Transcript Project Goals The Class E Inverter Improved

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