Higher accuracy of current sensing

To achieve power management performance of portal devices, high precision measurement of current is required. To measure the circuit current in general, first you insert current sense resistors in serial into a circuit then measure each voltage of both ends of the resistor. Like smartphones, however, when the current gap happens to be more than a hundred times between standby and operating APs, the current in both cases should be measured precisely and the current sense resistors required to be high precision of low T.C.R. , less affected by temperature change.

KOA’s thick film type low resistance resistors UR73, UR73V, SR73, WK73S and WU73 , offer a wide range of resistance values from 10mΩ to 10Ω, some of them offer T.C.R. of ±75 x10^-6/K. DC-DC converters for CPU power supply in laptops must have extremely high switching speed since the output stability that can follow the load changes on CPU is required. Accordingly, the parasitic inductance of electronic components used in the control circuits of the DC-DC converters makes a big influence. Therefore, the current detection resistor must have a low parasitic inductance.

KOA’s metal plate chip type low resistance resistors TLR series have extremely small parasitic inductance and enables high precision current sensing.

Reliability

Car electronics devices require high reliability as they are used in severe environment. Especially around the engine, they must function for long periods of time, being exposed to high temperature, humidity, strong vibration, dust and chemical substances. Therefore, resistors to be used in this application require excellent resistance to heat, moisture and sulfuration, as well as the high bonding reliability to PWBs that can withstand the continuous thermal stress and strong vibration.

Further, since the amplification factor is determined by the resistance ratio of the two resistors in an amplifier circuit for current detection or a sensor, it is important that the resistance ratio does not vary. In such a circuit, stable characteristics can be maintained for a long time by using thin film chip resistors RN73H and RN73R with high accuracy and excellent long-term reliability. Also, thick film type highly reliable resistors RS73 that meets the requirement for electrolytic corrosion resistance is now available. Its long-term stability is slightly inferior to that of metal film types, but is significantly improved compared to conventional thick film types.

High voltage resistant

All the power supplies for multifunction devices and X-ray analyzers use high voltage, thus the electronic components used in the drive circuit must have the high maximum working voltage. In addition, power supply circuits of EVs and HEVs also use high voltage, requiring the high maximum working voltage. High voltage type resistors,HV73, HV73V, RCR and P series series are suitable for above applications.

When measuring the high voltage, the voltage is divided by using a plurality of resistors. For that application, KOA has developed the high voltage dividers, HVD in which the resistors are integrated into 1 package. Since these products are designed based on the resistance ratio, it is possible to measure the voltage with high accuracy. And the similar T.C.R.s of each the resistor minimize the fluctuation of voltage division ratio even if temperature changes.

Issues to be solved of resistor

Mounting reliability of 008004 (inch code) resistors

In order to mount a very small component with high accuracy, not only the board design, high-precision solder paste printing technology and component mounting technology, but also the electrode structure and dimensional accuracy of the component are required. Further, the accuracy of the carrier tape also greatly affects the mountability.

High voltage resistance and high reliability

In order to realize higher voltage resistance and reliability, resistive material itself needs further development and improvement.

Technical Trend

The downsizing of resistors is approaching its limit, and the modularization is expected to spread in all types of equipment in the future. Modularization improves signal quality and enables high-speed transmission and processing.

Furthermore, since the wireless communication standard 5G that was introduced in 2019 enabled the high-speed communication, circuits that previously worked only with electrical signals now can work with optical signals. Thereafter, the development of modules with optical components have been boosted.

In the automotive field, modularization is rapidly advancing in order to secure the space for the installation of ADAS (Advanced driver-assistance systems), sensing systems for self-driving cars, image processing systems, and control systems.

The demand for higher density mounting increases the demand for higher power with the same conventional size. From that reason, the resistors that have been derated with a large margin are now used with tight derating in more cases.

In order to cope with this situation, it is necessary to reconsider the power rating, mounting, and land pattern recommendation, while improving the resistor itself. KOA has been promoting the use of terminal part temperature over ambient temperature for the derating curve of SMD resistors as a result of the reconsideration.