Resistors for current detection are used in a variety of applications and devices, including motor drive circuits, for overcurrent protection, and to detect remaining battery level. And although they have been widely adopted in the industrial equipment and PC markets, in recent years demand has sharply increased in the automotive sector with the proliferation of hybrid and electric cars – and in the industry as a whole due to the increased computerization and sophistication of vehicle systems.
To meet this increasing need ROHM expanded its family of automotive-grade low-ohmic resistors to include compact, high power types, ultra-low-ohmic models that minimize circuit power consumption, and high precision products featuring superior TCR (Temperature Coefficient of Resistance) suitable for harsh temperature environments. (See Figure 1)
Figure 1. ROHM’s Low-Ohmic Resistor Family
Key Automotive Applications (See Figure 2)
・Motor drive circuits
・DC/DC converter output block
・Battery charge monitoring circuit/remaining battery level detection
Figure 2. Sample Applications and Circuit Diagram
Current Detection Resistor Trend
Low-ohmic resistors used for current detection are typically connected in series with the load, and the potential difference between both terminals measured by an IC to determine the amount of current flow. In order to increase measurement accuracy a higher resistance value (within the low resistance range) is recommended. However, this will lead to greater heat generation and loss, making it essential to find the right balance. On the other hand, in recent years IC performance has improved dramatically, allowing for increased current detection accuracy by measuring even smaller potential differences. This enables lower resistances to be used, capable of detecting even large currents with lower power consumption than existing products. ROHM developed the PSR series of low-ohmic resistors that support large currents over 150A.
Low-Ohmic Resistor Technologies
Low-ohmic surface mount resistors are normally classified by material and structure. The most popular type is based on thick-film technology. Another option is metal strip or substrate type that uses a metallic alloy for the resistive element. Although selecting the type of resistor to use will largely depend on the performance requirements, thick-film resistors are typically offered in the tens of milliohm range, while metal strip and substrate resistors are available in the low resistance range of several milliohms, and are characterized by higher rated power. (See Figure 3)
Figure 3) The distribution map of Low-Ohmic Metal Type and Low-Ohmic Thick Film Type in ROHM line up
For chip resistors, 2 types are offered, based on where the electrodes are mounted – on the short (standard type) or long (wide terminal type) sides. Generally, wide terminal chip resistors provide higher junction reliability and thermal cycling characteristics. Additional advantages include superior heat dissipation properties and superior rated power compared with conventional short terminal products. (See Figure 4)
Figure 4. Characteristics Comparison: Wide Terminal vs. Short Terminal (Conventional) Chip Resistors
・Regarding electrode plating, nickel-tin is normally used. However, copper is an option when stable resistance measurement and improved resistance-temperature characteristics are concerns in the low resistance range.
Low Resistance Based on Thick Film Technology
・The main stage of device creation in thick-film chip resistors is pattern formation through screen printing. Screen printing techniques are used to form the resistive element and electrodes on the alumina substrate. In the case of low resistance models, although a low-ohmic resistive element material is used, a variety of factors must be considered to achieve optimum design, such as the susceptibility of thermal coefficient and temperature cycling characteristics based on electrode material formation and resistive element/electrode thickness vs. conventional products.
ROHM, the pioneer of thick-film chip resistors, developed the industry’s first thick-film chip resistors and offers an expanded lineup utilizing industry-leading technologies cultivated over many years.
・For the automotive market, which requires high junction reliability and rated power, ROHM offers the LTR series of wide terminal chip resistors. The wide terminal design delivers greater junction reliability and superior temperature cycling characteristics. In addition, excellent heat dissipation characteristics result in higher rated power than conventional thick-film resistors (i.e. 1W in the 3216 size).
・ROHM’s standard low-ohmic lineup includes the MCR series of general-purpose thick-film chip resistors, available from 47mΩ (as well as an expanded resistance range from 11mΩ). Also available is the UCR series, which features a longer electrode size and utilizes a different resistive element material for higher rated power and superior temperature characteristics. The UCR series also minimizes resistance variations during mounting by adopting a rear-mount configuration. (See Figure 5) Models are available in the ultra-compact 0603 size (UCR006 series), featuring an expanded temperature range (-55℃ to +155℃) ideal for the automotive market.
Figure 5. UCR Series Rear-Mount Design
・The higher rated power of the LTR and UCR series makes it possible to use smaller-sized resistors, replacing conventional MCR series models, contributing to end-product miniaturization.
Low Resistance Based on Resistive Metal
・Low-ohmic metal strip resistors feature a completely different construction compared to thick-film resistors, with resistive metal thicknesses ranging from the tens of micrometers to a few millimeters. This type of resistor is produced using a variety of process technologies, such as etching and machining, and achieves target resistance values and characteristics based on element formation. As a result, metal strip resistors offer higher rated power and more precise temperature coefficient of resistance than their thick-film counterparts. A relatively thick resistive metal is commonly used for the low-ohmic range from 0.2mΩ to 10mΩ.
・In the 4W and larger class ROHM offers the PSR series, which is available from 0.2mΩ to 3.0mΩ. Proprietary welding technology is utilized to join resistive metal and copper electrodes, resulting in excellent heat dissipation and heat capacity. (See Figure 6) In addition, the series features larger copper electrodes, making it possible to increase the rated power to 5W while maintaining thermal dissipation characteristics. Plus, the optimum resistive metal for each resistance value is selected to ensure excellent accuracy temperature coefficient of resistance.
Figure 6. PSR Series Resistors
・For rated power less than 2W, ROHM offers the PMR series, available from 1mΩ to 10ｍΩ. Although the PMR series is structurally different than the PSR series, both adopt a metallic alloy for the resistive element. In addition, the PMR series does not utilize trimming to adjust the resistance value, preventing heat concentration (hot spots) on the resistive element during usage. (See Figure 7)
Figure 7. Heat Concentration Comparison of Resistors of the Same Size and Resistance Value
The PMR series has been expanded to include the industry’s smallest class of resistors, with further miniaturization planned. ROHM is also broadening its lineup of wide terminal PMR series resistors featuring superior junction reliability and heat dissipation.
In order to support low resistances of more than tens of mΩ in the 1W+ region the resistive metal thickness should be around 100um. However, since the resistive element itself comprises the main body, chip formation is difficult. And although it is possible to join a resistive material to the substrate to form the main body, ROHM has not expanded its lineup to include these types.
In this sector many users have adopted leaded or thick-film resistors, but in recent years the demand for metal strip resistors has increased in response to a variety of requirements, such as increased detection accuracy, board miniaturization, and the reduction in the use of leaded components. In response, ROHM is positioning its lineup of metal strip resistors to meet these expected needs, developing new products with an eye towards greater compactness and higher heat dissipation.
As mentioned previously, with the anticipated increased demand for lower resistances in the automotive sector, ROHM will continue to expand its resistor lineup, focusing on improving rated power and thermal dissipation while further reducing size.