Design Considerations for an LLC Resonant Converter

Transcription

1 Design Considerations for an LLC Resonant Converter Hangseok Choi Power 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 High frequency operation Overlap of voltage and current Capacitive loss Reverse recovery loss

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 3

4 1. Introduction Series Resonant (SR) converter Q1 resonant network V in Q V d I p L r n:1 R o V O L m I ds Cr 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 (V d ) 4

5 1. Introduction Series Resonant (SR) converter Advantages Reduced switching loss and EMI through ZVS Improved efficiency Reduced magnetic components size by high frequency operation Drawbacks Can 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 condition Pulsating rectifier current (capacitor output): limitation for high output current application 5

6 1. Introduction Parallel Resonant (PR) converter Q1 resonant network V in Q V d I p L lkp n:1 R o V O C r I ds 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 (V d ) 6

7 1. Introduction Parallel Resonant (PR) converter Advantages No problem in output regulation at no load condition Continuous rectifier current (inductor output): suitable for high output current application Drawbacks The primary side current is almost independent of load condition: significant current may circulate through the resonant network, even at the no load condition Circulating current increases as input voltage increases: limitation for wide range of input voltage 7

8 1. Introduction What is LLC resonant converter? Topology looks almost same as the conventional LC series resonant converter Magnetizing inductance (L m ) of the transformer is relatively small and involved in the resonance operation Voltage gain is different from that of LC series resonant converter LC Series resonant converter LLC resonant converter Q1 resonant network Q1 resonant network V in Q V d I p L r n:1 R o V O V in Q V d I p L r I m n:1 I D R o I o V O L m L m I ds Cr I ds Cr 8

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 9

10 1. Introduction Integrated transformer in LLC resonant converter Two magnetic components are implemented with a single core (use the primary side leakage inductance as a resonant inductor) One magnetic components (Lr) can be saved Leakage inductance not only exists in the primary side but also in the secondary side Need to consider the leakage inductance in the secondary side Q1 Q1 Integrated transformer V n:1 I I o I in D V p I n:1 I o V in D d Q L r V O Lm I ds C I ds r Cr R o Q V d L lkp I m L m L lks R o V O 10

11 . Operation principle and Fundamental Approximation Square wave generator: produces a square wave voltage, V d by driving switches, Q1 and Q with alternating 50% duty cycle for each switch. Resonant network: consists of L lkp, L lks, L m and C r. 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 I p Square wave generator I m Q1 resonant network Rectifier network I ds V in Q V d I p L lkp I m L m n:1 L lks I D R o I o V O I D V d (V ds ) V in I ds Cr V gs1 V gs 11

12 . 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 (V d ) is applied to the resonant network. Fundamental approximation: assumes that only the fundamental component of the squarewave 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 I I sec p V d Resonant network I I sec p V d Resonant network 1

13 . Operation principle and Fundamental Approximation 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 (I ac ) and a square wave of voltage (V RI ) appears at the input to the rectifier. The equivalent load resistance is obtained as R ac V V 8 V 8 = = = = I I I F F RI RI o F ac ac π o π R o I ac I ac V RI F V RI pk I ac V o I o I = π V O F 4V o V RI VRI = sin( wt) π I ac R o o sin( wt) 13

14 . Operation principle and Fundamental Approximation AC equivalent circuit (LLLC) V in V d C r L lkp L lks V O M 4nV o F F sin( ωt) VRO nv RI π n V = = = = F F V 4 V d Vd in sin( ωt) Vin π o L m V RI R ac = 8n π R o n:1 R o ω LRC m ac r = ω ω jω L m n Llks Rac ωo ωp (1 ) ( ) (1 ) V d F C r L lkp L m n L lks R ac V RO F 8n Rac = R o π 1 1 ωo =, ωp = LC LC r r p r p = m lkp, r = lkp m //( lks ) L L L L L L n L Lr is measured in primary side with secondary winding short circuited Lp is measured in primary side with secondary winding open circuited 14

15 . Operation principle and Fundamental Approximation Simplified AC equivalent circuit (LLC) V in V d C r L lkp L m L L L n L r = lkp m //( lks ) = L L // L lkp m lkp L = L L p lkp m n:1 1: L L lks p L p V RI L r V O R o Assuming L lkp =n L lks nv O M = = V in Q = ω k ( ) ω k 1 p ω ω ( k 1) ω j( ) (1 ) Q (1 ) ω ω k 1 ω L r R o o p / C ac r k = L L m lkp C r V F in L r L p L r R V F ac RO Lr is measured in primary side with secondary winding short circuited Lp is measured in primary side with secondary winding open circuited Expressing in terms of L p and L r nv O M = = V in ω ( ) ω p L ω ω L ω j( ) (1 ) (1 ) ω ω ω p Q o o L r p p L L p r 15

16 . Operation principle and Fundamental Approximation Gain characteristics Two resonant frequencies (f o and f p ) exist The gain is fixed at resonant frequency (f o ) regardless of the load variation f p LLC resonantconverter Q=0. f o Q = L / C r R ac Q= 1 r 1.6 Q= 0.8 ω= ω o k 1 Lp = = k L L p r Gain Q= 0.6 Q= 0.4 Q= 0. Peak gain frequency exists between f o and f p As Q decreases (as load decreases), the peak gain frequency moves to f p and higher peak gain is obtained. As Q increases (as load increases), peak gain frequency moves to f o and the peak gain drops Q=1 1.0 k 1 Lp 0.8 M = = k Lp Lr freq (khz) 16

17 . Operation principle and Fundamental Approximation Peak gain (attainable maximum gain) versus Q for different k values.4..0 Peak Gain Q k=1.5 k=1.75 k= k=.5 k=3 k=4 k=5 k=7 k=9 17

18 3. Design procedure Design example Input voltage: 380Vdc (output of PFC stage) Output: 4V/5A (10W) Holdup time requirement: 17ms DC link capacitor of PFC output: 100uF PFC DC/DC Q1 I D V DL I p Np:Ns C DL Q V d L lkp I m L m L lks V O R o I ds Cr 18

19 3. Design procedure [STEP1] Define the system specifications Estimated efficiency (Eff) Input voltage range: hold up time should be considered for minimum input voltage V = V min in O. PFC P in T HU C DL 19

20 3. Design procedure [STEP] Determine the maximum and minimum voltage gains of the resonant network by choosing k ( k = L / L ) it is typical to set k to be 5~10, which results in a gain of 1.1~1. at f o m lkp M M min max VRO Lm n L L lks m Llkp k 1 = = = = max Vin Lm Lm k V = V max in min in M min M max Gain (M) Peak gain (available maximum gain) 1.36 for V in min M min 1.14 for V in max M k 1 = = 1.14 k f o f s 0

21 3. Design procedure [STEP3] Determine the transformer turns ratio (n=np/ns) max N p Vin n= = M N ( V V ) s o F min [STEP4] Calculate the equivalent load resistance (Rac) R ac = 8n Vo π Po 1

22 3. Design procedure [STEP5] Design the resonant network With k chosen in STEP, read proper Q from gain curves k = = max 7, M 1.36 peak gain = % = 1.5

23 3. Design procedure [STEP6] 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 N min p nv ( VF) = f ΔB A o min s e 3

24 3. Design procedure [STEP7] Transformer Construction Since LLC converter design results in relatively large L r, 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 N p =5T N s1 =N s =6T Bifilar Design value: L r =34uH, L p =998uH 4

25 3. Design procedure [STEP8] Select the resonant capacitor I r π I n( V V ) [ ] [ ] n 4 f L RMS o o F C o m V max max Vin C r RMS ICr π f C o r 5

26 4. Conclusion Using a fundamental approximation, gain equation has been derived Leakage inductance in the secondary side is also considered (LLLC model) for gain equation LLLC equivalent circuit has been simplified as a conventional LLC equivalent circuit Practical design consideration has been presented 6

27 Appendix FSFRseries Variable frequency control with 50% duty cycle for halfbridge resonant converter topology High efficiency through zero voltage switching (ZVS) Internal SuperFETs with Fast Recovery Type Body Diode (trr=10ns) Fixed dead time (350ns) Up to 300kHz operating frequency Pulse skipping for Frequency limit (programmable) at light load condition Simple remote ON/OFF control Various Protection functions: Over Voltage Protection (OVP), Over Current Protection (OCP), Abnormal Over Current Protection (AOCP), Internal Thermal Shutdown (TSD) 7

28 8 Appendix FSFRseries demo board

29 9 Appendix FSFRseries demo board