Continuous current capability is most frequently referred current rating, 80ARMS and 125ARMS for TMCS112x and TMCS114x, respectively. RMS (Root Mean Square) value is a statistical measure used to represent the equivalent DC value of an AC signal RMS according to thermals. So, 80ARMS and 125ARMS can be applied to DC current. This continuous current specification is determined on Evaluation Module (EVM) board’s safe operating area (SOA) curve at 25℃. The impedance of input leadframe plays an important role in continuous current capability. The typical input conductor resistance for TMCS112X is 0.67mΩ, and for TMCS114X it is 0.27mΩ, which contributes higher current boundary. As shown in Figure 2-1 and Figure 2-2, lower input conductor resistance greatly reduces top case temperature.

Figure 2-1 TMCS112X-50A Input, 1-minute Duration, 68.2°C Top Case

Figure 2-2 TMCS114X-50A Input, 1-minute Duration, 49.7°C Top Case

The real continuous current capability in the system is strongly influenced by the operating ambient temperature, as well as the PCB layout, cable gauge, air flow, heat sink structure and so on. Note that solar system does not usually have as excellent thermal design as the EVM and the operating ambient temperature is usually around 85℃. The continuous current capability of the current sensor can be derated in a real solar system application. As shown in Figure 2-3 and Figure 2-4, TMCS112X device is tested under 25°C and 85°C, there has great difference on top case temperature.

Figure 2-3 25°C Ambient Temperature, 50A Input, 1-minute Duration, 68.2°C Top Case

Figure 2-4 85°C Ambient Temperature, 45A Input, 1-minute Duration, 97°C Top Case

A real case was conducted by using TMCS112x to test MPPT current in a string inverter. The test conditions were 85℃ ambient temperature, 34ARMS continuous current and lasted 1h20mins. 115℃ was detected on TMCS1126 top case while top case temperature kept rising.

The continuous-current capability of in-package Hall-effect current sensor has a strong dependence upon the operating ambient temperature range expected in operation. The maximum continuous current-handling capability of TMCS112x when mounted on the TMCS1126xEVM is approximately 80ARMS. Current capability falls off at higher ambient temperatures because of the reduced thermal transfer from junction-to-ambient and increased power dissipation in the leadframe.

TMCS devices cannot verify electrical performance while operating in over 125℃ free-air temperature. Though by improving the thermal design of an application, the SOA can be extended to higher currents at elevated temperatures, and using larger and heavier copper power planes provides air flow over the board. Adding heat sinking structures to the area of the device can also improve thermal performance. TMCS112x devices with 0.67mohm level input conductor resistance are recommended to be used in string current sampling or MPPT current sampling with single string; TMCS114X devices with 0.27mohm level input conductor resistance could be used in MPPT current sampling with 2 or more strings tied together. See SDAA234 to estimate junction temperature of in package hall sensor.

Take 50kW 3-phase hybrid inverter, shown in Figure 2-5, as an example, there are multiple current sensing places shown in Figure 2-5. See Summary of Solar Application Scenarios Using In-Package Hall-Effect Current Sensors for more details.

Figure 2-5 An 50kW 3-Phase Hybrid Inverter Example

• String Current Sampling (location 1). Common PV panels support continuous current less than 20A. While some PV panels supporting 22A have gradually emerged with technological evolution in recent years.
• MPPT Boost Current Sampling (location 3). Continuous current range depends on how many PV strings are tied together per MPPT. Usually, it’s 1 ~ 8 strings per MPPT based on different power levels and inverter types. For single phase residential or hybrid inverter within 10kW power level, each MPPT has 1 string. For 3-phase residential or hybrid inverter within 50kW power level, each MPPT has 1 or 2 strings. With more than 50kW power level, each MPPT has 2 or 4 strings or even more. For those MPPT followed by over #2 PV strings, the real current flowing through MPPT Boost is usually limited to no more than 40A by software. In the solar inverter field application, full power yield of every string doesn’t happen all the time even during daytime because of light intensity changes, shadow and dust shielding, and so on. So, customers usually add more strings into MPPT as redundancy to verify solar inverters can generate full power most of the time. MPPT Boost has overcurrent protection and over current point is usually set at 1.5 times the rated current. But the real protection point is approximately two times the rated current due to control loop delay. Therefore, the linear current range must be selected to be about 2.5 times the rated current.
• Inverter Phase Current Sampling (location 4). This current rating depends on AC power rating and grid voltage level. Take 50kW 3-phase hybrid inverter as an example. Current rating is calculated as 50kW/380V/sqrt (3) = 76A. Some string inverters and hybrid inverters demand two times the overload capacity for 10s or longer. The linear current range and thermal should be carefully considered.
• Neutral Current Sampling (location 5). For a 3-phase unbalanced hybrid inverter, the neutral current usually does not exceed 20A. This current can also be calculated by software and not directly sampled for lower cost.
• Bi-directional Converter (BDC) Current Sampling (location 7). This current rating depends on power rating, battery voltage and interleaved phase quantity. Take 50kW 3-ph hybrid inverter as an example. Current rating is calculated as 50kw/400V/2ph=62A. Overcurrent point is typically set to approximately 1.5 times the rated current.
• Off-grid Emergency Power Supply (EPS) Current Sampling (location 6). Depending on various EPS function definitions, this current rating is equal or higher than rated AC current. For those higher than rated AC current scenarios, bypass switches current flow capacity plays an important role in current rating.