SPELLMAN.MAR–Spellman High Voltage Electronics Corp.–SC– — –##
How to specify today's high-voltage supplies
Keep in mind technology improvements that make overspecification unnecessary
BY DEREK CHAMBERS and CLIFF SCAPELLATI
Spellman High Voltage Electronics
Corp., Plainview, NY
Recent advances in technology have made the latest high-voltage power supplies smaller, lighter, and more efficient than was possible just a few years ago. Given the improvements in high-voltage supplies, designers more than ever should guard against overspecifying the key parameters–output power, ripple, temperature stability, and size. Overspecification can lead to unnecessarily high cost. It can also lower reliability because of increased complexity and greater power density.
The technological advances come from two sources. The first source is the availability of key power components, which have low losses while operating at high frequency. The power components include faster switching transistors, MOSFETs, insulated-gate bipolar transistors, and silicon-controlled rectifiers. The second source is the development of advanced resonant power conversion techniques. The newer conversion techniques make use of zero current and zero voltage switching, resonant, and quasi-resonant inverters.
New high-voltage supplies usually operate in the range of 20 to 100 kHz. Industry wide they have virtually replaced all units operating at line frequency, even at high-power levels.
Ask the factory
In specifying high-voltage supplies, designers should consult the factory if a particular parameter in the catalog specification is inadequate for an application. Eleven parameters are important to users of high-voltage supplies.
1. Input voltage
The input power source specified for a particular model is determined by many factors. These include the output power capability of the supply and the form of power available in the application. In general, low-power, high-voltage supplies having outputs between 1 and 60 W operate from 24 or 28 Vdc.
On the other hand, most high-power modules over 100 W and rack-mounted models operate from the ac power line (see Fig. 1), whose voltage varies depending on the user's location. For instance, the U. S. and Canada standardize on single-phase 115/230 Vac at 60 Hz, while continental Europe and other parts of the world standardize on 220 Vac at 50 Hz.
Most power supplies include transformer taps to cover this range. Some new designs cover the 90 to 130-Vac and 180 to 260-Vac ranges without taps. All countries in the European Economic Community will eventually standardize at 230 V at 50 Hz.
Power factor correction and universal input at power levels below 3 kW can be specified for most off-the-shelf, high-voltage power supplies. Higher-power units require custom engineering.
2. Output voltage
High-voltage power supplies are usually designed for continuous operation at the maximum output voltage specified in the data sheet. Laboratory bench models and high-power rack units are normally adjustable from zero to the maximum specified output voltage.
Modular high-voltage supplies, on the other hand, may have either a preset output voltage, or a narrow adjustment range. They may also include monitor terminals instead of meters for measuring the voltage. It is not usually cost effective to specify a power supply with an output voltage that exceeds 20% of the maximum voltage actually needed in a particular application.
3. Output current
Power supplies are normally designed for continuous operation at the full current specified in the data sheet. Current limiting is normally built into the supply to prevent overload current from increasing beyond about 110% of the rated maximum value of output current. Overload trip-out can usually be specified to disable the power supply when the normal output current is exceeded.
Current regulation, available on most high-power racks and modules, allows the output current to be controlled by a front-panel potentiometer or from a remote source. Current regulation also provides automatic crossover to voltage regulation when the load current is lower than the programmed value.
4. Ripple
Ripple may be defined as those portions of the output voltage that harmonically relate to both the input line voltage and the internally generated oscillator frequency. In high-frequency switching designs, ripple combines the frequencies of both the line frequency-related components and the switching frequency-related components. Total ripple is specified either as the rms value or the peak-to-peak value of the combined line frequency and oscillator frequency components. It is normally expressed as a percentage of the maximum output voltage.
The amount of ripple that can be tolerated in different applications varies. In photomultiplier and nuclear instrumentation applications it can be less than 0.001% peak-to-peak. In applications like E-beam welding, where the output can be integrated over time, ripple can be several percent.
When operating from an ac input source, line frequency ripple can represent a significant part of the total peak-to-peak ripple. Typically, the power supply is designed to have equal amounts of high-frequency and line-frequency ripple when operating at full output power. In most designs, the magnitude of the line-frequency ripple is attenuated and controlled by feedback in the regulation circuits.
Regulated supplies operating from a dc input exhibit no ripple, with the ripple frequency a function of the supply's switching or oscillator frequency. To reduce switching frequency output ripple, the designer can add filtering components such as shunt capacitors or resistors, or electronic ripple-canceling circuits.
5. Stability
Four factors affect the output stability of a regulated high-voltage power supply:
a. Drift in the reference voltage.
b. Offset voltage changes in the control amplifiers.
c. Drift in the voltage ratio of the feedback divider.
d. Drift in the value of the current sense resistor.
All these variations are a function of temperature. Stability in a properly chosen reference device is usually less than 5 ppm, and offset errors can be virtually eliminated by careful choice of the control amplifier. This leaves the voltage divider and the current sense resistor as the critical items affecting stability in the output voltage and current.
Because the voltage divider and current sense resistor are both sensitive to temperature variations, they are selected to operate at a fraction of their power capability. However, rising ambient temperatures cause small changes in the ratio of the voltage divider and the value of the current sense resistor which could affect stability.
6. Stored energy
The stored energy at the output of a high-voltage power supply can be dangerous to operating personnel, particularly at higher voltages. That's because its value is a function of the square of the voltage and the value of the capacitance across the output. Certain loads, such as X-ray tubes, are also easily damaged by excessive stored energy in the high-voltage power supply when an arc occurs.
High-frequency power supplies require much smaller values of smoothing capacitance than those operating at line frequency. Thus, the dangers of electrocution are reduced.
7. Pulsed operation
While some power supplies are designed for dc operation, others can be used in pulsed power applications. In most cases, an energy storage capacitor either inside or external to the supply generates the peak pulse current. The supply operates in the current mode during the pulse and recharging parts of the cycle, and returns to the voltage mode before the next load current pulse.
Pulsed loads usually fall into one of three categories:
a. Very narrow pulses (1 to 10 microsecond), with a duty ratio of less than 0.01% to less than 1%. Pulsed radar applications typically generate narrow pulses of having durations in microseconds, at repetition rates between 500 Hz and 5 kHz.
b. Longer pulses (100 microsecond to 1 ms), with a duty ratio between 0.05% and 0.2%. This category includes pulsed electromagnetic supplies or cable testing where most of the pulse load current is still provided by a capacitor connected across the output.
c. Very long pulses (50 ms to 5 s), with a duty ratio between 0.1% and 0.5%. The third category requires a power supply designed to provide more current than its average rated value for relatively long periods. Typical applications are medical X-ray systems, lasers and high-voltage CRT displays.
8. Line regulation
Line regulation is expressed as a percentage change in output voltage for a specified change in line voltage, usually over a +/-10% line-voltage swing. Measurement is made at maximum output voltage and full-load current unless otherwise stated. Line regulation of most high-voltage power supplies is better than 0.005%.
9. Load regulation
Load regulation is specified at full output voltage and nominal line voltage. It is expressed as a percentage change in output voltage for a particular load current change, usually no load to full load. Typical load regulation of most high-voltage supplies is better than 0.01%.
10. Dynamic regulation
The transient response characteristic of the output voltage of a power supply is the instantaneous excursion of the output voltage and recovery when a step change in load current is applied.
Recovery time is the time required for the voltage to return from the initial deflection point to within 10% of the new static level (see Fig. 2). This parameter is important in certain pulsed applications, such as CRT raster scan displays where blocks of video data can cause a step change in the load current.
11. Efficiency
The efficiency of a power supply is a measure of its output power relative to the power required at its input. Efficiency is usually specified as a percentage, calculated by dividing the output power by the input power and multiplying by 100.
Other considerations
Remember that the heart of any high-frequency power supply is the oscillator (or inverter) used to drive the output transformer. The specific designs used in the high-voltage power supply industry are too numerous to cover since each manufacturer has developed its own proprietary power switching circuits. However, the user must remember to prevent the capacitance existing across the secondary winding of the step-up transformer from being reflected directly across the power switching semiconductors.
Isolating the capacitance can be achieved by:
1. Using a flyback circuit.
2. Using an inductor or a series resonant circuit between the switching devices and the transformer.
3. Including sufficient leakage inductance between the primary and secondary windings of the transformer.
4. Operating as a self-resonant oscillator.
Specifiers of high-voltage supplies may also need auxiliary supplies to back up the primary high-voltage supply. For instance, an electron beam system may use a primary beam supply (for example, -60 kV) for the cathode electrode, plus a floating filament supply (like 10 V) and a floating bias supply (such as -2 kV) both referenced to the cathode potential. These normally regulated supplies provide independent current- or voltage-mode control.
A final warning: All high-voltage power supplies must be operated by personnel familiar with the dangers of high voltage. High-voltage sources can be lethal! A general guideline for Safety Practices is found in IEEE Standard 510-1983 “Recommended Practices for Safety in high-voltage and high-power testing.”
CAPTIONS:
Opening shot:
High-voltage supplies such as this multiple-output model use more efficient and higher performance components and power conversion techniques to reduce weight and improve performance.
CAPTIONS:
Fig. 1. Higher-power high-voltage supplies, like Spellman's series SL which comes rated up to 1,200 W, typically operate from ac line power.
Fig. 2. After load switching, the output voltage waveform is momentarily deflected but then recovers to within 10% of the new static level. The time when the waveform is first deflected and recovers is known as the recovery time.
Advertisement
