Active Clamp Flyback Using GaN Power IC forPower Adapter Applications
Linqxiao (Lincoln) Xue, Staff Applications Engineer, [email protected] Zhang, VP Applications, [email protected]
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, [email protected] Zhang, VP Applications, [email protected]
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