An Overview of Glassman's Power Supply Technology
The technology and topologies developed and employed by Glassman allow us to offer compact and reliable HV power supplies that have the capability of being easily adapted for most applications while being the easiest in the industry to maintain. Almost all Glassman supplies employ air as the primary insulating medium and utilize a high frequency PWM off line converter.
While not suitable for ultra miniaturized modules operating in severe environmental conditions, air insulation offers a lightweight repairable structure that minimizes parasitic capacitance losses for most applications. We have developed HV structures that incorporate equipotential grading and electrostatic shielding of sensitive components so that we achieve excellent stability and accuracy. All of our HV assemblies are based on the well-known Cockcroft-Walton voltage multiplier concept, (or variations, thereof), to achieve high DC outputs while minimizing peak transformer secondary voltages.
The use of air allows for forced cooling of HV components when required. Forced air cooling allows us to incorporate an increased value of series protection resistance (where practical), which minimizes peak discharge currents when an arc or overload occurs. (NOTE: Some models or applications require external series protection resistance.) This not only protects the HV components and the customer’s load, but also reduces the discharge energy that occurs during an arc and minimizes the electromagnetic interference (EMI) pulse that can damage or disrupt sensitive controls and microcontrollers. All these techniques improve the reliability of the entire high voltage assembly, as well as the control and power elements of the complete power supply structure.
Above 150kV our designs utilize an open-air “stack” that eliminates the HV connector and cable that would be massive at these voltages. Toroidal terminals and equipotential surfaces are used to minimize the electrostatic fields. For units of 150kV and below, we mount the HV assembly in a proprietary HV insulated enclosure whose walls can withstand the full voltage. This enclosure is made of fire retardant materials and is designed to provide a uniform surface gradient to minimize corona. This, in turn, is mounted in a grounded chassis.
One of the problems with increasing the conversion frequency in HV supplies is the reflected parasitic capacitance. This is formed by the proximity of surfaces to ground. In a large HV structure, reflected parasitic capacitance can be sizable. If solid or liquid encapsulation is employed, this capacitance is much higher than in air since the dielectric constant of air is 1.0 while most encapsulants are on the order of 3-4.5. Capacitance is directly proportional to the insulation dielectric constant.
Our HV transformers typically have 6kV or less peak voltage on the secondaries and employ special universal winding techniques to produce a self-supporting large diameter winding which has the proper voltage gradients. In addition, we typically employ large window U-cores which give enough space for the proper gradients.
Glassman HV supplies utilize our proprietary PWM converter technology for the main power conversion. Typically, the AC mains line voltage is rectified and filtered into the DC rails directly off the line with no transformers. In many cases, a power factor correction boost converter is employed to provide a regulated 400VDC rail bus. This provides a power factor very close to unity, which virtually eliminates line harmonic currents, and reduces the VA drawn from the mains. The DC rail voltage is applied to the converter and coupled to the HV assembly via the HV transformers which provide line to ground isolation. The converter drive signals are coupled to the converter switching devices by isolation transformers which also provide line to ground isolation.
Most of our supplies use a converter that runs at switching frequencies between 30kHz and 70kHz and employs either FETs or IGBTs as the switching elements. Conversion efficiency is greater than 90%. The converter topology is well suited to drive large ratio step up transformers as it uses the energy stored in the stray and interwinding transformer capacitance to switch the secondary voltage rather than dissipate it in snubber or switching losses.
The converter is pulse width modulated and utilizes integrated magnetics to store the conversion energy. This is a zero current turn on topology that eliminates turn on losses. It runs at fixed frequencies which helps minimize the switching frequency ripple component and improves control loop response. This converter design is inherently current limited so that, even without any external control or protection, the converter can operate into a dead short continuously and can even withstand a dead short on the transformer secondaries indefinitely.
Because the conversion frequency is high, the stored energy in the HV assembly is low, making for a safer design both for personnel and for load stresses.
All Glassman supplies employ fast acting voltage and current feedback loop control with automatic crossover. In addition, techniques are used to ensure safe, well-controlled ramp up of voltage from any condition, including recovery from an arc, overload, or short circuit. This prevents dangerous voltage overshoot under any recovery condition.
All Glassman supplies employ redundant under voltage sensing so that the power supply is fully protected against any input line voltage perturbations all the way down to zero. This ensures safe operation during brownouts or large line dropouts. Bias rail voltages are all derived from a single source so that the rise and decay of the bias voltages during turn on and off remain in the same relationship as for normal running. This eliminates any possibility of the feedback operational amplifiers losing control and generating improper drive signals.
Glassman offers as an option an RS-232 serial interface that provides complete galvanic isolation between the host computer and the power supply of up to 3000VAC. This is very important in the high noise and transient environment in which HV power supplies operate. This technique completely isolates and protects sensitive computer circuits, both on the user end, as well as the power supply itself.
Most of the recent Glassman designs employ fast arc sensing and protection. Any time a high voltage power supply is discharged, the stored energy within the HV assembly is delivered to the series limiting resistors within the supply. These resistors are needed to limit the discharge current to a level that protects the HV diodes and capacitors and reduces generated EMI. Since most Glassman power supplies have a fast voltage recovery rise time, the power that will be dissipated in the series limiting resistors during repetitive arcing is proportional to the product of the energy and the arc repetition frequency. This could be many times the stored energy value.
Due to size and layout considerations, installing enough limiting resistors to handle all of this dissipation is not always practical. Even though the resistors are high-energy types and can withstand a short burst of arcs, they may not be able to withstand a continuous arcing condition. Protection is provided by an arc count circuit, which inhibits the HV generation when the number of arcs exceeds a safe limit within a specified period of time. This technique allows for reasonable average power dissipation in the limiting resistors. Our arc sense circuits respond within microseconds with a threshold that provides power supply protection without excessive “nuisance” trips. After the power supply trips off, automatic resetting is normally achieved within 5 seconds. As an option, the power supply can be latched off permanently. Resetting of the power supply can be done via an external signal. An arc quench feature inhibits the converter for a fixed time period after each arc. This allows the arc to extinguish.
Although the primary purpose for the arc sensing circuitry is to protect the power supply, in some applications it can also protect the load that the power supply drives. For example, for ion sources where an external series resistor is normally provided, the arc counting feature is not needed. However, the fast extinguishing of the arc via the “arc quench” feature protects the ion source from damage. The arc sensing feature’s inhibit duration, sensitivity and frequency can be custom modified for any application, as long as the parameters remain within the range required to maintain power supply protection. The factory should be consulted if an external resistor is used in series with the load, so that the arc sense sensitivity threshold can be properly adjusted.
|Arc response oscilloscope of 60 kV power supply.
Arc Response Characteristics:
The arc occurs at the point two cm to the left of the center graticule when the
HV output discharges. The HV output stored energy is discharged by the arc until
the arc extinguishes. In this photo, the arc extinguished at approximately 12kV.
The zero is the solid horizontal line.
The arc quench disables the HV output for 20ms, as shown by the delay before
the output begins to recharge. The output recharge characteristic causes the
output to charge to approximately 25% of rated very quickly (determined by
the current rating of the supply and the total output capacitance and load).
From 25% to rated, the output rises exponentially with a 50 ms time constant
Notice that the fast initial rise is 25% of rated, regardless of how far the
output initially discharged. If the output had discharged completely, the
fast initial rise would be the full 25%, in this case 15kV. However, since
the arc extinguished at 12kV, the fast rise is only 15kV – 12 kV = 3kV, as
The standard Glassman HV connector system used above 6kV employs a deep well
tube with a spring-loaded contact. The depth of the connector varies with
the voltage level. This depth is designed so that if the power supply is
operated without the insertion of the mating cable, personnel cannot come in
contact with dangerous voltages. The shield of the mating cable is
terminated at the chassis for safety.