LLC Resonant Half Bridge Converter

Transcription

1 LLC Resonant Half Bridge Converter Asia Tech-Day August 17 to 7, 009 Hong Huang Applications Engineer

2 Outline Introduction to LLC resonant half bridge converter Benefits Operation principle Design challenges Design method Transformer turns ratio selection Magnetizing inductor selection Resonant component selection Other design issues for LLC resonant converter Current limiting Soft start OVP and Burst Operation

3 Design Challenges for DC/DC Higher power conversion performance Higher efficiency, smaller heat sink Higher switching frequency, smaller magnetics Less energy storage capacitors, smaller size (e.g., for PFC holdup) Moderate frequency variations Wide input voltage variations AC-DC applications: holdup time requirement (PFC from 400V to 300V during holdup) Larger Energy Storage Capacitor high cost, large size, more space Converter ability to tolerate the variations DC-DC applications Telecom, 36 to 75V (3V to 78V) Some applications even asking 4:1 variations Wide output voltage trimming

4 Benefits of LLC Resonant Converter ZVS can be achieved by utilizing transformer magnetizing inductor Capacitor filter, less voltage stress on rectifiers Smaller switching loss due to small turn off current Variable switching frequency control, not sensitive to load change Frequency variations can be designed narrower compared to SRC Wide operation range without reducing normal operation efficiency

5 LLC Resonant Converter with Wide Operation Range Square Wave Generator to excite Resonant Circuit Rectifier to get DC Resonant frequency f 0 1 L r C r Transformer turns-ratio n Vin / V o f sw is set at resonant frequency at nominal input and output f sw is adjusted by feedback loop at other operation conditions

6 Operation Principles At Resonant Frequency Vg_Q1 t Q1 Vg_Q Vp t Vin Vp Q Cr Lr Lm n:1:1 * * D1 Vo RL * ir t D im is t t Q and D ON, Q1 and D1 OFF Magnetizing current in Lm, im Cr resonates with Lr, ir Cr and Lr deliver energy to output t0 t1 t t3 t4

7 Operation Principle Above Resonant Frequency V g_ Q1 V g_ Q Q1 V p i r Vin Q Vp Cr Lr Lm n:1:1 * * * D1 Vo RL D i m i s When switching frequency is above resonant frequency, circuit behaves as SRC Secondary current becomes CCM, reverse recovery loss increases t0 t1 t t3

8 Operation Principle Below Resonant Frequency Vg_ Q1 Vg_ Q Q1 Vp Vin Q Vp Cr Lr Lm n:1:1 * * D1 Vo RL ir * D im is When switching frequency is below resonant frequency, magnetizing inductor begins to participate in resonant and increases voltage gain Secondary diode becomes discontinuous t0 t1 t t3

9 LLC Resonant Converter Gain Function V I oe Re V rms os, 1 nv rms Ios,1 ( Io) / n o M L n f n ( f n L n f n 1) 1 jf n L n Q e

10 LLC Resonant Converter with Wide Operation Range Unity gain is reached at V r =(V Lr -V Cr )=0, where input voltage in phase with output voltage, and input voltage applies to load (R e ) directly Inductive Region, Ir lagging Vge Capacitive Region, Ir leading Vge Unity Gain, Vr=0 Voe in phase with Vge

11 LLC Resonant Converter with Wide Operation Range Working regions (Modes) - Inductive Region, if resonant network current is lagging input voltage - Capacitive Region, if resonant network current is leading input voltage - Resistive Region, if resonant network current is in phase with input voltage (boundary to divide Inductive and capacitive, by let the imaginary part zero of the input impedance) - Unity gain happens at (V Lr -V Cr )=0, where input voltage in phase with output voltage, and input voltage applies to load (R e ) directly

12 LLC Resonant Converter with Wide Operation Range Should operate in ZVS region (Inductive Region, I r lagging Vge) Avoid ZCS region (Capacitive Region, I r leading Vge) Hard switching of half bridge switches Reverse recovery losses in primary FET body diodes Large spikes on switch node Higher EMI levels Frequency relationship reversed Frequency increases as load increases

13 Gain Gain Characteristics with Ln and Qe Q e L r R / C e r Q e is related to output load Q e Increasing with Load Current Qe=0.0 Qe=0. Qe=0.8 Qe=1 Qe= Qe=10 ZCS/ZVS fn, (Ln =1) Qe increase with Ln constant (designed Lr, Cr, and operational RL), - Peak-gain becomes lower - Frequency at peak-gain moving to right - Better frequency-selective R f f e R L p 0 f n 8 1 L L f f 0 n 1 L r C r r m C r

14 Gain Gain Gain Impacts of Circuit Parameters L n =1 Qe=0.1 Qe=0. Qe=0.5 Qe=0.8 Qe=1 Qe= Qe=5 Qe=8 Qe=10 ZCS/ZVS L n =5 Qe=0.1 Qe=0. Qe=0.5 Qe=0.8 Qe=1 Qe= Qe=5 Qe=8 Qe=10 ZCS/ZVS L n =10 Qe=0.1 Qe=0. Qe=0.5 Qe=0.8 Qe=1 Qe= Qe=5 Qe=8 Qe=10 ZCS/ZVS fn, (Ln =1) fn, (Ln =5) fn Gain Change with Ln and Qe - Ln increase with Qe constant (designed Lr, Cr, and operational RL), - flat, - magnitude shift-up, - frequency value at peak-gain moving towards more left, - less frequency-selective - wide frequency variation from no load to full load (more discussion later) Ln somewhere 3 to 5 look best balance between peak gain and frequency change can be initially pick-up

15 LLC Resonant Converter Operation for design consideration Operation/design with no load and minimum gain at a1 Operation /design with full load and maximum gain at a3 All gain curves cross at unity at f n =1, or f=f 0 Qe design consideration at Heavy load, OCP, Short Circuit

16 Design Goals for LLC Resonant Converter Minimize RMS current under normal operation condition Ensure ZVS operation Ensure desired operation range

17 Design Consideration -1: Primary RMS Current at Normal Operation I P, RMS I r I m I Re FHA nv L O m I o n Primary current can be easily calculated from the phasor circuit Primary side RMS current is summation of magnetizing current and load current Larger Lm is better for less conduction losses

18 Design Consideration -: Secondary winding RMS Current I I RMS _ S RMS _ S ( I I Re n) / Io I 4 n Io I Re peak _ S peak _ S I I 4 O, O, center bridge tapped Secondary side current is the difference between resonant tank current and magnetizing current

19 Design Consideration -3: Zero Voltage Switching I m,pea k 1 ( L m L ) I r 1 m, peak (Ceq) ZVS conditions: - Enough H-field energy to balance E-field Energy in less than half cycle - Enough time to make the energy conversion - Worst operation for ZVS, -V o,min, I m,peak becomes small -V in,max, more C eq energy needs to discharge V in I m, peak m, peak tdead nv L I C L m m o eq T t 16C T 4 V dead eq in

20 Trade-off Design of Dead Time L m t dead Lm tdead Smaller turn off current Smaller magnetizing current Increase RMS current due to duty cycle loss Smaller duty cycle loss Larger magnetizing current Larger turn off loss

21 Trade-off Design of Dead Time Normalized Primary RMS Current Conduction loss Dead time (ns) Loss (%) Total loss Dead time (ns) Trade-off between the switching loss and conduction loss on a case of 100ns dead time Turn off current Dead time (ns) Switching loss

22 Peak Gains with Ln and Qe Initially select Ln in the range of 3 to 5 (gain curve not very flat and able to narrow down the frequency change while there is still enough gain) Find proper Qe to get enough peak gain

23 Design Flow Chart for LLC Resonant Converter Converter Specifications n V V in O Magnetizing Inductance L L m n L r choose an L n and Q e Check max gain against graph Change L n and Q e Resonant Inductor 1 Lr f sw Cr Resonant Capacitor No Peak gain enough? Yes 8n 8 Re n R L V P o out C r Q R e 1 e f sw Calculate Re

24 Design - Over Current Protection Q e Lr Cr R e Required Gain Max load Overload? During over load condition, check if the converter enters ZCS region

25 Design - Soft start Soft start is achieved by frequency control

26 Design OVP and Burst Operation Secondary OVP Feedback loop fault may cause output over voltage. Slow loop response may also cause OVP. Independent OVP circuit is needed. Burst Operation To cover no load operation, smaller Ln is needed which increases circulating (magnetizing) current, leading more conduction losses. To reduce switching losses, ZVS is still needed at no load. This requires higher magnetizing current, too. To maintain output regulation, burst operation at light load and no load is an alternative to balance switching losses and conduction losses.

27 Gain (db) Gain (db) Phase (Deg) Feedback Loop Design G ( S) K c dc S 1 C1 R koptos S ( f p _ opto 1) UCC5600 EVM Modulator Plot (Test) Frequency (Hz) Gain of Type I Compensator Gc( f) 0 0 c( f) Measure G m (jω) 40 - Design G c (jω) based on G m (jω) measurement f

28 Summary LLC resonant converter is able to achieve wide operation together with high efficiency Due to low switching losses, LLC resonant converter is able to operate at high switching frequencies, while maintaining high efficiency LLC resonant converter design needs to find a suitable magnetizing inductor to ensure small conduction losses and switching losses By choosing a suitable Ln and Qe value, desired voltage gain can be achieved to input and output voltage variation range

29 UCC5600 Resonant Half Bridge Controller Complete system features Programmable soft start Programmable dead time Programmable maximum/minimum switching frequency 0.4A source, 0.8A sink driving capability Simple ON/OFF control Burst operation at light load condition Precise timing control 3% accuracy on minimum switching frequency setting with only external resistor ±50ns matching on dead time Soft start timer range from 1ms to 500ms Complete protection functions Two levels over current protection, auto recovery and latch off Bias voltage UVLO and OV protection Over temperature protection Soft start enabled after all fault conditions 8 pin SOIC package, simplifies design and layout

30 Application Circuit UCC5600 Programmable dead time Frequency control with minimum/maximum frequency limiting Programmable soft start with on/off control Two level over current protection, auto-recovery and latch up Matching output with 50ns tolerance

31 Test based on EVM (UCC5600EVM)

32 Test with EVM: Resonant Tank TP TP13 Ch3: Lr Current Scale: 3.4A/div Primary Peak Current:.7A Secondary Peak Current (Nt=16.7): 3.4A/div x 0.8div x 16.7 = 45.4A Load Current: 5A Io/Is = 5/45.4 =0.55 Compare: half wave rectifier Q Vds TP4 TP6

33 Test with EVM: Ripple and Hiccup

34 Test with EVM: Freq and Feedback Frequency Variation Feedback Loop Bode Plots

35 Test with EVM: Efficiency and Load Regulation Efficiency Load Regulation

36 Thank You!