Designed at very high voltages

06/26/2019

For engineers who use design time for the unit number, low voltage world, the phrase "high voltage" may produce a two-digit voltage, possibly as high as 24V or 48V DC, or even a three-digit voltage. The line voltage is 120/240 VAC. However, a huge and important engineering design world must be completed at 1000V, 1500V and higher.


Designing products for the region requires very different thinking, component selection and interconnection, often in areas where low-voltage product designers don't even have to think about it. These issues apply to passive components, connectors, wiring interconnects, MOSFET / IGBT, layout, and of course safety and regulatory issues. When your voltage potential is high, it is a difficult and ruthless world. Trivial negligence can suddenly become a major device and life-threatening event. Remember: Rule 1 stops and thinks before you do anything; rule number 2 calls rule number 1, again, perhaps multiple times.




Need high pressure

Given design challenges and risks, why should design engineers even consider using these voltages? This may be because the engineer has no choice or because it is a very good and necessary idea. Applications are divided into two categories:


Within the scope of “engineers have no choice”, scientific, medical, and physical instruments require high voltages in specialized equipment such as X-ray machines to develop high-intensity fields, ionize atoms, and accelerate electrons and other particles. The same applies to vacuum tubes that still require high power broadcast or even medium power microwave and millimeter wave transmitters. In more common applications, even commercial neon lights require a few volts to ionize the internal inert gas. Note that many of these applications require more than a kilovolt, but at a relatively modest current of approximately 100 mA.


Designed at very high voltages: everything is changing, especially your way of thinking Figure 1

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Figure 1: Many scientific experiments require several kilovolts of potential at low currents to stimulate particles, or to control and accelerate their motion.


Engineers are designing power and efficiency when using high voltage is a "very good and necessary idea." When the power supply or motor needs to generate a lot of power, the power supply must provide watts, which is the product of voltage and current. However, at lower voltages, the current is significantly higher, so IR (current X resistance) losses in conductors, connectors, switches, and active devices can result in inefficiencies, losses, and I 2 R heating.


Designed at very high voltages: everything is changing, especially your way of thinking Figure 2

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Figure 2: To minimize IR losses in cables, connectors, magnetic components and active components, the motor is designed to use the power supply at very high voltages.


The way to minimize these losses is to increase the voltage, thereby reducing the current, thereby reducing IR losses and I 2 R heating. This is why, for example, an electric locomotive operates at 20 kV, while an electric company's AC feeder can operate at 100 kV or higher. If we operate this equipment at a lower voltage, the basic line and other losses - whether it is efficiency cost or heat dissipation - will be enormous and cannot be tolerated. In addition to their kV rating, these "power delivery" designs can be tens or hundreds of amperes compared to the scientific, medical, and physical instrumentation applications cited above.


Starting from physical size

The processing high voltage begins with the conductor spacing and associated dimensions. The key terms for spacing conductors at higher voltages are creepage distances and gaps.

• The creepage distance is the distance the arc measures on the surface, for example between two traces on a printed circuit board or on the surface of a connector or IC.

• The gap is the shortest distance the arc can travel through the air, for example from the pin or pin of the connector or IC.

Creepage distance and clearance requirements are a function of peak voltage; for sinusoidal AC signals, the peak value is 1.4 times the RMS value plus a substantial safety factor. While it is good to be able to call up specific creepage distances and gap size requirements at any given voltage, it is not possible because their size depends on many factors:

1. Whether it is a potential shock hazard or only a malfunction problem,

2. The region of the world: different regions have different standards,

3. Applications: eg science, industry or medical, even consumer goods,

4. Maximum working height and humidity (the lightning rating of dry sea at sea level is about 4kV / cm, or 10kV / inch),

5. PC board and other surfaces: potential contamination levels due to various contaminations; PCB material groups; and coatings (if any).

Therefore, some serious research is needed to determine the minimum creepage distance and gap value required, or the engineer may need to call an experienced consultant, especially if the final product requires formal regulatory approval for manufacturing and sales.


Go to passive components

Designers operating at lower voltages rarely need to look at the voltage ratings of their basic passive components; these are almost innumerable resistors, capacitors and inductors that support both ICs and discrete devices. However, each has a maximum operating voltage rating. Above this voltage, components may not function properly and may be "elegantly" degraded, prematurely failing, or suffering a catastrophic failure.


For example, the capacitor can be specified as "10μF / 15 VDC" and the rated voltage should be the maximum value allowed to see. Note that the length of time it can withstand this overvoltage depends on the vendor; it can be as short as milliseconds or as long as a few minutes, so engineers must look at the vendor definition. If used at 100V, arcing may occur between the inner layers of the capacitor, shorting them and destroying the capacitive function. Most designers prefer to use a factor of two to three times their expected maximum voltage, so designers of 1kV DC circuits will choose passive devices with a rated voltage of 2 to 3kV.


For example, the AVX SXP molded radial multilayer capacitor (Figure 3) has a variety of maximum rated voltages up to 3000 V. The largest member of the series, SXP4, has a voltage range of 100 pF to 2200 pF and a size of 22.4 x 16.3 x 5.84 mm with a lead pitch of 19.8 mm (approximately the length of a standard paper clip).


Designed at very high voltages: everything is changing, especially your way of thinking Figure 3

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Figure 3: This capacitor in the AVX SXP series is rated at 3000V with a lead spacing of no more than 20 mm. 




Connectors and cables

What about connectors and cables? Although they are generally not considered with "passive" components such as resistors, capacitors and inductors, they are also critical links in the high voltage chain and have many of the same parameters as the basic passive components. Like layout and routing, creepage and clearance are the main factors when choosing a high voltage interconnect. However, there are differences between wiring and layout issues compared to connectors and wires: circuit and system designers typically do not design connectors; they bought them. Whether standard, off-the-shelf or custom-designed components, the connector manufacturer determines and defines the rated voltage of the connector for different applications and situations.


Almost all high voltage connectors are targeted at specific industries and needs, not for general purpose high pressure applications. For example, a supplier can refer to a given connector as “2000 V DC rated for medical applications according to standard IEC60601,” which provides a statement of suitability that system designers need when selecting connectors.


For example, the TE Con nectivity HVTT and HVTE cable assemblies are high voltage interconnect cables and connectors for electric rail vehicles, rated for 15/25 for automotive and passenger car roof lines and equipment connections, depending on the model. kV. In addition to the basic DC operating ratings, they also have an AC withstand voltage of 50 / 90kV and a pulse withstand voltage of 125 / 175kV. Of course, these are large connectors with a diameter of 90 to 135 mm and a creepage distance of 650 to 1000 mm. Their terminals include heavy duty, flexible shrink tubing to maintain moisture and containers in the exposed final assembly.




High voltage active devices are also needed

The high voltage design requires not only a high potential wiring current. The design also involves controlling and switching current at high voltages. IGBTs and MOSFETs are the most commonly used devices used here, although vacuum electronic devices (VED) - often referred to as vacuum tubes - still play an amazingly large role in this field because they can handle and consume a lot of power, especially at RF Spectrum.


In the first review, it was usually a difficult decision to use MOSFETs or IGBTs. In general, IGBTs are better suited for combinations of higher voltages, higher currents, and lower switching frequencies. The MOSFET is suitable for a combination of lower voltage and lower current, but with a higher switching frequency.


Regardless of which discrete power device is chosen, the package is determined by three related factors: voltage, creepage distance and gap issues; current, with larger lead size to reduce IR (current x resistance) drop; and power consumption, including Low thermal resistance from the chip to the case to maximize internal heat generation, whether due to the on-resistance RDS(on) in the MOSFET or the diode drop in the IGBT, outside the chip and package.


For example, International Rectifier's IRG7PK35UD1 IGBT is rated at 1400 V and is suitable for use in high-power, single-ended, parallel resonant power converters in furnace-top induction heating systems and microwave ovens (Figure 4).


Designed at very high voltages: everything is changing, especially your way of thinking Figure 4

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Figure 4: The International Rectifier IRG7PK35UD1 IGBT is optimized for home appliance applications and is available in a standard through-hole TO-247 package, which reduces cost and simplifies installation on the PC board.


In addition to the 1400V rating, the IGBT also supports 40A continuous collector current with a switching speed of 8 to 30kHz, which is very fast for IGBTs. With a voltage, current and maximum power consumption rating of 167W, it is available in an industry standard TO-247 package. Each of the three package leads has a width slightly greater than 1 mm and a minimum lead pitch of approximately 5 mm, which is comparable to the 1400V / 40A rating (Figure 5).


Designed at very high voltages: everything is changing, especially your way of thinking Figure 5

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Figure 5: The TO-247 dimension plot shows how it must comply with the creepage distance and clearance requirements of the rated IGBT voltage when dealing with two-digit currents. 


Selecting high-voltage IGBTs is a viable candidate for MOSFETs, and there is an additional aspect: the commercial availability of silicon carbide (SiC)-based MOSFETs, not just traditional silicon. In SiC devices, a wider bandgap and other detailed physical properties result in a breakdown voltage that is 10 times higher than silicon. As a result, despite other limitations in SiC devices, SiC MOSFETs that are thinner and smaller and capable of carrying more current with less loss can be fabricated. Furthermore, SiC has a much higher thermal conductivity than silicon, resulting in an excellent power density. For critical maximum operating temperature parameters, SiC devices can operate at junction temperatures in excess of 150 °C.


Cree offers the C2M family of 1220V and 1700V SiC MOSFETs, also in a TO-247 package, illustrating this shift. The C2M0160120D is rated at 1.2kV at 17.7A, only 160mΩ RDS(on), and consumes 125W. Their C2M0160120D is also a 1.2kV device, but the current is up to 90A, only 25mΩ RDS (on), the maximum power consumption is 463W. . This series is ideal for solar inverters, high voltage DC/DC converters, motor drives, switch mode power supplies (SMPS) and uninterruptible power supplies (UPS) designs. Cree claims that their SiC MOSFETs have three times the power density of silicon-based IGBTs with a loss of only 20% - both have significant improvements (Figure 6).


Designed at very high voltages: everything is changing, especially your way of thinking Figure 6

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Figure 6: SiC-based MOSFETS provides better high voltage/high current efficiency and density than roughly equivalent silicon MOSFETs and IGBTs; 300A SiC is more powerful than 600A IGBTs 


Despite the many challenges in design - even just around these high pressures, they are an indispensable aspect of many products. That's why engineers must be familiar with the relevant design aspects and basic high-pressure related issues, as well as safety and regulatory issues, in order to establish an appropriate perspective, while considering what high voltage can do and why they are needed.