High-voltage pulse resistant and highly stable filament power supply

The paper focuses on how to solve the problem of how to resist the impact of 30 kV high-voltage feedback pulse of filament power supply device when the high-current switch of high-power device is in action. Through the design of resisting high voltage and strong current impact, the low-voltage power supply provides high-stability power output for the filament of high-current switch; the high-voltage isolation transformer is used to isolate the influence of high-voltage trigger feedback pulse on surrounding instruments through the power supply; the choke is used to block the inflow of feedback high-voltage peak current, which effectively reduces the damage of instantaneous strong current to filament heating power supply after discharge of high-power device; in the high-voltage and strong-current environment, a reliable DC heating power supply with resistance to high voltage and strong current is provided for the heating of high-current switch filament.

In order to ensure that the filament heating power supply can work normally, the filament heating power supply itself must have the functions of resisting high voltage and strong current impact. This requires that the filament heating power supply should not only provide 4-way high-stability power output for the filament heating of the high-power device and high-current switch, but also have the characteristics of resisting the feedback pulse high voltage and strong current impact of the high-current switch, and isolating the mutual interference with the mains (220 V).

1 Design concept of filament power supply device

High-voltage and high-precision filament power supply should have the following characteristics:

(1) Because the level of filament voltage directly affects the triggering quality of the high-current switch, for example, if the filament voltage is too low, the cathode emission capacity is insufficient and the gain will be reduced; if the filament voltage is too high, the cathode active material will evaporate excessively, which will shorten the life of the high-current switch. Therefore, the filament power supply must provide a highly stable voltage output.

(2) The filament of a high-current switch has the characteristics of small cold resistance and large hot resistance. The filament power supply is easily impacted by surge current (more than ten amperes) at the moment of power on, which will affect its life. Therefore, the filament power supply must have the ability to resist large current impacts.

(3) After the high current switch is triggered, a high voltage pulse with an amplitude of nearly 30 kV pulse voltage and 100 kA pulse current will be fed back, which will directly damage the power supply itself and affect other surrounding instruments. Therefore, the filament power supply must also have the ability to resist the impact of high voltage feedback pulses.

In order to meet the above requirements, a high-voltage resistant and high-precision filament power supply is studied using the method shown in Figure 1.

1.1 Isolation of high voltage trigger feedback pulse interference technology

The design of the high-voltage isolation transformer is to use the AC impedance formed by the capacitance between the primary poles of the high-voltage isolation transformer and disconnect the ground loop to isolate the impact of the high-voltage pulse. At the same time, a high-voltage bypass capacitor is also connected to the input end of the high-voltage isolation transformer, so that the impact of the high-voltage trigger feedback pulse on the subsequent test instrument through the power supply can be isolated.

Adding a high-voltage isolation transformer between the power supply and the instrument can block the coupling path. The ground loop can be disconnected after connecting the high-voltage isolation transformer, as shown in Figure 2. Moreover, this connection has a very low impedance to the normal transmission current, but it has a very high impedance to the longitudinal noise current, that is, the fundamental wave component of 50 Hz can pass almost unimpeded, while the high-frequency component is weakened, so in the filament power supply device, the high-voltage isolation transformer is indispensable.

Connecting a high-voltage isolation transformer isolates the equipment power supply from the incoming power supply, cuts off the path of noise interference, and thus achieves the effect of suppressing noise interference. It can effectively suppress noise interference that enters the AC power supply. The isolation transformer is an inductive load, which can suppress the sudden change of current, effectively reduce surge current, reduce the sudden change of voltage and power supply fluctuation, and suppress the interference of high-voltage pulses introduced from the power line on the power supply; it can fundamentally prevent the power supply from malfunctioning due to ground potential disturbance.

1.2 Technology of high current impact resistance and high stability power supply

The filament of a high-current switch has the characteristics of low cold resistance and gradually increasing hot resistance after power is turned on. Therefore, the DC power supply is susceptible to surge current (more than ten amperes) when the power is turned on, which will affect the life and reliability of the filament heating power supply. At the same time, in order to ensure the reliability of the high-current switch, stable triggering and its life, the filament heating power supply is required to provide a highly stable voltage output.

To this end, the low-voltage power supply part adopts slow start and integrated voltage stabilization technology. The integrated voltage stabilization technology is used to provide a highly stable power output for the filament of the high-current switch; the slow start technology is used to prevent the impact of the instantaneous surge current (greater than 10A) on the low-voltage power supply during startup.

In order to reduce the impact of surge current and avoid premature damage to the low-voltage switching power supply, measures are taken in the design of the low-voltage switching power supply circuit, that is, the low-voltage switching power supply circuit is combined with the slow-start circuit design. The filament voltage is slowly increased to the rated value, and the current is also slowly increased, thereby avoiding the impact of surge current. The slow-start circuit adopts the method of smoothly increasing from zero, and uses the principle of corresponding changes in its output voltage to achieve the purpose of smoothly increasing the output voltage from zero (see Figure 3).

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Since the output working current of the low-voltage switching power supply reaches 1.6 A, the DC impedance of the choke itself is required to be very small, so that its own DC voltage drop is very small; at the same time, in order to increase the voltage drop of the pulse high voltage on it, its AC impedance is required to be very large. In order to obtain a higher AC impedance, when selecting the choke core, a core with high magnetic permeability should be preferred.

When the operating frequency is much higher than the cut-off frequency, the resistance increment is much larger than the reactance increment, and the impedance increment is close to the resistance increment. At this time, the choke is close to a resistor, which can not only suppress but also absorb the energy of the feedback pulse.

According to the circuit connection shown in Figure 5, L is a choke (self-made) made of 2 m long φ1 mm high-strength enameled wire wound on magnetic cores (φ50 mm×30 mm×20 mm) with μ0=2 kH/m and μ0=7 kH/m respectively. When the input voltage is a 5 V sine wave signal, the voltage drop of the AC impedance on L can be obtained by measuring the output voltage value. Because the pulse width of the main pulse is 10 μs, the frequency should be 100 kHz, but the only SG503 signal source does not have a 100 kHz range, so the existing 50 kHz and 220 kHz ranges of the signal source can only be used for experiments. The experimental data are shown in Table 1.

If the AC impedance is high, the voltage drop on L is large, and the output voltage value Vo is low. By comparing the experimental data, it can be seen that the AC impedance of the magnetic core with μ0=7 kH/m is better than the AC impedance of the magnetic core with μ0=2 kH/m in the test frequency band.

1.4 Anti-interference technology

(1) The output DC current of the low-voltage switching circuit reaches 1.6 A, so the DC resistance of the high-voltage pulse choke is required to be very small to make its DC voltage drop very small; in order to increase the voltage drop of the feedback high-voltage pulse on it, its AC impedance is required to be very large. In order to obtain a higher AC impedance, the material of the choke core should be selected with a high magnetic permeability.

(2) A parallel double choke with an inductance of 10 mH is connected in series between the low-voltage switch circuit and the hydrogen thyristor filament, so that the 100 kHz frequency signal can form an impedance of about 6.3 kΩ. Therefore, about 3/4 of the peak voltage of the high-voltage trigger feedback pulse is dropped on the high-voltage pulse choke.

(3) A large number of high-voltage bypass capacitors are used between the input/output lines of the low-voltage switch circuit, and between the input/output and the ground, forming a high-voltage resistance component to contain and discharge the impact of high-voltage feedback pulses to prevent excessive peak voltage from forming on the low-voltage switch circuit and damaging the components of the low-voltage switch power supply. At the same time, the grounding also uses the "floating ground" method to suppress environmental interference.

(4) When winding the high-voltage isolation transformer, the primary and secondary are wound separately and shielded to reduce the distributed capacitance to improve the anti-interference ability. After the 220 V AC power supply passes through the isolation transformer, a filter circuit is installed. This filter has a certain effect on filtering out interference frequencies. Because L has a certain impedance to higher frequencies, the capacitor C has a small impedance to high frequencies, so it can provide a loop for the interference frequency, which is effective in filtering out interference.

When the high current switch is in action, the high voltage isolation transformer is equivalent to a large capacitor C; due to the bypass effect of the high voltage capacitor, the AC impedance of the low voltage switch power supply module at this time tends to zero; the high voltage pulse choke is equivalent to the inductor L. In short, the AC impedance of the entire circuit should be as large as possible to make the high voltage peak current flowing in tend to zero. The entire circuit can be equivalent to a Γ type filter circuit, as shown in Figure 6.

2 Experimental verification

The anti-interference technology in this paper was used to conduct experiments. The results are as follows:

(1) Parallel dual choke: The high voltages at both ends of the choke are measured to be 16 kV and 6 kV respectively. Therefore, the high voltage trigger feedback pulse has a peak voltage drop of about 10 kV on the high voltage pulse choke.

(2) Bypass circuit: It is measured that the high voltage on the low-voltage power supply has been discharged to several hundred volts. Through the redundant design of the withstand voltage parameters of the low-voltage power supply components, the filament voltage is guaranteed to work normally in the special application environment of high voltage and high current, meeting the reliability design and use requirements.

In order to improve the reliability of the power supply, key component screening and aging, high-voltage spark prevention, electromagnetic shielding, high-frequency high-voltage isolation and system stability design technologies are also adopted to ensure the stability and reliability of the power supply. At the same time, in the design of the whole machine, reasonable circuits and processes are adopted, especially grounding and electromagnetic shielding, to isolate the interference of high-voltage pulses generated by the back stage on the front stage instruments.

3 Conclusion

Due to the particularity of the use environment, the filament power supply device is required to not only provide 4 independently adjustable and highly stable DC outputs, but also be able to withstand and isolate the impact of 30 kV peak voltage. Practical applications have proved that in special application environments with high voltage and high current, the filament power supply has the characteristics of good stability, resistance to high voltage feedback interference, and strong strong current impact capability, providing a reliable DC power supply that can resist high voltage and high current impact for instruments and equipment in high voltage and high current environments (especially single high voltage and high current environments).