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EET426 Power Electronics II
Isolated Forward Converter
1EET 426 – Power Electronis II
Prepared by : Mohd Azrik Roslan
2
DC Transformer conceptIsolated buck converter circuitIsolated converter advantages and disadvantagesIdeal and real transformer reviewLeakage inductanceOverlap loss
What you should know after this lecture
EET 426 – Power Electronis II
EET 426 – Power Electronis II 3
DC Transformer Concept
4EET 426 – Power Electronis II
Buck Converter
Ei n
Vout C
R
L
Basic converter have several limitations:• Single input Single output• No isolation Can cause prob during fault• Output voltage relative to the input (based on duty cycle only)
5EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
TRANSFORMERINSERTION
POINT
6EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
NOT REQUIRED:
DC TRANSFORMER CONCEPT
7EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
ON
winding polarity (dot notation) ensures D1 forward biased when mosfet is onDfwd reverse biased when mosfet is on
resulting in forward transfer of energy
D1
Dfw
d
8EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
LOW-SIDE mosfet: easier drive requirements
D1
Dfw
d
9EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
D1
Dfw
d
10EET 426 – Power Electronis II
ProblemWhen the transistor switch is off the
transformer core must be fully reset by the end of the switching cycle.
This is to avoid core saturation and the resulting increase in switch current the next time it turns on. The time available for reset reduces as the
switch duty cycle increases hence reset must be possible during the minimum switch off -time.
11EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
D1
Dfw
d
Drese
t
12EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
D1
Dfw
d
Drese
t
ON
+ +
+
OFF
OFF
ON
MOSFET is ON
13EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
L
sw
D
on
on
off D V V V
p s r
E in
D r D off
n n
n r p s
p inV E
MOSFET is ON
s ss p in
p p
n nV V E
n n
r rr p in
p p
n nV V E
n n
,sw sw onV V
Dr in rV E V
14
ISOLATED BUCK CONVERTER
EET 426 – Power Electronis II
Ei n
Vout C
R
L
D1
Dfw
d
Drese
t OFF
winding polarity (dot notation) ensures Dreset forward biased when mosfet is offD1 reverse biased when mosfet is off
ON
15EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
Ei n
Vout C
R
L
D1
Dfw
d
Drese
t
OFF
+
+
+
ON
ON
OFF
MOSFET is OFF
16EET 426 – Power Electronis II
ISOLATED BUCK CONVERTER
L
sw
D
off
off
on
on
D
D
V V V p s r
E in
D r
I p,swon
n n n r p s
pp in
r
nV E
n
MOSFET is OFF
s ss r in
r r
n nV V E
n n
r inV E
1 psw in p in
r
nV E V E
n
,Dr DrV V on
17EET 426 – Power Electronis II
nr < np nr = np nr > np
Dsw < 0.5
Dsw = 0.5
Dsw > 0.5
Dsw >>0.5 requires reduced nr
→ increased Vp → Vsw ↑ → limit
nr = np OK for Dsw ≤ 0.5
limit
p
rsw
n
nD
1
1
o
V p t
D<0.5
E in
mag.
de mag
o
V p t
D<0.5
E
-E
in
in
mag.
demag. o
V p t
D<0.5
E in
mag.
demag.
o
V p
t D=0.5
E in
mag.
de mag
o
V p t
D=0.5
E
-E
in
in
mag.
demag o
V p
t D=0.5
E in
mag. demag ex
o
V p t
D>0.5
E in
mag.
demag. o
V p
t D>0.5
E
-E
in
in
mag.
demag. excess o
V p t
D>0.5
E in
mag.
demag. excess
inr
pE
n
n
inr
pE
n
n
inr
pE
n
n
inr
pE
n
n
inr
pE
n
n
inr
pE
n
n
18EET 426 – Power Electronis II
Possible to have multiple outputs
No common input-output connection (increase safety) Indirect converter
Output voltage polarity choiceDepending upon winding polarity
Output voltage can be varied dependent uponTurns ratioDuty cycle
Isolated converter advantages
19EET 426 – Power Electronis II
ExtraCostSizeWeight
Increase lossesWinding resistanceCore loss
Leakage inductance overlapReduce output voltageProduce transient voltages
Possible to have core saturationNeed core reset
Isolated converter disadvantages
EET 426 – Power Electronis II 20
Leakage Inductance Overlap
21EET 426 – Power Electronis II
Ideal Transformer
VPRIM VSEC
IPRIM ISEC NP NS
secP
Sprim V
N
NV
secsecprimprim IVIV
POWER BALANCE
=100%
P
S
prim
secN
N
V
V
secS
Pprim I
N
NI
22EET 426 – Power Electronis II
FunctionTransfer energyScale current and voltage
Magnetic device
Provide electrical isolation
No energy storage
Ideal Transformer
23EET 426 – Power Electronis II
Have some energy storage
Magnetizing inductance (within core)Can be minimized by usinggapless coreHigh permeability core
Leakage inductance (external to core)
Low frequency core saturation
SMPS frequency core loss
REAL TRANSFORMER
24EET 426 – Power Electronis II
REAL TRANSFORMERNP NS
RC LM
IMAG
practical magnetic cores have finite permeability
magnetising current IMAG required to establish core flux
effect represented by magnetising inductance LM
CORE LOSS represented by RC
25EET 426 – Power Electronis II
REAL TRANSFORMER
NP NS
RC LM
RP RS
winding copper loss
represented by PRIMARY & SECONDARY winding resistance
EET 426 – Power Electronis II 26
Leakage InductanceParasitic Element
27EET 426 – Power Electronis II
NP NS
RC LM
RP RSLLP LLS
TRANSFORMER LEAKAGE INDUCTANCEinductive parameter of transformers (& inductors )
due to imperfect magnetic linking between windings
magnetic flux that does not link primary-secondary windings
represented as primary & secondary series inductive impedances
P
leak,PPleak,P I
NL
S
leak,SSleak,S I
NL
28EET 426 – Power Electronis II
Leakage Inductance Effects
ISOLATED FORWARD CONVERTER
Vout Ei n
C
R
L
SCH1
SCH2
VS,2 VS
VP
LL.P LL.s
NP: NS
NORMALLY STEP DOWN
29EET 426 – Power Electronis II
LEAKAGE INDUCTANCE MEASUREMENT
LCR meter primary : LL,P +LPRIM
NP NS
LPRIM
LL,P
LCR meter
OPENCIRCUIT
LEAKAGE INDUCTANCE is an INTEGRAL PROPERTY of the
TRANSFORMER
IMPOSSIBLE to measure DIRECTLY
30EET 426 – Power Electronis II
LEAKAGE INDUCTANCE MEASUREMENT
LCR meter primary : LL,P
NPNS
LPRIM
LL,P
LCR meter
PERFECT
SHORTCIRCUIT
31EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER
Vout Ei n
C
R
L
SCH1
SCH2
VS,2 VS
VP
LL.P LL.s
NP: NS
NORMALLY STEP DOWN
VS < VP IS > IP
32EET 426 – Power Electronis II
IMPEDANCE TRANSFORMATION
NP NS
VSECVPRIM
ISECIPRIM
ZSEC
ZPRIM
2
prim
secprimsec N
NZZ
STEP DOWN CONVERTERS
2
sec
primsecprim N
NZZ
33EET 426 – Power Electronis II
IMPEDANCE TRANSFORMATION
STEP DOWN CONVERTERS
LL(reflected)< LL,P
LL,P
VSECVPRIM
NSNP
VSECVPRIM
LL(reflected)
2
p
sP,L)reflected(leak N
NLL
34EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER
VS < VP IS > IP
Vout Ei n
C
R
L
SCH1
SCH2
VS,2 VS
VP
NP: NS
NORMALLY STEP DOWN
LL.P LL.r
35EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER: OVERLAP
0Vgs
Ids0
ISCH1 0
ISCH20
Vout
Ei n
C
R
L SCH1
SCH2
VS,2 VS
VP
LL,
r
NP
NS
2 overlap intervals
SCH1 & SCH2 ON
LEAKAGE INDUCTANCE prevents
instantaneous commutation between SCH1 SCH2
SCH2 ON VS,2 = 0
36EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER: OVERLAP
0Vgs
Ids0
ISCH1 0
ISCH20
0VSEC
Vs,2
VL,r
0
0
Vout
Ei n
C
R
L SCH1
SCH2
VS,2 VS
VP
LL,
r
NP
NS
OVERLAP INTERVAL 2NO EFFECT on Vout
DUTY CYCLE REDUCTION
OUTPUT VOLTAGE LOSS DUE to OVERLAP
OVERLAP INTERVAL 1
DswTsw
0 tovlapTsw
37EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER: OVERLAP
0Vgs
0VSEC
Vs,2
VL,r
0
0
Vout
Ei n
C
R
L SCH1
SCH2
VS,2 VS
VP
LL,
r
NP
NS
DswTsw
0 tovlapTsw
sec
swoutr,L
sw
ovlapoverlap,sw V
fIL
T
tD
p
sdsin
swoutr.Loverlap,sw
n
nVE
fILD
ovlap
outr,Lr,Lr,Lsec t
IL
dt
dILVV
during overlap
sec
outr,Lovlap V
ILt
Effective Duty Cycle LOSS
38EET 426 – Power Electronis II
ISOLATED FORWARD CONVERTER: OVERLAP
0Vgs
0VSEC
Vs,2
VL,r
0
0
Vout
Ei n
C
R
L SCH1
SCH2
VS,2 VS
VP
LL,
r
NP
NS
DswTsw
0 tovlapTsw
swoutr,Lsw
ovlap
ovlap
outr,L
sw
ovlapovlapave.out fIL
T
t
t
IL
T
tVV
ovlap
outr,Lovlap t
ILV
during overlap
Vout LOSS NON EFFICIENCY RELATED Vout LOSS
EET 426 – Power Electronis II 39
Isolated Forward Converter
Analysis
40EET 426 – Power Electronis II
input voltage Ein
transformer primary turns np
transformer secondary turns ns
secondary current maximum Is(max)
primary current max Ip(max)
transistor switch duty cycle Dsw
transistor on-state resistance rdson
transistor power loss Pmosfet
output current Iout
inductor ‘resistance’ rind
inductor ‘rind’ power loss Pr,ind
rectifier D1 and D2 voltage drops VF DATA SHEETrectifier D1 and D2 current max Iak,max
combined D1 and D2 duty cycle Drects 1
combined D1 and D2 loss Prects
transformer primary loss Pprim
transformer secondary loss
Psec
pnsn
(max)sI
dsonpsw rID 2
max
indoutindrmsind rIrI 22,
outak II max,
Fakrects VID max,
primpsw rID 2
max
sec2
max rID ssw
41EET 426 – Power Electronis II
total power loss Ploss,total
input powerPin
output powerPout
efficiency
‘ideal’ output voltage
Vout(ideal)
‘real’ output voltage
Vout real
sec, PPPPP primrectsindrmosfet
maxpswininavin IDEIE
lossin PP
in
out
P
P
pnsn
swin DE
)(idealoutV
42EET 426 – Power Electronis II
txer leakage inductance (ref prim)
Lleak(prim)
txer leakage inductance (ref sec)
Lleak(sec)
average output voltage ‘overlap’ loss
output voltage Vout
2
)(
p
sprimleak n
nL
overlapV swoutleak fIL (sec)
)( )( overlapidealout VV
43EET 426 – Power Electronis II
‘effective’ duty cycle loss
output voltage Vout
PNsN
inE
swfoutIleakL
(sec)
pnsn
swswin DDE )(
overlapswD ,
44EET 426 – Power Electronis II
Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine
i. the isolated forward converter output voltageii. the isolated forward converter efficiencyiii. the mosfet voltage requirement
Example 1
Ein
200 V
100 : 100 : 10
L
r ind
esr
C Vout
f SW = 50 kHz
R
Iout =10A D1
D2
200 V
Dsw = 0.3
L
rind Rds(on) Lleakage
(ref to secondary) VF :
D1 D2
2.0 mH
0.04 0.3 2.0 H 0.9 V
45EET 426 – Power Electronis II
Example 2
Ein
200 V
200 : 200 : 20
L
r ind
esr
C Vout
f SW = 40 kHz
R
Iout =10A D1
D2
rind VF : D1 D2
0.04 0.9 V
200 V
Dsw = 0.4
Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine
i. the isolated forward converter output voltageii. the isolated forward converter efficiency