GB2140236A - Pulse generators - Google Patents

Abstract

A line type modulator producing short high power pulses at a high repetition rate uses an active device (TRI) (field effect transistor, triode or transistor) to switch the power to the pulse forming network (PFN) and uses a passive device, namely a saturable reactor (SRI), preferably an inductance coil with a core of very square B/H material, to control the transfer of the energy stored in the PFN to the load. Variation of the conduction period of TRI may vary the amplitude of the output pulse. <IMAGE>

Description

SPECIFICATION Pulse generators This invention concerns pulse generators, and relates in particular to pulse generators of the variety known as "line-type modulators".

In the electronics field there are numerous occasions, particularly to satisfy the needs of high energy physics and of radar systems, when it is necessary to generate high power short duration electrical pulses-that is, pulses of up to 100 MW, but most commonly of from 1 to 1 00 kW, peak power for time durations of from 50ns to 20yes. A well-known generator for such pulses is the circuit arrangement known as a line type modulator.

The general concept of a line-type modulator (also known as a "line-type pulser") is described in some detail in "Pulse Generators", Glasoe and Lebacqz, 1 sot sot(1948) Edition, McGraw-Hill, together with the advantages and disadvantages thereof. Briefly, however, it may here be described as follows.

A line-type modulator is an energy-storage device in series with a switch and a load (so that closing the switch applies across the load any energy stored in the storage device), and the energy storage device itself is in essence a lumped-constant transmission line. The latter both stores the energy and "shapes" the pulse, and is thus referred to as a "pulse forming network" (PFN), and the invention concerns that type of PFN which stores the energy as an electrostatic field (and is thus a "voltage-fed" network).

The line-type modulator is based on two fundamental parameters of the transmission line, namely its characteristic impedance and its delay time. The theorem of maximum power transfer from a generator states that for maximum power output the load resistance must equal the generator internal resistance. By extension of this theorem, and with the application of general transmission line theories, there can be envisaged a simple pulse generator (line, switch and load in a series loop) that is the basic idea behind the many forms of the line-type modulator.

Practical realization of this type of pulse generator means paying careful attention to: 1) the nature of the transmission line itself; 2) the method of initially charging the transmission line, and the supply of energy for this purpose: 3) the nature of the electronic switch used to connect the charged transmission line to the load; and 4) the method of coupling the load to the transmission line and switch.

So far as concerns the line, in practice it is not often satisfactory to use an actual transmission line, primarily because of the length of line that would be needed and because of the limited range of line impedance available. For example, if the cable was a polyethylene dielectric cable and a 1 ,tbS pulse was required then a length of approximately 100 metres would be needed, which would clearly be unmanageable in a compact equipment, while most line cables have an impedance Zo in the 50 ohms to 75 ohms, and this is frequently not satisfactory when related to available switch requirements.

Generally, then, the transmission line-the PFN-is produced by using lumped values of inductance and capacitance chosen by calculation to simulate any desired pulse shape characteristic.

In particular this allows freedom to choose such parameters as the impedance Zo at will, and also to control rise and fall times in a manner most suitable for use with the desired load.

So far as concerns the energy supply and the charging method, this is conveniently provided by connecting the transmission line to a DC power supply unit (PSU) via a choke and series unidirectional electronic switch. With the switch "closed" (and assuming the transmission line uncharged), the line capacity will be resonantly charged to twice the DC supply voltage in a time equal to

(where L is the choke inductance and Cn the PFN capacity), with a peak current demand from the DC supply of half sinusoidal form and peak value Edc

(this assumes no losses, but is in practice accurate, within a few per cent, even for moderate losses up to 10% or so). When the line voltage reaches 2Edc the current tries to reverse, the unidirectional switch then effectively becoming "open".

In respect of the nature of the electronic switch connecting the charged transmission line to the load, suitable switches-that is, devices that can be triggered "on" (after which the trigger electrode has no control over device operation) and then remain fully conducting until the current through them drops below some critical (and low) value at which they automatically reset themselves into the "off" state-are hydrogen thyratrons, silicon-controlled rectifiers (SCRs, sometimes called thyristors) or spark gaps.Apart from their limited control characteristics, electronic switches are characterised by high hold-off voltage (Vf) ratings together with high peak current ratings (Ipk). The attendant power that can be switched relates to the factor Vflpk P=~ 2 (which in practice has to be reduced to ailow for imperfections and working factors of safety). The price paid for high power switching capability is generally a long recovery time after each pulse, the ability only to turn the device on (to trigger it), not to turn it off, and the need to apply the forward voltage relatively slowly to ensure that random and spurious turn-on does not occur.

Although it is possible to consider using an active device--thus, a device, such as a valve transistor or field effect transistor (FET), that can be turned on and off at will by an external control signal, and so does not usually have rate of change of voltage or recovery time problems-rather than a passive one, it is not feasible to employ such a device because the "P" rating (see above) is very much lower (this is generally the result of substantially reduced current ratings). Thus, while a present-day State of the Art pulse modulator thyrister could have a 1 000V 250A peak current rating, a similarly priced Power Transistor of similar switching speed and average power handling could be only 500V 20A rating.

Coming, finally, to the method of coupling the load to the line and switch, it may be-and usually is the case that the optimum Vf and IpK do not easily match the voltage and current relationships (i.e. impedance) of the device to be pulsed, and if this is so then a transformer can be introduced to match the load impedance to the transmission line. Suitable transformer designs are well known; they are usually referred to as pulse transformers, and are characterised by controlled, and low, values for leakage inductance and shunt capacity, by high voltage outputs and by large step-up ratios with quite frequently 1-turn primaries.

As will be apparent from the preceding description, one major problem associated with this variety of line type modulator is the need to employ an electronic switch to connect the charged line to the load. With present-day requirements for very short (less than 1 microsecond) high power pulses to be repeated at relatively short intervals (every 20 microseconds) it is difficult if not impossible to find a switch that can recover in time for the next pulse without being prematurely triggered by the very rapidly increasing forward voltage during PFN recharge.

The invention seeks to solve this difficulty, for line type modulators producing short high power pulses at a high repetition rate, by using an active device to switch the power to the pulse forming network and by using a passive device, namely a saturable reactor, to control the transfer of the energy stored in the PFN to the load.

In one aspect, therefore, the invention provides a line type modualtor in which the power to the pulse forming network is switched thereto via an active device, and the subsequent transfer of the stored energy from the pulse forming network to the load is controlled solely by a saturable reactor.

Apart from the use of an active device in the circuit powering the pulse forming network (PFN), and the use of a saturable reactor as the sole "switch" in the circuit connecting the PFN to the load, the line type modulator of the invention may be any such device used or suggested for use in the art. The general nature of these devices has already been described hereinbefore, and there is no need to repeat the points here.

The inventive modualtor uses an active device to switch the power from the power supply unit (PSU) to the PFN. The active device is a device that either conducts electricity or not depending upon the signal applied to a control electrode, the triode valves (where the grid is the control electrode), transistors (where the base is the control electrode) and field effect transistors (FETs-where the gate is the control electrode) are such devices. For the present invention the active device is preferably one of the relatively new type of high power field effect transistors.

One particular such device is that sold by International Rectifier under the name HEXFETS.

The active device switching the power from the PSU to the PFN may be at any convenient point in that part of the circuit. Most preferably, however, it is on the earthed side of the PFN, for this significantly minimises the problems of attaining a suitable electrical environment for a transistor active device.

The line type modulator of the invention employs a saturable reactor in the circuit from the PFN to the load to "switch"-to control the transfer of-the energy stored in the PFN to the load. In the context of this invention a saturable reactor is a device whose electrical impedance changes sharply from high to low as some property of the device saturates under increasing "pressure". For the purposes of the present invention, the saturable reactor will (almost inevitably) be an inductance coil with a magnetic core of a very square B/H loop material; such a reactor exhibits a very high inductance (and thus impedes the flow of current through the coil) while it is driven from -B rem. to +B sat., but upon saturation has a markedly lower inductance (and thus no longer impedes current flowing through the coil) which it retains until "reset".During excitation from -B rem. to +B sat. the impedance any reactor exhibits is a function of the rate at which the magnetic flux is established in the core.

By choosing a very square B/H loop material for the core the impedance can, for a very short period, be suitably high. Accordingly, a saturable reactor-in this particular case the defined inductance with the square B/H loop core-will, for a short period (of the order of a few microseconds), act as a switch, blocking the passage of current therethrough. Thus, provided the period during which energy is to be prevented from transferring from the PFN to the load is short enough, as it can be if the charging of the PFN (the transfer of energy thereto from the PSU) is itself switched by an active device, then the control of the subsequent transfer of the PFN-stored energy to the load may very satisfactorily be exercised solely by a saturable reactor.

As stated, a typical saturable reactor is an inductance with a core of very square B/H loop material. Such a core is a thin tape torroidal one of 50% Ni 50% Fe composition-as, for example, that available from Telcom under the designation HCR (Heavy Cold Reduced).

One particular advantage of the modulator of the invention lies in that it allows the charging circuit to be switched off early in the charging period, and thus the amplitude of the output pulse may within limits be varied as desired. It should be noted, however, that earlier switch-off necessitates the use of a one-way leakage link completing the circuit from the PFN back to the inductance normally used to regulate the charging period (an example of this is discussed hereinafter with reference to the accompanying drawings) else the rise in potential across the active device as the flux in the inductance collapses is likely to result in the device's destruction.

The invention extends, of course, to a high power pulse producing system whenever employing a line type modulator as described and claimed herein.

The invention is now described, though by way of illustration only, with reference to the accompanying drawings in which: Figures 1A andB are respectively a circuit diagram of a Prior Art line type modulator and a series of graphs showing its various inputs/outputs plotted against time; Figure 2 is a modified version of the modulator of Figure 1A; Figure 3 is a circuit diagram for a line type modulator according to the invention; and Figures 4A and B show respectively a more practical version of the circuit of Figure 3 and the input/output time plots relating thereto.

The circuitry shown in Figure 1 is a simple representation of a Prior Art line type modulator connected between its PSU and the load. The circuit has two distinct halves-one (the charging haif) comprising the PSU, the inductance L1, switch SCR, and PFN, the other (the discharging half) comprising the PFN, switch SCR2, impedance matching transformerTt and load-and these are decoupled by the two switches.

Described with reference to Figure 1 B, the method of operation is as follows: With SCR2 open (so separating the load from the PFN) SCR1 is triggered closed (at t,) and the PSU charges up the PFN via inductance L,. The charging current (icy) is half of a sine wave (as determined by the resonant frequency of the LC circuit comprising L, and Cn), and the capacity Cn of the PFN is charged up to E, twice the PSU voltage EdC. Charging is complete at time t2, and a short time thereafter (at T3) SCR2 is triggered closed and the PFN stored energy is discharged (via the impedance transformer T,) to the load, generating a short pulse of voltage thereacross.

Typical figures for a working system are times of t1 =0, t2 =6, t3 =10, t4 =11, t5 = 11.05 and te = 20 microseconds, with PSU voltages of about 300V charging the PFN to 600V at a peak current of about 4A and delivering a pulse of 1 OkV to the load.

The Figure 1 A circuit working from a DC supply of, say, about 300V, could be required to produce 50nS pulses at a pulse repetition rate of 50,000 pulses per second each with a peak power of 100 kW into a load such as magnetron (in a radar transmitter) which itself would represent a working resistance of, say, 1,000 ohms.

This particular example demonstrates the limitations which the invention seeks to overcome.

The major limitation is the "turn-off," or recovery, time of the switches SCR, and SCR2. When an electronic switch (such as a thyration, spark gap or -in this instance- a silicon-controlled rectifier, (SCR)) is triggered on it can pass a substantial current. However, after the cessation of the current the switch does not recover its 'off' state for some time, and thus it is necessary to allow a certain time before forward voltage is re-applied to the device. These times are represented in Figure 1 e by t2 to t3 and t5 to t2 for SCR, and SCR2 respectively.

As well as the turn-off problem, however, the switch SCR2 also has a turn-off problem, at least when very short pulse lengths are required. At these short pulse lengths a partial answer to the turn-on problem is to associate the switch with a saturable reactor. A corresponding circuit is shown in Figure 2; it is identical to that of Figure 1A save for the addition (in series with SCR2) of the saturable reactor SR,.The saturable reactor may be an inductance with a magnetic core of very square B/H loop material which exhibits a very high inductance while it is going from -B rem. (to which it had previously been reset by, say, a constant current source fed via a blocking choke) to +B sat., the reactor being designed so that this process takes some fairly small time (such as 0.5us). The small inductance of the saturable reactor in its saturated state is by design part of the PFN inductance system.

Unfortunately, the loss in the saturable reactor is not usually negligible at such short pulses, and this loss makes the design and specification of both SCR2 and the PFN very much more difficult since the energy loss provides an unacceptable drain of the PFN capacitors before saturation is reached, which in turn requires the charging of the PFN capacitors to a much higher voltage than they would normally need.

The essence of the invention is that by further modifying the circuit so as to replace SCR, with an active device (TR, in Figure 3) and exclude SCR2 altogether the operation of the active device TR can be utilised both to charge the PFN and to switch the saturable reactor at the end of the charging period.

Figure 3 shows the modulator of the invention using an active device (TR,) in the charging circuit and solely a saturable reactor (SR,) in the discharging circuit. As a result, the switch recovery time (t2 to t3 in Figure 1 B) can be eliminated, since the active device TR, can be turned off sharply and rapidly (relatively) at t2, and the time until the next pulse (t5 to t6) can also be noticeably reduced (if desired) since the reactor SRt can usually be reset in a time far less than the recovery time of switch SCR2.

Because recovery time in electronic switches generally and SCR's in particular can be a very variable parameter a circuit which eliminates reliance upon these parameters as this invention does is highly desirable.

Simple mathematical analysis can show that the active device TR, is itself switching less power in terms of Vf and Ipk than the original SCR2 in Figure 1.

The analysis (assuming no losses) is as follows: PFN = 600V charging SR original had Vt of 600V-,uS If the transfer takes 2,us this will represent the same AB.

In this equation the left hand term is the power the active device needs to handle, while the Ep. p of the right hand term is the output pulse power the device is controlling. With the values tp = 50ns and tch = 2ys, Edc.lpk 7d 50 x 10 9 = -. ------- = 0.03 Ep. Ip 2 2 x10-6 Thus the load power is being "switched", albeit indirectly, by the switching within the active device by an operation at approximately 1/30 of the power level.

This theoretical value will not usually be achieved in practice (because of factors such as loss and load mismatch effects), but experience has shown that a figure of 0.04 to 0.07 at worst is certainly attainable. Furthermore, in this circuit the saturable reactor will be magnetised at a rate approximately half that in the previous circuit, reducing its losses appreciably.

A somewhat more practicai circuit for an inventive line type modulator is shown in Figure 4A, with the corresponding input/outputs versus time plots in Figure 4B.

As in the circuit of Figure 3, TR, is turned on for the duration of the period t, tot2 during which a nominal 2 sine wave of current, determined by the resonant frequency of L and C, flows from the supply Edc and charges up Cn to 2EdC (assuming no losses). At t2 the active device is turned off, and the reactor SR (which was previously fully reset, and is designed to support flux under the rising value of En until t2 is reached), saturates, and couples the charged PFN to the load.

Diode D4 limits reverse voltage across the active device, should this be necessary, and diode D3 prevents the PFN partly discharging via L and D4 back to Edc during the output pulse period t2 to t3. The network D1 and D2 constitutes a backswing network which minimises the forward voltage on the switch TR1.

From t3 to t4 there may be a small'reverse voltage on the PFN-either due to load mismatch or to the need to reset SR and T1 by methods which are understood. At any time after t4 (core reset) the cycle can be repeated. As shown, t5 is the commencement of the next cycle.

Because an active device is controllable at its input it can be turned off during the period t, to t2, say at tx. Thus (as also shown in Figure 4A), if a diode Dx is connected across the LC components of the charging circuit then the current in Lch will continue into the PFN even when TR, is open, but will result in the PFN being charged to a lower value since for part of the cycle Edc is not in series with L,, and the PFN (the diode Dx is necessary, of course, to ensure a unidirectional path for the current in Lch after the turn-off of TR1-i.e., after tax}.

Thus, variation of the time t, to t2 will vary the PFN final voltage, and if desired this process can be used as a means of varying the voltage on Cn and consequently the value of the output pulse amplitude.

Claims (1)

1. A line type modulator in which the power to the pulse forming network is switched thereto via an active device, and the subsequent transfer of the stored energy from the pulse forming network to the load is controlled solely by a saturable reactor.

2. A modulator as claimed in claim 1, wherein the active device is a high power field effect transistor.

3. A modulator as claimed in either of the preceding claims, wherein the active device is positioned on the earthed side of the pulse forming network.

4. A modulator as claimed in any of the preceding claims, wherein the saturable reactor is an inductance coil with a magnetic core of a very square B/H loop material.

5. A modulator as claimed in any of the preceding claims, wherein, to enable utilisation of the ability of the charging circuit to be switched off early in the charging period, whereby the amplitude of the output pulse may within limits be varied as desired, a one-way leakage link is provided to complete the circuit from the pulse forming network back to the inductance used to regulate the charging period.

6. A line type modulator as claimed in any of the preceding claims and substantially as described hereinbefore.

7. A high power pulse producing system whenever employing a line type modulator as claimed in any of the preceding claims.

Superseded claim 1 New or amended claim CLAIM

1. A line type modulator in which the power to the pulse forming network is obtained directly from a DC power supply, and is switched to the pulse forming network via an active device, and the subsequent transfer of the stored energy from the pulse forming network to the load is controlled solely by a saturable reactor.