Linear fast shutdown technology based on self-resonant constant voltage clamping

SUN De-li，SONG Shuang，TENG Fei

(College of Instrumentation & Electrical Engineering, Jilin University, Jilin 130061, China)

Abstract：Transient electromagnetic method(TEM)has been widely used in the field of medium and shallow underground detection due to its high detection efficiency and large detection depth.However, due to the long turn-off time of the transmitting current caused by the inductive characteristics of the transmitting coil, the early signals will be overwhelmed by primary field.Since the early signals contain most of shallow geological signals, it is necessary to reduce the long turn-off time to get shallow layer signal.Due to lack of a reliable and effective clamping method for high-power transmission at present, we design a TEM transmitter fast turn-off circuit, combining self-resonant zero-voltage switching technology with the corresponding timing control circuit to solve this problem effectively.A transient electromagnetic transmitter based on self-resonant constant voltage clamping technology was fabricated to charge the clamping capacitor.The rated transmitting current of the transmitter is 20 A, and the turn-off time is continuously adjustable from 550-50 μs.Moreover, the current drop process is approximately linear rather than exponential attenuation.Compared with the existing clamping methods, the proposed clamping method solves the problems that transient voltage suppressor(TVS)clamping cannot be used in high-power occasions and has a high failure rate.It also solves the problem of long pre-charge time in traditional capacitor clamping methods due to insufficient inductance of the small size transmitting coil.The proposed method can provide a reference for fast shutdown of large current.

Key words：transient electromagnetic method(TEM)；constant voltage clamp；zero voltage switch(ZVS)；linear turn-off

0 Introduction

Recently, more and more cities have begun to develop underground space to solve transportation and residential problem[1-2].With the acceleration of urbanization in China, China’s urban population is increasing rapidly.Therefore, an efficient large-scale detection method is required to conduct pioneering work for urban development, then expanding to the surrounding urban-rural junction.Traditional methods used in urban underground space detection mainly include ground penetrating radar(GPR)method, shallow seismic method, and transient electromagnetic method(TEM).Due to the deepening of urban underground space, the GPR has been unable to meet the need of detection depth.The shallow seismic method has strong limitations in the application of urban detection due to its operating characteristics, therefore the TEM has gradually become the most popular one of these detection methods.The TEM has the advantages of being less affected by terrain and a large detection depth.It is widely used in hydrology and water conservancy surveys, underground rock formation and geological structure surveys and so on[3].Besides, in non-destructive detection of ancient ruins, TEM also plays an irreplaceable role in prevention and rescuing of disasters, such as landslides and debris flow.

The principle diagram of TEM is shown in Fig.1.

Fig.1 Principle diagram of transient electromagnetic method

When an alternating current is conducted in the transmitting antenna in a time-sharing manner, a field will be excited in the space.When the primary field encounters an underground anomalous body, it induces a current inside the anomalous body which generates a secondary induced field[4].The acquisition system receives the secondary field signal in space, records the signal as a series of voltage sequences, and supplements it with related data processing algorithms to finally obtain relevant information such as apparent resistivity and anomalous body depth.

Transient electromagnetics consist of two parts, transmitting system and receiving system.The transmitting system is mainly composed of drive circuit and ransmitting bridge.Excitation is achieved by the current inside the transmitting coil.Two important quotas are duration and linearity of the falling edge of the transmitting current.They will significantly affect the signal quality and data validity of the transient electromagnetic detection system.Since the transmitting coil or ground loop mostly shows inductivity, which causes the linearity and fast closing of the transmitting edge fault, it is a major problem in transient electromagnetic detection systems.In addition, the acquisition system often suffers from problems such as saturation of the receiving amplifier circuit, difficulty in correction, and difficulty in field elimination, etc.All these make the application of TEM limited or difficult in shallow detection.

Transient voltage suppressor(TVS)diode clamping is universally used to solve the problem of non-linear phenomenon of the transmitting current[5-8].The two ends of the transmitting coil are clamped at both ends of the TVS diode with a higher clamping voltage.The linearity is maintained by maintaining the same voltage.This method can optimize the linearity of the emission current and shorten the turn-off time to a large extent.However, this method has its own inherent limitations.First of all, TVS diodes should not be used in series, thus it is difficult to achieve a high clamping voltage.Due to the inevitable deviation of the distribution parameters, there will be a problem that individual TVS diodes are subject to excessive back pressure in series application.which will cause frequent damage to instrument; secondly, TVS diodes are difficult to withstand large current, therefore TVS diodes cannot be used for clamping in scenarios where large current is emitted.

Another active clamping scheme is to use the inductor of the transmitting coil itself and an external clamping capacitor to form a boost circuit[9-10].The energy storage capacitor is charged by the transmitting coil, and a control circuit needs to be designed to maintain the voltage of the energy storage capacitor at a relatively stable level to achieve clamping.However, under some special application conditions such as small self-inductance of coil and low emission current, the charging speed to the clamping capacitor will be very slow, therefore a new transient detection system based on active constant voltage clamping is developed to meet new detection requirements and new detection scenarios, which can achieve linear optimization and freely adjustable turn-off time, and finally improve of the quality of the transmitted waveform.

1 Basic principle

1.1 Principle of clamping absorption

The basic bridge circuit of TEM transient electromagnetic transmitting system is shown in Fig.2.It mainly turns on and off the bridge arm switch transistor to invert DC power to AC power at a specific frequency.In this design, an H-bridge is used to invert the DC power into a designated bipolar transient electromagnetic excitation signal.

Fig.2 Structural drawing of basic transmitting bridge

L and R in the Fig.2 are the equivalent load of the coil.VT1, VT2, VT3 and VT4 are turned on according to a specific sequence, the DC voltage is inverted to an AC signal, and a field in the space is excited to achieve the underground detection.The transmitting current waveform will directly affect the quality of detection.When no absorption is used, the emission current is shown in Fig.3.

Fig.3 Free shutdown current waveform

This method is called free shutdown.This structure is simple to be set up and widely used.However, there are many limitations in its use.Firstly, due to the resistance-inductive characteristic of the coil itself, the current after the switch-off does not attenuate linearly but exponentially[11].The off-time of free shutdown is determined by the time constant of the second-order system consisting of the load and the entire bridge.Because fast linear turn-off cannot be achieved, detection in the middle and shallow layers becomes an impossible mission based on this structure, and the signal of secondary field is submerged seriously by the primary field in this structure.

In addition to the basic inverter circuit in Fig.4, there is a passive constant voltage clamping circuit based on a TVS diode.

Fig.4 Structural drawing of TVS absorption

TVS is clamp-type.When its two ends are subject to instantaneous high energy impact, it can change the impedance value between the two ends from high to low at a very high speed.At the same time, the TVS diode absorbs the instantaneous large current, so as to clamp the voltage at both ends at a predetermined value.Assuming that the clamp voltage of TVS isE, the coil current and voltage should meet the equation as[12]

(1)

whereRis the resistance of the transmitting coil, andLis the equivalent inductance of the coil.Because the load is a coil, the equivalent resistance is very small.If resistance effect is ignored, Eq.(1)can be simplified as

(2)

whereELis the induced electromotive force of the inductor.This method can achieve better linearity and turn-off time than free shutdown, but its limitations are also very obvious.First of all, the power that TVS diode can tolerate is generally limited, and it is also easy to be damaged by multiple breakdowns.Secondly, the breakdown characteristic of TVS diodes is not repeatable, which means that the turn-off time is different.Thirdly, the clamping voltage of a single diode of a TVS diode is not very high, and the reliability of series connection cannot be guaranteed.Therefore it is not suitable for clamping with TVS diodes in high energy or high level voltage scenarios.

This paper presents a fast turn-off technology based on active constant voltage clamping, which mainly uses self-resonant oscillation technology to precharge the clamping capacitor, so that the turn-off time can be effectively controlled and reduced.

1.2 Principle of ZVS circuit

Zero voltage switch(ZVS)is a technology when the switch is turned off, the voltage at its two ends is already 0.In this way, the switching loss of the switching transistor can be minimized[13-15].Thus ZVS can achieve very high efficiency.The schematic diagram of the ZVS circuit is shown in Fig.5.

Fig.5 Structural drawing of ZVS circuit

At the moment of power on, energy is sent to the gate poles of both Q1 and Q2.The voltage between gate poles and the source poles is clamped by the zener diode at 12 V.Thus the two MOS transistors are turned on simultaneously.Because of the discreteness of component parameters such as gate-source(GS)voltage and trans-conductance of MOS transistor, non-strict symmetry of primary winding of the transformer, difference of wiring length, etc., the drain-source(DS)current values of the two transistors are not the same at the moment of power on.Assuming that the current through the lower MOS transistor Q2 is slightly larger, it meansIL2>IL1.Because L1 and L2 are wound on the same magnetic core, there is magnetic coupling between two inductors.The excitation current to the core is the sum ofIL1andIL2.From the perspective of the transformer tap, the current directions ofIL1andIL2are opposite, therefore the excitation current to the magnetic core isIP=IL1-IL2.In this way, it can be equivalent to the excitation effect of only the L2 coil.

As shown in Fig.6, at the moment of power on, the equivalent exciting currentIPin L1 and L2 is indicated by the grey dotted line.Therefore, the voltage at pointawill be greater than the voltage at pointc.Then a positive current of from pointb(+)to pointa(-)negative will be generated on the transformer.Because the current will charge C1, which leads toVGS2>VGS1.Finally, a positive feedback through diodes D1 and D2 is formed, Q2 is turned on and Q1 is completely turned off.After entering the steady state, Q2 continues to conduct.The current direction of T1 at this time is still from pointbto pointc.The current cannot be abruptly changed due to there is an inductor, Lin.The current of T1 will continue to charge C1, thus C1 gradually becomes a capacitor pointa(+)and pointc(-), and then gradually increases following the sine curve while the current gradually decreases.At this time, the voltage of pointcis pulled down to 0 V by Q2, therefore the voltage at pointagradually increases, and the gate voltage of Q2 is clamped by diode D1, and Q2 is always turned on.

Fig.6 Principle of ZVS circuit starting vibration

As shown in Fig.7, with the voltage across C1 increasing, the inductor current decreases.After the voltage of C1 reaches the maximum, it starts to discharge from pointcto pointathrough T1.At this time, Q1 is still on, the voltage of C1 at pointcgradually decreases, and the current of T1 gradually increases from pointcto pointa.

Fig.7 Principle of ZVS circuit reversal

As shown in Fig.8, when the voltage of C1 gradually drops to about the threshold voltage of the MOS transistor, Q1 will be brought into the amplification region through D2.At this time, the discharge current of C1 to T1 reaches the maximum from pointcto pointa.When Q1 enters the amplification region, the voltage at pointagradually increases.Q2 also enters the amplification region through D1.Because T1 continues to charge C1, the voltage on C1 is positive at pointaand negative at pointc.The gate voltage of Q2 increases through two diodes, and the gate voltage of Q1 gradually decreases.Finally, positive feedback is formed, Q2 is turned on, and Q1 is turned off.It enters the next metastable state, and then it will oscillate again similar to the first flip.

Fig.8 Principle of ZVS circuit reversal

1.3 Design of ZVS circuit

First of all, the power stage characteristic is analyzed.The first step is to normalize the transformer parameters to the primary side.The transformer primary side is an inductor with a tap.The total inductance of two coils wound in the same direction on the same magnetic core is the sum of the two inductances and two double mutual inductance of two coils.The two primary leakage inductances of Lp1 and Lp2 will affect the coupling between the inductances of pins 1-2 and 2-3 in Fig.9.The self-inductance and mutual inductance of the two coils are very difficult to calculate accurately, but when there is an open circuit between pin 4 and pin 5, the equivalent resonance inductanceLrcan be obtained by measuring pin 1 and pin 3 using an LCR measuring instrument.This inductance already includes the self-inductance and mutual inductance of the primary side.The equivalent model of the circuit shown in Fig.9.Ls is the leak inductance of the secondary coil.When the secondary coil is under load, the actual resonant inductance will change due to of change the load.For simplicity, the secondary circuit is left open.Therefore, the equivalent circuit of the entire transformer can be regarded as a resonant inductor with a set of 1∶1 transformers.The secondary can be seen as an ideal 2∶ntransformer.

Fig.9 Normalized equivalent schematic diagram of transformer

After the circuit is split, the equivalent circuit is shown in Fig.10, whereI1andI3correspond to the currents of pins 1 and 3 of the transformer, respectively.Iinflows through Lin for the input current of the entire ZVS power loop.Because the resonant frequency of the entire circuit is high, when the input inductance is sufficiently large, the ripple current on the point inductance Lin is so small that it can be regarded as a current source.The simplified equivalent circuit of state 1 is shown in Fig.11.

Fig.10 Equivalent circuit diagram of state 1

Fig.11 Simplificed equivalent circuit diagram of state 1

Because the effect of 1∶1 auto-transformer which divides the input current into two is taken into account, the size of the current source is half of the input current.If the center tap is not considered, one terminal can sink current withIin/2; and the other is a current source with the same magnitude, therefore it is equivalent to a current source ofIin/2.

From the first time the two transistors were turned on alternately, Supposing Q1 is turned off and Q2 is turned on, at this time, Q2 and Q1 are all in the linear region, and capacitors and inductors resonate.Starting with the shutdown of Q1, at this time, Q2 has been turned on, the current of Q2 is approximately equal to the input current, and the current values of the inductor and capacitor are equal.A moment before the start of state 1, the current on Q1/D1 isIin/2-ILrand the current on Q2 isIin/2+ILr.In state 1, resonance of Lr and Cr will cause the voltageVCron Cr continuously rising.WhenVCrreachesVO/2×n, the primary reflected voltage of the output voltage transformer, state 1 ends and state 2 occurs.

As shown in Fig.12, at the beginning of state 2, the transformer primary side begins to take over the current, Cr begins to not pass current at all, andVCris clamped to the level of the reflected voltage.

Fig.12 Equivalent circuit diagram of state 2

During this period, the input current and inductor currentILrpower the load together, and the secondary rectifier diode is turned on to receive energy from the primary side.The equationIP=ILr=Iin/2-ILrholds, whereIpis the primary current, as shown by the arrow in Fig.13.

Fig.13 Simplificed equivalent circuit diagram of state 2

Initially, the inductor and input current power the load together, and the inductor currentILrstarts to decrease.WhenILrcrosses zero, the input current powers the load and simultaneously charges Lr.ThenILrandIPcontinue to decrease, actuallyILris increasing in the other direction.UntilIPcrosses zero, the secondary rectifier is closed and state 3 occurs.

As shown in Fig.14, after the beginning of state 3, Cr takes overILr=Iin/2-ILragain.At this time, Cr resonates with Lr,VCrbegins to fall in a sine wave law, andILrgradually becomes positive and becomes gentle.

Fig.14 Equivalent circuit diagram of state 3

In this state, the current of Q2 is equal to the input current.WhenVCris equal to 0, state 3 ends.The simplified diagram of state 3 is the same as that of state 1, but the initial state is different as shown in Fig.15.

Fig.15 Simplificed equivalent circuit diagram of state 3

When state 4 occurs,VCrreaches 0 V, and the inductor current is still positive, the same as state 3 acting from top to bottom in Fig.16.

Fig.16 Equivalent circuit diagram of state 4

The current will open a new path in order to find a way out, and D1 will turn on at this time.Then the current on D1 isIin/2-ILr, and the current on Q2 isIin/2+ILr.In state 4,VCrhas been clamped to nearly 0 V, which creates the conditions for ZVS(VDS1=VCrwhen Q2 is turned on).Q2 can be turned on at any time.After Q2 is turned on, most of the current passes through the MOS channel of Q1.The left portion of the current passes through the body diode D1, and state 4 is maintained until Q2 is turned off, and then the current enters the state 1 repeatedly.Before state 1 begins, the currents in Lr and Lin remain basically unchanged.

According to the boundary conditions of these four states, the time of states 1-3 and the conversion ratio of the power stage can be determined under ideal lossless conditions.The results are as follows.

(3)

wheret1,t2andt3correspond to the time occupied by states 1, 2 and 3, respectively, and the time required by state 4 can be changed according to different switching frequencies;Mis the ratio of the output voltage to the input voltage;ωris the angular frequency of the resonance;Zris the characteristic impedance;A1andA2are symbolic constants defined to simplify the equation and have no practical physical meaning.

The previous analysis has not considered the leakage inductance of the transformer secondary.This leakage inductance will generate a spike on both ends of the resonance capacitor in state 2.The reason is that the resonance capacitor Cr and the secondary leakage inductanceLleakresonate.The initial conditions are the same as that of state 2, the formula of the excess voltage peak caused by leakage inductance can be obtained.Lleakis the leakage inductance measured at the secondary coil, which is the inductance value read when the primary is all short-circuited.The actual ZVS circuit is shown in Fig.17.

Fig.17 Schematic diagram of self-stop function

Since the current after passing through the ZVS circuit is actually high-frequency alternating current, when the capacitor is stored with energy, the entire circuit should be provided with a self-stop function due to the presence of inductance.The self-stop function is implemented by a sampling resistor and a hysteresis comparator, and its implementation structure is shown in Fig.17.When the voltage across the capacitor reaches the set voltage, the main circuit of the ZVS circuit will be cut off to stop charging.

2 System design

The transmitting circuit principle is shown in Fig.18, where VT1, VT2, VT3 and VT4 constitute the main transmitting bridge, VT5 and VT6 are used to control the discharging and charging of the capacitor as well as the charging voltage.In order to elimate the influence of the high-frequency chopping of the ZVS circuit on the transmission circuit, an independent power supply system is used to supply power to the ZVS part.In addition, when the capacitor voltage exceeds a set threshold, the hysteresis comparator module will make VT5 conductive through Rdc.The extra energy on the capacitor is fed back to ZVS power, which reduces the overall power consumption and increases the working time of the instrument.

Fig.18 Transmitting circuit principle diagram

The launch sequence diagram is shown in Fig.19 and the specific steps are as follows.

Fig.19 Launch sequence diagram

1)After the system is powered on, the ZVS circuit starts to pre-charge the clamping capacitor duringt0-t1.The remaining switches are closed except VT6.The charging voltage can be adjusted by settingVref.When the voltage at both ends of the capacitor reaches the preset threshold, the main charging circuit is cut off by VT6.Besides, VT6 is turned off after the clamp capacitor has been charged during the whole working period.

2)Int1-t2period, VT1 and VT3 are turned on, and the remaining switching transistors are turned off to perform coil charging operation.The voltage across the coil is clamped by the power supply to the supply voltageE.The current in the coil can be regarded as a step response signal, thus the expression of the current in the coil is

(4)

3)At the beginning oft2-t3, all the switches are turned off, and the coil is clamped by the clamp capacitor through the IGBT freewheeling diode.At this time, the coil current is poured into the capacitor to charge the capacitor, and the voltage across the capacitor will rise.The current in the inductor can be obtained by

(5)

whereVCrefers to the voltage at capacitor C.After the energy in the coil is completely consumed duringt2-t3, the next charging state occurs, and then VT2 and VT4 start to conduct.At the same time if the capacitance voltage reaches the upper limit threshold, VT6 is on and the energy originally stored in C is fed back to the power supply of the ZVS.

Duringt3-t4, the coil is reversely charged, and all the watch transistors are turned off duringt4-t5just like duringt2-t3.The coil injects energy into the capacitor through the freewheeling diode.This cycle is performed until the voltage of the capacitor exceeds the set threshold again.During the coil charging cycle, controller controls the VT6 turning on and the energy of the coil is fed back to the ZVS power supply until the voltage of the clamp capacitor is lower than the lower limit threshold to stop discharging.The charging and discharging operations of the capacitor and the main circuit operation are performed simultaneously, without affecting the main circuit operation.

3 Experiment and analysis

The transmitting system is powered by a 12 V lead-acid battery.The DC resistance of the transmitting coil is 0.124 Ω, the inductance is 314 μH(measured at 2 kHz frequency), and the transmitting current is 20 A.Current control is performed by adding a series resistor to the circuit.The clamping voltage is adjustable.The core of the entire control system uses a CPLD chip for timing control.The off-time of the entire circuit can be adjusted by adjusting the charging voltageVCof the capacitor C, and the off-time can be obtained by making equation 3 equal to 0, that is

(6)

According to the above calculation process and simulation results, the transmitting circuit was actually manufactured and debugged.The emission current is adjusted to a rated value of 20 A, and the clamping voltages were 75, 100 and 150 V, respectively.The current was sampled by a current sampler and passes through the amplifier circuit to collect the current[16].There is an important reason why current transformer cannot be used for collection.The bandwidth of main models of current transformer which is capable of measuring high power is currently not more than 200 kHz.It is difficult to accurately measure relatively short falling edges and according reflect the current spike in the current due to the bandwidth problem of the current transformer.Thus we use the shunt to measure the current.Its structure is shown in Fig.20, and its physical photo is shown in Fig.21.

Fig.20 Structure diagram of current sampling

Fig.21 Current sensing resistor 40 A/75 mV

Using ZVS boost circuit and constant voltage clamp control transmitter circuit can obtain stable current in flat top section and rapid linear drop in falling section.The transmitting waveform is close to the ideal trapezoid wave, which can effectively excite the underground abnormal body.In the process of constant voltage clamping with large capacitance, the clamping voltage is basically stable during the whole transmitting process.Fig.22 compares the off-time and the free off-time when using 12 V clamp.

Fig.22 Comparison between free shut-off and constant voltage clamp

Fig.22 ill shows the turn-off time for the current drop at different clamping voltages.The test environment is shown in Fig.23, with coil resistance of 0.124 Ω, inductance of 314 μH, and current of 20 A.

Fig.23 Actual test coil and transmitter

Fig.24 shows the cut-off duration of current drop with different clamping voltage.It can be seen from Fig.24 that the higher the voltage, the faster the shutdown speed and the higher the linearity.Under the condition of constant emission current, the whole shutdown process is shortened from 450 to 60 μs, and the shutdown speed is obviously accelerated.With the increase of clamp voltage, the turn off time is significantly reduced, and the linearity is good.Compared with the traditional TVS clamp system, the ZVS clamp system can withstand greater current, and the current repeatability is better.Compared with the traditional scheme of using the emission coil to charge the clamp capacitor, the low-frequency charging problem of small coil is solved, and the system efficiency is effectively improved.The test results show that the whole launch system basically meets the design and actual engineering needs.

Fig.24 Comparison between different voltage clamps

4 Conclusion

A new type of electromagnetic transmitting circuit was realized by combining two methods of ZVS self-resonant circuit and constant voltage clamping control, and the feasibility of the circuit was verified by actual measurement.This new circuit can provide a reference to solve the problem of rapid current drop in high-current or high-power transient electromagnetic detection system.Compared with the traditional constant voltage clamping method, the charging time of the clamping capacitor is shortened by adding a TVS charging circuit, and the voltage of the clamping capacitor is more stable by the active clamping method.Furthermore, the turn-off speed is accelerated, and the receiving system can better receive the shallow secondary field signal.When the transmitting current is 20 A, the turn-off time of the transmit current is 1/10 of the original.In addition, by using a controllable boost scheme, the turn-off rate of the falling edge of this system can be adjusted, which can be applied to different detection requirements, and the structure of the entire circuit is very simple, which effectively reduces the manufacturing cost.