DC/DC Converters with 20kVDC Isolation
2019/02/05
This whitepaper discusses the applications and hazards of using high DC voltages and explains how high isolation DC/DC converters can be used to enhance safety.
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1. Introduction
There are many DC high voltage applications in industrial, medicine, materials science and electronics production. Security at border controls relies heavily on X-ray scanners for checked-in luggage and parcels, mass-spectroscopy systems for ‘sniffing’ out explosive compounds and compact linear accelerators (LINACs) to scan through vehicles to detect smuggled goods, stowaways and illegal weapons.
In medicine, not only are high voltage DC supplies required for X-Ray and CT diagnostics, but also for oncology treatment, 3D imaging for reconstructive surgery and image-guided operations. As the population ages, diagnostic tools, such as nuclear imaging, bone densitometry and dental imaging are becoming increasingly vital to investigate dementia, osteoporosis and tooth disease - all common age-related illnesses. Industrial grade X-ray detectors permit liquid and solids level measurement in production plants, without the risk of contamination or leaks caused by puncturing the tank to insert a sensor, which also makes them useful for hygienic food production. In research institutes, sputtering, ion deposition, and laser and plasma beams allow novel compounds and structures to be created or analyzed. Not only are HV DC supplies needed for the beam electronics, but also for the high-vacuum molecular pumps that these systems depend upon.
In medicine, not only are high voltage DC supplies required for X-Ray and CT diagnostics, but also for oncology treatment, 3D imaging for reconstructive surgery and image-guided operations. As the population ages, diagnostic tools, such as nuclear imaging, bone densitometry and dental imaging are becoming increasingly vital to investigate dementia, osteoporosis and tooth disease - all common age-related illnesses. Industrial grade X-ray detectors permit liquid and solids level measurement in production plants, without the risk of contamination or leaks caused by puncturing the tank to insert a sensor, which also makes them useful for hygienic food production. In research institutes, sputtering, ion deposition, and laser and plasma beams allow novel compounds and structures to be created or analyzed. Not only are HV DC supplies needed for the beam electronics, but also for the high-vacuum molecular pumps that these systems depend upon.
2. The need for very high isolation DC/DC converters
All of these different detection systems have one thing in common: a high voltage DC supply. The most common method used to generate a continuous high voltage DC supply is to use a regulated low voltage source to feed a high voltage multiplier (Figure 1). The high output voltage is sensed by a long divider chain and fed back to control the LV power supply. The output current is measured between the chassis common ground and the floating ground at the input to the multiplier.
Fig. 1: Typical HV DC supply with voltage feedback and ground current monitoring
This topology is simple, cheap and reliable as long as the chassis grounds are all well connected and at the same potential. However, if an external sensor is added to the system, for example, to measure the hardness of the vacuum inside the application, then there is a possibility of a secondary leakage current flow through the ground-referenced sensor output. This current is deducted from the return current, so it gives a false reading (Figure 2). Additionally, a potentially hazardous leakage voltage could develop across any potential differences between the chassis ground and the sensor ground.
Fig. 2: False return current reading due to leakage currents
A solution to this problem would be to isolate the sensor output so that the leakage current path is completely blocked, but this requires ...
Fig. 1: Typical HV DC supply with voltage feedback and ground current monitoring
This topology is simple, cheap and reliable as long as the chassis grounds are all well connected and at the same potential. However, if an external sensor is added to the system, for example, to measure the hardness of the vacuum inside the application, then there is a possibility of a secondary leakage current flow through the ground-referenced sensor output. This current is deducted from the return current, so it gives a false reading (Figure 2). Additionally, a potentially hazardous leakage voltage could develop across any potential differences between the chassis ground and the sensor ground.
Fig. 2: False return current reading due to leakage currents
A solution to this problem would be to isolate the sensor output so that the leakage current path is completely blocked, but this requires ...
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