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Posted on 02 July 2019

Optimized IPM for low Power Applications

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In light industries and consumer applications like air condition, inverters optimized for specific applications are necessary. 3 phase motors have to be smaller, more efficient and high reliable compared to the conventional AC motors. In order to achieve these requirements a trade-off between costs and the mentioned improved performances need to be found. The small intelligent power module (Small IPM) by Fuji Electric combines the desired benefits in one package using optimized technologies.

By Daniel Hofmann, Application Engineer, Fuji Electric Europe 

In general an IPM is a power module with extensive functions compared to a regular insulated gate bipolar transistor (IGBT) module. While IGBTs always need a gate driver to operate properly the IPM does not because it’s already integrated. In consequence of the embedded control integrated circuits (IC) driving and protection functions are already implemented. As a basic principle the built-in ICs lead to the best handling of the device.

On the one hand, it drives the IGBT under optimally configured conditions. On the other hand, it protects the transistor from breakdown by over current, short circuit, under voltage supply or temperature. In addition, the wiring length between drive circuit and IGBT is kept short which reduces the internal stray inductance.

Detection of the collector current prevents over current and short circuit failure of each IGBT caused by the load. The protective function against power supply voltage drop is built-in every control IC. To secure the IGBT from overheating the temperature is monitored by an implemented diode at the center of the die area and it even works at high speed heat-up very well which ensures a real time temperature monitoring.

For every protection function of the mentioned failure cases the IPM sends out an alarm signal and initiates the safety shut-down of IGBT by communicating to a driver interface that controls the IPM. Moreover, the failure mode can easily be analyzed by the pulse length of the output signal. There are three cases that can lead to a shutdown of the IPM module shown in figure 1. Starting from the left to the right, wave forms have been recorded for over current, under voltage and over temperature failure cases. The turquois colored signal indicates the alarm output signal. In order to distinguish the failure mode the pulse width is 2ms for an over current case, 4ms for under voltage failure and 8ms if over temperature is detected.

Alarm output for protection functions

This compact intelligent power module called Small IPM is designed for light industries to save energy and improve efficiency. The new package diminishes temperature rising by using a substrate which realizes low thermal resistance [1]. This insulated metal substrate (IMS) is built up of a copper pattern on top of an electrically insulated dielectric layer which is bonded to a metallic substrate. Hence, the thermal management is improved.

To Increase the reliability even more a 600V Field Stop-IGBT (FS IGBT) with trench gate technology is applied. In combination with 600V low loss and high speed freewheeling diode (FWD) a reduction of 25% of the total loss at light load has been achieved when compared to Non-Punch-Through IGBT (NPT IGBT). The light load is the main operation mode in the annual performance factor (APF) which is the major index of energy-saving standards.

The Small IPM consists of six IGBTs and six FWD bridged to a 3 phase inverter (figure 2). Upper IGBTs are driven by three independent high voltage integrated circuits (HVIC) equipped with internal isolation circuit. Above all no external optocoupler is needed due to the galvanic separation inside the HVIC. However, the HVIC needs an own power supply that is given by implemented charge pump circuits and bootstrapping diodes (BSDs). Bootstrapping guarantees that part of the output power is used for the startup of the HVICs.

Block diagram of internal circuit

The low voltage integrated circuit (LVIC) is connected to the lower IGBTs making an independent power supply unneeded. Besides that HVIC and LVIC are equipped with monitoring and output functions so that every single IGBT is protected and an error signal is transferred to outside.

As displayed in figure 3 the cross section of the module as well as the outside view of the Small IPM package is visible. IGBT and FWD are assembled at an IMS covered by resin mold. Control ICs and BSDs are located at an internal lead frame. This separates control ICs and power semiconductors from the assembling area thus accomplishes a better thermal performance. As can be seen in figure 3, the proven Aluminum bond wire technology is used. The advantage of this bond wire conjunction in combination with separation of ICs and semiconductors reflects itself in the temperature of the terminals (or lead frame). Although it is not the latest bonding technology it achieves 10K less terminal temperature compared to current competitor products. Due to this new structure and added functions in a minimized IPM package less additional components at the integrated printed circuit board (PCB) are necessary. In addition, high electromagnetic compatibility because of minimum current loop inside the package has been achieved [3].

Cross-section of Small IPM

In order to achieve a low loss power module the chip technology for IGBT and FWD need to be optimized for such applications. Figure 4 demonstrates the cross section of two IGBT structures which are used in Small IPM modules. The NPT-planar gate structure IGBT contains a p+-layer in combination with the drift region (n--drift) and a horizontal gate structure. Fundamentally the FS trench gate IGBT has the same structure as NPT type but instead of a planar gate this IGBT is designed with a cut into the n--drift layer, so called trench gate. An added n+-doped region where more negative charge carriers are implemented accomplishes the field-stop behavior. Thereby the die itself is optimized and switching characteristics are improved.

NPT-planar gate IGBT compared to trench gate IGBT

Figure 5 shows the forward I-V characteristics. The graph describes the relationship between a typical Collector-Emitter voltage in saturation and the corresponding collector current. Moreover, conduction power loss of FS trench gate and NPT planar gate IGBT can be calculated by multiplying On-state voltage and collector current at different temperatures. Black lines indicate the FS IGBT while red emphasizes the NPT IGBT. The squares refer to 25°C, circles to 100°C and triangles to 125°C. Focusing at a collector current of 1 A/mm2 at 25°C a decrease of approximately 0,3V in the voltage drop is realized which means decreased conduction loss. This reduction is desirable due to the fact that the light load condition takes place at 1 A/mm2.

On-state voltage drop temperature dependency of NPT and FS IGBTs

Since the IGBT On-state behavior has been demonstrated the FWD is required to have low VF (forward voltage drop) and fast switching characteristics to reduce the total loss even more. Consequently on the right hand side of figure 5 the VF is shown for room temperature and high temperatures, showing that the temperature dependency impacts barely the forward voltage drop.

Turn-off energy loss vs collector current of NPT and FS IGBTs

Regarding to the switching behavior of the Small IPM, switching tests have been performed under the following condition:

VDC = 300V
I = 5A
L = 2mH
VGE = 0/+15V

At the rated current of 15A the turn-off energy loss is about 36% less than the conventional NPT type (figure 6). Furthermore, at light load condition which is below 3A the energy saving performance increases to 50%. According to these results the FS trench gate IGBT structure can achieve turn-off characteristic superior to NPT IGBT.

The corresponding turn-off switching wave forms are indicated in figure 7. In order to turn-off the IGBT the gate voltage (red) is set from +15V to 0V. Hence, the collector-emitter voltage (black) rises while the collector current (green) decreases. The spike voltage at rising blocking voltage is small in both cases. The conventional IPM has got a relatively large tail current which is responsible for larger turnoff loss. Instead the latest IPM chip instead has the advantage of a short tail current which contributes to the lower total loss of the module package.

The advantage of a better turn-off characteristic is clearly visible while taking a look at the total loss (figure 8). Here one has to distinguish between three conditions: light load, rated load and maximum load where the diode loss is almost constant for all cases.

Turn-off comparison of the latest IPM and a conventional IPM

In the former case of light load with a rated current of roughly 2A the saturation loss (Psat) are similar to each other. Turn-on loss is improved by the new IPM die structure and the biggest amount of saving loss occurs during the turn-off so that 33% of total energy can be saved.

Going to higher currents like the rated current the total loss reach almost the same level where 12% of energy improvement is achieved due to a bigger contribution of turn-off loss of the Small IPM.

At a maximum load of 8A the turn-off loss of Small IPMs is still roughly 50% less than the conventional IPM but the diode loss intervene and lead to a total loss improvement of 13%.

Loss comparison for light load (2A), rated load (5A) and maximum load (8A)

Summary

By applying new and known technologies to the Small IPM package the efficiency raised. It has been proven that the total energy loss is reduced which leads to higher energy saving in light industry applications. The 3 phase inverter with individual HVICs for upper arm IGBTs guarantee a galvanic isolation making an external optocoupler unnecessary. Thus the Small IPM module accomplishes an improvement of energy saving of the inverter system.

References:
[1] T. Yamada et al. “Latest Small Intelligent Power Module For Energy-Saving”, PCIM Europe, May 2012, Paper 183.
[2] T. Yamada et al. “Novel Small Intelligent Power Module For RAC”, PCIM Asia, June 2012.

 

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