In the synchronous output experiment of two pulse power sources, the trigger control system is the key to ensure the correct and effective synthesis of the sources. On the one hand, the control system generates the working sequence of two light sources, and at the same time, through the design of synchronous consideration, it controls the laser trigger switch to generate the trigger signal, thus achieving a certain power combination efficiency. Because power MOSFET has the characteristics of unipolar, voltage-driven, fast switching speed, high input impedance, good thermal stability, low required driving power and simple driving circuit, MOSFET is used to design the external trigger control system of laser trigger.
Structure and working principle of 1 system
Figure 1 is the structural block diagram of the synchronous control system of laser-triggered pulse power supply. The single source is SpitLight 1200 laser from InnoLas, Germany, and the trigger signal is divided into multiple channels to control the unit switch to turn on. The working principle of the laser trigger system is that the energy storage units of two pulse power sources are charged to the set values, and the control system sets the trigger time interval of the two sources according to the target position, and sends instructions to the laser trigger systems of the two sources respectively. The laser generated by the trigger system is injected into the main switch to control the disconnection of the two main switches, and the electric energy stored in the primary energy system is fed to the load through the switches.
The design parameters of laser external trigger system are as follows:
(1) generates a flash trigger signal. Pulse amplitude 5 v ~ 15 v, pulse width
≥ 100 μs, working frequency 50 Hz, load 50 Ω;
(2) generating a pulse box trigger signal. The pulse amplitude is 5 v ~ 15 v, and the pulse width is ≥ 100? S, pulse rising edge ≤5 ns, load 50 Ω, working frequency 50/n (n = 1, 2, …, 50). The delay between the signal and the flicker signal is adjustable;
(3) Anti-interference protection measures such as isolation and shielding are adopted between the external trigger circuit, the laser and the pulse power supply to ensure that the trigger system can work normally in the harsh environment of high voltage, large current and strong radiation of the power supply.
2 theoretical design and analysis
The laser external trigger system consists of two parts: control signal generation and control signal trigger, which are connected by common multimode fiber (working wavelength is 820 nm). Among them, the functions of setting the working parameters of the control system (such as working frequency and working times), generating control signals, isolating and converting output signals (electricity/light) are realized in the control signal generating unit, which is located in the working area where the operator is located; Located at the laser side of the pulse power source is the control signal trigger unit, which completes the functions of input signal conversion (optical/electrical), amplification, fast rising edge signal formation and isolated trigger output transmitted through optical fiber.
2. 1 Design of control signal generating unit
The control signal generating unit is divided into two parts:
(1) pulse trigger signal generator. It is used to generate pulse trigger signals to control the operation of power MOSFET devices and power transistors. It has the ability to adjust the number of output pulses, pulse width and frequency, and the output is TTL level. Using industrial PC, built-in NI timing/counting card PCI-6602, using LabVIEW development system to write computer man-machine interface, set working parameters, and program to generate control signals needed for laser external trigger work. Among them, PCI-6602 provides 8 timing/counting channels, the 32-bit source frequency is 80 MHz, and the rising edge of the output pulse signal is about 10 ns.
(2) Optical fiber isolation circuit. It is used to isolate the trigger signal of TTL level from the output voltage of power MOSFET, and has the characteristics of fast response and no distortion. The optical fiber transmitting equipment is HFBR- 14 14, with a bandwidth of 5 MHz and a pulse width of several hundred? Transmission requirements of purple laser trigger pulse signal.
2.2 Design of control signal trigger unit
The control signal generating unit is divided into four parts:
(1) photoelectric conversion circuit. Using HFBR-24 12 optical fiber receiver, the optical signal transmitted to the control signal trigger unit through multimode optical fiber is converted into TTL electrical signal.
(2) Power MOSFET driver/power transistor driver circuit, the former is used to boost the TTL signal of low level to a level that can be used to drive power MOSFET devices, thereby generating a laser Pockel box trigger signal with a pulse rising edge of ≤5 ns. The latter is used to generate a flash trigger signal.
(3) Power MOSFET devices. MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a voltage-controlled device. Because MOSFET has a positive temperature coefficient, it can avoid the damage of the device due to the continuous temperature rise. At the same time, because there is no upper limit of its on-resistance in theory, the energy loss during on-state can be very small. Its advantages are: very fast opening and closing ability (ns order); Very low trigger energy; Able to work at high repetition rate (MHz); Long service life (average 109 times); High efficiency and adjustable pulse width (output is determined by input trigger signal). -choose the power MOSFET device IRLML2803 of IR company. The drain-source breakdown voltage VDSS is 30 V, the DC current ID is 1.2 A, the maximum output current under pulse is 7.3 A, the on delay time Td(on) is 3.9 ns, and the off time Toff is 9 ns.
(4) Power supply part. Lithium battery pack is used to provide low-voltage power supply for optical fiber isolation circuit and power MOSFET drive circuit. Equipped with special protection board, it has the functions of overcharge, overdischarge, overvoltage, undervoltage, overcurrent short circuit and reverse connection protection, further ensuring the safe work of battery control part. This effectively eliminates the high voltage breakdown and other dangerous factors that may occur in the high voltage working loop of the trigger unit, the front control signal generation unit and the rear power supply due to the grounding of the power supply.
As shown in Figure 2, the converted TTL level is shaped, driven by power MOSFET/ power transistor, isolated by pulse transformer and output to laser. In order to ensure the normal operation of the trigger unit, it is necessary to add a high withstand voltage (5 kV) pulse transformer for electrical isolation before its output to the laser.
2.3 Selection of power MOSFET devices and their driving circuits
Fig. 3 is a schematic circuit diagram of the working principle of the power MOSFET device. In fig. 3(a), RG and CGS are the main parameters that affect the turn-on delay of MOSFET; The drain-gate capacitance CGD is the main parameter that causes the gate voltage to be disturbed during switching operation. The drain-source capacitance CDS is the main parameter that affects the turn-off time. MOSFET devices have two conversion processes: on conversion and off conversion. Fig. 3(b) shows the relationship between the drain-source voltage VDS, the drain current iD, the gate-source voltage VGS and the gate current iG and the time t during the switching process. The conduction conversion process is divided into four stages, each of which is as follows:
(1) T0 ~ T 1 stage: the gate drive current iG charges CDS and CGS, so that the voltage on CGS rises from 0 to the MOSFET turn-on threshold VGS(th).
(2) T 1 ~ T2 stage: the gate-source voltage VGS continues to increase exponentially, exceeding the MOSFET turn-on Value VGS(th) to reach va, and after VGS exceeds VGS(th), the drain current begins to increase, reaching the final output current Io. In this process, MOSFET consumes the most power because of the overlap of voltage and current.
(3) t2-t3 stage: Starting from t2, the drain-source voltage VDS of MOSFET begins to drop, and Miller capacitance effect is generated from the drain to the gate, so that VGS cannot rise, and a plateau appears, and the drain-source voltage drops to the minimum value at t3.
(4) T3-T4 stage: During this period, the gate-source voltage VGS rises from the platform to the final driving voltage. The rising gate voltage reduces the drain-source resistance RDS(on), and the MOSFET enters the conductive state after t4.
The turn-off conversion process of MOSFET devices is the opposite of the above process. As can be seen from the above analysis, the requirements for the gate drive circuit mainly include:
The leading and trailing edges of the (1) driving signal should be steep.
(2) The time constant of the power MOSFET gate charge-discharge circuit should be small to improve the switching speed of the power MOSFET device.
(3) The driving current is the charging and discharging current of the gate capacitor, and the driving current should be larger to make the rising and falling edges of the switching waveform faster.
Select MOSFET IRLML2803 and check its characteristic curve. It can be concluded that when VDS= 15 V and VGS= 12 V, the total gate charge QG≈3.7 nC, then the gate capacitance c = qg/vgs = 3.7 NC/12v ≈ 0.3 nf = 300 pf.
The switching speed of MOSFET is related to the charging and discharging speed of MOSFET gate capacitor. The relationship among MOSFET gate capacitance, on and off time and driving current of MOSFET driver can be expressed as follows:
dT=(dV×C)/I
Where dT is that on/off time, dV is the gate voltage, c is the gate capacitance (from the gate charge value), and i is the peak drive current (for a given voltage value).
The on/off time of IRLML2803 is 4 ns, so I = qg/dt = 3.7 NC/4 ns ≈ 0.9 A, that is, the peak driving current obtained from the above formula is 0.9a. The external resistor used between the MOSFET driver and the power MOSFET gate needs to be considered, which will reduce the peak charging current of the driving gate capacitor, so the driver with the peak output current greater than 0.9a is selected. The system adopts the noninverting driver TC4424A with a peak output current of 4.5 A, and meets the requirements of fast rising edge signal output through experiments.
3 Test results and analysis
3. 1 Trigger signal optical fiber transmission conversion test
The laser external trigger system adopts optical fiber transmission and transceiver technology. Because it is made of insulating material, it has good high voltage isolation ability and strong anti-interference ability. The synchronization of multi-channel optical fiber signal transmission is also very good, which meets the requirements of high voltage isolation and synchronization of signals.
Fig. 4 is a waveform diagram of a signal generated by an external trigger unit of a laser. In fig. 4(a) and fig. 4(b), channel 2 shows the laser flicker trigger signal with the working frequency of 50Hz (the former is a pulse sequence with the output number of 50, and the latter is a single output pulse), which is programmed by the PC in the control signal generation unit, isolated by the pulse transformer, converted by electricity/light, transmitted by optical fiber and input to the trigger unit, and then amplified by the light/electricity conversion and power transistor drive, and driven by the high withstand voltage pulse transformer.
In fig. 4(a) and fig. 4(b), channel 1 is the laser Pauker box trigger signal (the display mode is the same as that of channel 2), and the working frequency is 50 Hz(50/N, N= 1). In the control signal generation unit, the signal generation mode is the same as the flash trigger signal, but it is processed by the power MOSFET and high-speed MOSFET driver in the trigger unit. Finally.
In the experiment, the pulse widths of the laser flash trigger signal and the Pockel box trigger signal are both 160 μs, and the latter lags behind the former by about 250μ s. Both of them are adjustable, and the output frequency of the Pockel box trigger signal is also adjustable, which meets the requirements of the laser.
3.2 Influence of laser external trigger operation on power supply
The main switch system with low jitter and high power repetition rate is the core and difficulty in the development of power synchronous control system. In order to realize the low jitter of the pulse power synchronization system, the source of jitter in the system is analyzed at first. The working flow of the synchronization system is as follows: the external trigger system of the laser generates a signal with a fast rising edge and sends it to the laser, and the laser generates a pulse laser to be injected into the laser switch, and the laser switch is closed to form a line to discharge the diode through the induction superposition module to generate an electron beam. During this process, the following jitter may occur:
Circuit jitter of (1) laser external trigger system J 1. The jitter comes from the different chip delays in transmission lines and conversion lines and the jitter of the chip itself, and the measurement is less than 2 ns.
(2) Laser jitter J2. The jitter comes from the working process of the laser, and the jitter of the laser is less than 3 ns under the trigger of the fast leading edge signal (tr≤5 ns).
(3) The laser switch dithers JBOY3. The jitter comes from the physical process of laser-triggered plasma discharge, and the design index is less than 5 ns.
Fig. 5 shows the load waveform synthesized by four inductance superposition modules in the pulse power source, with a repetition frequency of 25 Hz and a planar diode as the load. The figure shows the superposition of 25 waveforms (channel 1 is the diode current signal waveform and channel 2 is the diode voltage signal waveform). Experiments show that the output waveform of the load is consistent with the laser external trigger system, and the switching jitter is low when the repetition frequency is 25 Hz, which meets the design requirements.
3.3 Anti-interference considerations
Laser external trigger unit is the control link in synchronous operation and the key to whether the device can work normally. The requirements for the trigger circuit are steep pulse front, sufficient amplitude and pulse width, good stability and anti-interference performance. However, high voltage generators are prone to produce various instantaneous spikes, which are called "burrs". When its amplitude and energy reach a certain level, it will easily lead to abnormal operation of the system. In the previous debugging process of synchronous operation test, due to the limitation of experimental site conditions, the primary charging power supply of laser power supply and pulse power supply was * * *. When the power supply was running, the trigger signal output by the laser external trigger system to the laser Pucher box produced a peak interference pulse ahead of the set time, which could not guarantee the normal operation of synchronous operation test. In order to eliminate the interference caused by power supply, power filter and high frequency capacitor are added, and the effect is improved. The next step is that the laser and its external trigger system use the same power supply, and the power supply and pulse power supply are completely separated to ensure the safe operation of the synchronous system.
The experimental results show that it is effective to use power MOSFET and its high-speed driver, and it is feasible to use optical fiber transceiver to convert transmission and isolate high withstand voltage pulse transformer. The performance of laser external trigger circuit affects the conversion efficiency of synchronous output of pulse power supply switch. The on-off state of the power MOSFET switch can be controlled by the trigger pulse, and the rising edge of the output pulse signal can be controlled below 5 ns by selecting the MOSFET driver of the high peak output circuit. The laser external trigger system is adopted, and the single pulse power supply switches repeatedly. The parameters are: working voltage 150 kV, current 30 kA, jitter.
≤5 ns, repetition rate 25 Hz. It lays a certain technical foundation for further developing the stable, reliable and accurate synchronous output of two or more pulse power supplies.
In addition, in the printed circuit board that triggers the control circuit, the control circuit is easily disturbed by the power supply loop, so the trace length of MOSFET driver and MOSFET should be as short as possible to limit the oscillation effect caused by inductance. The inductance between the driver output and the MOSFET gate will also affect the ability of the MOSFET driver to keep the MOSFET gate low under transient conditions. The problems existing in laser trigger experiment, such as reducing waveform wavefront and enhancing anti-interference ability, need further study.
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Zhang, Li Jin. Experimental study on solid-state switching technology based on MOSFET [J]. High Power Laser and Particle Beam, 2004( 1 1).
Yee H P. A MOSFET driver for EMI suppression [A]. Proceedings of the Conference and Exhibition on Applied Power Electronics [C]. Twelfth year, 1997:242-248.
Saethrer, KIRBIE H, CAPORASO G, et al. Optical control, diagnosis and power supply system for solid-state induction modulators [A]. Proceedings of the 1 1 IEEE International Conference on Pulse Power [C]. Baltimore, Maryland,1997:/kloc-0.