VSWR adaptive power amplifier system

Abstract: An adaptive power amplifier system for use with VSWR loads includes an adaptive power supply that indirectly monitors the power dissipated by a RF power amplifier and adapts bias conditions of solid state components in the RF power amplifier to protect the components from excessive power dissipation typically experienced with VSWR loads, even while maintaining full forward power output.

Description: TECHNICAL FIELD The invention relates to solid state amplifiers and power supplies for solid state amplifiers. BACKGROUND OF THE INVENTION A high power, solid state amplifier, when driving uncertain load impedances having a high voltage standing wave ratio (VSWR), may suffer from serious degradation in performance due to mismatched loads on the amplifier. For example, a radio frequency (RF) amplifier driving a transmission system with 50 ohms nominal impedance may be required to provide full forward power output into a load with a 3:1 VSWR. A class B amplifier operating under these conditions suffers from thermal and electrical stress, such as increased junction temperatures in its transistors, gain compression, and input mismatch conditions to the driver stage. Elevated junction temperatures in the solid state devices caused by high power dissipation in excess of that under normal operating conditions seriously reduces the device's reliability, with the resulting stress permanently damaging the critical solid state devices. One technique for mitigating the adverse effects of abnormal load impedances from a high VSWR load is isolating the load. An example of this technique involves placing a magnetic isolator at the output of each solid state device. Under high VSWR conditions, power reflected by the load is absorbed in a dump port termination. Other techniques use a VSWR detection circuit to monitor the load status and provide a control signal to initiate the reduction of the power output of the amplifier to a safe operating value. In high power systems requiring constant forward output power, these approaches are not feasible because a constant forward output power from the amplifier is not maintained. An approach that protects the solid state components from high power dissipation but that also maintains full power output substantially independent of the load VSWR is most desirable. DE-A-38 27 579 discloses a power supply for a VSWR adaptive power amplifier, wherein the power supply includes a power stage for generating a power signal having an adjustable voltage level, a sensor for measuring the current level supplied from the power stage to the power VSWR adaptive amplifier and a controller for calculating the adjustable voltage level in response to the measured current level and a voltage level output from the power VSWR adaptive power amplifier. The voltage level of the output of the amplifier together with the current level of the output of the power stage result in a value signifying the power used by the load. Thereafter, complex and time consuming calculations are necessary to obtain the power dissipated by the amplifier. The system of the DE-A-38 27 579 has a substantial drawback in that measuring of the voltage level output from the amplifier is, under certain circumstances, not suitable for dynamically adapting the amount of power supplied to the amplifier, specially in cases of rapid changes of operation conditions. Therefore, it is not possible to avoid, for example, elevated junction temperatures in solid state devices caused by high power dissipation in excess of that under normal operating conditions. SUMMARY OF THE INVENTION This invention relates to a power supply as claimed in claim 1. In accordance with the invention, solid state components of an amplifier are protected from excess power dissipation with minimum affect on the forward power output of the amplifier. A dynamic feedback system and a power source comprise an adaptive power supply for the amplifier. The dynamic feedback system monitors the power drawn by the amplifier and, thereby, indirectly monitors the solid state components for a high or excessive power dissipation resulting from VSWR load conditions, that, in turn, can cause elevated junction temperatures and poor efficiency. The adaptive power supply, when a high dissipation condition is sensed, dynamically adapts the amount of power supplied to, and thereby the bias voltages of, the solid state devices to reduce dissipation and maintain nearly constant operation efficiency. The selection of the proper feedback response maintains optimum power input and, consequently, full forward output power of the amplifier as the bias condition of the solid state components are adapted. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a radio frequency (RF) power amplifier. FIGURE 2 is a block diagram of a VSWR adaptive power supply for the power amplifier of FIGURE 1. FIGURE 3 is a graphical representation of the dc supply voltage versus the dc supply current characteristic of the adaptive power supply of FIGURE 2. FIGURE 4 is a waveform illustrating test measurements of the power dissipated by one amplifier transistor in the RF power amplifier of FIGURE 1, both with (line B) and without (line A) the VSWR adaptive power supply of Figure 2, plotted against the phase of the VSWR load. FIGURE 5 is a graph of test measurements plotting RF power output of one of the amplifiers using the VSWR adaptive power supply, against the phase of a VSWR load. DETAILED DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a RF power amplifier that is designed to operate in the frequency range of 225 to 400 MHz. It includes a pair of 90° hybrid coupled amplifiers, 10 and 12. The RF signal to be transmitted is received on line 14 by hybrid coupler 16, which in turn provides the signal to each of hybrid amplifiers 10 and 12. Within each hybrid amplifier, second hybrid couplers 18 and 20, respectively, split the RF signals again such that the RF signals supplied to transistors Q1 and Q3 are in-phase and the RF signals supplied to transistors Q2 and Q4 are in-phase, but 90° out of phase with the RF signals supplied to transistors Q1 and Q3. The outputs of transistors Q1 and Q2 are combined with hybrid coupler 22, and the outputs of transistors Q3 and Q4 are combined in a hybrid coupler 24. Hybrid coupler 26 combines the outputs of the two hybrid couplers to form an amplified RF signal that is coupled to the load. The RF power amplifier of FIGURE 1 is an example of a type of power amplifier utilized in connection with the invention to be subsequently described. As will be apparent, the invention may also be used with an amplifier having a single solid state device, as well as with one having multiple, parallel stages. Referring now to FIGURE 2, a block diagram of an adaptive power supply in accordance with the teachings of the invention, for use with the RF power amplifier described in Figure 1 is depicted. It comprises two power supplies 204 and 206. Power supply 204 provides an output voltage coupled to transistors Q1 and Q3 operating in phase, and power supply 206 provides an output voltage coupled to transistors Q2 and Q4 operating in phase. Alternating current (AC) power at 400 Hz is supplied on lines 202 to each of the power supplies 204 and 206. A power stage 208 or 210 in each power supply 204 and 206 respectively, converts the 400 Hz AC power to a DC voltage. In the preferred embodiment, the power stages are switching power converters. Associated with each power stage 208 and 210 are control circuits 212 and 214 respectively. A control circuit forms part of a dynamic control loop that controls the voltage (Vdc) of the output of the power stage associated with the particular control circuit. The control circuits 212 and 214 sample Vdc of power stages 208 and 210 by lines 216 and 218, respectively. The control circuits 212 and 214 also form part of current control loops that sample the dc load currents (Idc) of the output power signals on lines 223 and 225 using current sampling devices 224 and 226, respectively. The current control loops provide a current feedback signal representative of Idc to control circuits 212 and 214 by connecting lines 228 and 230, respectively. In response to the feedback voltage on lines 216 and 218, and the current feedback on line 228 and 230, control circuits 212 and 214 generate control signals such that Vdc of the output of the respective power stages 208 and 210 follow a predetermined voltage and current characteristic. The use of dual power stages is intended to be illustrative of only one possible application of an adaptive power supply in accordance with the present invention. For example, only a single stage power supply is required for a power amplifier with transistors that are not operating in quadrature phase. The single power stage may supply the Vdc signal to one or a plurality of parallel transistors operating in phase with one another, the number of transistors being limited only by ability of the power stage to supply the required power. Now referring to the graph of FIGURE 3, each control circuit controls the output voltage level of a power stage in accordance with the voltage and current characteristic shown by the graph. The voltage feedback loop of each circuit maintains a steady voltage at 28 volts for Idc between 0 and 10 amps. At 10 amps, as indicated by a "break point", the current feedback loop "foldbacks" Vdc as Idc increases the break point. The graph demonstrates a linear relationship between Vdc and Idc during the foldback that maintains power supplied to the RF power amplifier at a fixed wattage level. The RF power amplifier shown in FIGURE 1, with the adaptive power supply of the present invention generating this voltage and current characteristic, has been shown to maintain a 250 watt power output into a 3 to 1 VSWR load. The Vdc and the Idc at breakpoint and the slope of the foldback are chosen for a particular application depending on the particular RF power amplifier and the expected VSWR load condition on the output of the power amplifier. The foldback characteristic is adaptable to any VSWR condition. Furthermore, the voltage and current characteristic need not necessarily be linear. Although it is shown to be piece-wise linear, the voltage and current characteristic may be customized for the particular application with a non-linear function or programmed with an empirically determined voltage and current relationship that avoids the deleterious affects of excessive power dissipation while maintaining the desired power output. Referring now to the waveform in FIGURE 4, this waveform plot illustrates power dissipated by the transistor Q1 or Q3, of the power amplifier shown in Figure 1, versus a load phase under a 3 to 1 VSWR load. The curve designated "A" represents power dissipated by a transistor that is supplied by a non-adaptive power supply without the voltage foldback feature. The curve designated by "B" represents power dissipated by a transistor when the voltage to the transistor is supplied by the adaptive power supply operated according the present invention. The plot demonstrates a reduction in the amount of power dissipated by a transistor supplied by the adaptive power supply and relatively constant power dissipation for providing constant transistor efficiency. Referring now to FIGURE 5, the waveform plot illustrates the RF power output of transistor Q1 or Q3 of the RF power amplifier depicted by Figure 1 with the RF power amplifier coupled to the adaptive power supply of the invention, versus the load phase. The load is a 3 to 1 VSWR load and the dual output of Q1 and Q3 equals 125 watts total. The plot shows an RF power output substantially identical to that of an RF power amplifier, with a non-adaptive power supply, thus, demonstrating that use of the adaptive power supply in accordance with the present invention does not reduce the forward power output of a RF power amplifier. While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but, on the contrary, it is intended to cover alternatives, modifications, equivalents as may be included within the scope of the invention as defined in the appended claims.

Claims: A power supply (204) having a dynamic feedback system for indirectly monitoring the power drawn by a VSWR (voltage standing wave ratio) adaptive power amplifier (Q1, Q3), wherein the power supply includes a power stage (208) for generating a power signal having an adjustable voltage level, the power signal is applied to the VSWR adaptive power amplifier (Q1, Q3), a controller (212) having a first input and a second input and a sensor (224) responds to the load current level supplied by the power stage (208) to the power VSWR adaptive amplifier (Q1, Q3), characterized by: the controller (212) having the first input connected to the power stage (208), and receiving the voltage level of the power signal, the second input of the controller (212) connected to the sensor (224) for sampling the load currents of the VSWR adaptive power amplifier (Q1, Q3), the controller (212) including a control circuit responsive to the sampled load currents and the voltage level of the power signal to generate a feedback control signal coupled to the power stage (208) to maintain a substantially constant voltage level for a load current level below a preselected threshold current level, and thereafter folding back the voltage level of the power signal to decrease in proportion to an increase in the load current level. The power supply according to Claim 1 further characterized by the controller (212) including means for generating a voltage feedback signal representative of the sensed output voltage level of the power signal. The power supply according to Claim 2 further characterized by the controller (212) including means for generating a current feedback signal representative of the sensed output current level of the power signal.

Inventors: BURKE DOUGLAS RONALD, JACKSON CHARLES R, BURKE, DOUGLAS RONALD, JACKSON, CHARLES R.
Applicants: E SYSTEMS INC, E-SYSTEMS INC.
Application year: 1991
Main classification: H03F1
IPC codes: H03F1