A Solid-state Power Supply/Modulator System

A Solid-state Power Supply/Modulator System

MagCap Engineering Inc.
Canton, MA

New advances in high power insulated-gate bipolar transistor and FET technology have made the design of solid-state high power transmitters possible. Thyratrons have been used traditionally as the main high power switch in radar modulators. Recently, solid-state switches in 250 to 1000 kW magnetron and klystron transmitters have become a reality.

The weather radar system manufacturers have generated requirements for compact, high reliability radar systems that are achievable only by utilizing state-of-the-art solid-state technology. One of the main criteria of weather radar systems is the extremely high regulation of the transmitted pulse’s energy, which directly affects the Doppler shift resolution. In simple terms, the system requires power supply regulation to be better than 0.001 percent and the magnetron’s pulse jitter to be < 10 ns. These requirements are necessary to limit the magnetron’s frequency shift due to amplitude changes or pulse jitter.

The three main weather radar systems that have been the target of these development efforts are a 250 kW X-band system, a 350 kW C-band system and a 1 mW S-band system. Table 1 lists the key specifications for these radar systems. The basic system block diagram consists of a solid-state modulator section, a high voltage power supply and a system control panel. The control panel contains all of the necessary system control and monitor functions. Additional control information can be provided depending on the specific system requirements.

The Power Supply

The system’s power supply is a state-of-the-art, high efficiency, series resonance converter type that achieves power regulation by modulating the internal frequency of the power supply. Effectively, the power supply’s resonant frequency is fixed and the pulse repetition frequency (PRF) is varied up or down depending on the power requirements.

The X- and C-band systems are powered by the same power supply since their power requirements are similar. The power rating of this supply is set to 1.5 kW. The S-band system is equipped with a 3 kW power supply due to the significantly higher power demand. The voltage level of the X- and C-band systems is set at 700 V DC while the voltage level of the S-band system is set at 1 kV DC. Input voltage for all of the systems is 220 V AC single phase at 50 to 60 Hz. Figure 1 shows the complete block diagram of the system’s power supply section. Figure 2 shows the power supply’s basic electrical circuit. All of the power supplies are equipped with power factor correction front ends that rectify, boost and preregulate the input voltage to 360 V DC. The power factor correction circuit is equipped with a soft-start capability to limit the rush of input current to the capacitor bank on the DC bus.

Initially, a packaged control system was sought to run the resonance inverter but no appropriate system was found that could meet all of the performance requirements. For example, one requirement is to run the inverter frequency from zero to maximum so the output voltage can vary from zero to maximum. That capability was difficult to find in an existing IC.

The series resonance control circuit or SRI control has a kick-start circuit to start the system and a dual-driver output stage that becomes locked in as soon as initial current begins to flow in the system. The initial current is paired with the driver that is on during the half-cycle of the series resonance converter’s cycle. The second driver takes over for the next power cycle. At the end of each power cycle the energy remaining in the system due to leakage inductance in the inverter transformer and energy in the resonant circuit are fed back. The basic configuration is a half-bridge circuit with provisions for energy recovery at the end of each resonant cycle.

The efficiency of the power supply including the inverter section is 98 percent at the 1.5 kW power level. A small fan is all that is needed in the system since the main circuit elements remain quite cool due to the power supply’s efficiency.

The output of the power supply is rectified and fed to the hard-tube modulator circuit, which is housed in an oil-filled container. The power supply is equipped with a modulator control circuit to provide protection and to prevent the PRF and the output voltage from reaching the modulator’s output terminals in the case of potential problems. The magnetron’s peak current is monitored and the PRF is inhibited if the current exceeds a preset limit.

A select switch allows a maximum number of magnetron peak overcurrent occurrences. In addition, the power supply is turned off at the end of the fault operation cycle, requiring a manual reset.

A maximum power supply current is established and monitored to protect the magnetron. In addition, the power supply is protected from short circuiting due to its series resonance configuration.

The hard modulator control circuit is responsible for generating the proper pulse width for the system. Four pulse widths are selected using the control panel. In addition, the system can be modified to provide a continuously variable pulse width from 0.25 to 5 ms. The input PRF enters the power supply at the rear and ends up in the modulator control where it is processed to provide the proper pulse width. This routing allows the modulator circuit to inhibit the PRF in case of system faults.

The power supply is turned on by a +24 V DC signal originating from the control panel, which energizes the power supply system’s front-end two-pole input relay. The power supply voltage and current monitors are located at the output rectified section of the inverter transformer. The power supply’s front panel contains all of the low voltage monitors for the modulator section along with the manual reset and fault indicators.

The Modulator Section

Figure 3 shows the system’s modulator section. Figure 4 shows the modulator’s electrical equivalent circuit used for circuit analysis and evaluation of critical parameters. The front end of the modulator contains the driver circuit, including the PRF input. The energy storage capacitors are located close to the main switch to minimize the circuit’s series inductance.

A high voltage pulse transformer is located between the solid-state switch and the magnetron load. Pulse energy is limited through the primary of the pulse transformer due to the saturating characteristics of the magnetic circuit. This configuration also maintains the power supply voltage at a low level for safety reasons. A current monitor is placed in the pulse transformer’s secondary winding to monitor the peak output current and to provide information for the peak overcurrent-protection circuit.

The inverter’s representative waveforms are shown in Figure 5 . Figure 6 shows actual magnetron voltage and current waveforms for an X-band short pulse with the peak RF current set at 24 A. The RF pulse width at the 50 percent point is 864 ns. The pulse rise and fall times are 46.9 and 250 ns, respectively.

Conclusion

This solid-state power supply/modulator represents a new level of reliability and efficiency that is unattainable with older tube-type systems. The resulting radar systems now can be made smaller and lighter with improved performance and less overall heat dissipation.

MagCap Engineering Inc. , Canton, MA (617) 828-1142.