Capabilities of power devices
Courtesy of
Wikipedia (Cyril
Progress in IGBTs
Courtesy of
Infineon 2011
Evolution of power semiconductor
devices
Active devices are a large fraction of the total system cost actual design try to minimize the number of active devices used
and their maximum ratings (cost)
Progress in Power devices DRIVE changes in circuit choices and
market adoption.
Examples:
• power MOSFETs —> switched-mode power supplies
• IGBT -> Energy efficient motor drives with inverters
Next
New materials: SiC, GaN -> Class D audio amplifier, inverter for
motion control – AC-DC and DC-DC power supply
Alternative semiconductors
Bandgap at Room T
(eV)
Thermal conductivity
(W/(cm K))
Max Temp. (C)
Max Electric Field
(V/m)
Saturation velocity
(cm/s)
Si
GaAs
SiC
GaN
1.12
1.43
2.2-3
3.4
1.5
0.5
5
1.3
150
3e5
300
4e5
600-1000
4e6
400
3e6
1e7
2e7
2.5e7
2.5e7
GaN and SiC have better DC
figures of merit
AlGaN/GaN
Courtesy of Transphorm Inc.
Power versus frequency
Courtesy of
Infineon 2011
SiC
• SiC diodes, SiC JFETs, SiC MOSFETs
• SiC JFET (Infineon)
GaN-AlGaN MIS-HEMT
Only 2 producers now: IRF and EPC
• No pn junctions (only majority carriers)
• Lateral device (reduced capacitances, high fields in the upper
layers)
DC-DC Converters
DC-DC Converters
Typical uses:
• DC Power supplies
• DC Motor drive
Types of converters
• Step-down (buck)
• Step-up (boost)
• Buck-boost
• Cuk
• Full-Bridge
Ideal concept of step-down
converter with PWM* switching
(* Pulse Width Modulation)
input
output
load
Assumptions: Switches, L, C are lossless, DC input has zero internal
impedance, load is an equivalent R
This cannot work: 1. Load is inductive and can destroy switch by
dissipating all stored energy, 2. output voltage must be continuous
Step-down (buck) converter
DC power supplies, DC motor drives -- Vo < Vd
Low-pass filter keeps
output voltage
constant
Note: 2nd order non
dissipative filter
1 1
=
≪ 2 Diode avoids voltage
spike on switch (when
switch is off, diode
provides current to L)
Continuous-conduction mode
Limit of continuous conduction
If the ripple amplitude ≡
= , the converter is at the limit
of continuous conduction (i.e. = 0)
! " #$
! 1 " #
≡
=
=
= %& 4# 1 " #
2
2
2
Discontinuous-conduction mode
with constant Vd Motor drives
()*+ =
()*+
()*+
!
"
#$
=
∆- $
!
#
=
# + ∆-
! $
#∆=
# + ∆-
#∆= 4/01*2
# + ∆-
()*+ # + ∆- $
$
=
2
= 4/01*2 #∆-
#$
!
∆- $
#
= # + /(4%& )
Limits of continuous-discontinuous
conduction (constant Vd)
Continuous
> 4# 1 " #
/01*2
=#
!
Discontinuous
< 4# 1 " #
/01*2
!
#
=
# +
4/01*2
Discontinuous-conduction with
DC voltage
constant Vo
supply
At the limit of continuous conduction
$
(1 " #)
=
= /01*2 1 " #
2
We can write D explicitly from:
()*+ =
∆- $
= 2/01*2 ∆-
()*+ # + ∆ =
= /01*2 ∆- # + ∆2
/01*2
=
!
#
1"
!
#=
!
! %&
# + ∆=
#
1"
!
9- Discontinuous-conduction with
DC voltage
constant Vo
supply
Continuous: > # >1"
%&
#=
!
Discontinuous: < # <1"
#=
%&
! %&
1"
!
9- Output voltage ripple
First order calculation:
The average iL flows in the
load, and the ripple
component in C.
Additional charge:
1 ∆ $
∆: =
2 2 2
Current ripple:
∆ = ( /) 1 " # $
Voltage ripple:
∆:
∆=
=
$
1 " #
8
∆
=
(1 " #) 2
Step-up (boost) converter
• DC power supplies
• Regenerative breaking of
DC motors
Output voltage always larger
than the input
Switch on diode off, output
isolated, L accumulates energy
from input
Switch off diode on, load
receives energy from input
and from L
Continous-conduction mode
Periodic conditions:
<= ! <>> ! " +
=0
if <= = #$
and
<>> = (1 " #)$
$
!
+ $
1 " #
1
=
1"#
!
No losses:
= ! !
=0
Continuous-discontinuous boundary
Average current in L
= ripple :
! =
2
1 " # $
#
=
2
Average output
current at the limit:
= 1 " #
$
1 " # #
=
2
?@ AB
,
C
? A
%& = @ B
D
ILB is max if D=0.5 %& =
IoB is max if D=1/3 =
D
E
1 " # #%&
Discontinuous conduction mode
(constant Vo)
Periodic conditions:
#$
! ∆- $
! " +
=0
# !
=1+
=
∆- !
Average current in L
#$
! # + ∆- $
! $
=
2
Average output current
$
!
∆=
#∆ = !
2
# + ∆27
!
!
=
%& #
4
" !
4
#=
27
!
!
"1
%&
Continuous-discontinuous mode
(constant Vo)
Continuous mode:
> 27 1 " # #
= %&
4
!
# =1"
Discontinuous mode:
< 4
#=
27
!
!
"1
%&
Losses and ripple
Losses: inductor, capacitor, switch, diode
Ripple: first order assumption: when the switch is on the C is
discharged through the load
∆
∆: #$
#$
=
=
=
G
∆
$
=#
H
Buck-boost converter
Negative DC power supply
Switch on: inductance
accumulates energy, diode off,
C supplies the load
Switch off: diode on,
inductance transfers energy to
the capacitance and to the load
Periodic conditions in
continuous conduction mode:
#$
!
1 " # $
"
=0
Digitare l'equazione qui.
!
#
!
=
=
1 " # = + ! =
1"#
Continuous-discontinuous boundary
Current in L at the boundary
#$
!
=
2
Output current at the boundary:
$
= 1 " # =
1"#
2
= %& 1 " #
= %& 1 " #
Discontinuous conduction
Periodic conditions:
# ! $
∆- $
"
=0
# !
=
=
∆- !
Average current in L:
! #$
# + ∆- $
$
=
2
Therefore:
#
! $
=
# # + ∆ = 1 +
∆2
!
= #∆= #
%&
!
→#=
!
%&
Continuous-discontinuous mode
Continuous operation
> = %& 1 " #
#=
" Discontinuous operation
> !
#=
!
%&
Output voltage ripple
When the switch is ON, C is discharged through the load
∆
∆: #$
∆
$
=
=
→
=#
G
H
Cuk DC-DC converter
Negative DC power supply
DC analysis: Y- = ! + note: ( Y- > ! )
Assumption: Large C1 (Voltage almost constant)
Switch OFF: C1 is charged through L1 and the input, Diode ON,
L2 supplies energy to R (currents in L1 and L2 decrease)
Switch ON: L1 receives energy, Diode OFF, C supplies current to
R, C1 gives energy to L2 (currents in L1 and L2 increase)
Cuk
Periodic conditions in L1
! #$
+ 1 " # $
! "
Y-
=
Y-
=
Y-
=0
!
1"#
Periodic conditions in L2
Y- " #$
" V< 1 " D $
= 0
#
Therefore
#
=
1"#
!
Pro: currents
in L1 and L2 ripple free
Con: C1 must be large
Full bridge DC-DC converter
When switch TA+ is on:
> 0: through TA+
< 0: through DA+
a)
=
dutycycle($`
[\
!
When switch TB+ is on:
< 0: through TB+
> 0: through DB+
a)
=
dutycycle($b
\
!
= [\ " \
Four quadrant operation
on , PWM with bipolar
voltage switching
When defg > defh ,
TA+ and TB- are ON
Duty cycle
1 defg 1
#- = +
i
2
2
efh
When defg < defh ,
TA- and TB+ are ON
# = 1 " # = [\ " \ = #- ! " #
= 2#- " 1 !
=
!
i
efh
defg
!
PWM with unipolar
voltage switching
When defg > defh ,
TA+ and TB- are ON
Duty cycle
1 defg 1
#- = +
i
2
2
efh
When "defg < defh ,
TA- and TB+ are ON
# = 1 " # = [\ " \
= #- ! " # !
= 2#- " 1 !
=
!
i
efh
defg
Less ripple
w.r.t. the
bipolar case
because
frequency of
Vo is double
PWM signal generation