electronics
16
Chapter 4: Switch realization
The dc-3øac voltage source inverter (VSI)
ia
Vg
+
–
ib
ic
Switches must block dc input voltage, and conduct ac load current.
Fundamentals of Power Electronics
17
Chapter 4: Switch realization
Bidirectional battery charger/discharger
D1
L
+
+
Q1
vbus
vbatt
D2
spacecraft
main power bus
–
Q2
–
vbus > vbatt
A dc-dc converter with bidirectional power flow.
Fundamentals of Power Electronics
18
Chapter 4: Switch realization
4.1.3. Voltage-bidirectional two-quadrant switches
• Usually an active switch,
controlled by terminal C
i
• Normally operated as twoquadrant switch:
i
1
+
on
v
v
C
off
off
(diode
blocks voltage)
(transistor
blocks voltage)
–
0
BJT / series
diode realization
instantaneous i-v
characteristic
• can conduct positive on-state
current
• can block positive or negative
off-state voltage
• provided that the intended onstate and off-state operating
points lie on the composite i-v
characteristic, then switch can
be realized as shown
• The SCR is such a device,
without controlled turn-off
Fundamentals of Power Electronics
19
Chapter 4: Switch realization
Two-quadrant switches
1
i
+
switch
on-state
current
i
v
on
–
0
v
1
i
+
off
off
(diode
blocks voltage)
(transistor
blocks voltage)
switch
off-state
voltage
v
C
–
0
Fundamentals of Power Electronics
20
Chapter 4: Switch realization
A dc-3øac buck-boost inverter
φa
iL
+
vab(t)
–
Vg
–
+
φb
+
vbc(t)
–
φc
Requires voltage-bidirectional two-quadrant switches.
Another example: boost-type inverter, or current-source inverter (CSI).
Fundamentals of Power Electronics
21
Chapter 4: Switch realization
4.1.4. Four-quadrant switches
switch
on-state
current
• Usually an active switch,
controlled by terminal C
• can conduct positive or
negative on-state current
switch
off-state
voltage
Fundamentals of Power Electronics
22
• can block positive or negative
off-state voltage
Chapter 4: Switch realization
Three ways to realize a four-quadrant switch
1
1
i
i
1
+
1
i
+
+
v
v
v
–
–
–
+
i
v
–
0
0
Fundamentals of Power Electronics
0
23
0
Chapter 4: Switch realization
A 3øac-3øac matrix converter
3øac input
3øac output
ia
van(t)
+
–
vbn(t)
+
–
ib
+
–
vcn(t)
ic
• All voltages and currents are ac; hence, four-quadrant switches are required.
• Requires nine four-quadrant switches
Fundamentals of Power Electronics
24
Chapter 4: Switch realization
4.1.5. Synchronous rectifiers
Replacement of diode with a backwards-connected MOSFET,
to obtain reduced conduction loss
i
1
+
i
1
i +
1
+
i
C
v
v
v
–
–
–
ideal switch
conventional
diode rectifier
Fundamentals of Power Electronics
v
off
on
0
0
0
on
(reverse conduction)
MOSFET as
synchronous
rectifier
25
instantaneous i-v
characteristic
Chapter 4: Switch realization
Buck converter with synchronous rectifier
iA
+
vA
L
–
iL(t)
Q1
Vg
+
–
–
vB
+
C
C
Q2
• MOSFET Q2 is
controlled to turn on
when diode would
normally conduct
• Semiconductor
conduction loss can
be made arbitrarily
small, by reduction
of MOSFET onresistances
iB
• Useful in lowvoltage high-current
applications
Fundamentals of Power Electronics
26
Chapter 4: Switch realization
4.2. A brief survey of power semiconductor devices
●
●
●
●
●
●
●
Power diodes
Power MOSFETs
Bipolar Junction Transistors (BJTs)
Insulated Gate Bipolar Transistors (IGBTs)
Thyristors (SCR, GTO, MCT)
On resistance vs. breakdown voltage vs. switching times
Minority carrier and majority carrier devices
Fundamentals of Power Electronics
27
Chapter 4: Switch realization
4.2.1. Power diodes
A power diode, under reverse-biased conditions:
v
+
–
p
+
+
+
{ {
low doping concentration
n-
n
–
–
E
–
v
+
–
depletion region, reverse-biased
Fundamentals of Power Electronics
28
Chapter 4: Switch realization
Forward-biased power diode
v
+
–
i
{
conductivity modulation
n-
p
+
–
+
+
+
n
+
–
+
–
minority carrier injection
Fundamentals of Power Electronics
29
Chapter 4: Switch realization
Typical diode switching waveforms
v(t)
t
i(t)
tr
0
t
di
dt
area
–Qr
(1)
(2)
Fundamentals of Power Electronics
(3)
(4)
30
(5)
(6)
Chapter 4: Switch realization
Types of power diodes
Standard recovery
Reverse recovery time not specified, intended for 50/60Hz
Fast recovery and ultra-fast recovery
Reverse recovery time and recovered charge specified
Intended for converter applications
Schottky diode
A majority carrier device
Essentially no recovered charge
Model with equilibrium i-v characteristic, in parallel with
depletion region capacitance
Restricted to low voltage (few devices can block 100V or more)
Fundamentals of Power Electronics
31
Chapter 4: Switch realization
Characteristics of several commercial power
rectifier diodes
Part number
R ated max voltage
Rated avg current
V F (typical)
tr (max)
Fas t recov ery rect i fi ers
1N3913
400V
30A
1.1V
400ns
SD453N25S20PC
2500V
400A
2.2V
2µs
Ul t ra-fas t recov ery rect i fi ers
MUR815
150V
8A
0.975V
35ns
MUR1560
600V
15A
1.2V
60ns
RHRU100120
1200V
100A
2.6V
60ns
MBR6030L
30V
60A
0.48V
444CNQ045
45V
440A
0.69V
30CPQ150
150V
30A
1.19V
S chot t ky rect i fi ers
Fundamentals of Power Electronics
32
Chapter 4: Switch realization
4.2.2. The Power MOSFET
Source
• Gate lengths
approaching one
micron
Gate
n
p
n
n
p
nn
n
• Consists of many
small enhancementmode parallelconnected MOSFET
cells, covering the
surface of the silicon
wafer
• Vertical current flow
• n-channel device is
shown
Drain
Fundamentals of Power Electronics
33
Chapter 4: Switch realization
MOSFET: Off state
source
n
p
–
n
• p-n- junction is
reverse-biased
n
p
n
• off-state voltage
appears across nregion
depletion region
nn
drain
Fundamentals of Power Electronics
+
34
Chapter 4: Switch realization
MOSFET: on state
• p-n- junction is
slightly reversebiased
source
n
p
n
n
p
• drain current flows
through n- region
and conducting
channel
channel
nn
drain
Fundamentals of Power Electronics
n
• positive gate voltage
induces conducting
channel
drain current
35
• on resistance = total
resistances of nregion, conducting
channel, source and
drain contacts, etc.
Chapter 4: Switch realization
MOSFET body diode
• p-n- junction forms
an effective diode, in
parallel with the
channel
source
n
p
n
n
p
Body diode
• negative drain-tosource voltage can
forward-bias the
body diode
• diode can conduct
the full MOSFET
rated current
nn
• diode switching
speed not optimized
—body diode is
slow, Qr is large
drain
Fundamentals of Power Electronics
n
36
Chapter 4: Switch realization
Typical MOSFET characteristics
=1
DS
• On state: VGS >> Vth
V
V
=2
DS
V
10A
0V
00V
• Off state: VGS < Vth
V DS
ID
=2
• MOSFET can
conduct peak
currents well in
excess of average
current rating —
characteristics are
unchanged
on state
5A
V DS =
off
state
1V
V DS = 0.5V
0A
0V
5V
10V
VGS
Fundamentals of Power Electronics
37
15V
• on-resistance has
positive temperature
coefficient, hence
easy to parallel
Chapter 4: Switch realization
A simple MOSFET equivalent circuit
D
• Cgs : large, essentially constant
• Cgd : small, highly nonlinear
Cgd
G
• Cds : intermediate in value, highly
nonlinear
Cds
• switching times determined by rate
at which gate driver charges/
discharges Cgs and Cgd
Cgs
S
Cds(vds) =
Fundamentals of Power Electronics
C0
Cds(vds) ≈ C0
v
1 + ds
V0
38
C '0
V0
vds = vds
Chapter 4: Switch realization
Characteristics of several commercial power MOSFETs
Rated max voltage
Rated avg current
R on
Qg (typical)
IRFZ48
60V
50A
0.018Ω
110nC
IRF510
100V
5.6A
0.54Ω
8.3nC
IRF540
100V
28A
0.077Ω
72nC
APT10M25BNR
100V
75A
0.025Ω
171nC
IRF740
400V
10A
0.55Ω
63nC
MTM15N40E
400V
15A
0.3Ω
110nC
APT5025BN
500V
23A
0.25Ω
83nC
APT1001RBNR
1000V
11A
1.0Ω
150nC
Part number
Fundamentals of Power Electronics
39
Chapter 4: Switch realization
MOSFET: conclusions
●
●
●
●
●
●
●
A majority-carrier device: fast switching speed
Typical switching frequencies: tens and hundreds of kHz
On-resistance increases rapidly with rated blocking voltage
Easy to drive
The device of choice for blocking voltages less than 500V
1000V devices are available, but are useful only at low power levels
(100W)
Part number is selected on the basis of on-resistance rather than
current rating
Fundamentals of Power Electronics
40
Chapter 4: Switch realization
4.2.3. Bipolar Junction Transistor (BJT)
Base
• Interdigitated base and
emitter contacts
Emitter
• Vertical current flow
n
p
n
n
• minority carrier device
• on-state: base-emitter
and collector-base
junctions are both
forward-biased
nn
• on-state: substantial
minority charge in p and
n- regions, conductivity
modulation
Collector
Fundamentals of Power Electronics
• npn device is shown
41
Chapter 4: Switch realization
BJT switching times
vs(t)
Vs2
–Vs1
VCC
vBE(t)
0.7V
RL
iC(t)
iB(t)
vs(t)
+
–
RB
+
vBE(t)
–
–Vs1
+
iB(t)
IB1
vCE(t)
0
–
–IB2
vCE(t)
VCC
IConRon
iC(t)
ICon
0
(1) (2) (3)
Fundamentals of Power Electronics
42
(4)
(5)
(6)
(7)
(8)
(9)
t
Chapter 4: Switch realization
Ideal base current waveform
iB(t)
IB1
IBon
0
t
–IB2
Fundamentals of Power Electronics
43
Chapter 4: Switch realization
Current crowding due to excessive IB2
Base
Emitter
–IB2
p
–
–
n
–
+
+
–
–
n-
–
p
can lead to
formation of hot
spots and device
failure
n
Collector
Fundamentals of Power Electronics
44
Chapter 4: Switch realization
BJT characteristics
IC
n
gio
e
r
ve
acti
10A
ation
atur
asi-s
qu
CE
V CE = 5V
saturation region
slope
=β
5A
V CE = 200V
V = 20V
VCE = 0.5V
• Off state: IB = 0
• On state: IB > IC /β
• Current gain β decreases
rapidly at high current.
Device should not be
operated at instantaneous
currents exceeding the rated
value
VCE = 0.2V
cutoff
0A
0V
5V
10V
15V
IB
Fundamentals of Power Electronics
45
Chapter 4: Switch realization
Breakdown voltages
BVCBO: avalanche breakdown
voltage of base-collector
junction, with the emitter
open-circuited
IC
increasing IB
BVCEO: collector-emitter
breakdown voltage with zero
base current
IB = 0
open emitter
BVsus BVCEO
BVCBO
VCE
BVsus: breakdown voltage
observed with positive base
current
In most applications, the offstate transistor voltage must
not exceed BVCEO.
Fundamentals of Power Electronics
46
Chapter 4: Switch realization
Darlington-connected BJT
• Increased current gain, for high-voltage
applications
Q1
Q2
D1
Fundamentals of Power Electronics
• In a monolithic Darlington device,
transistors Q1 and Q2 are integrated on the
same silicon wafer
• Diode D1 speeds up the turn-off process,
by allowing the base driver to actively
remove the stored charge of both Q1 and
Q2 during the turn-off transition
47
Chapter 4: Switch realization
Conclusions: BJT
●
●
●
BJT has been replaced by MOSFET in low-voltage (<500V)
applications
BJT is being replaced by IGBT in applications at voltages above
500V
A minority-carrier device: compared with MOSFET, the BJT
exhibits slower switching, but lower on-resistance at high
voltages
Fundamentals of Power Electronics
48
Chapter 4: Switch realization
4.2.4. The Insulated Gate Bipolar Transistor (IGBT)
• A four-layer device
Emitter
• Similar in construction to
MOSFET, except extra p
region
Gate
n
p
n
n
n-
n
p
minority carrier
injection
• compared with MOSFET:
slower switching times,
lower on-resistance, useful
at higher voltages (up to
1700V)
p
Collector
Fundamentals of Power Electronics
• On-state: minority carriers
are injected into n- region,
leading to conductivity
modulation
49
Chapter 4: Switch realization
The IGBT
collector
Symbol
gate
Location of equivalent devices
emitter
C
n
Equivalent
circuit
n
p
i2
n
p
n
i1
n-
G
i1
p
i2
E
Fundamentals of Power Electronics
50
Chapter 4: Switch realization
Current tailing in IGBTs
IGBT
waveforms
iL
Vg
vA(t)
iA(t)
C
curr
e
nt ta
}
il
0
0
iL
t
diode
waveforms
iB(t)
0
0
t
vB(t)
G
i1
–Vg
i2
pA(t)
E
Vg iL
= vA iA
area Woff
t
t0
Fundamentals of Power Electronics
51
t1
t2
t3
Chapter 4: Switch realization
Characteristics of several commercial devices
Part number
Rated max voltage
Rated avg current
V F (typical)
tf (typical)
S i ngl e-chi p dev i ces
HGTG32N60E2
600V
32A
2.4V
0.62µs
HGTG30N120D2
1200V
30A
3.2A
0.58µs
M ul t i pl e-chi p p ow er m o dul es
CM400HA-12E
600V
400A
2.7V
0.3µs
CM300HA-24E
1200V
300A
2.7V
0.3µs
Fundamentals of Power Electronics
52
Chapter 4: Switch realization
Conclusions: IGBT
●
●
●
●
●
●
●
Becoming the device of choice in 500-1700V applications, at
power levels of 1-1000kW
Positive temperature coefficient at high current —easy to parallel
and construct modules
Forward voltage drop: diode in series with on-resistance. 2-4V
typical
Easy to drive —similar to MOSFET
Slower than MOSFET, but faster than Darlington, GTO, SCR
Typical switching frequencies: 3-30kHz
IGBT technology is rapidly advancing —next generation: 2500V
Fundamentals of Power Electronics
53
Chapter 4: Switch realization
4.2.5. Thyristors (SCR, GTO, MCT)
The SCR
construction
symbol
equiv circuit
K
G
Anode
Anode (A)
n
Q2
p
n-
Q1
Gate
Q2
p
Cathode
Fundamentals of Power Electronics
n
Q1
Gate (G)
Cathode (K)
K
A
54
Chapter 4: Switch realization
The Silicon Controlled Rectifier (SCR)
• Positive feedback —a latching
device
iA
forward
conducting
• A minority carrier device
• Double injection leads to very low
on-resistance, hence low forward
voltage drops attainable in very high
voltage devices
increasing iG
reverse
blocking
• Simple construction, with large
feature size
forward
blocking
iG = 0
vAK
reverse
breakdown
• Cannot be actively turned off
• A voltage-bidirectional two-quadrant
switch
• 5000-6000V, 1000-2000A devices
Fundamentals of Power Electronics
55
Chapter 4: Switch realization
Why the conventional SCR cannot be turned off
via gate control
K
G
• Large feature size
K
–iG
• Negative gate current
induces lateral voltage
drop along gate-cathode
junction
n
+
–
–
–
–
p
–
n
–
+
n-
• Gate-cathode junction
becomes reverse-biased
only in vicinity of gate
contact
p
iA
A
Fundamentals of Power Electronics
56
Chapter 4: Switch realization
The Gate Turn-Off Thyristor (GTO)
• An SCR fabricated using modern techniques —small feature size
• Gate and cathode contacts are highly interdigitated
• Negative gate current is able to completely reverse-bias the gatecathode junction
Turn-off transition:
• Turn-off current gain: typically 2-5
• Maximum controllable on-state current: maximum anode current
that can be turned off via gate control. GTO can conduct peak
currents well in excess of average current rating, but cannot switch
off
Fundamentals of Power Electronics
57
Chapter 4: Switch realization
The MOS-Controlled Thyristor (MCT)
Anode
• Still an emerging
device, but some
devices are
commercially available
Gate
n
• p-type device
• A latching SCR, with
added built-in
MOSFETs to assist the
turn-on and turn-off
processes
Q3 channel
Q4 channel
n
pn
• Small feature size,
highly interdigitated,
modern fabrication
Fundamentals of Power Electronics
n
p
Cathode
58
Chapter 4: Switch realization
The MCT: equivalent circuit
Cathode
• Negative gate-anode
voltage turns p-channel
MOSFET Q3 on, causing
Q1 and Q2 to latch ON
Q1
Q4
Gate
Q3
Q2
• Maximum current that can
be interrupted is limited by
the on-resistance of Q4
Anode
Fundamentals of Power Electronics
• Positive gate-anode
voltage turns n-channel
MOSFET Q4 on, reversebiasing the base-emitter
junction of Q2 and turning
off the device
59
Chapter 4: Switch realization
Summary: Thyristors
• The thyristor family: double injection yields lowest forward voltage
drop in high voltage devices. More difficult to parallel than
MOSFETs and IGBTs
• The SCR: highest voltage and current ratings, low cost, passive
turn-off transition
• The GTO: intermediate ratings (less than SCR, somewhat more
than IGBT). Slower than IGBT. Slower than MCT. Difficult to drive.
• The MCT: So far, ratings lower than IGBT. Slower than IGBT. Easy
to drive. Second breakdown problems? Still an emerging device.
Fundamentals of Power Electronics
60
Chapter 4: Switch realization
4.3. Switching loss
• Energy is lost during the semiconductor switching transitions, via
several mechanisms:
• Transistor switching times
• Diode stored charge
• Energy stored in device capacitances and parasitic inductances
• Semiconductor devices are charge controlled
• Time required to insert or remove the controlling charge determines
switching times
Fundamentals of Power Electronics
61
Chapter 4: Switch realization
4.3.1. Transistor switching with clamped inductive load
iA
+
vA
iL(t)
–
physical
MOSFET
+
–
vB
gate +
driver
+
–
Vg
–
DTs
Ts
transistor
waveforms
L
Vg
vA(t)
iL
iA(t)
ideal
diode
0
0
iL
iB
t
diode
waveforms
Buck converter example
iB(t)
0
0
t
vB(t) = vA(t) – Vg
i A(t) + iB(t) = iL
vB(t)
transistor turn-off
transition
–Vg
pA(t)
V g iL
= vA iA
W off =
1
2
area
Woff
VgiL (t 2 – t 0)
t0
Fundamentals of Power Electronics
62
t1
t2
t
Chapter 4: Switch realization
Switching loss induced by transistor turn-off transition
Energy lost during transistor turn-off transition:
W off =
1
2
VgiL (t 2 – t 0)
Similar result during transistor turn-on transition.
Average power loss:
Psw = 1
Ts
Fundamentals of Power Electronics
pA(t) dt = (W on + W off ) fs
switching
transitions
63
Chapter 4: Switch realization
Switching loss due to current-tailing in IGBT
+
vA
iL(t)
–
iL
physical
IGBT
vB
+
–
Vg
–
+
–
DTs
Ts
L
gate +
driver
Vg
vA(t)
iA(t)
curr
e
nt ta
ideal
diode
il
}
iA
IGBT
waveforms
0
0
iB
iL
t
diode
waveforms
Example: buck converter with IGBT
iB(t)
0
0
t
vB(t)
transistor turn-off
transition
–Vg
pA(t)
Psw = 1
Ts
Vg iL
= v A iA
pA(t) dt = (W on + W off ) fs
switching
transitions
area Woff
t0
Fundamentals of Power Electronics
64
t1
t2
t3
t
Chapter 4: Switch realization
4.3.2. Diode recovered charge
iA
+
vA
fast
transistor
+
–
–
+
–
Vg
iL(t)
–
vB
+
L
iA(t)
transistor
waveforms
Vg
silicon
diode
iL
vA(t)
0
iB
0
t
diode
waveforms
iL
iB(t)
vB(t)
• Diode recovered stored charge
Qr flows through transistor
during transistor turn-on
transition, inducing switching
loss
0
0
t
area
–Qr
–Vg
tr
• Qr depends on diode on-state
forward current, and on the
rate-of-change of diode current
during diode turn-off transition
Fundamentals of Power Electronics
Qr
pA(t)
= vA iA
area
~QrVg
area
~iLVgtr
65
t0
t1 t2
t
Chapter 4: Switch realization
Switching loss calculation
Energy lost in transistor:
iA(t)
transistor
waveforms
Soft-recovery
diode:
Qr
Vg
WD =
0
switching
transition
With abrupt-recovery diode:
iL
vA(t)
vA(t) i A(t) dt
(t2 – t1) >> (t1 – t0)
0
t
diode
waveforms
iL
iB(t)
vB(t)
0
WD ≈
0
t
Vg (iL – iB(t)) dt
area
–Qr
switching
transition
Abrupt-recovery
diode:
(t2 – t1) << (t1 – t0)
–Vg
= Vg i L t r + Vg Q r
tr
pA(t)
• Often, this is the largest
component of switching loss
= vA iA
area
~QrVg
area
~iLVgtr
Fundamentals of Power Electronics
66
t0
t1 t2
t
Chapter 4: Switch realization
4.3.3. Device capacitances, and leakage,
package, and stray inductances
• Capacitances that appear effectively in parallel with switch elements
are shorted when the switch turns on. Their stored energy is lost
during the switch turn-on transition.
• Inductances that appear effectively in series with switch elements
are open-circuited when the switch turns off. Their stored energy is
lost during the switch turn-off transition.
Total energy stored in linear capacitive and inductive elements:
WC =
1
CiV 2i
Σ
2
capacitive
WL =
elements
elements
Fundamentals of Power Electronics
1
L jI 2j
Σ
2
inductive
67
Chapter 4: Switch realization
Example: semiconductor output capacitances
Buck converter example
Cds
+
–
Cj
+
–
Vg
Energy lost during MOSFET turn-on transition
(assuming linear capacitances):
W C = 12 (Cds + C j) V 2g
Fundamentals of Power Electronics
68
Chapter 4: Switch realization
MOSFET nonlinear Cds
Approximate dependence of incremental Cds on vds :
Cds(vds) ≈ C0
C '0
V0
vds = vds
Energy stored in Cds at vds = VDS :
V DS
W Cds =
vds i C dt =
vds C ds(vds) dvds
0
V DS
W Cds =
0
C '0(vds) vds dvds = 23 Cds(VDS) V 2DS
— same energy loss as linear capacitor having value
Fundamentals of Power Electronics
69
4
3
Cds(VDS)
Chapter 4: Switch realization
Some other sources of this type of switching loss
Schottky diode
• Essentially no stored charge
• Significant reverse-biased junction capacitance
Transformer leakage inductance
• Effective inductances in series with windings
• A significant loss when windings are not tightly coupled
Interconnection and package inductances
• Diodes
• Transistors
• A significant loss in high current applications
Fundamentals of Power Electronics
70
Chapter 4: Switch realization
Ringing induced by diode stored charge
iL(t)
L
vi(t)
+
–
vL(t)
vi(t) +
–
silicon
diode
iB(t)
+
vB(t)
–
V1
t
0
–V2
C
iL(t)
• Diode is forward-biased while iL(t) > 0
• Negative inductor current removes diode
stored charge Qr
• When diode becomes reverse-biased,
negative inductor current flows through
capacitor C.
0
t
area
– Qr
vB(t)
t
0
–V2
• Ringing of L-C network is damped by
parasitic losses. Ringing energy is lost.
Fundamentals of Power Electronics
71
t1
t2
t3
Chapter 4: Switch realization
Energy associated with ringing
t3
Recovered charge is
Qr = –
iL(t) dt
vi(t)
V1
t2
t
0
Energy stored in inductor during interval
t2 ≤ t ≤ t3 :
t3
WL =
vL(t) iL(t) dt
t2
Applied inductor voltage during interval
t2 ≤ t ≤ t3 :
di (t)
vL(t) = L L = – V2
dt
Hence,
t3
t3
diL(t)
WL =
L
iL(t) dt =
( – V2) iL(t) dt
dt
t2
t2
–V2
iL(t)
0
vB(t)
t
0
–V2
W L = 12 L i 2L(t 3) = V2 Qr
t1
Fundamentals of Power Electronics
t
area
– Qr
72
t2
t3
Chapter 4: Switch realization
4.3.4. Efficiency vs. switching frequency
Add up all of the energies lost during the switching transitions of one
switching period:
W tot = W on + W off + W D + W C + W L + ...
Average switching power loss is
Psw = W tot fsw
Total converter loss can be expressed as
Ploss = Pcond + Pfixed + W tot fsw
where
Fundamentals of Power Electronics
Pfixed = fixed losses (independent of load and fsw)
Pcond = conduction losses
73
Chapter 4: Switch realization
Efficiency vs. switching frequency
Ploss = Pcond + Pfixed + W tot fsw
Switching losses are equal to
the other converter losses at the
critical frequency
100%
dc asymptote
fcrit
90%
Pcond + Pfixed
fcrit =
W tot
80%
η
This can be taken as a rough
upper limit on the switching
frequency of a practical
converter. For fsw > fcrit, the
efficiency decreases rapidly
with frequency.
70%
60%
50%
10kHz
100kHz
1MHz
fsw
Fundamentals of Power Electronics
74
Chapter 4: Switch realization
Summary of chapter 4
1. How an SPST ideal switch can be realized using semiconductor devices
depends on the polarity of the voltage which the devices must block in the
off-state, and on the polarity of the current which the devices must conduct
in the on-state.
2. Single-quadrant SPST switches can be realized using a single transistor or
a single diode, depending on the relative polarities of the off-state voltage
and on-state current.
3. Two-quadrant SPST switches can be realized using a transistor and diode,
connected in series (bidirectional-voltage) or in anti-parallel (bidirectionalcurrent). Several four-quadrant schemes are also listed here.
4. A “synchronous rectifier” is a MOSFET connected to conduct reverse
current, with gate drive control as necessary. This device can be used
where a diode would otherwise be required. If a MOSFET with sufficiently
low Ron is used, reduced conduction loss is obtained.
Fundamentals of Power Electronics
75
Chapter 4: Switch realization
Summary of chapter 4
5. Majority carrier devices, including the MOSFET and Schottky diode, exhibit
very fast switching times, controlled essentially by the charging of the
device capacitances. However, the forward voltage drops of these devices
increases quickly with increasing breakdown voltage.
6. Minority carrier devices, including the BJT, IGBT, and thyristor family, can
exhibit high breakdown voltages with relatively low forward voltage drop.
However, the switching times of these devices are longer, and are
controlled by the times needed to insert or remove stored minority charge.
7. Energy is lost during switching transitions, due to a variety of mechanisms.
The resulting average power loss, or switching loss, is equal to this energy
loss multiplied by the switching frequency. Switching loss imposes an
upper limit on the switching frequencies of practical converters.
Fundamentals of Power Electronics
76
Chapter 4: Switch realization
Summary of chapter 4
8. The diode and inductor present a “clamped inductive load” to the transistor.
When a transistor drives such a load, it experiences high instantaneous
power loss during the switching transitions. An example where this leads to
significant switching loss is the IGBT and the “current tail” observed during
its turn-off transition.
9. Other significant sources of switching loss include diode stored charge and
energy stored in certain parasitic capacitances and inductances. Parasitic
ringing also indicates the presence of switching loss.
Fundamentals of Power Electronics
77
Chapter 4: Switch realization