Advantages of using IPM design and matters needing attention in selection

Advantages of using IPM design and matters needing attention in selection

Norman Day

brief introduction

Because the power circuit structure driven by single or three-phase motor has been quite mature and stable, the power module integrating the power switch and driving circuit of this part has had a revolutionary impact on the concept of frequency converter system design after its introduction. With the maturity of module packaging technology and the rapid decline of cost,There is a trend to gradually replace traditional components and become the mainstream of system design.This integrated power module has a name with high liquidity, which is called integrated / intelligent power module, which is abbreviated as IPM [1].Unfortunately, most designers still regard such components as a black box. They either abandon them for fear of being unable to grasp the problems that may arise from them, or they can only accept the influence of some leading manufacturers and form some specious concepts.However, neither the former nor the latter believe that designers should hold an attitude towards the trend of power integrated packaging. Only by selecting the design concept with competitive advantage and fully mastering the characteristics and limitations of the components used can we ensure that they will not be eliminated by the rapidly changing market.

Advantages of using IPM design

In terms of the cost of a single component, IPM is indeed difficult to compete with the already standardized and mass-produced separated packaging elements.However, only considering the overall product design with a single material cost is not the vision that a designer should have, so the discussion level of this topic must be expanded. Here, the author divides it into three levels: performance, reliability and price to explore the differences caused by using IPM as system design.

Performance:

(1) Greatly reduce the number of components and the area required by PCB

(2) Provide solutions with high insulation and good heat dissipation performance

(3) Greatly reduce the complexity of line layout

(4) Reduce the stray inductance effect of power crystal connection and driving circuit

(5) Internally integrated crystals have similar electrical properties

(6) It can respond to all kinds of abnormal protection in real time

The benefits of the first item are obvious.Figure (1) provides a quantitative data for reference.The second advantage comes from the differences provided by different processes.When using the traditional separated element as the design, because the source or collector of the internal power crystal is directly connected with the bare metal shell, in order to achieve the design of high insulation and easy heat conduction, in addition to selecting the insulating gasket with high unit price, it also makes the production operation complicated, the abnormality of the assembled finished product is difficult to control, and it is very expensive

Assembly time.IPM breaks the thinking that traditional design can only turn around in the above problems, and provides a real solution with high insulation and good heat dissipation performance.The diagram in figure (2) presents the above conclusion more specifically.

Figure (3) shows the circuit design diagram using traditional components, and figure (4) shows the circuit design diagram using IPM.Both achieve the same function, but the circuit obtained by using IPM design system is relatively simple.In fact, the circuit diagram in figure (IV) above mainly highlights the simplified comparison between the power circuit and the driving circuit. If the further simplified results of the circuit using the auxiliary power supply and current sampling corresponding to IPM are also presented in figure (IV), the difference of circuit simplification will be greater.

In the design criteria of traditional components, reducing the leakage inductance of the connection between lines and shortening the loop between IGBT and driving IC as far as possible to obtain lower switching stress and line interference have always been the direction of system designers.However, taking the schematic diagram in figure (5) as an example, the connection of IGBT collector and emitter has stray inductance effects of different sizes. To reduce these effects, the path between traditional packages must be short and thick. Therefore, it is necessary to use multi-layer PCB or increase PCB area to meet such requirements,IPM can solve these stray effects well in the package level.The reason is that no matter how close the drive IC is to the power switch (IGBT or power MOSFET) by using the layout of PCB, it will not be closer than directly placing it next to the power switch in the form of bare crystal.Similarly, if each power switch is connected directly on the wire frame by wire bonding, it will be much smaller than the connection through the pin of the traditional component itself and then through the copper foil on the PCB.

The feature mentioned in the fifth item is that IPM can directly solve the problem of abnormal assembly control that puzzles the system designer in the wafer level.

Japanese manufacturers usually adopt a strict attitude towards system assembly. The practice is to measure the characteristics of each power crystal before on-line assembly. During production and assembly, components with similar characteristics must be assembled on the same PCB to reduce potential problems caused by the deviation of component parameters in the system during mass production.For example, if the turn-off delay and turn-off time of the upper arm are in line with the specification, but close to the upper limit of the specification, while the turn-on characteristics of the upper arm are just the opposite, if the nonlinear effect caused by the rise of the operating temperature is added, the possibility that the upper and lower arms are connected at the same time during switching will greatly increase, resulting in additional power consumption.Moreover, the original design basis of the heat sink was based on the assumption that the calorific value of each crystal was equivalent, but if the above assumption also occurred on the parameter V (CE), the uneven heat distribution may further cause thermal run away and lead to system failure.But in this case, because of the matching exception

The potential problems caused are either ignored by the designers or denied by the management due to the impact on benefits and costs.Even if there is such a concept, the designer can only use a higher design margin to overcome it, such as increasing the dead time and increasing the area of the heat sink to obtain a lower operating temperature, etc., but the relatively necessary payment is to reduce the performance of the system and increase the material cost.In fact, after each wafer is tested, there would have been a characteristic distribution diagram of each bare crystal, but the information of this distribution characteristic will disappear as each bare crystal on the wafer is packaged in the form of to220 or to247, and so will the high-voltage IC used to drive the power crystal.The manufacturing process of IPM starts from the whole wafer, so it can ensure the symmetry and matching of three-phase crystal characteristics in the same module in the die bonding stage by using the method that the characteristics of adjacent bare crystals are the closest.Using different process thinking, we can easily solve this difficult problem in traditional design.

The result of the sixth item comes from the improvement of the fourth item. The timeliness of protection comes from the difference of us or even ns level. Avoiding system malfunction and accelerating the timeliness of protection are often a dilemma for system designers.Therefore, the reduction of leakage inductance can not only reduce the transmission delay of the abnormal signal itself, but also reduce the constant of the filter line, so as to improve the response speed of the IC to the abnormal signal. In this way, it can also reduce the failure rate caused by the failure of the abnormal protection signal in time.

Reliability

(1) Significantly reduce the potential failure of production personnel caused by complex assembly process

(2) Provide a more robust structure than traditional packaging

(3) The whole system will have a lower failure rate

The improvement of the first item is very significant.The assembly method of traditional components is not only complex, but also repeated many times, so some alignment abnormalities, missing lock of isolation nut, dark crack of internal crystal, damage of insulating sheetIt's hard to prevent problems.In addition, these potential problems may not be effectively detected. Therefore, if the semiconductor components must be connected in parallel due to the rated relationship, the number of crystals may change from six to twelve or more, and the probability of potential failure caused by assembly is greater.Figure (6) shows the above explanation more clearly by illustration.

Generally speaking, the vibration stress of distributed components is easy to extend to the internal bare crystal through the pin no matter when locking the screw or folding the foot or even when the finished product is transported.The mechanical stress borne by bare crystals mostly comes from the strain caused by thermal changes in the internal chip or operating environment of the package.Therefore, whether directly in the assembly stage or indirectly due to the strain caused by the stress caused by thermal shock, IPM provides a more robust structural solution than the original separated components.

The third argument is based on the fact that if the failure rate of IPM is equivalent to that of traditional components, it is necessary to use 20 or 30 components to achieve equivalent functional mode. The possible failure rate of the whole system is naturally much higher than that of only one component.However, whether the basis of this merit is tenable or not involves a wide range of aspects. Perhaps a special topic can be set up for discussion in the future.

Overall cost

(1) Reduced quality cost due to increased reliability

(2) Greatly shorten the time of product development for designers

(3) Reduce the cost of heat sink drilling and PCB board

(4) Shorten the working hours for assembly and inspection of production personnel

It is not difficult to predict the above advantages, but the quantitative results must be further actuarial based on the development procedures and quality costs of each company. The author believes that although the results may not support you to abandon the traditional mature scheme and choose IPM, such an action is absolutely helpful to the thinking of scheme selection and design system.

Precautions for selecting IPM

Although there are many advantages in using IPM as the design, IPM is still not as easy to master as the traditional separated components in terms of the maturity of market verification and the complexity of the components themselves, so the IPM module is selected

The design of blocks should still be very careful. The following discussion can provide some references for designers.

Supply chain oriented considerations

(1) Ability of suppliers and manufacturers to control process exceptions

(2) Is there an alternative solution when the supplier is out of stock

(3) Supplier technical support and overall supply chain quality assurance mechanism for customers

(4) Improvement and control management of market application feedback

Consideration of module design

(1) Packaging structure

(2) Whether the layout of internal components is reasonable

(3) Whether the design of peripheral matching circuit is easy to master

(4) Strength of driving IC and power semiconducting crystal

Due to space limitations, the following only discusses the key points of the relevant parts of the module design.

Packaging structure

Characteristics of good power package design

A good power package design should have the characteristics of high structural strength, simple manufacturing process, high insulation, easy heat conduction and low thermal resistance.

Whether the strength of the structure is high or not determines whether the joint surface of the structure inside the module and the material system are prone to defects and failure under the conditions of rapid thermal change and long-term mechanical vibration.

The simple process shows that the process has a good control over the abnormality, and the potential defects in the process can be easily detected.

The requirement of easy heat conduction is that when the semiconductor element instantly generates high power consumption (such as short circuit or abnormal switching), the heat can be conducted instantly, so that the semiconductor element will not cause heat spot effect, resulting in instantaneous burning.

The purpose of low thermal resistance is to export the heat after the heating body reaches the steady state of thermal balance, so as not to cause heat accumulation and early failure of components.

Differences of various power packaging structures

Figure 7 Figure 8 figure 9

Figure (7), figure (8) and figure (9) are representative of several typical IPM packaging structures on the market.Next, we use the above mentioned conclusions to verify the advantages and disadvantages of these three structures.

The structure of figure (7) is that the driving IC and power semiconductor are placed on the lead frame on the same plane, the ceramic substrate is directly used as the material for insulation and heat conduction to the heat sink, and then the whole structure is coated with the molding compound similar to the packaging separated element.This packaging structure can be called simple and high strength, but several parts that need to be paid attention to are described below.

The first is that although the ceramic substrate is a highly insulating material, it is not a material easy to conduct heat, and the effect on dispersing instantaneous hot spots will be relatively poor. Therefore, whether the conductor frame carrying power semiconductor can meet the requirement of instantaneous heat conduction without forming hot spots is a problem that needs special attention.At the same time, the thermal resistance of ceramic substrate with the same thickness is much higher than that of aluminum, let alone copper. Therefore, the same heat sink temperature means that the temperature of power crystal inside the module is higher than that of module using aluminum or copper.In short, the scope of safe operation of the module will be relatively reduced.The only way for the designer to fully believe that the specification is not measured by the designer.

The second is the materials and technology used for the bonding surface of ceramic substrate, because it is related to whether the long-term use will cause delamination, which will cause the temperature of semiconductor to be unable to be exported normally and then burned.The designer can ask the supplier for the test conditions in this regard, and then contact itself

The conductor frame forms two different planes. The purpose of lowering the conductor frame is to make the plane of the power bearing crystal very close to the surface of the IPM colloid in contact with the heat sink. This structural design makes the conductor frame can be directly molded and encapsulated without other matching materials and derivative processes after the crystal planting of the conductor frame is completed.

In addition to the insulation thickness, it is also the most economical structure.It is suggested to match the heat sink of the system, the maximum possible operating temperature and the operating voltage of the actual system to find out the limited range of the module.

Whether the layout of internal components is reasonable

Check whether the internal component layout is reasonable, including whether the heat source (most of which are power crystals) is arranged on the conductor frame to achieve uniform heat distribution, whether the driving delay of three-phase power crystals is consistent and whether the current circuit of power crystals of upper and lower arms is symmetrical.

It is worth mentioning that designers can often see that IPM recommends in the specification (as shown in Table 1) that the voltage of the system DC link should not exceed 450V and the switching voltage should not exceed 500V.This is because there is equivalent leakage inductance formed by wiring and conductor frame in IPM,

The voltage drop may be much greater than the voltage measured on the IPM pin. In order to ensure that the voltage drop across the internal power crystal does not exceed the rated 600V, such a limit range is set.

But in fact, it is because the leakage inductance of conductor frame and wiring is about 10-20nh, and the current change rate of IPM switch rarely exceeds 400A / us (generally between 200A / us-300a / US).In this way, the voltage difference between the pin voltage and the internal IGBT due to voltage mutation should be less than 10V, and the actual measurement results are the same.In addition, almost all IGBTs with 600V rating have a margin of more than 100V. In short, it is unlikely that the IGBT end voltage inside the IPM will exceed its limit range and then collapse due to switching.It should be noted that in order to further reserve the area where the power consumption of IPM during switching can not exceed the safe operation, because the possibility of exceeding the power limit is much greater than that of exceeding the voltage limit.However, it can be expected that the higher the design margin is, the lower the failure rate of the field application will be. When it is difficult to estimate the instantaneous current, it is a necessary design criterion to lower the voltage across p-n.

In fact, the relationship between the definition and measurement of the safe operation area of power crystal and its filed application and failure rate is not only a proprietary topic of IPM, but also a deep and wide topic in mature separated packaging. Maybe another special article can be discussed next time.

Is it easy to master the design of peripheral matching lines

Most IPM peripheral matching circuit designs have little difference. Roughly, it is completed as long as the three floating power supplies and short-circuit protection resistors of the upper bridge arm are placed with the correct components according to the reference design, as shown in figure (11), and then the driving signals of the six bridge arms are hung.However, whether each module is really as easy to design as the reference manual says is not necessarily true.In fact, the judgment basis of this part must be determined by the semiconductor components selected by the module and the conditions of system application. In particular, it should be noted that it is necessary to prevent the problems that may be derived from the special drive IC selected by IPM. The discussion of this part is described in detail in reference [2].

In addition, it is worth mentioning that whether the driving logic is positive logic driving or negative logic driving is more reliable.The author met a quite senior design director, who believed in the statement of Japanese leading manufacturers and believed that positive logic drive would be far more reliable than negative logic drive.In fact, negative logic has high robustness

When the power supply of IPM is abnormal, the switch can be safely turned off. The two driving modes have their own advantages and disadvantages. It is impossible to say which is more reliable.

Referring to figure (XII), when the noise level exceeds the driving level recognized by IPM, the mechanism of negative logic is to turn off IGBT, but the positive logic is to turn on IGBT.Generally speaking, the switching states of the upper and lower arms are mostly complementary. Therefore, for the IPM driven by negative logic, turning off the IGBT that should have been turned on for several us at most only reduces the voltage utilization, but for the IPM driven by positive logic, there is the danger of conducting up and down at the same time, resulting in an arm shot through.Furthermore, for the design of direct drive, although the negative logic drive needs to be equipped with pull up resistor, the fan out capability of general MCU is usually weaker than the sink capability, which is about 1 / 5 ~ 1 / 10.Therefore, the risk of positive logic driving will occur when the output port of MCU requires high output current, and the driving level may be triggered

However, when the negative logic drive outputs the high level, the drive port is high impedance, and the drive current is provided by the power supply of the pull-up resistor, so there will be no problem of positive logic drive.

High reliability of driving IC and power semiconductor

Although IPM provides many advantages that traditional components cannot have by changing the packaging type, it should be noted that it does not change the function and characteristics of semiconductor essence.Therefore, if the selected driving IC and power semiconducting crystal have application limitations and defects, IPM

These limitations and defects are bound to exist.For example: if the IPM of manufacturer a is selected

The power switch of is IGBT with non penetration (NPT) technology, while the IGBT with penetration (PT) technology selected by manufacturer B, the characteristics of the two IPMS will be different.The recently popular trench technology is also a derivative technology of penetration type. Although it greatly improves the disadvantage of slow switching speed of traditional penetration type IGBT, it also has the characteristics of weak short-circuit current bearing capacity and sensitive parameters to temperature changes.The driving IC used is the same. Therefore, if we can fully grasp the limited range of the semiconductor selected for IPM for temperature change, di / DT, DV / dt and instantaneous negative pressure, so as to select the IPM suitable for the requirements of system application, then the design has been more than half successful.

As far as the author's own experience is concerned, the mixed environmental verification can easily detect the defects of system design because it has the stress combination of temperature, vibration, voltage and current, but it will cause great trouble to analyze the causes of failure.If the designer has been very clear about the impact of different component characteristics on the system in the design scheme of separated components, it is suggested that the supplier should provide the specification characteristics, reliability verification and market application experience of semiconductors selected in IPM in traditional packaging components, which is believed to be of great help to verify the reliability of IPM and the causes of separation failure.

conclusion

Although IPM provides three-phase motor driver or three arm topology uninterruptible power supply system, it is a solution to simplify system design and improve power density.However, due to its unique packaging specifications and manufacturing methods, it seems that it is still difficult to compete with the standardized and mass-produced separated packaging components in terms of the cost of a single component.However, under the iron law that the cost of any emerging technology is bound to be solved due to time, the trend of power integrated packaging replacing traditional packaging will not be avoided.In addition to clearly explaining the differences in performance, reliability and overall cost caused by using IPM as system design;Also combined

The thinking of module manufacturer designers and system designers provides readers with different levels of thinking directions when understanding and selecting IPM. At the same time, some specious ideas are further clarified in the article.It is hoped that readers who are interested in using or have used IPM will improve their understanding of such components

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