LOW COST 1000 WATT, 300 VOLT RF POWER
AMPLIFIER FOR 13.56 MHz
Prented at RF EXPO EAST 1995
A P P L I C A T I O N N O T
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Low Cost 1000 Watt, 300 Volt RF Power Amplifier for
化学推断题
13.56MHz
Kenneth Dierberger Lee B. Max
Applications Engineering Manager Independent Consultant
Advanced Power Technology Inc.6284 Squiredell Dr.
405 SW Columbia St.San Jo, California 95129 USA
Bend, Oregon 97702 USA
Bobby McDonald
UNI-WEST ENGINEERING
6329 Bethel Island Rd.
ekBethel Island, CA 94511 USA
ABSTRACT
This paper details the design, development , asmbly and performance of a low cost, high-efficienc
y, 1000Watt, 13.56MHz RF power amplifier (PA) operated from a 300VDC supply, with an efficiency of 80%. The PA is built around a “symmetric Pair” of low cost RF power MOSFETs from Advanced Power Technology (APT). The transistors are from a new generation of high quality, commercial, HF/ VHF, silicon, 900V RF power MOSFETs in TO-247 packages. The paper address both the theoretical design and physical construction of the amplifier. The paper also contains a technical description of the RF power transistors.
INTRODUCTION
Most transistorized RF Power Amplifiers operate from a DC to DC converter. This supply is usually low voltage, about 50V, and requires a down regulator when operated from AC mains. This converter is a significant portion of the overall cost of the RF amplifier system.
As a result of IEC555-2, all electronic equipment sold in Europe with a power draw of greater than 250W will require power factor correction (PFC). The addition of a PFC preregulator to the system could add 50 to 100% to the cost of the power supply portion. The requirement for PFC is soon to follow in the USA and the rest of the world.
The u of a new high voltage RF MOSFETs from Advanced Power Technology (APT) makes possib
le a new RF amplifier design which can be operated at 300V, allowing for the direct u of regulated output, thus eliminating the DC to DC converter, reducing the cost of the RF amplifier system.
The new devices, like their predecessors, utilize the high performance of APT’s Power MOS IV® technology and the “symmetric pair”package.
AMPLIFIER DESCRIPTION
The amplifier is a 1000 Watt, 13.56MHz design operating in class C with a 300VDC power supply. Efficiency of the amplifier is 80 percent. The power amplifier is built around two “symmetric pair” of ARF444/ARF445 900V RF power MOSFETs provided in TO-247 plastic packages. The devices are electrically identical, except that they are packaged in “mirror image”pairs to facilitate a symmetrical layout that helps maintain the electrical symmetry required for push-pull operation. Figure 1 shows the circuit diagram of the amplifier, with the parts list given in Table 1. The amplifier is a classical push-pull configuration of a straight forward nature, using a simple L-C network for impedance matching and transformer-coupling to achieve the required complementary gate drive signals. A wideband wire wound transformer output circuit is ud, with a conventional bifilar-wound RF choke for DC power supply isolation.
Short, low inductance interconnections are easily made using the ARF444/ARF445 devices, becau they can be mounted symmetrically in a common source configuration. In particular, the gate circuit should minimize inductance to avoid instability and loss when that inductance is combined with the high capacitance of the gates. Similarly, the frequency respon of the output circuitry is improved with minimum stray inductance due to interconnections[1].
The amplifier is operated directly from the PFC 300VDC power supply, eliminating the DC-DC converter, and is constructed on a heat sink sized for proper dissipation at the expected power levels. Figure 2 shows the component placement on the PC board and heat sink. The common source design of the package allows the device mounting to be accomplished without an insulator thus allowing good heat transfer to the heat sink with the u of thermal grea.
INPUT NETWORK
The input network provides a 50Ω impedance to the driver source and transformation of the MOSFET gate impedance, as well as balanced drive for push-pull operation. The input network compris capacitor C1, the input capacitance of the power MOSFETs and the ries gate resistors, both transformed by T1. The proper lection of C1 tunes the input network for minimum input return loss at maximum power output [2].
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hevaTransformer T1 provides a 9:1 impedance transformation of the MOSFET input impedance. It is constructed using two Fair-Rite cores #2643540002, µ=850 with 3 turns of stranded司法考试培训
Figure 1. Circuit Diagram of the 1000 Watt Class C Amplifier
Part Number Description
R1,R210Ω 1W
R3-R18 4.7Ω 1W
C1200pF Chip Capacitors
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C2-C50.1µF Chip Capacitors
C6-C100.1µF Disk Ceramic
C11, C120.01 Disk Ceramic
Q1, Q3ARF444
Q2, Q4ARF445
L1, L2VK200-19/4B
L3, L40.37µH: 6T, #18AWG, ID=0.438
RFC12T, #14 PTFE coated twisted pair on a Fair-Rite #2643665702 shield
bead, µi=850
T19:1(Z) conventional transformer; 3:1(T), #18 stranded PTFE coated
wire on two Fair-Rite #2643540002, µi=850
T21:1(Z) conventional transformer; 2:2(T), #14 stranded PTFE coated
wire on two stacks of three Fair-Rite #2643102002 shielded bead, µi-850 BFC16T, #18 Twisted pair stranded PTFE coated wire on three stacked
Indiana General Toroid #F624-19-Q1, µi=125
Table 1. Parts List for the 1000 Watt Power Amplifier
PTFE coated #18 wire on the primary and 1 turn of stranded PTFE coated #18 wire on the condary. The condary is coupled through the DC blocking capacitors C1-C2 and C3-C4 and res
istors R3 through R18 to the gates of the MOSFETs. The resistor-inductor combination R1-L1 and R2-L2 stabilize the push-pull amplifier at lower frequency and provide the MOSFETs with a DC ground reference to insure the gates do not float to a DC potential thus unbalancing the amplifier bias points. The parallel resistors R3-R6, R7-R10, R11-R14 and R15-R18 in ries with the gates of the MOSFET, prevent high frequency oscillation common when paralleling MOSFETs [3].
OUTPUT CIRCUIT
The 300VDC power input is delivered through a balanced feed choke [4]. The choke is designed
to create a zero DC magnetic bias in the core when both transistors draw the same average current.With the devices operating 180 degrees out of pha, the construction of the windings prents a high impedance at 13.56MHz to the drain of each MOSFET. The choke is constructed by winding 6 turns of #18 stranded PTFE coated twisted pair around three stacked Indiana General Toroids #F624-19-Q1, µi=125.
The output of the power devices is coupled to the output transformer T2 through two 0.37 µH inductors. The transformer is a wideband 1:1conventional transformer. No output filtering was ud in the test amplifier, which has the third harmonic 30db down and the cond harmonic 55db below the 1000 watt output power level.
The transformer is constructed by winding 2turns of #14 stranded PTFE coated wire for the primary and 2 turns of #14 stranded PTFE coated wire for the condary around two stacks of three Fair-Rite #2643102002 shield beads, µi=850.PERFORMANCE MEASUREMENTS The amplifier was operated under two conditions. First the amplifier was driven with a 13.56MHz RF signal, modulated by a 1kHz square wave, at a 50% duty cycle, up to a peak power out of 1200W. Second the amplifier was driven with a 13.56HMz CW RF signal up to a continuous power out of 1000W. Due to the clo correlation of the modulated data and the CW data, it was concluded that there is significa
nt thermal margin from using four 300W devices at 1000W CW. Figures 3 through 6 show the performance data for this amplifier. Figure 3 is a plot of P in versus P out and Figure 4 shows gain versus P out . The curves show the classical class C characteristics,with a low gain at low power output, improving as the output power increas. The gain peaks at 16.9db when the amplifier output is 800W, with a roll-off to 15.9db at 1200W.脑图英语
princess of chinaEfficiency versus P out is shown in Figure 5. As would be expected in class C, the efficiency is over 50% at power output above 300W. The efficiency ris to an outstanding 80.4% at 1000W,continuing upward to 84.4% at 1000W, continuing upward to 84.4% at 1200W output. Figure 6 is total amplifier power dissipation versus P out .
Figure 3. Input Power versus Output Power
大斌网Figure 4. Gain versus Output Power
