design considerations for an llc resonant converter ppt

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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