Post on 01-Jan-2017
www.fairchildsemi.com
Design Considerations for an LLC Resonant Converter
Hangseok ChoiPower Conversion Team
2
1. Introduction
Growing demand for higher power density and low profile in power converter has forced to increase switching frequency
However, Switching Loss has been an obstacle to high frequency operation
Capacitive loss Reverse recovery lossOverlap of voltage and current
High frequency operation
3
1. Introduction
Resonant converter: processes power in a sinusoidal manner and the switching devices are softly commutated
Voltage across the switch drops to zero before switch turns on (ZVS)• Remove overlap area between V and I when turning on• Capacitive loss is eliminated
Series resonant converter / Parallel resonant converter
4
1. Introduction
+
VO
-
Ro
Q1
Q2
n:1Ip
Lr
Lm
CrIds2
Vd
Vin
resonant network
Series Resonant (SR) converter
The resonant inductor (Lr) and resonant capacitor (Cr) are in series The resonant capacitor is in series with the load
The resonant tank and the load act as a voltage divider DC gain is always lower than 1 (maximum gain happens at the resonant frequency)The impedance of resonant tank can be changed by varying the frequency of driving voltage (Vd)
5
1. Introduction
Series Resonant (SR) converter
AdvantagesReduced switching loss and EMI through ZVS Improved efficiencyReduced magnetic components size by high frequency operation
DrawbacksCan optimize performance at one operating point, but not with wide range of input voltage and load variations Can not regulate the output at no load conditionPulsating rectifier current (capacitor output): limitation for high output current application
6
1. Introduction
+
VO
-
Ro
Q1
Q2
n:1Ip
Llkp
Cr
Ids2
Vd
Vin
resonant network
Parallel Resonant (PR) converter
The resonant inductor (Lr) and resonant capacitor (Cr) are in series The resonant capacitor is in parallel with the load
The impedance of resonant tank can be changed by varying the frequency of driving voltage (Vd)
7
1. Introduction
AdvantagesNo problem in output regulation at no load conditionContinuous rectifier current (inductor output): suitable for high output current application
DrawbacksThe primary side current is almost independent of load condition: significant current may circulate through the resonant network, even at the no load conditionCirculating current increases as input voltage increases: limitation for wide range of input voltage
Parallel Resonant (PR) converter
8
1. Introduction
What is LLC resonant converter?Topology looks almost same as the conventional LC series resonant converterMagnetizing inductance (Lm) of the transformer is relatively small and involved in the resonance operation Voltage gain is different from that of LC series resonant converter
+
VO
-
Ro
Q1
Q2
n:1Ip
Lr
Lm
CrIds2
Vd
Im
IDVin
resonant network
Io
+
VO
-
Ro
Q1
Q2
n:1Ip
Lr
Lm
CrIds2
Vd
Vin
resonant network
LC Series resonant converter LLC resonant converter
9
1. Introduction
Features of LLC resonant converter
- Reduced switching loss through ZVS: Improved efficiency- Narrow frequency variation range over wide load range- Zero voltage switching even at no load condition
- Typically, integrated transformer is used instead of discrete magnetic components
10
1. Introduction
Integrated transformer in LLC resonant converterTwo magnetic components are implemented with a single core (use the primary side leakage inductance as a resonant inductor)One magnetic components (Lr) can be savedLeakage inductance not only exists in the primary side but also in the secondary sideNeed to consider the leakage inductance in the secondary side
+
VO
-
Ro
Q1
Q2
n:1
Lr
Lm
CrIds2
Vd
IDVin
Io
+
VO
-
Ro
Q1
Q2
n:1Ip
Llkp
Lm
CrIds2
VdLlks
Im
IDVin
Integrated transformer
Io
11
2. Operation principle and Fundamental Approximation
Square wave generator: produces a square wave voltage, Vd by driving switches, Q1 and Q2 with alternating 50% duty cycle for each switch. Resonant network: consists of Llkp, Llks, Lm and Cr. The current lags the voltage applied to the resonant network which allows the MOSFET’s to be turned on with zero voltage. Rectifier network: produces DC voltage by rectifying AC current
Ip
Ids2
Vd(Vds2)
Vgs2
Im
Vin
ID
Vgs1
+
VO
-
Ro
Q1
Q2
n:1Ip
Llkp
Lm
CrIds2
VdLlks
Im
IDVin
Square wave generator
resonant network Rectifier network
Io
12
2. Operation principle and Fundamental Approximation
The resonant network filters the higher harmonic currents. Thus, essentially only sinusoidal current is allowed to flow through the resonant network even though a square wave voltage (Vd) is applied to the resonant network.
Fundamental approximation: assumes that only the fundamental component of the square-wave voltage input to the resonant network contributes to the power transfer to the output.
The square wave voltage can be replaced by its fundamental component
Resonant network
Vd
IpIsec
Resonant network
Vd
IpIsec
13
2. Operation principle and Fundamental Approximation
+VRI
-
Io
+
VO
-
Iac
pkacI
Iac
VRI
4 sin( )F oRI
VV wtπ
=Vo
)sin(2
wtII oac
⋅=π
Ro
VRIF
2 2
8 8F FoRI RI
ac oFac ac o
VV VR RI I Iπ π
= = = =
Because the rectifier circuit in the secondary side acts as an impedance transformer, the equivalent load resistance is different from actual load resistance.
The primary side circuit is replaced by a sinusoidal current source (Iac) and a square wave of voltage (VRI) appears at the input to the rectifier. The equivalent load resistance is obtained as
14
2. Operation principle and Fundamental Approximation
AC equivalent circuit (L-L-L-C)
VO
Lm
LlkpCr
Ro
Vin
VdF VRO
FLm
LlkpCr
Rac
Llks
n:1
n2Llks
Vd+
-
2
2
8ac o
nR Rπ
=
+
-
VRI
+
- 2
2 22
2 2
4 sin( ) 24 sin( )
2
(1 ) ( ) (1 )
oF F
RO oRIF F
ind d in
m ac r
m lks aco p
n V tV n Vn VM VV V Vt
L R C
j L n L R
ωπ
ωπ
ωω ωωω ω
⋅⋅⋅
= = = =
=⋅ − ⋅ + + −
2
2
2
8
1 1,
, //( )
ac o
o pr r p r
p m lkp r lkp m lks
nR R
L C L C
L L L L L L n L
π
ω ω
=
= =
= + = +
- Lr is measured in primary side with secondary winding short circuited
- Lp is measured in primary side with secondary winding open circuited
15
2. Operation principle and Fundamental Approximation
Simplified AC equivalent circuit (L-L-C)
VO
Lm
LlkpCr
Ro
Vin
VinF VRO
FLp-Lr
LrCr
Llks
n:1
Vd+
--
+
VRI
+
-
1: p
p r
LL L−
2//( )
//r lkp m lks
lkp m lkp
L L L n L
L L L
= +
= +
p lkp mL L L= +
acR
Assuming Llkp=n2Llks
2
2
2 2 2
2 2
( )12
( 1)( ) (1 ) (1 )2 1
pO
in
o o p
kkn VMkV j Qk
ωω
ω ω ωω ω ω
+⋅= =
+⋅ − ⋅ + −
+
2
2
2 2
2 2
( )2
( ) (1 ) (1 )
p r
p pO
pin
o o r p
L LLn VM LV j QL
ωω
ω ω ωω ω ω
−
⋅= =
⋅ − ⋅ + −
Expressing in terms of Lp and Lr
/r r
ac
L CQ
R= m
lkp
LkL
=
- Lr is measured in primary side with secondary winding short circuited
- Lp is measured in primary side with secondary winding open circuited
16
2. Operation principle and Fundamental Approximation
Gain characteristicsTwo resonant frequencies (fo and fp) existThe gain is fixed at resonant frequency (fo) regardless of the load variation
Peak gain frequency exists between foand fpAs Q decreases (as load decreases), the peak gain frequency moves to fp and higher peak gain is obtained. As Q increases (as load increases), peak gain frequency moves to fo and the peak gain drops
LLC resonant Converter
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
40 50 60 70 80 90 100 110 120 130 140
freq (kHz)
Gain
Q = 1
Q= 0.8
Q= 0.6
Q= 0.4
Q= 0.2
fofp
1 p
p r
LkMk L L+
= =−
Q=0.2
Q=1
/r r
ac
L CQ
R=
@1
o
p
p r
LkMk L Lω ω=+
= =−
17
2. Operation principle and Fundamental Approximation
Peak gain (attainable maximum gain) versus Q for different k values
1
1.2
1.4
1.6
1.8
2.0
2.2
2.4
0.2 0.4 0.6 0.8 1 1.2 1.4
k=1.5
k=1.75
k=2
k=2.5
k=3
k=4k=5k=7
Q
Peak
Gai
n
k=9
18
3. Design procedure
Design example- Input voltage: 380Vdc (output of PFC stage)- Output: 24V/5A (120W)- Holdup time requirement: 17ms- DC link capacitor of PFC output: 100uF
VO+
-
Ro
Q1
Q2
Np:NsIp
Llkp
Lm
CrIds2
VdLlks
Im
ID
PFC
VDL
CDL
DC/DC
19
3. Design procedure
[STEP-1] Define the system specificationsEstimated efficiency (Eff) Input voltage range: hold up time should be considered for minimum input voltage
min 2.
2 in HUin O PFC
DL
P TV VC
= −
20
3. Design procedure
[STEP-2] Determine the maximum and minimum voltage gains of the resonant network by choosing k- it is typical to set k to be 5~10, which results in a gain of 1.1~1.2 at fo
2min
max1
2
m lkpRO m lks
in m m
L LV L n L kMV L L k
++ += = = =
maxmax min
minin
in
VM MV
=
( / )m lkpk L L=
fo
1 1.14kMk+
= =
fs
Gain (M)
Mmin
Mmax for Vinmin
for Vinmax
1.36
1.14
Peak gain (available maximum gain)
21
3. Design procedure
[STEP-3] Determine the transformer turns ratio (n=Np/Ns)
maxmin
2( )p in
s o F
N Vn MN V V
= = ⋅+
[STEP-4] Calculate the equivalent load resistance (Rac)2 2
28 o
aco
n VRPπ
=
22
3. Design procedure
[STEP-5] Design the resonant network- With k chosen in STEP-2, read proper Q from gain curves
max7 , 1.361.36 110% 1.5
k Mpeak gain= =
= × =
23
3. Design procedure
[STEP-6] Design the transformer- Plot the gain curve and read the minimum switching frequency. Then, the minimum number of turns for the transformer primary side is obtained as
minmin
( )2
o Fp
s e
n V VNf B A
+=
⋅Δ ⋅
24
3. Design procedure
[STEP-7] Transformer Construction- Since LLC converter design results in relatively large Lr, usually sectional bobbin is typically used - # of turns and winding configuration are the major factors determining Lr- Gap length of the core does not affect Lr much- Lp can be easily controlled with gap length
Design value: Lr=234uH, Lp=998uH
Np=52T Ns1=Ns2=6TBifilar
25
3. Design procedure
[STEP-8] Select the resonant capacitor
2 2( 2 )[ ] [ ]2 2 4 2r
RMS o o FC
o m
I n V VIn f L
π + ⋅≅ +
maxmax 2
2 2r
RMSin Cr
Co r
V IVf Cπ
⋅≅ +
⋅ ⋅ ⋅
26
4. Conclusion
Using a fundamental approximation, gain equation has been derived
Leakage inductance in the secondary side is also considered (L-L-L-C model) for gain equation
L-L-L-C equivalent circuit has been simplified as a conventional L-L-C equivalent circuit
Practical design consideration has been presented
27
Appendix - FSFR-series
Variable frequency control with 50% duty cycle for half-bridge resonant converter topologyHigh efficiency through zero voltage switching (ZVS)Internal Super-FETs with Fast Recovery Type Body Diode (trr=120ns)Fixed dead time (350ns)Up to 300kHz operating frequencyPulse skipping for Frequency limit (programmable) at light load conditionSimple remote ON/OFF controlVarious Protection functions: Over Voltage Protection (OVP), Over Current Protection (OCP), Abnormal Over Current Protection (AOCP), Internal Thermal Shutdown (TSD)
28
Appendix - FSFR-series demo board
29
Appendix - FSFR-series demo board