Switching mode system to supply pulse modulators in radar applications Agnaldo Vieira Dias – Ernesto Ruppert School of Electrical and Computer Engineering - UNICAMP - Brazil agnaldo@dsce.fee.unicamp.br - ruppert@fee.unicamp.br Abstract- The study of high-voltage isolated DC/DC converters used to supply pulse modulators is presented on this paper. The Full Bridge topology was selected, focusing in problems caused by parasitic capacitances in the circuit. This characteristic, which appears sharply in high voltage transformers and in inductors of output filter, is highly undesirable when the circuit is operated in switching mode. These capacitances are handled using a soft commutation technique, where their stored energy are returned to the primary source in the end of each switching cycle, trying to get a low size, high efficiency and low heat dissipation in the converter. Moreover, a charging regulator circuit is used to ensure good pulse stability and avoid typical problems in the pulse modulators use, such as the backswing effect. The pulse modulator used as load for this converter operates with a supply voltage of 1kV, pulse repetition frequency (PRF) of 600Hz and consumes 2.5 kW. The converter input is supplied by a 280V unregulated DC bus. I. INTRODUCTION Pulse modulators are devices widely used in high power pulsed systems such as radar transmitters, particle accelerators and medical equipment. They are generally powered by step-up linear sources, which have high size and low efficiency. However, the development of power electronics created new circuit topologies that can be applied to improve performance and reduce size. A pulse modulator is a circuit that stores the energy provided by a DC power source during a long period in the capacitive elements that constitute its pulse-forming network (PFN) and discharge all that energy in the load in a short period of time. An example of electric circuit of a pulse modulator is shown in Fig. 1. Fig. 1. Pulse Modulator using Pulse-Forming Network A protection element should be inserted between the PFN and the source in order to limit the current that flow through the circuit during the charging time. It is very common to find a choke playing this role, called charging choke. The pulse- forming network is equivalent to a capacitor during the charging and the interaction with the choke generates an undesirable oscillatory behavior. To solve this problem, a diode called blocking diode is inserted in series with the charging choke. This diode maintains the modulator charged with the highest voltage reached during the charging process. This voltage is twice the input voltage. The equivalent circuit for pulse modulator during the charging phase is shown in Fig. 2, where C T represents the sum of all capacitors in PFN. Fig. 2. Equivalent Circuit of Modulator during the charging of the PFN This process of charging and discharging of the pulse modulator occurs at a particular frequency, called the pulse repetition frequency or PRF. The voltage and current waveforms of the pulse modulator are shown in Fig. 3. Observe that the input current of modulator has a pulsed characteristic. The peak value of this current can be calculated by (1) and the average value, by (2), where L C is the value of charging choke and V F is the output voltage of the modulator’s power source. Pulse modulators applied in radar systems demand peak currents that can be 5 to 10 times larger than the average current. Fig. 3. Voltage (A) and charging current (B) waveforms in the PFN during modulator’s operation