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Posted on 12 December 2019

Integration of Peripheral Functions in Power Modules

MiniSKiiP IPM with SOI driver IC directly mounted on the DBC

 

 

A few examples of the integration of peripheral functions in power modules are described below. These are arranged in order of degree of integration.

 

 

 

 

Modules with integrated current measurement

Firstly, current measurements in modules are taken to protect the power semiconductors from overcurrent, and secondly, the current signal is also required for current control loops. Rough monitoring is sufficient for the first task, which is why the semiconductor itself can be utilized as a current sensor (VCE(sat) monitoring). The latter must be very accurate (2%…5%), very dynamic (response times ~ 1 μs), and requires a frequency range from DC to some 10 kHz.

Current shunts

Current shunts for taking direct measurements are integrated into the emitter path (-DC) of IGBT modules or placed at the AC output. The three square components in Figure 1 show a solution for current measurement in the emitter path of three low-side IGBTs in a three-phase inverter module. Evaluation must be performed using a differential amplifier in the driver electronics stage. The connecting pads at the sides ensure low-inductance coupling with the main current path.

Current shunts in the emitter path of a MiniSKiiP IGBT module

Figure 1. Current shunts in the emitter path of a MiniSKiiP IGBT module

The problem with the use of shunts is the discrepancy between the low measurable voltage range in a disturbed environment and the losses in the shunt. A 5 mΩ shunt has a voltage drop of just 100 mV at 20 A, but 2 W losses. With such a power loss, the limits for PCB assembly have been reached. Shunts that are integrated in the DBC provide the advantage that the heat loss can be directly dissipated through the heat sink. This extends the usable current range on the PCB from approx. 20 A to approx. 50 A. The disadvantage here is that "precious" DBC space is lost.

Current sensors

For currents above 50 A, electrically insulated transmitters operating according to various principles are used (transformer, Hall effect, magneto-resistive effect). SKiiP, for example, uses compensating transducers (Figure 2), which are charaterised by high precision, a wide frequency range, and a high overload capability. Sensor evaluation is part of the IPM electronics and the protection concept. In the principle presented here, the magnetic field of the main current is measured in an air gap of the transmitter core and, with the aid of an amplifier, a current is impressed on an auxiliary winding which compensates the magnetic field to zero. The compensation current is a direct map of the main current. It is possible to measure DC current and detect the direction of current flow.

Operating principle of the compensation current sensor in SKiiP IGBT modules

Figure 2. Operating principle of the compensation current sensor in SKiiP IGBT modules

Sense IGBT modules

These IGBTs require special chip types for which a measurement current that is proportional to the main current is withdrawn via a small number of separately connected cells. These IGBTs require special chip bonding and signal conditioning which is not available in SEMIKRON modules. Compared to solutions with shunts in the emitter circuit, a much higher measuring resistance may be selected here. In contrast to overcurrent protection provided by VCE monitoring, either shorter dead times are required or none at all. A disadvantage here is the lack of precision as well as the temperature dependency of the measurement method, meaning that it can only be employed for protection purposes.

Sense IGBT

Figure 3. Sense IGBT

Modules with integrated temperature measurement

Modules with a high degree of integration increasingly use simple PTC (Positive Temperature Coefficient) or NTC (Negative Temperature Coefficient) temperature sensors in SMD designs or as chip sensors. The PTC sensor of type SKCS2Typ100 is used in MiniSKiiP, in some SEMITRANS, and SEMITOP modules, as well as in SKiM4/5 and SKiiP2/3. At 25°C, the sensor has a resistance of 1000 Ω and a typical temperature coefficient of 0.76%/K.

R(T) = 1000 \Omega \cdot (1 + A \cdot (T - 25^{\circ}C) + b \cdot (T - 25^{\circ}C)^2 )

Where A = 7.635 · 10 -3 °C -1 and B = 1,731 · 10 -5 °C -2

The measurement tolerance of the sensor in the measured current range 1 mA … 3 mA is max. ± 3% at 25°C, max. ± 2% at 100°C.

Characteristic curve of the PTC temperature sensor

Figure 4. Characteristic curve of the PTC temperature sensor

An NTC sensor is used in SEMiX components, some SEMITOPs, as well as in SKiM63/93 and SKiiP4. The individual product groups use sensors with different characteristics. For details on the characteristic parameters, see the datasheets. For example, the sensor in SEMiX IGBT modules has a resistance of 5 kW at 25°C and a resistance of 493 W at 100 °C. The measurement tolerance of the sensor in the measured current range 1 mA … 3 mA is max. ± 5% at 100°C. Owing to its exponential characteristic, the sensor is more suitable for protection than for temperature measurement.

 R(T) = R_{100} \cdot e^{B_{100/125} \cdot \Big ( \frac{1}{T} - \frac {1}{T_{100}} \Big )}

where R 100 = 0.493 kW (± 5%)
B 100/125 = 3550 K (± 2%)
T 100 = 373.15 K [T -in Kelvin]

NTC sensor characteristic in its relevant temperature range incl. tolerance

Figure 5. NTC sensor characteristic in its relevant temperature range incl. tolerance

In modules, the sensors are insulated and soldered on the DBC ceramic substrate close to the chips. For modules with base plate, the sensors approximately render the base plate temperature; sensors in modules without base plate approximately capture the heat sink temperature. Ideally, the vertical heat flow between the measuring point and the heat sink areas under the hottest chips is negligible. A suitable evaluation circuit provides static overtemperature protection by active driver control or by analogue signal processing. However, there are considerable dynamic time delays, represented for example in the long time constants of thermal impedance Zth(j-r) in the SKiiP3 (shares of t > 200 s for air-cooled systems, > 50 s for water-cooled systems). For this reason, an insulated temperature sensor cannot provide protection from short-time overload.

High quality protection in terms of dynamics is possible with IPM solutions. The protective function can be carried out at the secondary side of the driver and thus at high potential. The sensor can therefore be placed on the same copper pad as the power semiconductors, directly beside the heat source. Measurements are much closer to the chip temperature, although not yet equal. The biggest advantage, however, is that the greatest time constant of Zth(j-r) is now in the range of 1s, thus enabling much better protection from short-time overload. Digital signal transmission in the SKiiP4 nevertheless provides an analogue temperature sensor signal at the driver interface, which meets the requirements of reinforced insulation.

Temperature sensors in power semiconductor modules

Figure 6. Temperature sensors in power semiconductor modules, a) Insulated PTC chip in a MiniSKi- iP (0.7 mm clearance); b) Insulated NTC component in a SEMiX (1.6 mm clearance); c) non-insulated chip sensor on collector potential in SKiiP4 module

With the exception of the SKiiP4 described above, all temperature sensors have a basic insulation, but do not meet the requirements for reinforced insulation. The insulation clearances are between 0.7 mm and 1.6 mm; the required insulation voltage is achieved by filling the modules with silicone gel. Insulation clearances are so small that in the event of a fault, plasma - and thus high electrical potential - might be present at the sensor. For this reason, the insulation of the temperature sensor is regarded basic insulation only, as per EN 50178. Additional circuits are necessary to obtain the safety grade "Safe Electrical Insulation" described in this standard.

Possible voltage fl ashover to the insulated temperature sensor if a bond wire melts

Figure 7. Possible voltage flashover to the insulated temperature sensor if a bond wire melts as a  result of a fault

IPM (Intelligent Power Module)

In addition to IGBTs and freewheeling diodes, IPM modules are able to integrate more components for drivers and protective units (IPM minimal configuration) as well as complete inverter control units. Advantages are their high degree of integration and a higher degree of reliability in comparison to relevant discrete structures thanks to ASIC solutions. A disadvantage for the user consists in the fact that he of she will normally not be able to influence the switching features and logic functions. IPMs are therefore often designed specifically for the individual application (ASIPM = Application Specific IPM). SEMIKRON SKiiP and MiniSKiiP IPM are modules with integrated logic functions in two performance classes.

This driver is a SMD-PCB that is located directly above the power modules. Driving and power supply can be performed on potential of the superordinate control system. The SKiiP driver unit integrates the necessary potential separation, a switched-mode power supply and the driver output stages. SKiiPs feature current sensors in the AC outputs and temperature sensors, as well as DC-link monitoring (SKiiP3 optional). The driver acquires the signals transmitted by the sensors for the purpose of overcurrent, short-circuit, overtemperature, and overvoltage protection, as well as faults resulting from supply undervoltage. An error status signal and standardized analogue voltage signals for the actual AC output current value, the actual sensor temperature, and the DC-link voltage are available on separate potentials at the driver connector for evaluation in the superordinate control circuit. Overcurrent protection is ensured by the current sensors as well as bridge shorting protection by way of VCE monitoring. The logic for the control signals provides short-pulse suppression and interlocking of the control signals for a bridge arm.

Block diagram of SKiiP4 IPM driver functions using digital driver logic

Figure 8. Block diagram of SKiiP4 IPM driver functions using digital driver logic

MiniSKiiP IPMs are equipped with a high-volt driver IC in SOI (Silicon on Isolation) technology. A two-level "level shifter" enables the operation of 1200 V IGBT in addition to the customary 600 V type series. The IGBT may be driven with controller potential without further insulation.

MiniSKiiP IPM with SOI driver IC directly mounted on the DBC

Figure 9. MiniSKiiP IPM with SOI driver IC directly mounted on the DBC

"Down level shifters" also tolerate negative emitter potential up to –50 V, which may be caused by inductive voltage drops during switching. Without this additional feature, IPMs would often fail. Secondary side power supply is provided by means of a bootstrap circuit. Undervoltage monitoring has been integrated for protection. Measurement voltages of external shunts in the DC path can be evaluated by the driver for overcurrent monitoring.

 

For more information, please read:

Intelligent Power Modules Drive Public Transport

Comparing the Incomparable - Understanding and Comparing IGBT Module Datasheets

Successful Selection of Power Modules

Power Electronics Packaging Revolution

 

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