Demystifying active-clamp flyback loop compensation. Pei-Hsin Liu

Transcription

1 Demystifying active-clamp flyback loop compensation Pei-Hsin Liu

2 What will I get out of this session? Purpose: 1. Analyze the small-signal properties of CCM and TM operations of ACF 2. Address the benefit and stability issue of burst mode operation of ACF 3. Introduce design guides based on the analytical model and two simple ripple compensation methods to stabilize the burst control loop Relevant End Equipment: 1. High density AC adapter or charger 2. USB power delivery chargers 3. AC/DC or DC/DC auxiliary power supply

3 Content Small-signal model of current-mode controlled ACF Burst mode operation as light load operation of ACF Ripple compensation to stabilize burst mode control Serial damping for ACF with π output filter Summary

4 Quasi-resonance (QR) vs. active clamp (ACF) QR ACF V bulk Clamping loss Switching loss EMI Recycle L k energy w/o clamping loss Zero voltage switching (ZVS)

5 TI active clamp flyback (ACF) vs. existing solutions Power density (W/in 3 ) W/in 3 * 27 W 22 W/in 3 * 45 W 11 W/in 3 65 W 30 W/in 3 * 65 W Best OEM devices today 65W QR 65W ACF (94% pk) 9 W/in 3 15 W 7 W/in 3 45 W Power level (W) * Open frame power density

6 Efficiency Efficiency difference on 45W adapter (22W/in 3 ) Condition: (1) same RM8LP XFRM (2) same EMI filter (3) same output CLC filter (4) similar f SW range Si QR Si ACF GaN ACF 90V AC 92.12% (f sw =237kHz) 93.12% (f sw =206kHz) 94.14% (f sw =227kHz) 265V AC 89.93% (f sw =413kHz) 93.51% (f sw =285kHz) 94.63% (f sw =295kHz) 20V/45W efficiency with UCC28780: GaN ACF Si ACF Si QR Si ACF provides 3.6% improvement over Si QR at 265V AC V AC With same EMI filter, Si ACF is 1% lower than GaN ACF at 90V AC

7 Cont. conduction mode (CCM) vs. transition mode (TM) CCM V CST / R CS TM V CST / R CS I M I M PWML t D(L-H) t D(H-L) PWML t D(L-H) ZVS loop t D(H-L) PWMH PWMH f SW PWMH turns off by a programmable clock PWML turns on after t D(H-L) delay PWML turns off by a peak current loop (Legacy controller: UCC289x) PWMH turns off by a separate ZVS loop (like a negative valley current loop) PWML turns on after t D(H-L) delay PWML turns off by a peak current loop (New controller: UCC28780)

8 Small-signal property in CCM operation UCC289x V CST to V O transfer function: Gain (db) X RHP zero (right half plane) f SW /2 double pole f SW O X X ESR zero of C O O Need slope compensation to damp f SW /2 double pole to stabilize the peak current loop as duty cycle > 50% Phase delay of RHP zero limits system bandwidth Phase (⁰) Frequency Frequency

9 Small-signal property in TM operation UCC28780 V CST to V O transfer function: Gain (db) X ESR Zero of C O PWML off by peak current loop; PWMH off by ZVS loop Inherently stable; no need slope compensation No RHP zero results in a higher bandwidth design Phase (⁰) Frequency Frequency O

10 Proposed modeling methodology for TM ACF I SEC V CST /R CS difficult I QL I M Describe I QL mathematically I QL I M(-) 1 V N V ( I ) 2 R V N V CST PS O M ( ) CS BULK PS O I I I I V V V V V V SEC SEC SEC SEC CST BULK O I SEC V CST BULK O Small-signal perturbation BULK V O I QL From energy balance w/o describing resonance current

11 Small-signal model and simulation verification I M(-) K e R e V CST VBULK CSW LM N V / [2 R ( V N V )] PS BULK CS BULK PS O 2 2 RCS ( VBULK N PSVO ) 2 N PS VBULK ( VCST RCS I M ( ) ) 2 P ( V N V ) N V V I in BULK PS O PS O BULK M ( ) N V V PS O BULK VO ( s) ReRL 1 scorco Ke VCST () s Re R RR L e L 1 sco[ RCo ] R R R CS Phase (degree) Gain (db) e L X Model SIMPLIS O ESR zero LF pole f sw / k 10k 100k 1M Frequency (Hz)

12 Content Small-signal model of current-mode controlled ACF Burst mode operation as light load operation of ACF Ripple compensation to stabilize burst mode control Serial damping for ACF with π output filter Summary

13 DC/DC efficiency DC/DC efficiency Issue of ACF light load efficiency I M (Heavy load) Condition: P o(max) =30W, V o =20V, sec. Schottky 0.92 (Lighter load) i m(+) : Delivers energy to output i m(-) : Stores energy used for ZVS (no contribution to output power) Si FET 0.95 i i m( ) m( ) High ratio is good Lower ratio results in lower eff! GaN FET % V bulk =70V V bulk =325V % V bulk =70V V bulk =325V % 25% 50% 75% 100% Load % 25% 50% 75% 100% Current mode control only can not maintain light load efficiency Load

14 Optimize avg. eff. with burst setting of UCC28780 I M(+) I M(BUR) I M I M(BUR) I O PWML PWMH f SW I O R V BUR2 BUR 5 V R BUR1 R BUR2 ɧ ( I R ) K m( BUR) CS CST BUR 50~60% I O where V REF = 5V, K CST-BUR = 4 V/V The programmable burst mode is simple to optimize the efficiency

15 Adaptive burst mode (ABM) of UCC28780 Condition: P OUT(MAX) = 45 W, V O = 20 V, RM8LP, Q L & Q H (650V/500mΩ/GaN), SR (150V/9.3mΩ/Si) 100% ɧ 95% 90% 115V AC 230V AC 85% (A) COC DOE Avg. eff. 94.2% at 115V 93.6% at 230V PWML I pri V sw PWML I pri V sw PWML I pri V sw 10% load 25% load 25% Load 50% load

16 Simplified ABM loop with ripple regulator V O I FB I REF I REF I FB RUN PWML V RCS V CST - Feedback loop filters part of the switching-ripple and retains burst ripple for regulation - Down slope of output voltage ripple generated by the output load discharging the output capacitor - The down slope of feedback signal (I FB ) intersecting with I REF to trigger next burst packet

17 Content Small-signal model of current-mode controlled ACF Burst mode operation as light load operation of ACF Ripple compensation to stabilize burst mode control Serial damping for ACF with π output filter Summary

18 Stability issue from phase delay of feedback signal Regulation method: Compensation principle: Make I FB in-phase with V O burst ripple Note: not switching ripple V O I FB Consequence of large phase delay between I FB and V O : Subharmonic oscillation

19 Solution: passive ripple compensation I FB( s) CTR Z 0 s 1 ( s / Z1) 1 1 V ( s) R s 1 ( s / ) 1 ( s / ) 1 ( s / ) O BIAS1 P1 opto FB (2) R P1 (1) BIAS1 O( ABM ) I FB( ABM ) R DIFF CTR 1 C DIFF V 1 1 (3) opto Z1 ( RFB RFBI ) C opto ( R R ) C 1 (4) FB with 100~220pF of C ( R / / R ) C FB FB FBI FB 1 (5) Z 0 << crossover frequency of AAM ( R R ) C Vo1 INT INT ( I FB(ABM) =10~20μA) > Max burst frequency (35kHz) x 2 DIFF BIAS1 DIFF - R BIAS1 and C DIFF compensate phase delay from optocoupler - ω P1 and ω FB attenuate switching ripple but not burst ripple

20 Stability issue with low-esr cap - Power stage: primary resonance ACF in ABM of UCC Condition: V BULK =120V, V O =20V, I O =0.5A, C O =680μF Closed loop with electrolytic cap Closed loop with polymer cap PWML PWML V O V O I PRI I PRI Always 4 PWML pulses per burst packet 4~14 PWML pulses in different burst packets Burst-ripple magnitude with low-esr cap is too small to maintain consistent burst package, so the noise-sensitive burst loop impairs output ripple and aggravates audible noise

21 Stability issue with 2 nd -order filter Filter effect on burst ripple -Single cap -With CLC filter (π filter) Close loop test with π filter V O L O V O V O C O C O1 C O2 I FB I REF V O V O L O & C O1 resonance creates ringing on output ripple, which may trigger next burst package prematurely Every first L O & C O1 ringing reaches I REF, so the adjacent burst bundles together and results in higher voltage ripple, amplified low-frequency audible noise

22 Solution: active ripple compensation (ARC) Q COMP : 2N7002 (SOT-323); R COMP =1~2MΩ Used for ACF with low-esr output capacitor or 2 nd -order output filter I COMP to push the undesirable ripple and switching noise away from intersection point with I REF, so consistent burst packets can be obtained

23 ARC performance with low-esr output cap - Power stage: primary-resonance ACF in ABM of UCC Condition: V BULK =120V, V O =20V, I O =0.5A, C O =680μF using polymer capacitor No ARC: With ARC: C O PWML PWML V O V O I PRI I PRI 4~14 PWML pulses in different burst packets Always 3 PWML pulses per burst packet ARC is effective on ABM with low-esr output capacitor

24 ARC performance with output π filter - Power stage: secondary-resonance ACF in ABM of UCC Condition: V BULK =120V, V O =20V, I O =0.5A, C O1 =66μF, L O =1μH, C O2 =680μF (ceramic) (polymer) No ARC: C O1 With ARC & serial damping: L O C O2 Serial damping PWML PWML V O V O I PRI L O & C O1 resonance I PRI 4~14 PWML pulses in different burst packets Always 3 PWML pulses per burst packet ARC + weak serial damping stabilizes ABM with 2nd-order output filter

25 Content Small-signal model of current-mode controlled ACF Burst mode operation as light load operation of ACF Ripple compensation to stabilize burst mode control Serial damping for ACF with π output filter Summary

26 Issues of 2 nd -order filter w/o damping for ACF I PRI I V L K SEC L O BULK V O V CLAMP - + C CLAMP L M + - V Co1 C O1 C O2 (1) 1kHz audible noise (2) Erratic SR operation PWML PWMH V gs(sr) V Co1 Q H Q SR V Co2 V Co2 I M flows to the low-volt. side first I PRI I PRI N PS V Co1 > V CLAMP : N PS I PRI > I SEC N PS V Co1 < V CLAMP : N PS I PRI < I SEC Inconsistent resonance current every switching cycle Cause: large L O & C O1 resonance ripple changes V Co1 every cycle, so the resonance current on I PRI is not consistent

27 Concept serial damping technique Bode plot of V O (s) / I SEC (s): Gain (db) X X W/o damping With damping Damping ratio of L O C O1 double pole: 1 RDAMP C 2 1 ( L / L ) L O1 DAMP O O Phase (⁰) Serial damping can effectively reduce peaking of L O C O1 double pole

28 Design and trade-off of damping strength R DAMP L DAMP L O Strong damping: L DAMP 0.13 L O R DAMP L DAMP = 150nH results in 0.5% eff drop at 90V AC L C O O1 C O1 C O2 Design example: V AC =90V, V O =20V, P O =45W, Secondary-resonance ACF, C O1 =66μF ceramic, C O2 =680μF polymer L O =1μH Weak damping: L DAMP 0.13 L O R DAMP L DAMP = 680nH results in 0.15% eff drop at 90V AC Too strong damping traps AC current in the damping circuit and affects full load efficiency L C O O1

29 ACF with weak damped output π filter - Power stage: secondary-resonance ACF in ABM of UCC Condition: V BULK =120V, V O =20V, I O =0.5A, C O1 =66μF (ceramic), L O =1μH, C O2 =680μF (polymer) - Weak damping: L DAMP =680nH, R DAMP =0.68Ω Without serial damping (ARC only): I V PRI BULK V CLAMP - + C CLAMP L K L M Q H With weak serial damping : R L DAMP DAMP I SEC V O L + O V Co1 Q SR - C O1 C O2 V gs(sr) V gs(sr) V Co1 V Co2 V Co1 V Co2 I PRI I PRI Weak seral damping ensures consistent SR driving pulse and ABM operation

30 Component selection on the weak damping One small SMD inductor UCC W EVM R DAMP L DAMP L O C O1 C O2 L O C O2 L DAMP =680nH, R DAMP =0.68Ω R DAMP + L DAMP (1206 size Inductor) The intrinsic resistance of the chip inductor is a free R DAMP C O1 (1206 x 2)

31 Summary The efficiency advantage of ACF with TI s new ACF chipsets is demonstrated and compared with QR on a high-density 45W adapter operating > 130kHz A unique small-signal modeling technique for ACF is proposed and the distinctive plant characteristic under CCM and TM operations are compared The light load efficiency advantage of ACF in burst mode is demonstrated and the stability and SR operation issues are highlighted Two ripple compensation techniques and a serial damping method are described that effectively stabilize both burst control loop and SR operation

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