The Technology Behind the World s Smallest 12V, 10A Voltage Regulator

Transcription

1 The Technology Behind the World s Smallest 12V, 10A Voltage Regulator A low profile voltage regulator achieving high power density and performance using a hybrid dc-dc converter topology Pradeep Shenoy, Ph.D. Systems Engineer, DC Solutions 1 of 30

2 Power Delivery System Point-of-Load Voltage Regulators Intermediate Bus Architecture 2 of 30

3 Why Increase Switching Frequency? Inductors are usually the largest component. 1) Smaller size Converter Volume: 1,270 mm 3 Converter Volume: 157 mm 3 Inductor Volume: 232 mm 3 Inductor Volume: 19.2 mm 3 2) Faster Response 3) Lower BOM Cost 3 of 30

4 Current Density (A/cm3) Current Density Comparison as of Aug TPS54A20 60 Research Industry Rated Output Current (A) Series Cap Buck: 1.2 mm height TPS54A20 IC Inductors Conventional Buck: 4.8 mm height Current density of over 60A/cm 3 and power density of 1.25kW/in 3 4 of 30

5 Inductor Size Reduction: 10A Output 2-5MHz 500kHz ~$2-3 (1k price) ~$0.20-$0.30 (1k price) High frequency operation 15 times smaller inductors! 5 of 30

6 Agenda High Frequency Buck Converter Limitations Series cap buck converter prototype Series Capacitor Buck Converter Sample Experimental Results Design Considerations for a Series Cap Buck Converter TPS54A20 Series Capacitor Inductors 6 of 30

7 High Freq (HF) Buck Converter Limitations Buck Converter High switching loss Ploss f sw Switch Timing Diagram High side switch on-time is very short at HF 5 MHz 200ns period 10-to-1 voltage ratio 20ns high side on-time HF converters on the market today have low conversion ratios (<5-to-1) and low current (<1A) 7 of 30

8 Series Capacitor Buck Topology Series capacitor Two-phase, series cap buck converter Benefits Single conversion stage Switching at reduced V ds Series cap soft charge/discharge Automatic current balancing Duty ratio doubled Drawback 50% duty cycle limitation Theoretical: V IN,MIN = 4 V OUT Practical: V IN,MIN = 5 V OUT P. S. Shenoy, M. Amaro, J. Morroni and D. Freeman, Comparison of a Buck Converter and a Series Capacitor Buck Converter for High-Frequency, High-Conversion-Ratio Voltage Regulators, IEEE Trans. Power Electron., vol. 31, no. 10, pp , Oct of 30

9 Steady-State Operation: Interval 1 Inductor & series cap currents Series cap voltage (differential) Switch node voltages 9 of 30

10 Steady-State Operation: Interval 2 Inductor & series cap currents Series cap voltage (differential) Switch node voltages 10 of 30

11 Steady-State Operation: Interval 3 Inductor & series cap currents Series cap voltage (differential) Switch node voltages 11 of 30

12 Steady-State Operation: Interval 4 Inductor & series cap currents Series cap voltage (differential) Switch node voltages 12 of 30

13 Reduced Switching Loss Reduced switch voltage/current overlap loss Loss due to switch output capacitance reduced by 67% Enables higher frequency operation Energy loss per switching cycle 13 of 30

14 Efficiency (%) Measured Efficiency Comparison MHz, TPS54A20 530kHz, TPS Output Current (A) Conditions: 12V in, 1.2V out Room temp, no air flow Higher efficiency over the load range Inductors selected for equivalent DCR Higher peak efficiency at ~4 times the switching frequency 14 of 30

15 Inductor Current (A) Auto Current Sharing Series cap forms average current feedback mechanism Inductors charge/discharge cap Charge balance maintained Robust to variations in L, DCR Current Sharing: La 100nH, Lb 200nH 5 4 ILa 3 ILb Output Current (A) I LA (1A/div) I LB (1A/div) P.S. Shenoy, et al., Automatic current sharing mechanism in the series capacitor buck converter, in Proc. IEEE Energy Conversion Conf. Expo., Sept of 30

16 High Frequency Controller Adaptive constant on-time control Fast transient response Internal compensation Frequency synchronization by adapting on-time Fixed frequency in steady state Can use external clock or internal oscillator 16 of 30

17 Reference Design PMP15008 Tiny, Low Profile 10 A Point-of-load Voltage Regulator Board Image Total solution size is 135mm 2 and 1.25mm tall 17 of 30

18 Efficiency and Power Loss 2 MHz per phase, 1.2V OUT, room temperature, no air flow, two layer board Over 90% efficiency at 9V input, less than 3W loss at full load 18 of 30

19 Thermal Image Series Capacitor Inductors Integrated Converter Less than 35 deg. C temp rise at 12V input, 8A output 19 of 30

20 Load Transient Response V OUT (ac coupled), 20mV/div V OUT (ac coupled), 20mV/div ±25mV 10µs/div I OUT, 2A/div I OUT, 2A/div 10µs/div 2% variation in V OUT during 5A load change 20 of 30

21 High Bandwidth and Ample Phase Margin Over 50 degrees of phase margin Over 300kHz bandwidth Bode plot taken with 12V input, 5A output 21 of 30

22 Efficiency (%) Inductance Impact on Efficiency Inductance equation V L, MAX 2V I K O 2 IN O V IN, MAX VO f K = ΔI L /I L where I L is current at full load K is usually between 0.1 and 0.4 SW V IN, 1.2 V O, 2MHz/phase 250nH 330nH 470nH Output Current (A) Higher inductance tends to increase peak efficiency Lower inductance has higher full load efficiency 22 of 30

23 Efficiency (%) Inductor Size Larger inductors tend to result in higher efficiency Thicker wire Lower winding resistance Benefit seen in mid to high load current range Measured results for Same inductance Same vendor Same core material V IN, 1.2 V O, 2MHz/phase 4 x 4 x 2 mm 3.2 x 2.5 x 1.2 mm 3.2 x 2.5 x 1 mm 2.5 x 2 x 1 mm Output Current (A) 23 of 30

24 Efficiency (%) Inductor Vendor Finding the right inductor vendor matters Various core material, construction, etc. Should not judge an inductor by DC resistance alone Measured results for Same inductance Same size If possible, experimentally test inductors V IN, 1.2 V O, 2MHz/phase Vendor A Vendor B Vendor C Vendor D Output Current (A) 24 of 30

25 Series Capacitor Selection Select the cap value to keep voltage ripple <8% at full load, lowest V IN Ex: 10 A load, 2 MHz, 10.8 V IN, 1.2 V O C i DT 0.08 V out in V 1 10A 10.8V 2MHz V μF PGOOD Tradeoff: Startup delay to precharge the series cap 10 ma precharge current into 1 µf cap 625 µs to precharge to 6 V (V IN,typ /2) V O Precharge EN SCAP 25 of 30

26 Capacitance Change (%) Capacitance Change (%) DC Voltage and Temp Impact on Capacitance Capacitance varies with temperature 20 Capacitance decreases with DC voltage Examine capacitance at V IN / x1.6x1.15mm 2.0x1.2x1.25mm 2.0x1.2x0.85mm Temperature ( C) Select a capacitor taking capacitance variation into account x1.6x1.15mm 2.0x1.2x1.25mm 2.0x1.2x0.85mm DC Voltage Bias (V) 26 of 30

27 Temp Rise ( C) Series Capacitor Self Heating Capacitor temp rises with current kHz 500kHz 1MHz 3.3A, 15.8 C Current (A_rms) Ensure series cap temperature stays within limits Calculate RMS current Check datasheet/online tools Ex: 10.8V IN,MIN, 1.2V O, I L,RMS = 5.02A I 2 2 V VIN, O 2 SCAP, RMS I L, RMS MIN 2.2µF cap, 1206 (3.2 x 1.6 x 1.15mm) Result: 15.8 C temp rise 3.34A X7R capacitors with 125 C operating temperature rating recommended 27 of 30

28 Total Solution Size Inductor on 10A buck EVM 10.2x10.2x4.7mm = 489mm 3 10A series cap buck prototype 16x10x1.85mm = 296mm 3 The total solution size is 65% smaller in volume than just the inductor on a competitor s 10A evaluation module! 28 of 30

29 SUMMARY High frequency (HF) operation of switching converters enables size reduction and performance improvements Buck converters have fundamental limitations that limit HF operation The series capacitor buck converter has unique properties that support HF operation Design recommendations for an HF series cap buck converter demonstrate the ease of implementation 29 of 30

30 Additional Resources View the TPS54A20 product page. View the reference design Tiny, Low Profile 10A Point-of-load Voltage Regulator. Download the application note Introduction to the Series Capacitor Buck Converter. Watch the video training series Designing with TI s Series Capacitor Buck Converter. 30 of 30