Analysis of a new soft-switching high-voltage pulse capacitor constant current charging technology

In the technical fields of particle accelerators, laser pulses, radar transmission, etc., power pulse modulators are widely used. Pulse modulators are usually composed of a DC high-voltage charging power supply, a high-voltage energy storage capacitor or a pulse forming network (PFN) and a load. The high-voltage energy storage capacitor or PFN is first charged to the required voltage, and then discharged to the matching load through a discharge switch under the action of a timing signal to generate a power pulse of a certain width. For different applications, the pulse repetition frequency may vary from a single discharge to several kilohertz. In some applications, the charging voltage stability is often required to be better than 1% or even 0.1%. The most traditional charging method is the LC resonant charging method using an industrial frequency high-voltage power supply and a De2Q circuit. The energy storage capacitor can obtain a voltage value twice that of the high-voltage power supply. Although the technical route is relatively simple, it works in a low-frequency state, has a large volume and weight, and the ripple and stability are not satisfactory, especially when the grid voltage fluctuates.

The more advanced technical route is to use the switching conversion technology in power electronics. Due to the continuous progress of new power switching devices and new circuit topologies, the switching conversion technology has developed rapidly. Compared with the hard switching circuit, the resonant switching circuit works in the soft switching state, and the technology is more advanced. It has the advantages of small switching loss, small harmonic component, high frequency (such as tens of kHz), and small energy storage element size.

1 Technical indicators of high voltage pulse capacitor charging power supply

Figure 1 is the charge and discharge waveform of pulse capacitor or PFN, where Tc is the charging time, Tw is the discharge waiting time, and Tp is the charging repetition cycle. The average charging rate is CV2/2Tp, and the peak charging rate is CV2/2Tc. Charging rate, load capacitance range, voltage stability, ripple, power factor, efficiency, etc. are all important indicators for measuring power supply performance.

Figure 1 Charging and discharging voltage waveform

2 Basic constant current charging circuit analysis and calculation

In the resonant switch technology, the most suitable circuit for pulse capacitor charging is the series resonant switch circuit. The output is approximately a constant current source or "equal step charging". The outstanding advantages are high charging efficiency and inherent short-circuit protection capability. Figure 2 shows a series resonant switch full-bridge conversion circuit. The two switch tubes on the diagonal and the switch tube on the other diagonal are turned on alternately. The alternating conduction is a switching cycle Ts. In half a switching cycle, the resonant current completes a resonance through the switch tube and the freewheeling diode, and the load capacitor voltage increases by one step △V. Figure 3 shows the relationship between the switch tube gate drive pulse and the resonant current waveform.

Figure 2 Series resonant switch full-bridge conversion charging circuit

Figure 3 Switching tube gate drive pulse and resonant current waveform

Ignoring the loop resistance, the characteristic impedance of the circuit is Z =√ L/C, and the resonant period is T r = 2π √L/C, where C is the total capacitance formed by the series connection of the resonant capacitor Cr and the equivalent capacitance C' of the load converted to the primary side. Usually, due to the high transformation ratio of the high-voltage transformer, the equivalent capacitance C' is larger than Cr, so the total capacitance C and Cr after series connection are not much different. The complete charging current waveform envelope and charging voltage are shown in Figure 4, where t1 is the turning point, and before t1 it is linear equal step charging. The current waveform before t1 is enlarged as shown in Figure 5 (a), and the current waveform near the turning point is shown in Figure 5 (b).

Figure 4 Resonant current envelope and charging voltage waveform of a charging cycle

Figure 5 Resonant current waveform

By calculation, the amount of electricity charged by C' in each switching cycle (two resonances) in the linear stage can be obtained as:

The peak forward current is:

Vs is the supply voltage; N is the number of resonant cycles.

The reverse freewheeling peak value is:

From equation (3), we can see that the reverse freewheeling gradually decreases. When N = C'/4C, the freewheeling is cut off and the circuit loses the linear charging state. The turning point is:

The waveforms in Figures 4 and 5 are obtained by simulation using PSP ICE8.0. The component values ​​are as follows: Vs=500V, Cr=1.6uF, Lr=30uH, Ts=100us, (corresponding to a switching frequency of 10kHz), transformer step-up ratio 1∶40, Cload=0.4uF, and the charging power can reach 16kJ/s.

The output voltage control circuit is quite simple. When the pulse capacitor or PFN is charged to the set value, the drive output of the switch tube is turned off through sampling detection. The voltage stability depends on the amount of electricity in half a switching cycle (or a charging step). The circuit can be vividly described as working in "bang-bang" mode, that is, charging within Tc time and the switch circuit stops working within Tw time.

Figure 6 is a waveform diagram of a modulator charging power supply. The charging power of the power supply can reach 2kJ/s, the operating frequency is 12.5kHz, and the voltage is 30kV. The oscilloscope model used for measurement is Tektronix TDS3032. V s = 500V, C r = 0.4uF, L r = 158uH, T s = 80us, the secondary side of the transformer is a double winding, each winding has a step-up ratio of 1:40, and C load = 0.66uF. In the actual circuit, L r fully utilizes the leakage inductance of the transformer, and the leakage inductance is selected to be larger to reduce the distributed capacitance. Figure 6 (c) is the simulated waveform of the resonant switch current of the power supply, which is in good agreement with the measured waveform.

Figure 6 Measured and simulated waveforms of a charging power supply. (a) Charging voltage measurement waveform; (b) Resonant current measurement waveform; (c) Resonant current simulation waveform

3 High stability charging technology issues under high repetition frequency and small capacitive load

3.1 Problem Statement

In many cases, the series resonant switch circuit can achieve a higher charging stability with the simple control circuit mentioned above. For example, the injection impact magnet modulator newly developed by the National Synchrotron Radiation Laboratory 800M eV storage ring has a low repetition frequency (0.5Hz) and a large load energy storage capacitor (0.66uF). Through the design of specific parameters, it can achieve a stability of 0.1%. However, in some applications where the repetition frequency is high, the capacitance is small and the charging power is required to be large, this method will not meet the high stability charging requirements because the charging time is short and the amount of electricity in a charging step is too large. For example, the repetition frequency of a klystron modulator is 100Hz, the total PFN capacitance is 0.22uF, the charging power is 9kJ/s, and the charging time is only 10ms at most. If a 15kHz resonant switch is used, there are only 300 charging steps. If the stability is equivalent to the charging voltage of a step, it is difficult to achieve a charging stability of 0.1%. Some laser pulse modulators also require charging stability better than 0.1%.

To address the above issues, technical improvements can be made on the basis of the conventional series resonant switching circuit to adapt it to a wide range of repetition frequencies and changes in energy storage capacitor capacity, thereby achieving highly stable charging while maintaining the constant current source charging advantages of the original circuit.

3.2 Current situation at home and abroad

Maxwell and EMI in the United States have developed a series of switch-mode high-voltage power supplies suitable for pulse capacitor charging. They are all improved versions of series resonant switches and have their own technical patents. For example, the average charging power of EMI's commercial products reaches 30kJ/s (50kW in DC state), the voltage is 50kV, the overall size is 480×310×560, the weight is 84kg, the power density is 0.6W/cm3, the efficiency is 85%, and the power factor is 0.9. The linear accelerator modulators of Argonne National Laboratory and DESY Laboratory use EMI and Maxwell products.

Most of the domestic high-power pulse modulators use the traditional low-frequency LC resonant charging mode, and few use the high-power charging power switch mode, or use a voltage source with low charging efficiency. Reference [7] reports a medium-frequency high-power constant-current high-voltage power supply using thyristors, which is a good attempt.

3.3 Several improved technical routes

When the load capacitance is small and the repetition frequency is high, one design idea is to improve the control circuit so that the effect achieved is equivalent to: at the beginning of each charging cycle, use the main power supply with large resonant current to quickly charge to the preset voltage, then switch to the small current power supply for charging, and provide a small amount of power to the load in the Tw stage to reduce fluctuations; another technical route uses a dual-bridge phase-shift control circuit, as shown in Figure 7, which is very suitable for high-power charging applications. EM I's 30kJ/s, 50kV power supply uses this technology.

Figure 7 Schematic diagram of phase-shift control charging circuit of dual-bridge circuit

4 Summary

The series resonant switch circuit works in a constant current source state. Considering the charging efficiency, circuit implementation difficulty, volume, etc., this circuit is the most suitable for capacitor charging. Based on the basic circuit, the technical innovation is carried out to improve the charging stability, so that it can adapt to a wide range of repetition frequencies and changes in the capacity of energy storage capacitors. The application prospects will be more extensive, and it is an upgraded replacement for the traditional charging power supply.