Comparing Synchronous to NonSynchronous Converters Comparing Size Cost
- Slides: 21
Comparing Synchronous to Non-Synchronous Converters Comparing Size, Cost, Efficiency, & More Anston Lobo Texas Instruments 1
Synchronous and non-synchronous buck Non-Synchronous Buck Heavy Load Light Load Non-sync Synchronous Buck Sync-FPWM Sync-DCM (Diode Emulation) 2
Comparison – Solution size & cost Non-Synchronous Buck Solution Size Non-Sync Larger IC + Power Diode Smaller Both FETs Integrated IC costs less + Diode cost IC costs more Layout Synchronous Buck Cost Other Considerations § Integrated sync rectifier § More Sophisticated controller & driver § Needs a thermally optimized package 3
Comparison – Ease of use Non-Synchronous Buck Design Sync Non-Sync Easier Need to select Schottky diode No need Selection Considerations Ø Reverse voltage rating VR >VIN max Ø Forward current rating IFWD >Load max + Iripple/2 Ø Forward voltage drop VFWD, over current and temp Conduction loss = VFWD * IFWD Ø Reverse leakage current over temp Loss = VIN* Ileak Ø Parasitic capacitance Switching loss Ø Package for heat dissipation Ø Size increase with VR and IF Synchronous Buck External Diode 4
Power diode example – DIODES B 340, 40 V 3 A VFWD IREVERSE Package 5. 59 mm× 6. 6 mm 5
Comparison – Layout to optimize EMI Non-Synchronous Buck For Any Buck Converter Reduce Critical Path Area Reduce EMI Switching Current on the input side Synchronous Buck Switch Node Critical path Key of PCB Layout 6
Comparison – Layout to optimize EMI Non-Synchronous Buck Non-Sync Harder to optimize EMI by layout (Depends on pin out of IC) Easier (Depends on pin out of IC) Layout for EMI 7
EMI mitigation by PCB layout Critical Path Area Reduction SW 14. 5 V max 41 d. BµV/m VOUT 47 m. Vpp VIN VOUT 8 8
EMI mitigation by PCB layout Critical Path Area Reduction Comparison SW 18. 1 V SW max (V) Results max Smaller Area VOUT Larger 75 m. V Areapp Vout p 2 p (m. V) EMI peak (d. BµV/m) 44 d. BµV/m 14. 5 47 41 18. 1 75 44 9 Now shown with single CIN with 2. 5 times larger area
Comparison – Light load efficiency Non-Sync - FPWM Sync - DCM Negative Current? No Yes No HS Conduction Losses Lower IRMS HS: (IRMS-HS 2)*RON-HS*D Higher IRMS HS: (IRMS-HS 2)*RON-HS*D Lower IRMS HS: (IRMS-HS 2)*RON-HS*D LS Conduction Loss Diode: IDiode*VF*t. Diode*f. S LS: (IRMS-LS 2)*RON-LS*(1 -D) LS: (IRMS-LS 2)*RON-LS*t. LS*f. S Switching Frequency Reduced at very light load Constant over load Reduced at very light load Switching Losses Less More Less Light Load Current Waveform 10
Comparison – Heavy load efficiency Non-Sync – FPWM and DCM Low Side Conduction Losses Diode: IDiode*VF*(1 -D) LS: (IRMS-LS 2)*RON-LS*(1 -D) HS FET Switching Losses Less ½*(VIN*IOUT)*(trise+tfall)*fs Parasitic Body Diode Heavy Load Current Waveform More ½*(VIN*IOUT)*(trise+tfall)*fs + Qrr*fs*VIN Qrr: LS Body diode Reverse recovery charge 11
Comparison – Efficiency Synchronous Buck Parasitic Body Diode • Conduct during dead-time • Reverse Recover Charge increases switching loss • Increase ringing in SW node Parasitic Body Diode To have the best of both worlds Parallel a SMALL Schottky diode • • • Bypass body diode during dead-time No reverse recover charge Current rating can be a fraction of the power diode for a non-sync buck – only conducts during deadtime 12
Efficiency improvement with small Schottky Improvement depends on switching loss from Qrr 13
Comparison – Efficiency summary Non-Sync-FPWM Sync-DCM Sync + Small Schottky Light Load Efficiency Better Worse Better Heavy Load Conduction Losses More Less Heavy Load Switching Losses Less More Less Fixed Switching Frequency No Yes No No Lower VOUT Worse (VF) Better 14
Comparison – Thermal performance Non-Synchronous Buck Thermal Synchronous Buck Synchronous + small Schottky Non-Sync + Small Schottky Better Two packages Harder One package LMR 14030 LM 43603 Loss and heat reduced by Schottky diode LM 43603 Example VIN = 24 V VOUT = 5 V Load = 3 A 500 k. Hz IC IC IC Diode 15
Comparison – Noise VOUT Spike SW Ringing Peak VIN = 24 V VOUT = 5 V Load = 3 A 500 k. Hz VOUT Spikes VIN = 24 V VOUT = 5 V Load = 3 A 500 k. Hz Sync Non-Sync + Small Schottky More - Body diode reverse recovery Less - Schottky diode Less – small Schottky diode 8 V 4. 5 V 2 V
Comparison – Unloading transient Unloading Transient Overshoot Sync-FPWM Sync-DCM Non-Sync Good Allow neg current to discharge COUT Worse Depend on load to discharge COUT Example VIN = 8 V VOUT = 5 V Load = 3 A FS=2. 2 MHz Same as Sync-DCM LM 53603 set at FPWM and DCM modes 17
Comparison – Controllability Non-Sync Limited Only controls one FET Better Controls both FETs Info from both FETs Current Limit Peak current only Peak current Valley current Average current (depends on controller) OVP Cannot actively pull down VOUT Can actively pull down VOUT (depends on controller) Controllability Control architecture More options 18
Comparison – Current limit DC current limiting Fast slew rate Slow slew rate Sync Non-Sync Control of Peak current and Valley current more accurate DC current limiting Only has control of Peak current DC current limit depends on ripple 5 A 4 A 3 A 5 A
Comparison – Summary Non-Synchronous Size / Ease of use Larger size Smaller size, Easier to Use Light Load Efficiency Better Worse with FPWM, better with DCM Heavy Load Efficiency More conduction loss Less switching loss Less conduction loss More switching loss due to reverse recovery Adding small Schottky gives highest efficiency Lower VOUT Lower Efficiency (VF/VOUT) Higher Efficiency Thermal Better with two packages Harder with one package Adding small Schottky reduces power loss a lot Fixed FSW No Yes with FPWM, No with DCM Noise less More, due to body diode reverse recover charge Adding small Schottky diode reduces noise a lot Unloading Transient Worse Good with FPWM, worse with DCM Controllability No LS control More control flexibility 20
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