Active Clamp Flyback Using GaN Power IC forPower Adapter Applications

Linqxiao (Lincoln) Xue, Staff Applications Engineer, lingxiao.xue@navitassemi.comJason Zhang, VP Applications, jason.zhang@navitassemi.com

March 29th 2017

How to Improve Power Adapter Density?

2

Samsung 25 W5 W/in3

Apple 45 W7 W/in3

Traditional Travel Adapter and Chargers USB PD and Quick Charge

> 20 W/in3

• Added power in USB PD and Quick Charge requires dramatically higher power density (>20 W/in3)

• Higher efficiency and lower power loss are required in high density adapters• How to dramatically improve the power density?

Outline

• Limitations of standard flyback• Active clamp flyback’s benefits and drawbacks• Improvement of active clamp flyback (ACF)• GaN half-bridge power IC enables high density ACF• Experimental results and conclusion

3

Resonant for valley switching

QR Flyback Hits Performance Ceiling

4

• Frequency-dependent losses• Leakage inductance loss • Snubber/clamp losses • Partial hard-switching loss at high line• Slow turn-on loss to minimize EMI

• Difficult to improve efficiency at high frequency

iLr

VSW

S1

iLm

id

S1 ON S1 OFF

iLr

Vsw

id

iLm

Lossy RCD clamp Lossy hard switching

ACF Enables ZVS and High Frequency Switching

5

• No snubber losses, all leakage energy is recovered• ZVS soft switching over entire operation range• ZCS soft turn-off for output rectifier• Clean waveforms reduce EMI• Enable small adapter design with high-frequency switching

Vsw

S1

iLm

Lossless snubberZero-voltage switching

S2Vsw

iLm iLr

Zero-current switching

ACF Operation

6

• Lr resonates with Cr during S2 ON interval• ZVS is achieved by magnetizing inductance current• Rectifier current is the difference between iLm and iLr

• iLr returns to iLm by the end of S2 ON interval for rectifier ZCS

S1Vsw

iLm

iDiLr

S2Cr

VCrnVo

iLm

iLr iD/n

+-

Cr

VIN

VO

Zero-current Switching (ZCS)

S1 ON S2 ON

Vsw

iLm iLr

iDCr

VCr=nVonVo

iLm

iLr = 0

+-

t = ∞

n:1

VCr nVo

Drawbacks of Traditional ACF

• Difficult to shape the current to achieve ZCS and minimize rms current

• Conduction loss is high• Not compatible with SR controllers

7

QR ACF

iLr iLr

Rectifier current double dips, confusing SR

Increased primary RMS current

iLr (1 A/div) iD (5 A/div)

iD

New ACF Using Secondary Resonance

8

• S1 ON interval is the same as primary resonant• In S2 ON interval

• Co/n2 << Cr , Co resonates with Lr• iLr centers around a line lower but in parallel with iLm

• ZCS is easily achieved. No rectifier current double dipping

S1Vsw

iLm

iDiLr

VIN

VO

Zero-current Switching (ZCS)

S1 ONS2 ON

Vsw

iLm

iLr

iD

VCr

nVo

IO

Co/n2VCr

nVoiLm

iLr iD/n

+-

IO/n

n:1

S2Cr

Co

iLm-Io/n

Big Cr, small Co

VCrnVoiLm

iLr=iLm-Io/n

+-

IO/nt = ∞

Io/n

New ACF Simplifies SR and Reduces Current RMS

• iLr current can be easily shaped• No rectifier current dipping issue. Simplifies SR• Reduced rms current and conduction loss

9

iLm iLr

iD

iLr (1 A/div) iD (5 A/div)

Experimental Result of RMS Reduction

10

0.60.70.80.9

11.1

100 150 200 250 300 350

Curr

ent (

A)

Input Voltage (V)

TX Primary Current iLr(rms)*

Pri. Resonant

Sec. Resonant2.7

33.33.63.94.2

100 150 200 250 300 350

Cuur

ent (

A)Input Voltage (V)

TX Secondary Current iD(rms)*

Pri. Resonant

Sec. Resonant

*Measured results of 45W ACF

Sec. ResonantiLr

Pri. Resonant

Sec. ResonantiD

Pri. Resonant

24% reduction

11% reduction

2.42.62.8

33.23.43.63.8

100 150 199.9 249.8 299.75 349.7

Pow

er L

oss (

W)

Input Voltage (V)

Total Power Loss (W)

Efficiency Benefit of New ACF

11

Pri. ResonantSec. Resonant

• 0.4 W power loss reduction • ~0.8% efficiency improvement

92.0%

92.5%

93.0%

93.5%

94.0%

94.5%

95.0%

100 150 200 250 300 350Input Voltage (V)

Efficiency

Pri. Resonant

Sec. Resonant

100 150 200 250 300 350

*Measured results of 45W ACF

Advantages of Using GaN in ACF

• GaN ACF needs only 0.2A negative current for ZVS vs. Si’s 0.5A

• GaN ACF RMS is only 0.9A vs. Si’s 1.1A• GaN has no body diode loss• Low high-frequency gate-charge loss

12

GaN: NV6260

iLr(RMS) = 0.9A

iLr (1 A/div) VSW (100 V/div)VSR (20 V/div)1 μs/div

Si: IPA60R299CP

iLr(RMS) = 1.1A

iLr

iLr (1 A/div) VSW (100 V/div)VSR (20 V/div)1 μs/div

IPA60R299CP NV6260 (per FET)

Voltage Rating (V) 650 650

RDS(ON) 270 160

Co(tr) (pF) 120 50

Qg (nC) 22 2.5

Qrr (nC) 3900 0

iLr

-0.2A

-0.5A

Half-Bridge iDrive GaN Power IC

• Internal level-shift, bootstrap

• Range from 150-600 mOhm (650V)• Single component

• Ground-referenced PWM signals

• Active Clamp Flyback, Half-Bridge, LLC, etc.

13

GaN Power IC Simplifies ACF

14

NV6260

GaN IC

GaN Power IC Efficiency Advantage

15

• GaN reduces power loss by 0.6W• GaN boosts efficiency by 1.2%

91.9%91.5%92.0%92.5%93.0%93.5%94.0%94.5%95.0%

99.8 149.8 199.8 249.8 299.8 349.8

Input Voltage (V)

Efficiency: GaN vs. Si

3.3

3.9

1.5

2

2.5

3

3.5

4

4.5

99.8 149.8 199.8 249.8 299.6 349.55

Pow

er L

oss (

W)

Input Voltage (V)

Power Loss: GaN vs. Si

SiGaN Si

GaN

93.1%

100 150 200 250 300 350

*Measured results of 45W ACF

High Density 65W Adapter Using ACF and GaN2.63 x 1.32 x 0.62 inPower density 22 W/in3 (including case)

16

Diode bridge: 90.9°C

90VAC, 65W

92.6%

94.0%94.5%

94.4%

90.0%91.0%92.0%93.0%94.0%95.0%

90 120 150 180 210 240Input AC Voltage (V)

Efficiency at 65W

GaN IC84.1°C

90VAC, 65W

Tansformer75.1°C

90VAC, 65W

Conclusion

• USB-PD and QC demand high density solutions• QR flyback hits performance ceiling• ACF overcomes QR limitations and enables high frequency operation• New ACF using secondary resonance improves ACF’s operation and

efficiency • GaN is uniquely suitable for high frequency operation• Half-Bridge GaN Power IC simplifies ACF design and improves density• An example of 65W ACF adapter using GaN and secondary resonance

achieves 22W/in3 density, while meeting thermal and EMI requirement

Navitas Confidential 17

Active Clamp Flyback Using GaN Power IC forPower Adapter Applications

Linqxiao (Lincoln) Xue, Staff Applications Engineer, linxiao.xue@navitassemi.comJason Zhang, VP Applications, jason.zhang@navitassemi.com

March 29th 2017

Backup

19

Waveform: QR vs. ACF

20

QR flyback

Active clamp flyback

Startup in CCM and Hard-Switching

• Start-up in CCM: hard-switched, body diode reverse recovery..

21

Navitas GaNHalf Bridge IC

iLr (2A/div)Vsw (100V/div)

iLm

S1 OFFS2 ON

iSR

iLr

nVCo

VCr

iLm

nVCo

0

0

Lr Resonant Interval

22

iLm

iLr

iSR Co

n:1

Initially resonates up

Initially nVCo > VCr

Cr

Initially resonates down

Initially nVCo < VCr

S1 OFFS2 ON

VCr

0

0

VCr

nVCo

iSR

iLr

iLm

CrVCr

nVCo

+ -nVCo-VCr

iLm

iLr iSR

+-

• Cr << Co/n2

• SR double turn-on

Primary Resonant

• Cr >> Co/n2

• No SR double turn-on

Secondary Resonant

Co/n2VCr

nVCo

+ -nVCo-VCr

iLm

iLr iSR

+-

ACF Primary Current Dip

• Bigger Cj/Coss

bigger current dip• Why is current dip

important?23

S1 OFF(S2 ON)

S1 ON(S2 OFF)

Current dip

iLr

iLr

iLm

iLm

Coss

Coss

CjiLr

iLm

iLr

Cj

Coss

Coss

iD

iLm

iLmCj/Coss = 0.5Cj/Coss = 1Cj/Coss = 2

Current Dip Benefits

24

iLm

• Current dip SR double turn-on

SR double turn-on

iLr

• Current dip RMS value iLr(RMS)

less dip

iLr

More dipiLr

How to Maximize iLr Dip

Cj

ZVS current

Coss

Use better device: GaN

MaximizeCj/Coss

SR single turn-onSR single turn-on

25

iLr(500 mA/div) Vsw (100 V/div) VSR (20 V/div)

iLr(500 mA/div) Vsw (100 V/div) VSR (20 V/div)

1 μs/div

1 μs/div

Current Dip: Adding 2.2nF CjOriginal

Added 2.2nF

iLr(RMS) = 0.74A

iLr(RMS) = 0.66A