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Posted on 01 September 2019

Higher Functionalities in Power Electronic Modules

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Improvement is achieved in thermal design and space requirements

Modern inverter applications and sophisticated control schemes demand a precise current measurement. Different strategies can be utilized, each providing specific advantages and disadvantages. Of special interest is the possibility of higher integration providing high accuracy current measurement with digital output and galvanic separation.

By Dr. Martin Schulz, Technical Marketing Special IGBT-Modules, Dr. Peter Kanschat, Product Development, Xi Zhang, Business Development, Infineon Technologies, Warstein

 

Current Sensing in Power Electronic Modules

Current sensors are an inherent part of modern inverter technology and need to fulfill different demands depending on the application. Besides costs the space needed, the mounting technology utilized and the temperature range along with accuracy demands and response time are factors that influence the decision as to what sensor is to be used.

The classical method to measure currents is using shunts. The voltage drop across the shunt, caused by the current to be determined, is measured. This solution comes with the least expensive sensor but has two major disadvantages. The sensor does not provide galvanic separation and the resistor causes additional losses. However, from new developments in the areas of shunt technology, thermal management and signal processing circuitry it is necessary to reconsider shunts for current measurement in modern inverter designs.

Sensor Technology on Module Level

Integrating current sensors into industrial drives usually is done by mounting the sensor and the necessary processing circuitry to PCB. The thermal situation gets difficult, however, if currents in a range of up to hundred amperes occur. Due to the losses caused by the shunt in addition to heat coming from power electronic devices and from the PCB, temperatures beyond the level tolerable for PCB materials could be reached. Thus it is mandatory to place shunts in an area where the power losses can be dissipated easily. Including the shunts into a power module is an attractive alternative with excellent thermal properties. The results from a thermo graphic camera are visible in Figure 1.

Excellent power dissipation is achieved by mounting shunts into power modules

Despite the temperature swing coming from the losses, the shunt has to stay constant to provide a linear relation between measured voltage and measured current under all thermal circumstances. The linearity properties across the whole range of current and temperature are highly sufficient as can be seen from the graph in Figure 2.

Outstanding linearity is mandatory for highly accurate measurement

Even at higher base plate temperatures, the measurement has to work properly demanding that a temperature swing at the heat sink has no negative effect on the output accuracy. The measurement shown in Figure 3 demonstrates the excellent characteristics of the shunts in use:

Temperature independent shunts are a key component within the measurement

Galvanic Separation

Even with the thermal situation improved, the main disadvantage of the shunt remains its lack of galvanic separation. However, any equipment that connects the shunt to a microcontroller needs to be designed in a way that it separates control and power electronics. One common way to do so is to have the voltage across the shunt digitized using an A/D-converter that has its supply voltage referenced to the high side voltage. The A/D’s digital information is then passed to the control by means of optoelectronic compounds.

As these elements are known to age under high temperature, a further step of optimization is the improvement on the method of transmission. Here, the coreless transformer technology that is already in use in IGBT-Drivers is combined with Sigma-Delta data conversion technology forming an interface to the sensor. The task to be completed by this interface is to read out the electric value, transform it to digital information and transmit this information to the controller level providing galvanic isolation. The schematic for one bridge leg with shunt, and sigma delta converter based on coreless transformer technology looks as shown below.

To support different customer needs, the hierarchically designed MIPAQ™ module family was introduced. Based on standard module technology the MIPAQ™ base features the integrated current sensing shunts while the MIPAQ™ sense also includes the fully digital current measurement as drafted in Figure 4.

Different levels of integration

Sigma/Delta conversion for Current Measurement

Evaluating a measurement system can most efficiently be done by recording a step response. One of the features of the Sigma/Delta or Σ/Δ-conversion is the ability to trade speed for accuracy and vice versa. Utilizing a FPGA to digitally implement the sinc³-decimator necessary, the step response from I=0 to rated current I=INOM as depicted in Figure 5 is observed at 13bit effective resolution:

19µs Step-Response time using a Sinc³-Decimator

To be used as a short circuit detection, response times <<10µs are mandatory. Though this can not be achieved in form of a measurement, the sigma-delta-technology provides a “workaround”. An initial estimate is done in the converter’s decimator parallel to the real measurement. If this estimation detects an over current a special signal is generated allowing the user to determine a proper action. In the given example this signal is available about 2.8µs after the short circuit occurs. The according correlation is shown in Figure 6.

2.8µs to recognize short circuit

The new MIPAQ™ family utilizes new shunts, new interconnection techniques and new signal processing technologies to integrate a highly accurate, cost saving current measurement (Figure 7).

MIPAQ™ base in the well-known Econo3 Package

Improvement is achieved in thermal design and space requirements due to the integration of the sensor into the MIPAQ™ base. The measurement integrated in the MIPAQ™ sense provides accurate data for regulation purpose as well as information to handle over current situations. Galvanic isolation is provided by coreless transformer technology (Figure 8).

Fully digital current measurement using MIPAQ™ sense

Shunt based current measurement reappears to be a very attractive solution for a wide range of power electronic systems.

 

 

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