SCR PRIMING AND SWEEP-OUT CIRCUIT APPARATUS

Abstract: Losses are obviated or minimized by transferring energy resulting from printing and sweep-out back to the power supply, and wherein a single common storage inductance (L2) is used in a secondary winding circuit of a saturable-core inductor transformer, the primary (L1) of which is series-connected in the SCR-switching energy storage and discharge pulse gen­ erating circuit, to serve both in the priming and sweep-­ out functions, and wherein reset circuit inductance (Lr) does not participate in the energy transfer or storage func­ tions.

Description: The present invention relates to SCR-controlled or similar energy-switching circuits, hereinafter generically referred to as SCR switching devices or means, and more particularly to improved circuits for reducing losses and reverse-voltage transients in prior priming and carrier sweep-out networks. In U.S. Letters Patent US-A-4 230 955 of common assignee herewith, a significant improvement in obviating dissipation losses in both the saturable; inductor "priming" of SCR switching devices and in minority carrier "sweep-out" circuits that limit reverse current in SCR switching is disclosed, such improvement residing in the transferring of energy resulting from each of the priming process and the sweep-out process back to the power supplies by respective saturable inductor networks, these networks actively participating in energy exchange and storage functions in the resetting process (Fig. 4). While this has been found in practice admirably to solve these loss problems, the embodiments therein taught require the two separate function saturable inductors - one required to control the priming, and the other sweep-out. In accordance with the present invention, on the other hand, it has been found possible to control both functions with a single saturable inductor network and without the resetting network participation in energy exchange or serving as a storage inductor, as in said patent, with significant resulting simplification and reduction in cost and efficiency improvement, as later more fully explained. An object of the present invention, accordingly, is to provide a new and improved saturable inductor circuit apparatus for SCR priming and sweep-out circuit loss and reverse voltage transient elimination that has the above and other advantages. According to the invention there is provided a priming and sweep-out circuit for an SCR pulse generator comprising primary inductor means connected to one side of a DC power supply and transformer-coupled by means of a saturable core to a secondary inductor means for enabling energy storing and discharging under the control of an SCR switching means connected in series with said primary inductor means, said secondary inductor means being connected at each of its ends through respective diode means to said one side of said DC power supply, characterised in that a storage inductance is connected between said diode means and said one side of said DC supply, and that the secondary inductor means is constituted by a single continuous winding with an intermediate tap to which the other side of the said DC supply is connected. The invention will now be described with reference to the accompanying drawings, Fig 1 of which is a circuit diagram illustrating the invention in preferred form; and Figs 2A through 2E are waveform diagrams and Fig 2F a timing diagram explanatory of the operation of Fig 1. Fig 1 illustrates a portion of an SCR pulse generator which utilises the invention, the pulse generator employs a well known resonance charging principle employing a saturable-core inductor L₁ to deliver half-sinusoidal pulses of current from the DC power supply E dc to the capacitor C₁ and a load circuit, labelled "LOAD", which may be a resistor, a saturable-magnetic pulse compressor, another SCR circuit, etc. The saturable-core inductor L₁ serves as the primary winding "p" of a transformer the secondary winding S₁-S₂ of which is coupled to the primary and is connected at its terminals to respective diodes D₁ and D₂, and from its intermediate tap to the - terminal or side of E dc . The common terminals of the diodes D₁ and D₂ are connected through a storage inductance L₂ to the + side of the D.C. power supply E dc . The current pulse begins when a series-connected SCR₁ is gated at time t=0, Figs 2C and 2F, and continues for half a resonant period with t=π √ L₁C₁ seconds, when the natural oscillation reverses the direction of current through SCR₁ and turns SCR₁ off. At turnoff, the positive voltage e c₁ on the capacitor exceeds E dc , so that a net reverse voltage appears across SCR₁. The SCR will regain its forward voltage-blocking capability after a few 10's of microseconds of reverse bias. Once SCR₁ has recovered, the charged capacitor is isolated from the charging circuits, and its energy can be delivered to the load.Often, a negative voltage e c1 will remain on C₁ following the discharge into the load circuit. This negative voltage adds to the power supply voltage in driving the natural oscillation of inductor L₁ and C₁ when SCR₁ is gated. In "lossless" resonant charging, the capacitor voltage reaches a positive level which is twice the power supply voltage plus the negative of the residual capacitor voltage. Thus the peak-to-peak capacitor voltage during steady-state operation of the pulse generator may be many times the D.C. supply voltage E dc , shown as 6 E dc in Fig. 2A. The half-sinusoidal current pulses passing through SCR₁, Fig. 2C, are correspondingly large, often reaching thousands of amperes. The current-carrying and current-switching capability of an SCR is limited by the rise in junction temperature caused by resistive dissipation. Resistive losses are particularly high just after gating when the entire circuit voltage is applied to the small conducting area of the junction near the gate, and at turn off when the minority carriers stored in the junction are swept out by reverse voltage, and the conducting junction area contracts to zero. The circuit configuration of Fig. 1 provides the designer with a means of reducing the rate-of-change of SCR current (di /dt) during the critical turn-on and turn-off intervals. The circuit produces a large increase in the charging inductance above its saturated value L₁ during the two critical intervals, thereby reducing di /dt. At turn-on, the SCR junction is "primed" with a small ramp of current which expands the conducting junction area prior to the main current pulse. At turn-off, the SCR current in the reverse direction is similarly restrained while the reverse circuit voltage "sweeps out" the minority carriers stored in the conducting junction (see "Timing diagram", Fig. 2F) The reverse current terminates very rapidly, and the resulting di /dt in the associated circuit inductance causes a transient pulse of reverse voltage across the now insulating SCR junction. The reverse-current restraint provided by the circuit of Fig. 1 keeps this transient small, and obviates the need for prior art series RC snubber networks across the SCR with attendant loss. Returning to Fig. 2A illustrating the capacitor voltage e c1 following the gating of SCR₁, the previous discharge of C₁ through the load reversed its voltage from a positive level of 6E dc to a negative level of -4E dc , similar to that indicated by the dotted line to the right. These voltage levels are arbitrary, and serve only to illustrate the basic law of resonant charging mentioned above. Another initial condition is the magnetic state of the saturable core of inductor L₁. As indicated by the dots and the polarity or direction of the reset current I r in Fig. 1, the core of L₁ is fully reset between discharge cycles from a D.C. reset supply E r connected through L r to a reset secondary winding S₃ coupled to L₁ as a transformer. The reset inductance L r and the turns ratios of the windings on L₁ are adjusted such that the reset current I r remains essentially constant during the operating cycle to be described. The energy exchange during SCR priming and sweep-out, which is the feature of the invention, occurs across the saturable core transformer windings of L₁ -S₁ - S₂ into and out of the relatively small storage inductor L₂, via diodes D₁ and D₂. While the core of the saturable inductor is unsaturated, the wound core behaves like a closely coupled transformer. When saturation occurs, the mutual coupling between primary and secondary windings L₁ and S₁-S₂ largely disappears. Prior to the gate pulse at time t=0, Fig. 2 F, SCR₁ has a forward voltage of, say, 5E dc applied to it. When the gating occurs, the resistance of the SCR junction collapses almost instantly to near zero, and the 5E dc circuit voltage transfers to the "p"-turn primary of the saturable inductor L₁. Since the core is unsaturated in the dotted direction (Fig. 1) in which the voltage is applied transformer action occurs to the secondary S₁, Fig. 2B, across which an induced voltage 5 E dc (S₁/ p turns ratio) appears. Diode D₂ is back-biased, diode D₁ is forward-biased, and a net voltage (5 (S₁/ p ) -1) E dc is applied to the storage indicator L₂. Since for an inductor the terminal voltage E = L di /dt, the current is passing into the power supply through secondary S₁, diode D₁ and L₂ will rise at a rate given by: Via the transformer action of the saturable inductor, a similar rising current (p/S₁) times as large will pass through the primary "p", SCR₁ and into the load capacitor. As shown in Figs. 2C, 2B and 2D, the linear rises respectively in the storage inductor current i₂ and the SCR current continue until the square-loop core of L₁ saturates abruptly at time t₁, Fig. 2F. By design, the SCR current at the end of the priming interval (0 to t₁) is a few tens of amperes; small compared to the thousands of amperes of main-pulse SCR current to follow, but enough to promote enlargement of the conducting junction area in the SCR. When the core saturates, the mutual coupling between the primary and secondaries "p" (L₁) -S₁ -S₂ is abruptly reduced, as indicated in Fig. 2B. On the primary side, all the circuit voltage (slightly under 5E dc ) is applied to the saturated inductance L₁ of the primary winding, and the main-circuit sinusoidal re-charge of the load capacitor occurs over an interval of π √ L₁C₁ seconds. Back on the secondary side, at time t₁, the main circuit voltage coupled from the primary nearly disappears, leaving the (transformed) SCR priming current passing through the storage inductor L₂, the diode D₁, the small saturated inductance of secondary S₁, and into the power supply E dc . This loop current into the power supply falls to zero at a rate of approximately and accomplishes the desired end of returning the (transformed) SCR priming energy stored in L₂ to the power supply. By this design of the saturable inductor L₁, the primary-to-secondary coupling is small during the saturated interval t₁ to t₂, Fig. 2F, so that only a minor amount of the main discharge energy gets diverted into L₂, unlike the action of the circuit of said patent. The return of priming energy through D, is therefore completed well before the main discharge terminates at t₂, as shown in Fig. 2D. Similar circuit behavior occurs when the main resonant discharge current approaches zero on its way down from the pulse peak. The circuit voltage applied across the still-saturated primary L₁ of the saturable inductor is now negative with respect to the dots (Fig. 1) because the main-pulse di /dt is negative. When the core of L₁ is pulled out of forward saturation, the entire circuit voltage (now 6E dc from C₁ minus E dc from the power supply = 5E dc ) appears across the primary of L₁ in the negative direction, Fig. 2B. A current ramp into the power supply E dc is now initiated on the secondary side via secondary S₂, diode D₂ and storage inductor L₂. The SCR sweep-out interval starts when the primary current goes negative. So long as there are minority carriers remaining in the SCR junction, the voltage drop across the SCR remains low, the rising portion of the current ramp backwards through the SCR, and the corresponding secondary current ramp through D₂ persist, Fig. 2E. By design, the core of L₁ does not saturate in the reset direction before the reverse SCR junction current terminates at SCR turn-off. As turn-off begins, the SCR reverse current tapers rapidly toward zero, causing the primary di /dt in the saturable inductor to go from negative back to positive. In response, the storage inductor current transfers rapidly from D₂ to D₁ at a rate limited by the leakage inductance between secondaries S₂ and S₁, as shown in Figs. 2D and 2E. For a short time, both diodes D₁ and D₂ conduct at once, shorting the secondary of L₁. Simultaneously, the SCR junction is rapidly regaining its reverse voltage-blocking capability, thereby removing the circuit voltage applied to the primary of L₁. The diode D₁ and the storage inductor L₂ are left carrying a transformed replica of the SCR sweep-out current into the power supply. As in the priming case, this current flow is opposed by E dc , and the sweep-out energy is returned to the power supply. When conduction through D₁ ceases, only the reset current is left applying-magneto motive force to the saturable core. As shown in Fig. 2B, at the end of the secondary voltage waveform, a small relatively long pulse of voltage occurs as the core is reset. A useful feature of the circuit in Fig. 1 is that the two secondary windings S₁-S₂ on the saturable core need not have the same number of turns. A successful circuit of this type, for example, has used turns ratios of 12:5 for the priming circuit portion and 12:7 for the sweep-out. Thus the effective inductance referred to the primary during SCR priming can be different from that present during sweep-out, so that a measure of design independence exists between the peak priming circuit and the peak sweep-out current. An alternate circuit without this independence may use a center-tapped secondary with S₁ = S₂, or a conventional full-wave four-diode bridge across a single secondary (not shown). Thus circuit of Fig. 1 of the invention has some common objectives to that of the earlier-mentioned patent (Fig. 4 thereof) in (a.) reduction of SCR losses during turn-on and turn-off by reducing the voltage applied to the SCR in these critical intervals and (b.) accomplishing (a.) with much less dissipative loss than in prior-art circuits with resistors. A collateral advantage is that of substantial elimination of the reverse-voltage transient which occurs when SCR₁ turns off. The usual remedy, as before mentioned, is a series RC snubber networks across SCR₁, but these burn up a lot of power. Both circuits, moreover, reduce circuit di /dt at turn-off, so that the transient voltages, which are proportional to L₁ di /dt, are correspondingly less. Thus, among the more important and novel contributions of the present invention and its new circuit are the eliminating of one of the expensive and lossy saturable inductors, and the accomplishing of the return of priming and sweep-out energy to the power supply immediately. In the patent circuit, the reset inductor (L₃) retains the priming energy through the whole charging interval, causing some power loss (in R′₁). With the present invention, Fig. 1, the second inductor L₂ is but a small and inexpensive inductance as is reset inductor L r , which advantageously does not participate in energy exchange or storage, as earlier explained. Thus, Fig. 1 enables a single, common saturable inductance network to serve both the priming and sweep-out circuit needs of the SCR switching charging (or storage) and discharging energy-supply main circuit. If desired, a pair of such circuits may be used in push-pull, each with a saturable inductor (transformer) network L₁ -S₁-S₂ as above described.

Claims: A priming and sweep-out circuit for an SCR pulse generator comprising primary inductor means (L₁) connected to one side of a DC power supply and transformer-coupled by means of a saturable core to a secondary inductor means (S₁, S₂) for enabling energy storing and discharging under the control of an SCR switching means (SCR 1) connected in series with said primary inductor means (L₁), said secondary inductor means (S₁, S₂) being connected at each of its ends through respective diode means (D₁, D₂) to said one side of said DC power supply, characterised in that a storage inductance (L₂) is connected between said diode means (D₁, D₂) and said one side of said DC supply, and that the secondary inductor means (S₁, S₂) is constituted by a single continuous winding with an intermediate tap to which the other side of the said DC supply is connected. A priming and sweep-out circuit as claimed in claim 1 and in which said saturable-core inductor means (L₁) is further transformer coupled to a further winding (S₃) to provide re-set current through a further reset inductance (L r ). A priming and sweep-out circuit as claimed in claim 2 and in which said reset circuit inductance (L r ) and the turns ratios of the windings on L₁ are adjusted such that the reset current (I r ) remains essentially constant during the operating cycle.

Inventors: VER PLANCK, PETER
Applicants: MEGAPULSE INCORPORATED
Application year: 1987
Main classification: H03K17
IPC codes: H02M1, H03K17, H02M3