Ohmite EBW Current Sensing

Ohmite EBW Current Sensing

In high current applications such as hybrid power supplies, frequency converters, and automotive electronics, precision current detection resistors are the core components of feedback control and protection circuits. Ohmite's EBW series uses electron beam welding technology to precisely combine different alloys (Manganin or NiCr) with highly conductive copper terminals, achieving extremely low resistance values (starting from 0.2 m Ω), high power density (7 W continuous), and excellent thermal stability. However, engineers often face practical issues such as temperature resistance, pulse capability, welding process, and long-term reliability in selection and application. This article is based on the official technical specifications of the Ohmite EBW series, combined with practical design experience, to systematically sort out the characteristics, application points, and common problem elimination of this series of resistors, helping hardware engineers quickly complete selection and optimize PCB layout.

Overview of EBW Series Core Technologies

1.1 Advantages of Electron Beam Welding

Traditional current detection resistors often use integral alloys or solder connections, which have problems such as high contact resistance, concentrated thermal stress, and high inductance. The EBW series uses electron beam welding to fuse copper terminals with alloy resistors, which has the following advantages:

Extremely low contact resistance: The welding interface has no oxidation or mechanical pressure, and the contact resistance can be ignored.

Low inductance (<3 nH): Suitable for high-frequency switching circuits such as DC-DC converters and SiC/GaN inverters.

High mechanical strength: can withstand reflow soldering (350 ℃ for 30 seconds) or direct soldering on copper conductors.

Long term stability: After 2000 hours of load life testing, the resistance change is ≤± 1%.

1.2 Comparison of Two Alloy Systems

The EBW series offers two alloys, Manganin and NiCr, which are suitable for different TCR requirements and cost considerations.

Model prefix Alloy material Resistance range Power (W) TCR (20-150 ℃) Typical applications

EBWA-M Manganin 0.5, 1 m Ω 5 ± 75 ppm/℃ precision current detection, temperature drift sensitive

EBWA-N NiCr 2, 3, 4 m Ω 5 ± 100 ppm/℃ high resistance, cost optimization

EBWB-M Manganin 0.2, 0.5 m Ω 7 ± 100 ppm/℃ high current (180 A continuous)

EBWB-N NiCr 1, 2, 3 m Ω 7 ± 120 ppm/℃ High power, medium resistance

Key points:

Manganin has a lower TCR, but a lower resistance value (0.2-1 m Ω), making it suitable for applications that require high precision and low temperature drift.

NiCr alloy has a slightly higher TCR (± 100-120 ppm/℃), but can achieve higher resistance values (up to 4 m Ω), and the cost is relatively low.

1.3 Dimensions and Appearance

EBWA：L=10.5 mm，W=5 mm， The thickness varies between 0.32 and 0.88 mm depending on the resistance value.

EBWB：L=15.2 mm，W=7.5 mm， Thickness of 0.3-1.5mm.

The length of the copper terminals is 5.6 mm, which is convenient for welding or bolt fixing.

Key electrical and thermal characteristics

2.1 Power and derating curve

The PDF provides a derating curve chart. The rated power of EBWA and EBWB corresponds to different surface temperatures of resistors:

EBWA: rated power of 5 W at surface temperature below 85 ℃; Linear derating is required when the temperature exceeds 85 ℃, and the power drops to 0 when the temperature reaches 170 ℃.

EBWB: rated power of 7 W below 65 ℃ surface temperature; When the temperature exceeds 65 ℃, the power is reduced to 0 at 170 ℃.

Practical application calculation:

The surface temperature rise of the resistor is Δ T=P × R θ (thermal resistance). Although R θ is not directly given, it can be estimated through typical temperature rise: for example, when EBWB-M 0.2 m Ω passes 180 A, the power P=I ² R=180 ² × 0.0002=6.48 W, which is close to the rated 7 W, and the surface temperature is about 65 ℃. If the ambient temperature is above 65 ℃, it must be downgraded for use.

2.2 Pulse energy capability

The PDF provides pulse energy curves (Pulse Energy/Power) for EBWA and EBWB. This curve presents the relationship between the maximum pulse energy (J) and pulse time (ms) at different resistance values in logarithmic coordinates.

EBWA: The effective area is between 0.5 m Ω (maximum curve) and 4 m Ω (minimum curve). For the intermediate resistance value, the allowed pulse energy is between the two.

EBWB: Similarly, 0.2 m Ω provides the highest pulse tolerance and 2 m Ω provides the lowest.

Engineering interpretation:

The pulse energy capability is mainly determined by the alloy volume and heat dissipation conditions. The lower the resistance, the larger the cross-section of the alloy, and the higher the transient energy it can absorb.

For example, EBWA can withstand about 50 J at a pulse width of 1 ms for 0.5 m Ω, while 4 m Ω can only withstand about 2 J. Therefore, low resistance models should be preferred for high pulse applications such as capacitor pre charging and motor starting.

2.3 Long term stability

According to MIL/industry standard testing, the resistance change of EBW series after the following stress is ≤± 0.2% (load life is ± 1%):

Thermal shock (-65 ℃) ↔ 125 ℃, 25 cycles): ± 0.1%

Short term overload (5 times rated power, 5 seconds): ± 0.2%

Welding heat resistance (350 ℃ for 30 seconds or 250 ℃ for 10 minutes): ± 0.2%

Moisture resistance (10 cycles, high humidity, high low temperature): ± 0.2%

High temperature exposure (140 ℃, 250 hours): ± 0.2%

High frequency vibration (15g, 10-2000 Hz, 36 times): ± 0.2%

Load life (2000 hours, 90 minutes ON/30 minutes OFF): ± 1.0%

These data indicate that the EBW series has extremely high reliability in harsh environments such as automotive electronics and industrial control.

Selection guide: Manganin vs NiCr

3.1 Application scenario analysis

Reasons for recommending alloys based on application requirements

Battery Management System (BMS), with a current accuracy requirement of ± 1% Manganin and low TCR, with minimal drift across the entire temperature range

Motor driver, current detection is used for overcurrent protection NiCr or Manganin. If the ambient temperature changes greatly, Manganin is selected; If cost sensitive and software compensated, choose NiCr

Inverter output current feedback (high frequency) Manganin low inductance (both<3 nH), but Manganin lower thermal EMF

Welding machine/pulse power equipment Manganin low resistance high pulse energy absorption capacity

Automotive 12 V/48 V system, continuous 150 A+EBWB-M 0.2 m Ω extremely low loss, temperature rise controllable

3.2 Principle of Resistance Selection

Maximum Voltage Drop: Typically designed for full range of 50-100 mV to reduce losses. For example, using 0.5 m Ω for a current of 100 A generates a 50 mV voltage drop.

Power consumption: P=I ² R must be less than the rated power (considering derating). If the ambient temperature is 85 ℃, the EBWA power needs to be reduced to about 2.5 W (estimated linearly).

Thermal coupling: In high-density layouts, sufficient spacing should be left between multiple resistors to avoid thermal concentration.

3.3 Typical faults: premature drift or burnout

Phenomenon: Within the nominal current, the resistance value exceeds the tolerance or is open circuited.

troubleshoot

Check if it exceeds the pulse energy curve. Even if the average power does not exceed, transient surges (such as capacitor charging) may damage the resistance.

Measure the surface temperature of the resistor. If the temperature exceeds 170 ℃, the alloy will be permanently damaged.

Confirm the welding process. If the manual welding time is too long and the welding head directly contacts the resistor, it may cause deterioration of the internal welding interface.

Solution:

Choose low resistance models with higher pulse capability.

Increase the area of heat dissipation copper foil or add heat dissipation fins.

Strictly follow the reflow soldering curve (peak value of 245-260 ℃ is common, but EBW can withstand 350 ℃ for 30 seconds, and it is still recommended to follow standard J-STD-020).

PCB layout and soldering points

4.1 Pad Design (Land Pattern)

The PDF provides recommended pad sizes for EBWA and EBWB (see Land Pattern diagram in the document for details). Key parameters:

EBWA: The width of the solder pad is about 5 mm, and the length direction covers the copper terminal (5.6 mm) and extends 0.5-1 mm as a solder corner.

EBWB: Similarly, the pad width is 7.5 mm and the terminal length is 5.6 mm.

It is recommended to use a 4-wire Kelvin connection: the current terminal (large pad) carries the main current, and the voltage sampling terminal (small pad or independent wiring) is connected to the amplification circuit. Due to the fact that EBW resistors are two terminal components, Kelvin connections require shunting from the inside of the solder pad on the PCB, or using separate detection traces to lead out from the edge of the solder pad.

Common error: Directly connecting the two ends of the resistor with a wide copper sheet, causing the voltage sampling point to fall on the current path, resulting in the measured value including the contact resistance voltage drop.

Correct approach: Draw a thin wire from the inside of the resistor pad (near the resistor) to the operational amplifier, and avoid passing through high current paths.

4.2 Welding process

Reflow soldering: The EBW series can withstand 350 ℃ for 30 seconds or 250 ℃ for 10 minutes. The peak temperature of standard lead-free reflow soldering is about 245-260 ℃, far below the limit, so it has good compatibility.

Manual welding: Use a temperature controlled soldering iron (≤ 350 ℃) and control the welding time within 5 seconds to avoid heat transfer to the center of the resistor.

Directly welded to copper busbar: EBW's copper terminals can be welded to thick copper plates or busbars, suitable for ultra-high current (>200 A) scenarios. Suggest using a preheating platform for assistance.

4.3 Fault case: Resistance deviation after welding

Phenomenon: In mass production, some resistors have resistance values exceeding ± 1% of the specification after reflow soldering.

Reason: The PCB has a large thermal capacity or the peak value of the furnace temperature curve is too high (>350 ℃) and the time is too long, which causes the electron beam weld between the alloy and copper terminals to be affected by thermal stress, resulting in a slight increase in resistance.

Countermeasure:

Check the furnace temperature curve to ensure that the actual temperature of the resistor does not exceed 300 ℃ (with margin).

Verify using a temperature measuring board. If the temperature cannot be lowered, the EBW series with higher temperature resistance can be used instead (which is already good, or consider using other thick film resistors).

Pulse capability and surge protection

5.1 Interpretation of Pulse Energy Curve

The vertical axis of the double logarithmic curve in the PDF is Pulse Energy (J), and the horizontal axis is Time (millisecond). The curve provides the maximum allowable pulse energy (single, non repetitive) for different resistance values.

Actual calculation example:

Assuming the peak current surge in the circuit is 500 A, lasting for 2 ms, flowing through the EBWA-M 0.5 m Ω resistor. Pulse energy E=I ² × R × t=500 ² × 0.0005 × 0.002=0.25 J. Query curve: Approximately 30 J is allowed at 2 ms for 0.5 m Ω, which is much greater than 0.25 J and safe.

If it is a 4 m Ω model, under the same 500 A surge, E=500 ² × 0.004 × 0.002=2 J. However, the 4 m Ω curve allows for approximately 2 J at 2 ms, which happens to be at the boundary and requires caution.

5.2 Repetitive pulse

The PDF curve does not specify the ability to repeat pulses, but engineering experience shows that the average power of repeated pulses should not exceed the rated power (after derating), and the energy of each pulse should be less than 50% of the single pulse limit to ensure longevity.

5.3 Common faults: Surge causing resistance explosion

Phenomenon: At the moment of power on, the current detection resistor explodes or has an infinite resistance value.

Reason: The charging of the front-end capacitor generates extremely high peak currents (such as thousands of amperes), and although the time is very short, the energy exceeds the absorption limit of the resistor.

Countermeasure:

Use an oscilloscope in conjunction with a Rogowski coil to measure the actual surge current.

Calculate energy and compare it with the curve. If it exceeds the limit, NTC thermistor or pre charging resistor can be connected in series to limit the surge.

The EBWB 0.2 m Ω model is selected, which has the highest pulse energy capability.

Engineering Application Q&A

Q1: Can the EBW series be used for AC current detection (such as 50/60 Hz)?

A: Okay. Manganin and NiCr alloys are both non-magnetic materials with inductance<3 nH, and have no inductive impedance effect on power frequency and kHz level currents. However, it should be noted that the effective value of AC current corresponds to heating, and the temperature rise should be calculated based on the derating curve.

Q2: How to improve the accuracy of current measurement?

A：

Adopting Kelvin connection.

Select low bias voltage and low temperature drift models for operational amplifiers (such as ± 0.5 μ V/℃).

Pay attention to the resistance TCR: For Manganin (± 75 ppm/℃), the resistance changes by 0.75% at a temperature rise of 100 ℃. If higher accuracy is required, temperature compensation can be done in the software.

Perform single point calibration before use (such as adding 50 A current and recording ADC values).

Q3: Can EBWA and EBWB be used in parallel to expand current?

A: Parallel connection is possible, but due to the dispersion of resistance values, it is recommended to independently connect a small inductor (such as a magnetic bead) in series with each resistor or use a matching resistor. The simplest method is to use a higher power single resistor, EBWB-M 0.2 m Ω already supports 180 A continuous current, which is usually sufficient.

Q4: Is the resistance stable after being stored for many years?

A: The EBW series uses metal alloys with no aging mechanism. However, long-term exposure to high humidity and corrosive gases may cause oxidation of copper terminals, affecting weldability. It is recommended to store in a dry, sulfur free environment.

Alternative and upgrade suggestions

When a certain model of the EBW series is discontinued or difficult to purchase, the following alternative solutions can be considered:

Recommended Precautions for EBW Model Replacement

EBWA-M 0.5 m Ω Vishay PMA2415 or Bourns CSM2F inspection package size and TCR

EBWA-N 2-4 m Ω Isabellenh ü tte BVS or PBV series, pay attention to power and pulse capability

EBWB-M 0.2 m Ω Ohmite's own LVK series (lower power) or Riedon's SSA series ensures continuous current capability>180 A

Re evaluation is required when replacing:

PCB pad compatibility

Pulse energy curve

Thermal resistance difference