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Download Project Goals The Class E Inverter Improved
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LEES A VHF Resonant Boost DC/DC Converter Justin Burkhart - Advisor: David Perreault Department of Electrical Engineering and Computer Science : Project Goals Improved Inverter • Design a resonant boost converter using TI LBC5 process LDMOS devices • Targeted at automotive applications with: • 11-16 VDC input • 30 VDC output Completed Converter Design The long transient response of the Class E Inverter limits its usefulness in VHF DC/DC converter applications where regulation is performed using on/off modulation. To improve the transient response, the input choke inductor can be made resonant. To see what effect this has on inverter operation, L1 is broken into 2 hypothetical inductors, one that only carries DC current and one that carries only AC Current, as shown in Figure 3. The improved Class E Inverter and resonant rectifier are joined together to form a DC/DC Converter. The schematic of the converter is shown in Figure 6, and waveforms are shown in Figure 7. This converter operates at 75MHz and delivers 15 Watts with 12VDC input voltage and 30VDC output voltage. This converter was tuned specifically to make up the shunt capacitor of the rectifier entirely from the diodes parasitic capacitance. Figure 3. Illustrative Schematic of the Improved Class E Inverter In this configuration L1-AC is in parallel with C1. Their equivalent impedance at the switching frequency can be made it look capacitive and is given by: Figure 6. Schematic of the DC/DC Converter • 10-20 Watt output power • Highest possible switching frequency with efficiency greater than 80% (if possible) The Class E Inverter The DC/DC converter design will use at its core a variant of the Class E Inverter. Figure 1 shows a schematic of a varient of the Class E Inverter that has been adjusted to deliver power at both AC and DC. Figure 2. shows an example of inverter waveforms. Figure 7. DC/DC Converter Operating Waveforms 0 Voltage (Volts) 10 20 30 Time (ns) 40 50 • Faster transient response • Lover minimum output power • Flexibility in the choice of L1 and C1 V(t) Vo(t) 40 Resonant Rectifier 20 86 22 85.5 20 85 18 84.5 16 84 14 83.5 0 0 10 20 30 Time (ns) 40 50 Figure 2. Class E Inverter operating waveforms A DC/DC converter can be formed by rectifying the output of the Class E Inverter. A resonant rectifier must be used since the losses incurred by a hard switched rectifier at VHF frequencies are too high V +V to maintain good efficiency. 40 • Assume load has high enough Q such that io is sinusoidal • When the switch opens, circuit is designed such that the voltage V(t) rings back to zero DT later, thus providing a ZVS opportunity Cons • High peak device voltage • Large choke inductor limits transient response • Sensitive to load changes • Limited minimum output power when C1 is constrained Figure 4. Resonant Rectifier Schematic • Diode turns on when Vdiode(t) goes > Vout • Diode turns off when Io(t) goes < 0 • Initial conditions are known; thus, equations for Io(t) and Vdiode(t) can be derived • Using initial conditions ton and toff can be solved for • Close form solutions is not easily arrived at since equations are non-linear • Thus, rectifier tuning is the preferred design method Voltage (Volts) • Switch is opened and closed periodically 12 11 12 13 14 15 Vin (Volts) 16 17 83 18 Figure 8. Efficiency and Output Power of the DC/DC Converter DC 20 Figure 9. Loss Breakdown by component 0 -20 0 10 20 Time (ns) Vdiode(t) 30 40 References 50 0 -50 0 Current (Amps) • L1 is large choke with only DC current Vdiode(t) Voltage (Volts) AC Pros • Zero Voltage Switching • Only one ground referenced switch is required 24 Efficiency (%) This modification results in: I 0.5 -0.5 0 Figure 1. Schematic of the Class E Inverter io(t) 1 Power Out (Watts) Current (A) 1.5 10 20 Time (ns) I o(t) 30 20 Time (ns) 30 40 4 2 0 -2 0 10 40 Figure 5. Resonant Rectifier Waveforms [1] N.O. Sokal and A.D Sokal. Class E – a new class of high-efficiency tuned single-ended switching power amplifiers. IEEE Journal of Solid-State Circuits, SC-10(3):168-176, June 1975. [2] W.A. Nitz, W.C. Bownam, F.T. Dickens, F.M Magalhaes, W. Strauss, W.B. Suiter, and N.G. Zeisses. A new family of resonant rectifier circuits for high frequency DC-DC converter applications. Third Annual Applied Power Electronics Conference Proceedings, pages 12-22, 1988. [3] Anthony Sagneri, Design of a Very High Frequency dc-dc Boost Converter, M.S. Thesis, Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, Feb. 2007