10 Tips for High Voltage Resistor Design - Technical Article (eepower.)com)
An optimized strategy prioritizes the definition and testing of critical components early in the design cycle. High-voltage resistors are one such component. Here are 10 tips for designing with high-voltage resistors.
An optimized strategy prioritizes the definition and testing of critical components early in the design cycle. "High voltage" is a term that can have several meanings, but here we are considering circuits with voltages ranging from 1 to 100 kV.
An example at the lower end of this scale is an automated external defibrillator, in which a capacitor is charged up to 5 kV and then delivered to the patient with a precisely calibrated surge that could save a life. Sticking to the healthcare topic, but at the other end of the scale, we have medical X-rays, which are created by accelerating electrons at about 70 kV and then abruptly stopping them with a metal target. Careful control of this voltage variation allows adjustments to be made to the system to capture images of soft tissues or different bone thicknesses. These are just two examples of high-voltage applications where resistors, often the simplest commercially available components, are elevated in importance to provide critical protection and precise control in demanding applications. Based on years of support for designers who need high-voltage resistors, this article proposes ten tips, not only in the medical field, but also in the industrial, transportation, and scientific fields.
1.Know the voltage rating
The primary voltage rating of a resistor is its limiting element voltage (lev), sometimes referred to as the operating voltage. This is the maximum continuous voltage that can be applied on a resistor with ohmic values greater than or equal to the critical resistance. Below this value, the maximum voltage is limited by the power rating ((p) to , sqrt[2 , p cdot r). Typically, it is DC or AC RMS, but the datasheet of a high-voltage device may define it as a DC or AC peak. Related to this is the overload voltage rating, which is usually 2 to 25 seconds lev 2 or 5 times. In general, higher peak voltages can be withstood for short periods of time, as shown in the Pulse Performance section of the datasheet. The final rating is the isolation voltage, which is the maximum continuous voltage that can be applied between the resistor and the conductor in contact with its insulator.
2.The voltage division of a discrete resistor
The voltage divider requires a high-value resistor r1 to be connected in series with a low-value resistor r2, as shown in Figure 1.
The voltage ratio is given by the formula.
It should be noted that the voltage ratio is not the same as the resistance ratio r1, 12 but is offset by 1. For example, to get a voltage ratio of 1000, you need to define a resistance ratio of 999. For discrete resistor designs, it is best to choose standard values, and Table 1 gives some examples of decimal voltage ratios.
Once the nominal value has been selected, the next consideration is the required tolerance. The tolerance of the resistance ratio is simply the sum of the tolerances of the individual resistors. These are not necessarily the same; Often, it is most economical to choose tighter tolerances on low-pressure components. For example, the high voltage R1 is at 1% and the low voltage R2 is at 0At 1%, the resistance specific tolerance is 11%。For voltage ratios over 50:1, the tolerance of the voltage ratio is actually the same as the tolerance of the resistance ratio.
3.Specify the integrated voltage divider
The high-voltage dividers1 and R2 with integrated R can be integrated into a three-terminal assembly, as shown in TT Electronics' HVD series (Figure 2). This method has a number of precision advantages. For example, the target voltage ratio can be precisely defined without being constrained by the selection criteria.
The values specified for the integrated divider are usually the low value R2 and the total value R1+ R2In addition, the tolerance of the voltage ratio can be controlled directly by the fine-tuning process and can therefore be much tighter than the absolute tolerance of the resistance value. For example, R1 and R2 can be defined with an absolute tolerance of 2%, but the voltage ratio can be adjusted to 05% tolerance. A similar advantage applies to the temperature coefficient of resistance (TCR), where tracking the TCR determines the temperature stability of the voltage ratio, which may be lower than the absolute TCR of the resistive element. In addition, voltage dividers that extend this matching element to the lifetime drift and voltage resistivity (VCR) regions can be designed, although this often requires a custom design.
4.Evaluate TCR and VCR errors in the crossover
The R1 value is high enough and the voltage is low enough that there will be a low level of self-heating within the divider. If this is the case, it is relatively easy to measure the TCR and VCR effects separately. The TCR effect was calculated using an environmental chamber and the resulting figure of merit was defined as the voltage specific temperature coefficient (= FRAC}) in ppm °C, where VRHT and VRLT are the voltage ratios at high and low temperatures, and HT and LT are high and low temperatures.
The corresponding figure of merit for the VCR effect is likewise defined as the voltage-specific voltage coefficient (= frac}) in ppm °V, where VRHV and VRLV are the voltage ratios at high and low voltages, and HV and LV are high and low voltages.
If self-heating is not negligible, then in the TCR test, the chamber temperature should be adjusted to give the correct HT value, and time should be allocated for the temperature to stabilize. The VCR test should be of short duration to minimize temperature rise. Alternatively, an environmental chamber can be used to measure low voltages at higher temperatures and vice versa, thus counteracting temperature-dependent changes in resistance.
5.Calculate the value of the bleeder resistor
EAK bleeder resistors are used to discharge capacitors to a safe voltage level after a power failure. Bleeder resistors can be switched across the capacitor for fast discharge without static dissipation, or they can be permanently connected for high reliability and low cost. In the latter case, there is a trade-off between the time it takes to reach a safe discharge and the static power loss. Select the maximum suitable ohmic value by means of exponential discharge calculations:
where Td is the discharge time, C is the capacitance value assuming the maximum positive tolerance, Vt is the safe threshold voltage, and VO is the initial voltage. The highest standard value of the permissible tolerance is lower than that of Rmax should be used.
For the selected value r, the initial power is given by p o= vo2 rFor on-off bleers, this is peak power. In the case of permanently connected bleeders, it is continuous dissipation and the resistor chosen must be rated accordingly.
6.Choose the right balancing resistor
All aluminum electrolytic capacitors generate leakage current when connected to a DC voltage. This can be modeled by the leakage resistance in parallel with the capacitor. This resistance is nonlinear, that is, its value is a function of the applied voltage. In this case, the value is not well defined and varies greatly from one capacitor to another. When building a capacitive reservoir for a high-voltage DC bus, it may be necessary to use a series combination of two capacitors, each rated at half the bus voltage. If the capacitors are the same, the bus voltage will be evenly distributed between them. In practice, however, the leakage resistance will vary, resulting in uneven distribution and possible voltage overload on capacitors with higher leakage resistance.
The solution is to connect the balancing resistors (shown in Figure 3) in parallel with each capacitor. These are high-resistance resistors with a suitable voltage rating, with values matched to within a few percent. This value needs to be as high as possible to minimize power dissipation, but it is generally chosen to be no more than 10% of the minimum value of the leakage resistance at the capacitor's rated voltage. In this way, the effect of the leakage resistance of the unbalanced internal capacitor is drowned out by the effect of the balancing resistance, and the voltages are approximately equal.
7.Withstands high-pressure surges
Sometimes, designers do this when they are studying high-voltage resistors because their circuits must be able to withstand high-voltage transients. If a high-voltage rating is not required for continuous voltage stress, then a low-voltage but surge-resistant component is likely to be the best solution. For example, EAK's 5W wirewound high surge resistance WH5S does not have a high voltage rating but can withstand up to 10 percent of a peak of 1kV2 50 s, while the surge tolerant 2512 chip resistor HDSC2512 has a lev of 500V but can withstand peak voltages up to 7kV.
8.Designed to meet safety standards
When designing equipment to meet the requirements of electrical safety standards such as IEC 60664, it is necessary to consider the relevant creepage and clearance requirements at an early stage. These not only affect the PCB layout design, but in some cases also affect component selection. When a resistor is connected to a high voltage level, it is important to check the distance between its terminations and, in the case of a heat sink mounting part, the distance between the resistor and the metal thermal interface. This can be defined in two ways. First of all, the creepage distance is the shortest distance to pass through the insulating surface. This reduces the likelihood of wet and contaminated conditions, allowing the surface to flicker at a high enough energy to be tracked. Secondly, the gap is the shortest distance in the air. This solves the risk of flashover. If these two dimensions are not visible from the datasheet, they should be obtained from the manufacturer.
Another piece of information that may be required is the material that forms the insulating surface, as this determines the comparative tracking index (CTI), which classifies the tendency of organic materials to support processes that lead to tracking. For example, if a resistor is designed to bridge an isolation barrier to provide a current connection to prevent excessive electrostatic charge build-up, the IEC 60065 safety standard requires the resistor to withstand the specified high-voltage surge test. Since this is becoming a traditional standard, continuous certification of resistors is no longer important. Nonetheless, designers who follow the IEC 62368-1 hazard-based safety engineering approach will be aware that there are products that meet the requirements of IEC 60065.
9.Design of potting and oil-filled components
Two limiting factors in high-voltage designs may be the risk that contaminated organic surfaces tend to support tracking and discharge in the air, especially around small radius surfaces. Both of these limitations can be addressed by potting or soaking in mineral oil, which prevents contaminants from entering and replaces air with a substance with a higher dielectric strength. This, in turn, reduces creepage distances and clearance limits, which in turn reduces the size of the assembly. When selecting a resistor for such a component, it is important to choose a component that is insulated in a way that avoids the risk of outgassing. Any air bound to the component can form voids in which partial discharges can occur, leading to long-term degradation of the insulation. This precludes the use of parts with insulating sleeves or rough or porous coating finishes. Epoxy coatings, whether printed or powder-impregnated, are generally ideal, and manufacturers can advise on suitability.
In many cases, a resistor can be considered the simplest component of a circuit, and the designer does not need to pay special attention to it other than choosing the right ohmic value and power rating. However, high-voltage circuits often require specialized components from manufacturers that can provide experience and expertise. Designers are advised to prioritize these as key components to define and test in the early stages of the project, and to examine whether a custom or semi-custom approach can add significant value.