DESIGN AND APPLICATION OF A DIODE-DIRECTED SOLID-STATE
G.E. Dale^, H.C. Kirbie, W.B. Haynes, C.E. Heath,
T.A. Lopez, F.P. Romero, and R.M. Wheat Jr.
Los Alamos National Laboratory, PO Box 1663, Mail Stop J-570
Los Alamos, NM, USA
Researchers at Los Alamos National Laboratory
(LANL) are developing a new solid-state high-voltage
Marx modulator for the generation of pulsed power. The
initial application of the LANL modulator is to provide
power to a magnetron that requires a 46-kV, 160-A, 5-ps
rectangular pulse. This modulator technology is also
being developed for other applications, including portable
millimeter wave sources, a beam energy corrector for
induction accelerators, and space-based power systems.
The LANL solid-state modulator has several benefits,
including wave shape control, switch protection,
efficiency, and compactness. The present paper describes
this source technology and its design.
The traditional Marx generator, named for its inventor.
Professor Erwin Marx , produces a single high-voltage
pulse by switching precharged capacitors into a series-
connected string using gas-insulated spark gaps . The
Marx generator is a rugged, low-impedance source of
electrical energy that has served well in a wide variety of
high-peak-power applications for the past 75 years. Marx
generators are now undergoing a renaissance due to the
use of modern solid-state switches [3-8]. The use of
insulated gate bipolar transistors (IGBT) in place of spark
gaps, for example, gives simple Marx generators the
ability to produce square-shaped output pulses at very
high rates. The on/off switching capability of the IGBT
also allows the output pulse to change width from one
pulse to the next, enabling the generator to adapt rapidly
to changing load requirements. Currently, Marx
generators using solid-state switches are unable to equal
the high peak voltage and peak power capacity of
generators using spark gaps, but the operational
advantages gained in pulse control and high average
power have transformed the single-shot Marx generator
into a versatile modulator.
A schematic of a typical resistively charged Marx
generator is shown in Figure 1. Other Marx configurations
use inductors in place of the charging resistors. Our Marx
modulator replaces these linear charging elements with
fast-recovery diodes , as shown in Figure 2. The diodes
provide a low-loss, low-impedance path for the Marx
charging current between pulses and a high-impedance
path when the Marx bank is erected.
Figure 1 . Simplified diagram of a four-stage Marx
modulator with resistive charging elements.
Figure 2. Simplified diagram of a four-stage Marx
modulator with diode charging elements.
The charging inductor shown in Figure 2 is necessary to
complete the charging pathway when the Marx bank can
not be charged through the load (such as when used to
power a magnetron). Supply current enters the Marx
circuit through the charging inductor, trickles through the
Marx assembly via diode routing, and returns to the
power supply near the grounded end. The charging
* Work sponsored by Los Alamos National Laboratory under US DOE contract W-7405-ENG-36.
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Design And Application Of A Diode-Directed Solid-State Marx
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See also ADM002371. 2013 IEEE Pulsed Power Conference, Digest of Technical Papers 1976-2013, and
Abstracts of the 2013 IEEE International Conference on Plasma Science. IEEE International Pulsed Power
Conference (19th). Held in San Francisco, CA on 16-21 June 2013., The original document contains color
Researchers at Los Alamos National Laboratory (LANL) are developing a new solid-state high-voltage
Marx modulator for the generation of pulsed power. The initial application of the LANL modulator is to
provide power to a magnetron that requires a 46-kV, 160-A, 5-is rectangular pulse. This modulator
technology is also being developed for other applications, including portable millimeter wave sources, a
beam energy corrector for induction accelerators, and space-based power systems. The LANL solid-state
modulator has several benefits, including wave shape control, switch protection, efficiency, and
compactness. The present paper describes this source technology and its design.
15. SUBJECT TERMS
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19a. NAME OF
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
inductor is generally sized to shunt no more than 10% of
the load current at several times the maximum expected
Employing diode charging and solid state switching in a
Marx architecture produces significant benefits in
efficiency, wave shape control, switch protection, and
A Marx bank with resistor charging can be no more
than 50% efficient because at least half of the charging
energy is dissipated in the charging resistors. In
comparison, diode charging of a Marx dissipates very
little of the charging energy resulting in a very efficient
charging arrangement. Using inductors for Marx charging
can also be made to be very efficient, but suffers from
pulse width limitations due to saturation. Diode charging
has the added benefit of not limiting the pulse width since
the diodes can remain reverse biased indefinitely (when
used, an external charging inductor can limit the pulse
width and so needs to be designed appropriately).
Another efficiency benefit of using diode charging is
that it facilitates the recycling of energy that would
otherwise be lost. When the switches shown in Figure 2
are closed, the diodes are reversed-biased. When the
switches open, energy stored in the charging and load
inductances causes the diodes to become forward biased,
directing this inductively stored energy back into the stage
capacitors. Although the charging inductor siphons off
some of the Marx output pulse, this inductively stored
energy is given back (recycled) after each pulse.
B. Wave-Shape Control
The combination of solid-state switches and diode
charging adds the new and powerful dimension of
tailoring the output pulse for optimal performance at the
load through the unique ability to operate each stage
independently. Stages not receiving a switching command
are bypassed by the charging diodes, an arrangement
producing an output pulse voltage equal to the charge
voltage times the number of active stages. Similarly, the
independent stages can be switched on and off within a
single pulse envelope to produce a digitally synthesized
C. Switch Protection
The diode architecture that increases the Marx
efficiency in our modulator also intercepts destructive
transient energy and returns it safely to the energy-storage
capacitors for reuse. The switches used in modern Man-
type modulators are steadily increasing in peak and
average power-handling capacity, but the drawback is
their susceptibility to switch damage from energy
transients. The traditional approach to addressing this
problem is to absorb and dissipate the incoming transient
energy by connecting snubbing circuits in parallel with
the endangered switches. A higher level of protection is
provided by our Marx architecture by directing the
transient energy around the vulnerable switches into the
stage capacitors through the charging diodes. We believe
this approach may eliminate the need for snubbing
circuits entirely in future versions of the modulator.
D. Size and Weight
Our fully solid-state modulator is inherently compact
and lightweight. The basic architecture of the modulator
will allow it to keep pace with the demands of research,
the military, and industry for high-peak and high-average-
power sources in ever-smaller and ever-lighter packages.
Our Marx modulator does not use a large, heavy
transformer to produce high voltage. Moreover, since all
charging, switching, and gate-control components are
solid-state, we have the option of purchasing their silicon
interiors (dies) instead of encapsulated components.
Therefore, each Marx stage can be made very small and
light, either by combining the dies in a multichip module
or combining their functions within a single application-
specific integrated circuit (ASIC). This possibility leaves
the volume of the energy storage capacitors as the
dominant factor determining the dimensions. Since the
history of capacitor technology demonstrates steadily
increasing energy density, in the future it is likely we will
be able to reduce the size of our Marx modulators even
further . Use of all-solid-state construction in a Marx
generator has the added benefit of minimizing the heat
generated during operation, a result that allows reduction
in the size and complexity of the attendant cooling
system. In our first application, the relatively low output
voltage has allowed more weight reduction through the
use of sulfur hexafluoride gas (SF 6 ) for the insulating and
II. SYSTEM DESCRIPTION
We developed our Marx modulator to fill the power-
source needs of several applications, each possessing its
own set of parameters for weight, size, output voltage,
current, pulse width, repetition rate, and average power.
The first of these applications is a compact, gas-insulated
power source designed to deliver a 46-kV, 160-A, 5-ps
rectangular pulse to the cathode of a magnetron. The
magnetron is an S-band e2v Model 6028 with the
performance parameters listed in Table 1.
Table 1 . Magnetron performance parameters.
Energy per pulse
RF peak power
~ 3.5 MW
Rate of voltage rise
< 130 kV/ps
Average power limit
7 kW (input)
The magnetron produces a nominal load impedance of
288 Q and requires a pulse that is reasonably flat (<5%)
when averaged over the pulse width. The modulator is
designed to obtain the pulse flatness by storing about 15
times the energy needed by a single pulse. The droop can
be further reduced by either adding more stage
capacitance, using a passive droop compensation network
, or active compensation by wave-shape control.
The key parameters of the modulator are shown in
Table 2. Each stage consists of a single IGBT, two 8-pF
capacitors connected in parallel, two fast-recovery diodes,
a single gate driver, and an isolated power supply deriving
its energy from a ferrite isolation transformer. The IGBT
and gate-control elements are shown in Figure 3.
Table 2. Marx modulator parameters.
Max. stage voltage
Number of stages
48 (57.6 kV open circuit)
Capacitance per stage
Total Marx capacitance
Max. stored energy
Figure 3. Schematic illustration of a single Marx stage.
The IGBTs are triggered independently by fiber optic
signals received through an optical interface on each
stage. We selected the IXYS model IXBT42N170 for the
Marx stage switch because it is a good compromise
between operating voltage, pulsed current, and
commutation speed . We chose the IXYS model
DSDI60 fast-recovery diode for the charging elements
and a self-healing film capacitor from Aerovox for energy
Four Marx stages are collected on a single circuit board
measuring 12.7 cm x 30.5 cm. Figures 4 and 5 show two
views of a single circuit board. Each stage is powered by
a separate winding on a common ferrite transformer core
with a single high-voltage wire serving as the primary
winding for the ferrite transformer. The primary winding
is powered from an Fl-bridge power converter operating at
The entire modulator assembly consists of twelve of
these circuit boards. The circuit boards are rack-mounted
on rails in a folded arrangement that saves space. The
centers of each ferrite transformer share a common axis
through which the primary winding is threaded. The
boards are supported on the assembly rails by modified
circuit card connectors. These circuit card connectors also
serve to electrically interconnect the Marx boards in series
using a copper strip.
Figure 4. Front view of the circuit board showing four
optical fiber ports, four switches, and four metal
enclosures containing the gate drive electronics.
Figure 5. Rear view of the circuit board showing eight 8-
pF, 1500-V capacitors and a single ferrite-core
Two views of the assembled modulator are shown in
Figures 6 and 7. The assembled modulator weighs 11 kg,
and measures 76.2 cm x 30.5 cm x 17.8 cm.
Figure 6. Top view of the assembled modulator showing
support rails and edge connectors. Board spacing on the
rails is generous to allow single stage measurements.
The modulator has been extensively tested into a 300 O
resistive load. Detailed performance results are contained
in a companion paper .
Figure 7. Top view of assembled modulator showing
fiber-optic trigger cables.
We have produced an efficient power converter that
takes in low-voltage dc power and puts out a steady
stream of high voltage pulses in a package with an ultra¬
compact future . In addition to building this
modulator to power a magnetron, we are also developing
this modulator technology for other applications. These
future applications include modulators for portable
millimeter wave sources, an active tuning system for
pulse forming networks, a beam energy corrector for
induction accelerators, and spaced-based power systems.
The key element of the modulator is its unique Marx
circuit architecture that manages the outflow of power to
the load with advanced solid-state switches and uses
diodes to direct the flow of all incoming power, transient
or otherwise, to the capacitors. The charging diodes
provide high shunt impedance to the outflow of power
(when the diodes are off) and converts to a very low shunt
impedance to the inflow of power (when the diodes are
on), regardless of whether that power comes from a dc
power supply or a high-power transient.
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