Design of 200W LLC Resonant HB

Sober Hu (胡炎申)世纪电源网-版主（斜阳古道）

Texas Instruments SemiconductorDec 2009

Technical Challenges of Front-End AC/DC Converter

Efficiency and Power Density

Hold-Up Time

Limitation of PWM Converter

Operation of LLC Resonant HB

Operation Principles (fsw=fr)

At resonant frequency, maximum efficiency is expected

Cr Lr

Lm

n:1:1

Vin

Vo

RL

Q1

Q2

D1

D2

**

*

t0 t1 t2 t3

Vg_Q1Vg_Q2

Vp

ir

im

is

When switching frequency is below resonant frequency, magnetizing inductor begins to participate in resonant and increase voltage gain

Secondary diode becomes discontinuous

Operation Principles (fsw<fr)

When switching frequency is above resonant frequency, circuit behaves as SRC

Secondary current becomes CCM, reverse recovery loss increases

Operation Principles (fsw>fr)

FHA of LLC Resonant HB

)//(/1//4

4

2

||1

1

RacsLpsLrsCrRacsLp

VV

V

Vn

n

VinnVoG

S

P

S

P

++=

=== π

π

12

21 VsVinVinVs ππ

=⇒=

poorp Vn

VVnnVV4

41

ππ

=⇒==

2222

22

2

22

2

22

)1()]11(11[

1

1]1)[(1)(

)(

)1(]1)([

)1(

)()(1

)()(

//)(1//)(

4

4

2

),,(

nn

nn

rrr

r

nn

ffQ

fL

RacCrLr

LrLmj

LrLrLm

LrLm

LrCrLmjRacCrLrLmLmCrRac

LmRacjCrjLrCrRacLmLr

CrLm

LmRacj

RacLmjRacLmjLrj

Crj

RacLmjRacLmj

RacLmjLrjCrj

RacLmj

VV

V

Vn

n

VinnVoQLfM

F

d

F

Ro

F

d

F

Ro

−⋅+−⋅+=

⋅⋅⋅−⋅+−+

⋅

⋅=

−⋅⋅+−+=

+−

+−=

+++

+=

++=

===

ωω

ωω

ωω

ωω

ωωωω

ωωωω

ω

ωωω

ω

ωω

ωωω

ω

π

π

LrLmLn =

r

swn f

ff =

RacCrLrQ 1

⋅=

out

out

IVnRac ⋅= 2

28π

LrCrfr

π21

=

CrLrLmfp

)(21+

=π

Voltage Gain Equation

1=nL 5=nL

10=nL 15=nL

Voltage Gain of Different Ln

Ln ↑ → Traditional SRC → Enter into ZCS @ fsw<fr → lost gain monotony and MOSFET ZVS operation→ much wider fsw range, not good hold-up time performance

Ln ↓ → if low inductance → Im ↑ → conducted and switched-off losses ↑→ if high inductance, fixed resonant frequency → Cr ↓ → Vcr ↑

“A Novel Precise Design Method for LLC Series Resonant Converter”, Teng liu, etc., INTELEC ‘06

Larger Q value gives smaller start up current with less frequency range

Start Up Current Consideration

Advantages of LLC Resonant HB

Primary ZVS can be achieved with wide load rangeSecondary ZCS can be achieved when fs<frSmaller switch-off loss due to small turn off currentCapacitor filter w/o output inductorLess voltage stress on rectifiers>1 voltage gain during hold-up time

Step by Step Design of 200W LLC

Minimize RMS current under normal operation conditionEnsure ZVS operationEnsure desired input voltage operation range

Design Targets

fsw=fr @ normal operation)sin(2)( _ Φ+= tIti oPRMSLr ω

⎪⎪⎩

⎪⎪⎨

⎧

>−−=

<+−=

2),

2()(

2,)(

_

_

TstTstLmnViti

TsttLmnViti

omLmLm

omLmLm

4Ts

LmnVi o

Lm ⋅=

2

]4

)sin(2[

2

)]()([ 20 _2

0

Ts

dttLmnVTs

LmnVtI

Tsdttiti

nRV

Tsoo

oPRMS

Ts

LmLr

L

o∫∫ −⋅+Φ+

=−

=ω

22

224

_ 424

1 π+=m

SL

L

oPRMS L

TRnnRVI

2

)]()([20

2

_ Tsdttintin

I

Ts

LmLrSRMS

∫ ⋅−⋅=

42

2242

_ 12)485(24

3 ππ

π+

−=

m

SL

L

oSRMS L

TRnRVI

4)sin(2)0( _

TsLmnVIti o

PRMSLr ⋅−=Φ=

fsw<fr @ hold-up time

Nominal Vin 440VMinimum Vin 360V

Maximum Vin 54VOutput Power 200WResonant Frequency 100kHz

Electrical Parameters

F

PFC

VVoutVn

+⋅=

12

1. Winding Turn Ratio of TX

fsw=fr at nominal condition，M=1，So

n=4

2. Magnetizing Inductance

42

2242

_ 12)485(24

3 ππ

π+

−=

m

SL

L

oSRMS L

TRnRVI

22

224

_ 424

1 π+=m

SL

L

oPRMS L

TRnnRVI

The magnetizing inductance impacts conducted and switched losses，So

On the other hand，the maximum magnetizing current also impacts ZVS operation.

j

oPFC

oLmp

jPFCLmp

CtdTsLm

nVV

TsLmnVI

tdCV

I

162

4

2

⋅<⇒

⎪⎪⎪

⎩

⎪⎪⎪

⎨

⎧

=

=

>

jswj Cftd

CtdTsLm

1616=

⋅=

td=175ns

Cj=195pF, SPP20N60C3

Lm=530

3. Maximum Peak Gain

minmax V

VG PFC= Gmax=1.222

4. Decision Ln and Q

L

mon

n

RnLfQL

LrLmL

RacCrLr

Q

⋅=⇒

⎪⎪

⎩

⎪⎪

⎨

⎧

=

=

2

282

π

π

1 3 5 7 9 11 13 15 17 19 200.05

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.951

Ln

Q

1.3

1.3

1.3

1.3

1.6

1.61.6

1.61.6

2

22

22

2.52.5

2.52.5

33

33

Constant Lm

IntersectionFrom the intersection of curves of LnQ and required maximum peak gain to get the appropriate Ln and Q.

Ln=6

Q=0.3

5. Calculation of Lr and Cr

nLLmLr =

LrfCr

o2)2(

1π

=

Lr=68uH, additional 20uH leakage inductor in main transformer

Cr=15nF*2，two capacitor structure to reduce input ripple current

Converter Specification

Primary switching Device

O

in

VVn2

=

Magnetizing inductor

j

deadm C

tTL16⋅

=

1 3 5 7 9 11 13 15 17 19 200.05

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.951

Ln

Q

1.3

1.3

1.3

1.3

1.6

1.6

1.6

1.61.6

2

2

2

22

2.52.5

2.5

2.5

33

3

3

Constant Lm

Lm decreases

1 3 5 7 9 11 13 15 17 19 200.05

0.15

0.25

0.35

0.45

0.55

0.65

0.75

0.85

0.951

Ln

Q

1.3

1.3

1.3

1.3

1.6

1.6

1.6

1.61.6

2

2

2

22

2.52.5

2.5

2.5

33

3

3

Constant LmConstant Lm

Lm decreasesLm decreases

Peak gain enough? Yes

Choose cross section of required gain and

constant Lm

No

Reduce Lm

Resonant inductor

n

mr L

LL =

Resonant capacitor

rrCLf

π21

0 =

L

mn

RnLf

QL

2

20

82

π

π=

Flow Chart of Optimal Design

Check gain monotony,min and max fsw, Vcr etc.

Test of 200W LLC Resonant HB

Benefits:•Interleaved PFC with UCC28061•LLC Resonant HB with UCC25600•93% Efficiency w/o Secondary SR

Eff=96.61%

Possible Technical Issues

time/礢ecs 200nSecs/div

23 23.2 23.4 23.6 23.8 24 24.2 24.4 24.6 24.8 25

A

-4

-2

0

2

4

6

8

time/礢ecs 200nSecs/div

19.6 19.8 20 20.2 20.4 20.6 20.8 21 21.2 21.4

A

-4

-2

0

2

4

6

8

time/礢ecs 200nSecs/div

20.2 20.4 20.6 20.8 21 21.2 21.4 21.6 21.8 22 22.2 22.4

A

-4

-2

0

2

4

48% and 52% 49% and 51%

50% and 50%

Asymmetrical duty cycle makes resonant tank current unbalanced

Load current will be concentrated in one diode and increase conduction loss and switching loss- Controller should provide well matched PWM signal- Different secondary leakage inductance

LLC Issue-Imbalance Current

LLC Issue-Over Current

Add Clamping Diode

Lr

Lm

Q1

Q2

Cr/2

VinRoCo

Cr/2

Lr

Lm

Q1

Q2

Cr/2

VinRoCo

Cr/2

Iin

IQ1

VCR

Iin

IQ1

VCR

ID1

Over Load Operation

Part of resonant tank energy could be feedback to source, which helps limit output current

LLC Issue-MOSFET Failure @ Light Load

To achieve ZVS operation, the body diode of primary MOSFETs conducts first, and then the channel circulates the current.At light load, the reverse voltage between body diode is much small once the channel circulates the small current, such that the reverse recovery time in body diode is much long, resulting in primary MOSFETs shoot-through.

• Bing Lu, “Investigation of High-density Integrated Solution for AC/DC Conversion of a Distributed Power System”, Ph. D. dissertation, Virginia Tech, 2006http://scholar.lib.vt.edu/theses/available/etd-06262006-111218/

• Bo Yang, “Topology investigation of front end DC/DC converter for distributed power system “ Ph. D. dissertation, Virginia Tech, 2003 http://scholar.lib.vt.edu/theses/available/etd-09152003-180228/

References

Back-up

TI UCC25600 8 Pin Resonant Half Bridge Controller

1

2

3

4 5

6

7

8 GATE1

VCC

GND

GATE2

Td

RT

OC

SS

FeaturesAdjustable Soft start (1ms to 500ms)Adjustable dead timeAdjustable Fswmax & Fswmin (3% accuracy)

Io = +1A /-1.5AEnable (ON/OFF control)

Protection functionsTwo levels over current protection

auto recovery latch

Bias voltage UV and OV protectionOver temperature protectionSoft start after all fault conditions

SOT 8 pin package= Easy design and layout

Ready to Order

Application Circuit360~400V

DC_OUT

Vcc

11

11

11

11

Td

RT

OC

SS

Gate1

Gate2

Vcc

GND

Programmable dead timeFrequency control with minimum/maximum frequency limitingProgrammable soft start with on/off controlTwo level over current protection, auto-recovery and latch upMatching output with 50ns tolerance

Q & A

Thanks!