COMPACT HIGH-VOLTAGE NANOSECOND PULSED-POWER GENERATOR

Abstract

A pulsed-power circuit includes first, second, third and fourth compression stages. The first and second stages each include at least one pre-charged capacitor and at least one inductor in series, and at least one switch operative to pump a DSRD (drift-step-recovery diode). The pre-charged capacitor of the second stage is pre-charged in negative direction with respect to the pre-charged capacitor of the first stage. The third and fourth stages each include at least one DSRD. The switches of the first and second stage are operative to drive (pump and then pulse) the DSRDs of the third and fourth stages.

Description

FIELD OF THE INVENTION

The present invention relates generally to pulsed-power generators, and particularly to a compact high-voltage nanosecond pulsed-power generator.

BACKGROUND OF THE INVENTION

High peak-power pulses from nanosecond pulsed-power generators are useful for a variety of applications such as pollution control by ozone production, ignition system for internal combustion engines, biological research, underground detection, and beam steering in accelerators. They are also useful for driving fast, 100-ps, avalanche diodes.

Drift-step-recovery diodes (DSRDs) are highly-compact opening switches that can be used with an inductor to commutate HV nanosecond pulses into a load. The design and production of these switches is currently very limited.

A prior art circuit using a DSRD is shown in FIG. 4, taken from J. D. Taylor, Ultra-Wideband Radar Technology. New York: CRC Press, 2001.

Initially capacitor C₁ is charged and capacitor C₂ is discharged. In the first step switch S₁ closes, which results in DSRD pumping (forward direction current). In the second step, switch S₂ closes, which results in DSRD pulsing (reverse direction current). When the total charge that was stored in the DSRD junction during the forward (pumping) cycle is removed by the reverse (pulsing) cycle, the DSRD stops conducting the current and a high-voltage pulse appears at the load, R_L. Switch S₂ should pass the total current and the prior art uses a magnetic switch for this purpose.

The DSRD operation requires a prime switch to pump its junction with carriers in the forward direction, and then to pulse it quickly in the reverse direction and discharge the stored carriers. Fast pulsing is necessary in order to achieve a high compression ratio between the load peak voltage and the supply voltage. The prior art has used metal-oxide semiconductor field effect transistors (MOSFETs) for this purpose.

SUMMARY OF THE INVENTION

The present invention seeks to provide a compact circuit design for producing nanosecond high-voltage (HV) pulses, as is described more in detail hereinbelow. The present invention provides a compact high-voltage (>5 kV) pulsed-power generator with a rise time of the order of a few nanoseconds (<5 ns). The method is based on a series inductor-capacitor (LC) compression with a fast high-voltage recovery diode, such as a drift-step-recovery diode (DSRD).

In contrast with the prior art, the present invention uses commercially available off-the-shelf (COTS) components, which is advantageous for all of the above-mentioned applications, and which provides significant savings over the prior art, as well as being much more compact in design than any of the prior art designs.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

FIG. 1 is a simplified schematic of a pulsed-power circuit for a compact high-voltage nanosecond pulsed-power generator (e.g., 10 kV, 1.85 ns rise-time), in accordance with a non-limiting embodiment of the invention.

FIG. 2 is a graphical illustration of load peak voltage versus time for the circuitry of FIG. 1, wherein in graph (a), HV₁=850 V, HV₂ =−320 V. and in graph (b), HV₁=800 V, HV₂=−200 V and wherein V_B=−4.0 V (solid line) and V_B=0 V (dashed line).

FIG. 3 is a simplified graphical illustration of load peak voltage versus HV₁ for various values of HV₂.

FIG. 4 is a simplified illustration of a prior art circuit using a DSRD.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1, which illustrates a pulsed-power circuit for a compact high-voltage nanosecond pulsed-power generator, in accordance with a non-limiting embodiment of the present invention.

The compact pulsed-power circuit may be made of COTS solid-state switching components. The circuit may have four compression stages. The first stage consists of a pre-charged capacitor and an inductor in series, where a switch, such as an IGBT (insulated-gate bipolar transistor) is used for the DSRD pumping. The second stage is similar to the first one, with the exception of its capacitor being pre-charged in the negative direction. The pre-charge in the negative direction allows an enhanced performance for the DSRD pulsing, without any need for a magnetic switch of the prior art. Accordingly, in contrast with the prior art, in the present invention, capacitor C₂ is pre-charged in the negative direction with respect to the pre-charging of capacitor C₁. This allows an enhanced performance in pulsing of the DSRD, resulting in a high DC to peak pulse compression ratio, in a compact generator.

Further compression can be achieved if two DSRDs are operated in a cascaded operation mode. Accordingly, in one embodiment, there are third and fourth stages that include DSRDs operated in a cascaded compression mode, where a small bias voltage allows the balancing of their pumping charges. The switches of the first and second stages drive (pump and then pulse) the DSRDs of the third and fourth stages.

It is noted that cascaded operation is described in A. S. Kesar, L. M. Merensky, M. Ogranovich, A. F. Kardo-Sysoev, and D. Shmilovitz, “6-kv, 130-ps rise-time pulsed-power circuit featuring cascaded compression by fast recovery and avalanche diodes,” Electronics Letters, vol. 49, no. 24, pp. 1539-1540, 2013 (herein “Kesar et al.”). In Kesar et al., both DSRDs in the cascade have their anode on a high voltage bias (high means that the voltage bias is of the order of the high-voltage supply). In contrast, in the present invention, the first DSRD is grounded and the second DSRD has a small voltage bias (for example, only a few volts, which is small with respect to the initial charging of capacitors C₁ and C₂). This small bias is sufficient for a balanced-charge operation.

The circuit size, not including its power supplies, was 100×50×30 mm. Its output to a 50Ω load was above 10 kV (2 MW) with a rise-time of 1.85 ns. The invention is not limited to this size and output.

The pulsed-power generator is an order of magnitude more compact with respect to commercial products, as shown in Table 1.

TABLE 1
Comparison of commercial products to the
pulsed-power generator of the invention
	Company
	FID		The present
	Technology	Megaimpulse	invention
Product	FPM 10-N	NPG 15-2000
HV output	10 kV to	15 kV to	10 kV to
	50 Ohm	75 Ohm	50 Ohm
Rise-time	1-5 ns	4 ns	Approx. 2 ns
Dimensions [mm]	200 × 120 × 60	248 × 90 × 250	100 × 50 × 30
Specific power	1.39	0.72	14
[W/mm{circumflex over ( )}3]

Referring again to the circuit of FIG. 1, positive, +HV₁, and negative, -HV₂, high-voltage supplies may be used for charging the circuit capacitors. The circuit may include four compression stages, as mentioned above. In the non-limiting illustrated circuit, the first stage consists of an IGBT (S₁=IRG7PH50UPbF by International Rectifier), two capacitors (C_1a=C_1b=20 nF) pre-charged to HV1+|HV2|, and two inductors (L_1a=L_1b=200 nH). The second stage consists of two IGBTs (S_2a=S_2b=IRG7PH50UPbF), two capacitors (C_2a=C_2b=20 nF) pre-charged to -HV₂, and two inductors (L_2a=L_2b=150 nH).

As shown in FIG. 1, the safe parallel operation of S_2a and S_2b is due to the splitting of their capacitors and inductors, namely, S_2a operates with C_1a, L_1a, C_2a, and L_2a, and S_2b operates with the capacitors and inductors with the subscript “b”. The design of two branches in parallel is due to the estimated current in the second compression stage being more than twice the current in the first stage.

The third and fourth stages consist of DSRDs, where the current is compressed via an inductor (L₃=70 nH). The second DSRD is biased by a capacitor (C₃=2 μF) pre-charged to a small bias voltage, V_B, which allows the balancing of their pumping charges. The DSRDs were built using 1-kV cell rectifier diodes (CN25M by EIC). These diodes were found to have a snappy reverse recovery when operated in DSRD mode. DSRD₁ consists of ten stacks in parallel, each stack containing four diodes in series. DSRD₂ consists of seven stacks in parallel, each stack containing seven diodes in series.

In order to pump the DSRDs, S₁ is turned on. This IGBT is driven at time t₁ by its driver (MIC4452 by Micrel). In order to pulse the DSRDs, IGBTs S_2a and S_2b are turned on simultaneously at t₂=t₁+ΔT. These IGBTs are driven via an isolating transformer (not shown in FIG. 1). The transformer consists of two primary coils and two secondary coils, each consisting of five turns on a toroid ferrite core (outer diameter 20 mm, inner diameter 9.5 mm, height 10 mm, relative permeability ≈1500). MIC4452 drivers were connected to the primary coils and each secondary coil was connected to the gate and emitter of its respective IGBT. The output pulse was measured on the load (R_L=50Ω).

The circuit shown in FIG. 1 was operated with a delay time between the first and second IGBTs of ΔT=150 ns. The high-voltage supplies were HV₁=850 V, and HV₂=−320 V. The bias voltage of DSRD2 was V_B=−4.4 V. The result is shown in FIG. 2(a). The load peak voltage was 10.3 kV, and its rise-time (10-90%) was 1.85 ns.

The effect of the bias voltage, V_B on the output is shown in FIG. 2(b). In this graph HV₁ was 800 V, and HV₂ was −200 V. The load voltage versus time when V_B was −4.0 V and 0 V is shown by the solid and dashed lines, respectively. As shown, when V_B was optimized, the load peak voltage was 9.4 kV. When V_B was zero, two peaks are seen, where the higher one was 7.2 kV.

The load peak voltage versus the positive high-voltage, HV₁, is shown in FIG. 3, for various values of the negative high-voltage, HV₂, of −320 V (circle marks), −200 V (square marks), −100 V (diamond marks), and 0 V (triangle marks). As seen in this graph, the negative high-voltage (which charges C_2a and C_2b) results in enhanced performance. When HV₂ was zero, the peak load voltage in the case of HV₁=900 V was 6.1 kV. When HV₂ was −320 V and HV₁ was 500 V, e.g. the total energy in the capacitors was lower than in the previous case, a higher load peak voltage of 8.7 kV was obtained, the load peak power having doubled. Since the IGBTs were rated to 1200 V, the maximum allowed voltage on charging C1a and C1b was HV1+|HV2|≤1200 V.

Accordingly, the embodiment of FIG. 1 is a highly compact, 100×50×30 mm, circuit with an output of 10.3-kV, 1.85-ns rise-time that uses COTS components. Its output power per circuit volume is 14 W/mm³, which is a factor of ten greater than prior art circuits which use a magnetic switch. It is noted that a more compact circuit size is possible with a custom-designed DSRD.

The capacitors of the second compression stages, C_2a and C_2b were pre-charged to a negative high-voltage. This feature enhances the peak voltage at the load, FIG. 3, and eliminates the need for a magnetic switch. Since the current in the second compression stage is estimated at more than twice the current in the first stage, two IGBTs were used in parallel. An additional design feature is the introduction of a small bias voltage (compared to the HV supplies), at the anode of DSRD₂. Only a few volts were required for balancing the cascaded operation of the DSRDs, FIG. 2(b).

The invention may be utilized to produce higher peak load voltages by increasing the number of the IGBTs. For example, three more IGBTs in a similar configuration can be connected to point “A” in FIG. 1 in order to drive the DSRDs at a higher current. In this case, the DSRDs should be changed in order to support a higher peak power.

Claims

1. A pulsed-power device comprising: a pulsed-power circuit comprising first, second, third and fourth compression stages, the first and second stages each comprising at least one pre-charged capacitor and at least one inductor in series, and at least one switch operative to pump a DSRD (drift-step-recovery diode), wherein the pre-charged capacitor of the second stage is pre-charged in negative direction with respect to the pre-charged capacitor of the first stage; and the third and fourth stages each comprise at least one DSRD, and wherein said switches of the first and second stage are operative to drive (pump and then pulse) the DSRDs of the third and fourth stages.

2. The pulsed-power device according to claim 1, wherein said DSRDs are operated in a cascaded compression mode.

3. The pulsed-power device according to claim 2, wherein a bias voltage is used to balance pumping charges of said DSRDs.

4. The pulsed-power device according to claim 1, wherein said at least one switch of the second stage comprises two switches in parallel.

5. The pulsed-power device according to claim 1, wherein said at least one switch of each of the first and second stages comprises an IGBT (insulated-gate bipolar transistor).