10 Tips for High Voltage Resistor Design
1. Understand the rated voltage
The primary voltage rating of a resistor is its limiting element voltage (LEV), sometimes called the operating voltage. This is the continuous voltage that can be applied across a resistor whose ohmic value is greater than or equal to the critical resistance.Below this value, the voltage is limited by the rated power
(Pr)to2√Pr?R limit. Typically, it is DC or AC rms, but the data sheet of the high voltage component may define it as DC or AC peak. Related to this is the overload voltage rating, typically 2 or 2.5 times LEV for 2 to 5 seconds. Typically, higher peak voltages can be tolerated 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 continuous voltage that can be applied between a resistor and a conductor in contact with its insulation.
2. Discrete resistor voltage divider
Figure 1. Voltage divider.Image courtesy of Hakata Electric Power Systems
Voltage division requires a high-value resistor R 1 and a low-value resistor R 2 in series, as shown in Figure 1.
The voltage ratio is given by V input V output = (R1 + R2) / R2 = R1 / R2 + 1
Note that the voltage ratio is not the same as the resistance ratio R1/R2, but is offset by one. For example, to obtain a voltage ratio of 1000, you need to define a resistance ratio of 999. For discrete resistor designs, choose standard values, some examples of ten-fold voltage ratios are listed in Table 1.
Table 1. Ten-fold voltage ratio using standard resistor values
Target voltage ratio R 1 / R 2R 1 (E12) R 2 (E24 or E96) Actual voltage ratio nominal error 10982K9K110.01+0.1%
After selecting a nominal value, the next consideration is the required tolerance. The tolerance of the resistor ratio is simply the sum of the tolerances of the individual resistors. These are not necessarily the same; often it is economical to choose tighter tolerances on low-voltage components. For example, a high voltage R of 1% and a low voltage R of 0.1% results in a resistance ratio tolerance of 1.1%. For voltage ratios over 50:1, the tolerance for the voltage ratio is effectively the same as the tolerance for the resistance ratio.
3. Specify integrated voltage divider
A high-voltage voltage divider can be used that integrates R1 and R2 into a three-terminal assembly, as shown in TT Electronics' HVD series (Figure 2). This approach has many accuracy advantages. For example, the target voltage ratio can be defined without being restricted by the selection of standard values.
The values specified for integrated crossovers are usually the low value R2 and the total value R1 + R2. In addition, the tolerance of the voltage ratio can be controlled directly through the trimming process and therefore can be tighter than the tolerance of the resistor value. For example, R1 and R2 can be defined with a 2% tolerance, but the voltage ratio can be adjusted to a 0.5% tolerance. Similar advantages apply to the temperature coefficient of resistance (TCR), tracking TCR determines the temperature stability of the voltage ratio, which may be lower than the TCR of the resistor element. Additionally, voltage dividers can be designed to extend this matching element into the lifetime drift and voltage coefficient of resistance (VCR) domains, although this typically requires custom design.
4. Evaluate TCR and VCR errors in dividers
If the R1 value is high enough and the voltage is low enough, the degree of self-heating in the voltage divider will be lower. If this is the case, it is relatively easy to measure the TCR and VCR effects separately. The TCR effect is calculated using a temperature chamber, and the resulting quality factor is defined as voltage ratio temperature coefficient =
In ppm/°C, where VRht and VRlt are the voltage ratios at high and low temperatures, and HT and LT are the high and low temperatures.
The corresponding figure of merit for the VCR effect is similarly defined as the voltage coefficient of the voltage ratio =
the unit is ppm/° V where VRhv and VRlv are the voltage ratios of high voltage and low voltage, HV and LV are high voltage and low voltage.
If self-heating is not negligible, the chamber temperature should be adjusted to give the correct HT value during TCR testing and time should be allocated for the temperature to stabilize. The duration of the VCR test should be short to minimize temperature rise. Alternatively, a temperature chamber can be used to measure low voltages at higher temperatures and vice versa, thus canceling out temperature-related resistance changes.
5. Calculate the value of the bleeder resistor
A bleeder resistor is used to discharge the capacitor to a safe voltage level after a power outage. The bleeder resistor can be connected across the capacitor for fast discharge without static dissipation, or it can be connected for high reliability and low cost. In the latter case, there is a trade-off between time to reach safe discharge and static power loss. Choose a suitable ohm value by exponential discharge calculation:
where Td is the discharge time, C is the capacitance value assuming positive tolerance, Vt is the safety threshold voltage, and Vo is the initial voltage. Taking tolerances into account, standard values lower than R max should be used.
For a selected value of R, the initial power is given by P o = V o 2 /R. For a switching bleeder, this is peak power. With a connected bleeder it is a continuous dissipation and the resistor chosen must be rated accordingly.
6. Select the appropriate balancing resistor
All aluminum electrolytic capacitors experience leakage current when a DC voltage is connected to them. This can be modeled by a leakage resistance in parallel with the capacitor. This resistor is nonlinear, that is, its value is a function of the applied voltage. In this case, the value is poorly defined, with a large degree of variation from one capacitor to another. When building a capacitive accumulator for a high-voltage DC bus, it may be necessary to use a series combination of two capacitors, each rated for half the bus voltage. If the capacitors are the same, the bus voltage will be divided equally between them. In practice, however, the leakage resistance will vary, resulting in uneven sharing of capacitors with higher leakage resistance and possible voltage overloading.
Figure 3. TT Electronics' WPYP series is designed to be mounted directly on a capacitor.Image courtesy of Hakata Electric Power Systems
The solution is to use balancing resistors in parallel with each capacitor (as shown in Figure 3). These are high value resistors, rated for the appropriate voltage, and matched in value to within a few percent. This value needs to be as high as possible to minimize power dissipation, but is usually chosen so that it does not exceed 10% of the leakage resistance value at the capacitor's rated voltage. In this way, the effect of the unbalanced internal capacitor leakage resistance is swamped by the effect of the balancing resistance, and the voltages are approximately equalized.
7. Resistant to high voltage surges
Designers sometimes do this when considering high voltage resistors because their circuits must withstand high voltage transients. If continuous voltage stress does not require high voltage ratings, then low voltage but surge tolerant components are likely the solution. For example, TT Electronics' 5W wirewound high surge resistor WH5S does not have a high voltage rating but can withstand 1.2/50μs peaks up to 10kV, while the surge tolerant 2512 chip resistor HDSC2512 has an LEV of 500V but can withstand up to 7kV the peak voltage.
8. Design meets 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 will not only affect PCB layout design, but in some cases component selection. When a resistor is connected to a high voltage level, it is important to check the distance between its terminals and, in the case of a heat sink mounted component, the distance between the resistor and the metal thermal interface. This can be defined in two ways. First, creepage distance is a short distance across an insulating surface. This reduces the likelihood that in wet and contaminated conditions the surface will flash with high enough energy to be tracked. Second, gaps are short distances in the air. This solves the risk of flashover.
Another piece of information that may be needed is the material forming the insulating surface, as this determines the relative tracking index (CTI), which classifies the tendency of organic materials to support processes that lead to leakage. For example, if a resistor bridges an isolation barrier in a design to provide a galvanic connection to prevent excessive electrostatic charge build-up, the IEC 60065 safety standard requires that the resistor be able to withstand specified high-voltage surge testing. Since this is becoming the legacy standard, the ongoing resistor is no longer relevant. Nonetheless, designers who follow the hazard-based safety engineering approach of IEC 62368-1 will be helped by knowing that there are still products that meet the requirements of IEC 60065.
Figure 4. TT Electronics' T44TUH is suitable for oil immersion, doubling its LEV to 28kV.Image courtesy of Hakata Electric Power Systems [PDF]
9. Optimize PCB layout
PCB layout is critical to maintaining the safety of high voltage designs, especially in the case of high voltage resistor miniaturization and surface mount device (SMD) form. A good example is TT Electronics' HVC range, which includes 2512 size chip resistors rated at 3kV. Traces or vias under or in close proximity to components should be avoided, as well as any features that may trap or promote ionic contamination during manufacturing or use. One special measure that can be used to increase creepage distances and avoid trapped contamination is to cut a slot in the PCB underneath the component.
10. Design of Potted and Oil-Filled Components
Two limiting factors in high-voltage designs can be the tendency of contaminated organic surfaces to support tracking and the risk of airborne discharges, especially around small-radius surfaces. Both limitations can be overcome by potting or immersing in mineral oil, which prevents the ingress of contaminants and replaces the air with a substance with higher dielectric strength. This in turn reduces creepage distances and clearance limits, thereby reducing component size. When selecting resistors for such components, it is important to choose insulating components that avoid the risk of outgassing. Any air bound to components can create voids where partial discharges can occur, leading to long-term degradation of the insulation material. This precludes the use of components with insulating sleeves or with rough or porous coating finishes. Epoxy coatings (printed or powder dip) are often ideal and manufacturers can advise on suitability.
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