Design of high-power repetitive pulse laser power supply

 Abstract：   This paper proposes a design method for a high-power pulsed power supply for YAG laser systems, which is capable of realizing high-energy repetitive pulse with flexible voltage regulations. The power supply consists of an LC resonant converter as the charging network and a thyristor-based pulse forming network. Based on the proposed design approach, a 7 kW prototype is developed, which is able to output maximum voltage of 2.2 kV with a repetitive frequency ranged from 1 to 10 Hz. Moreover, the developed power supply is able to release up to 700 J of the energy on the xenon lamp. Key words：   YAG laser, series LC resonant converter, pulsed laser power supply, high pulse energy outpu

With the development of science and technology, yttrium aluminum garnet (YAG) lasers have been used in industry, medicine and scientific research due to their advantages of high reliability, low cost and long life
It is widely used in research and military fields [1-4]. The YAG laser needs a pulsed laser power supply to ignite the xenon lamp load and absorb the light energy emitted by it to form excitation radiation
and produce lasers [5-7]. In recent years, the application of high-energy pulsed laser has developed rapidly, and its core energy component pulsed laser power supply has been used in pulse energy, pulse width and Higher requirements are put forward in various indicators such as modular design. Although there are many studies on high-power YAG pulsed laser power supply in China, there is an average output
The power is low, the output voltage lacks the ability to be regulated, and there are no disadvantages such as modular design [8-13]. In addition, pulses with a single pulse energy greater than 500 J can be continuously output
Punching laser power supplies is rarely reported. A pulsed power supply with a maximum output voltage of 120 V and a single pulse energy of 40 J was designed in Ref. [14]. Ref. [15] designed
A pulsed power supply with a maximum output voltage of 1 kV, a single pulse energy of 50 J, and an average output power of 2 kW is not suitable for high-pulse energy YAG lasers
application scenarios. In order to solve this problem and application requirements, a 7 kW high-energy pulsed laser power supply for YAG laser is designed to achieve output
The voltage is flexible and adjustable from 0~2.2 kV, with a single pulse output energy of up to 700 J, a repetition rate of up to 10 Hz, and a current pulse width of 350 μs. This article will start from the power supply
The topology and working principle, power supply design and experimental test are introduced in detail.

1 Power topology and working principle
In this section, we will introduce the topology and operating principle of the pulsed laser power supply that has been developed. As shown in Figure 1, the pulsed laser power system is mainly composed of a rectifier network and a charge
It consists of an electrical network, a pulse-forming network, and a xenon lamp load. Among them, the rectifier network adopts the three-phase uncontrolled rectification topology, and the charging network adopts the series LC resonant converter topology, which has the advantages of wide voltage gain, soft switching and low loss. The discharge network selection is based on the thyristor-triggered LC discharge circuit, which can achieve a single high pulse
Punch energy output. The YAG laser pump, which is composed of a xenon lamp load, has the advantages of high brightness, long life and low cost, when the xenon lamp load is pulsed to form a network
The light emitted after the network is ignited is uniformly and efficiently concentrated onto the gain medium YAG crystal, resulting in laser light.

1.1 How it works
The charging network consists of DC voltage source Vdc, switching device Q1~Q4, resonant inductance Lr, resonant capacitor Cr, high-frequency transformer T, and full-bridge rectifier diode
D7~D10 and energy storage capacitor C2, as shown in Figure 2.

In order to achieve zero voltage switch and zero current switch of the series LC resonant converter, it is possible to reduce it
The switching loss of the switching device requires the switching frequency fs and resonant frequency FR to meet the fs<0.5fr, so that the circuit works in the underresonant mode. Figure 2 shows the tandem
Four modes of operation of the LC resonant converter in under-resonant mode.
The pulse-forming network consists of an energy storage capacitor C2, a wave-modulated inductor L, a power thyristor, and a xenon lamp load, as shown in Figure 1. To obtain a single greater than 700 J
The energy of the sub-discharge pulse energy is suitable for the application scenario of high-power YAG laser, and the energy of the energy storage capacitor C2 needs to be completely released at one time during the discharge process.
Therefore, the thyristor of a semi-controlled power device with high flux capability was selected as the discharge switch. When the power supply is working, the rectifier network realizes AC to DC conversion.
The series LC resonant converter outputs a constant current to charge the energy storage capacitor C2. When the controller sends a discharge signal to trigger the power thyristor to turn on, the energy is stored
The energy in vessel C2 is fully discharged through the LC discharge circuit, generating a pulsed current and igniting the xenon lamp.

2 Power Supply Design
Since the power of the pulsed laser power supply is greater than 7 kW, three-phase alternating current is chosen as the power supply input. The target parameter for the power supply design is the maximum single pulse
The punch energy is 700 J, the discharge current pulse width is 350 μs, and the maximum charging voltage and repetition rate are 2.2 kV and 10 Hz, respectively. In order to make the series LC resonant
The changer operates in the under-resonant state and the switching frequency fs is set to 22 kHz.

2.1 Charging network parameter design
The input of the series LC resonant converter is a three-phase alternating current to obtain a DC voltage of about 500 V. The maximum charging voltage of the energy storage capacitor is 2.2 kV,
Considering that there is a 20% charging margin, the maximum charging voltage Uout, max is set to 2.6 kV, and on this basis, the transformer turns ratio n is calculated

where: Udc is the input DC voltage of the series LC resonant circuit; Uout, max is the maximum charging voltage set. The ratio of transformer turns can be obtained by equation (1).
n is 1:5.2. In the case of high voltage output, the turns ratio of 1:5.2 will lead to an increase in the number of turns of the secondary winding, and the transformer will be larger. Therefore, three transformers are used
The modular structure design of primary side parallel and secondary side cascade can reduce the number of single transformer windings, reduce the current stress of the transformer winding and the secondary side rectifier
Voltage stress, the number of turns ratio n0 of a single transformer is calculated as 1:1.73. In order to achieve a maximum single pulse energy of 700 J and taking into account a charging voltage of 2.2 kV, the storage
The formula for calculating energy capacitor C2 is as follows

where: W is the energy of the single pulse of the energy storage capacitor C2; Uout is the charging voltage. The energy storage capacitor C2 calculated from equation (2) is 300 μF. Charging network adoption
Constant current charging mode, for fast charging at 10 Hz repetition rate, the average charging current Io, avg is as follows

where f is the maximum discharge repetition rate. According to Eq. (3), the average value of the constant current charging current is about 7.33 A. According to the series LC resonant converter
The switching frequency and constant charging current are calculated as follows

where: Udc is the output DC voltage of the three-phase rectifier, and the calculated value is 500 V; n is the ratio of transformer turns; Cr is a resonant capacitance. The resonant electricity is calculated from Eq. (4).
The capacitance is 0.44 μF. According to the under-resonant working conditions of the series LC resonant converter, the value of the resonant inductance Lr is 28 μH.

2.2 Pulse formation network parameter design
In order to achieve the shortest discharge current pulse width and the minimum reverse voltage of the LC discharge circuit to achieve the maximum energy conversion efficiency and prolong the service life of the xenon lamp,
Pulse-forming networks are to operate in a critically damped state. At the moment of xenon discharge, the xenon lamp is equivalent to a small resistance resistance. According to the characteristic parameters of the xenon lamp, it can be counted
The lamp constant K0 and the equivalent resistance Ro of the xenon lamp are calculated. By measuring the characteristic parameters of the xenon lamp, the formula for calculating the constant K0 of the xenon lamp is as follows

where: p is the air pressure inside the xenon lamp; l is the length of the xenon lamp; d is the inner diameter of the xenon lamp. From Eq. (5), the lamp constant K0 is 35.
The equivalent resistance of a xenon lamp at the moment of discharge can be calculated from the lamp constant K0 and the discharge voltage Uload of the xenon lamp, and the calculation formula is as follows

where: K0 is the lamp constant of the xenon lamp; Uload is the discharge voltage; RO is the equivalent resistance of a xenon lamp at the moment of discharge. According to equation (6) the xenon lamp is calculated at 2.2 kV
The equivalent resistance Ro is 0.75 Ω at the discharge voltage.
In actual operation, it is necessary to carry out discharge experiment correction to calculate the equivalent impedance value of xenon lamp, and draw it by comparing the experimental conditions of different repetition rates and discharge voltages
The instantaneous equivalent impedance of the xenon lamp discharge. As shown in Figure 3, the equivalent resistance of the xenon lamp at the moment of discharge is about 1 Ω, which is close to the theoretical calculated value
Guide the pulse formation network parameter design, so that the discharge circuit works in the critical damping state and reduces the reverse current. The pulse width of the discharge current in the design index is 350 μs,

The discharge oscillation period T is calculated as 1 ms. The formula for the critical damping oscillation of the discharge inductor L is calculated as follows

where: T is the period time of discharge oscillation; C2 is an energy storage capacitor. The discharge inductance L calculated from Eq. (7) is 80 μH. When the discharge circuit works at critical resistance
The relationship between the xenon resistance and the energy storage capacitor C2 and the discharge inductance L is as follows

where: Ro is the equivalent resistance of the xenon lamp at the moment of discharge; C2 is an energy storage capacitor; L is the discharge inductance. According to equation (8), the equivalent resistance Ro of the xenon lamp is calculated
1.03 Ω, which is close to the actual value, verifies that the discharge circuit operates in the critical damping state.

2.3 Control Strategy
As a charging network for high-power and high-energy pulsed laser power supply, the series LC resonant circuit adopts a fixed duty cycle and switching frequency modulation mode, which can be realized
Fast and reliable charging control is now available. By comparison, it is found that at the switching frequency of 22 kHz and the duty cycle of 0.3, the series LC resonant converter can achieve the maximum constant current
Stream fast charging to meet charging requirements at a maximum repetition rate of 10 Hz. Finally, the switching frequency is set to 22 kHz and the duty cycle is 0.3.
In order to realize the repetition frequency of the pulsed laser power supply, a control method of charging and discharging complementarity is proposed to ensure that the charging and discharging of energy storage capacitors will not interfere with each other
stalk, to achieve isolation. The existing control strategy is that before the power supply works, the controller sets the charging voltage and repetition rate, and the pre-ignition circuit pre-ignites the xenon lamp. Be in control
When the generator sends a charging signal and outputs a PWM signal, the series LC resonant converter starts to work, and the voltage at both ends of the energy storage capacitor begins to rise linearly at each end. voltage
The sampling circuit samples the voltage values at both ends of the capacitor in real time and sends it to the controller for calculation. When the set charging voltage value is reached, the controller shuts down
PWM signal, the series LC resonant converter stops working. Until the discharge signal arrives, the energy on the energy storage capacitor is released at one time during the discharge process.
The xenon lamp is lit by a pulsed current. Repeating the above process can realize the repetition rate of the pulsed laser power supply.

3 Experimental results and analysis
In order to verify the design method and control strategy of the proposed pulsed laser power supply
The feasibility of building an experimental prototype test platform is shown in Figure 4. Key parameters .It is given in Table 1.

3.1 Single-supply output test
In the experiment, the charging voltage of the energy storage capacitor was set to 1.8 kV and respectively
At 2.2 kV, the xenon lamp is discharged at a repetition rate of 1 Hz, as well as at different weights at 2.2 kV
Experimental waveforms of output currents at complex frequencies and at different discharge voltages at 1 Hz are as follows
Figure 5. Under the action of pulse current, the xenon lamp is ignited, and the pulse width of the discharge current is about The value is 350 μs, and the parameter design meets the requirements of the power supply.

3.2 Laser output energy test
In order to verify the matching effect of the power supply and the laser, a laser energy meter was used to test the energy output of the YAG laser at different voltages, as shown in Figure 6
Show. As can be seen from Table 2, the fluctuation of the laser energy output remains at about 0.5% at different voltages, indicating that the pulsed laser power supply can be repeatedly stabilized
The xenon lamp is loaded and provides a stable energy output, which can meet the application needs of YAG lasers.

4 Conclusions
In this paper, a design method of high-power and high-energy pulsed laser power supply is discussed. The charging network uses a series LC resonant converter to realize energy storage capacitors
Constant current charging. The pulse forming network uses a power thyristor to trigger the LC discharge circuit and output a high-energy pulse current. The experimental results show that the pulsed laser power supply
The proposed pulse excitation is verified by the ability to achieve a repetition rate output with a flexible voltage adjustment in the 2.2 kV range and a stable output energy under continuous operation
Feasibility and effectiveness of optical source design methods.