Compare the questions of a European university!

Original address: http://bbs.eeworld.com.cn/archiver/tid-93155.html compare a European university exam questions, domestic engineers please!

Note, measure the test taker's knowledge about domestic engineers.

1. What is the difference between the structure of a common diode and a diode used in power electronics? Tip: what is the role of the S layer in the PSN structure? It can be analyzed by impurity doping concentration.

2. In modern switch devices, the application of punch through technology is often seen. Introduce the principles and advantages of this technology (compared to devices that do not use this technology ).

3. The IGBT has the characteristics of "Conductivity Modulation". The IGBT is more advantageous in applications with high current and low frequency. What is Conductivity Modulation?

4. Thyristor and GTO are similar in structure, but the latter can achieve active shutdown. Introduce the differences in production processes.

5. In large current applications, multi-tube parallel connections are sometimes required. Currently, you can choose between IGBT and MOS. Which can be used for Parallel Connection and which cannot? Why?

6. In common dcdc converter, such as Buck converter or boost converter, the reverse recovery time of the diode has a great impact on energy loss. Two methods are provided to improve the loss. Tip: topology of the new device chicken.

 

Original post author's explanation
1. What is the difference between the structure of a common diode and a diode used in power electronics? Tip: what is the role of the S layer in the PSN structure? It can be analyzed by impurity doping concentration.

A: Generally, in order to increase the voltage of the diode, the PN junction can be increased theoretically and the impurity concentration can be reduced. However, the disadvantage is that the normal phase conduction loss increases.
By adding a low impurity S layer, the performance is improved: When the positive phase is turned on, the S layer is completely turned on, which is similar to a short circuit and is directly connected with Pn, so the pressure drop is small; when the reverse phase voltage is used, because the impurity concentration of the S layer is low, the conductivity is low, or the maximum electric field e that the S layer can withstand is large, the S layer can withstand large voltage without being broken down.

2. In modern switch devices, the application of punch through technology is often seen. Introduce the principles and advantages of this technology (compared to devices that do not use this technology ).

A: The S layer used to increase the voltage of the switch device is not fully utilized, because when the voltage of one end of the S layer is Emax, the other part is 0, that is, E is not evenly distributed on the S layer, umax ~ 0.5 (Emax + Emin) -- Note that the device withstand voltage only depends on the credits of e for the L length in the S layer. Punch through is to directly add a layer of high impurity doping to the S layer and the PN junction, so that the electrical field in this layer changes from Emax to Emin = 0, in the real S layer, the field strength is close to Emax everywhere.
Theoretically, with punch through technology, the width of the S layer can be reduced by 50%-without changing the withstand voltage. In reality, because the S layer must have a certain conductivity, the width will be slightly greater than 50%.

3. The IGBT has the characteristics of "Conductivity Modulation". The IGBT is more advantageous in applications with high current and low frequency. What is Conductivity Modulation?

A: Conductivity Modulation is actually very simple, that is, the equivalent circuit model between UCE is a diode close to the ideal. The saturated voltage does not increase with the increase of current, and the conduction loss is p ~ I; while the DS equivalent circuit of the MOS is a resistor, the loss is p ~ I ^ 2. Therefore, in the case of the main factors (such as motor drive), the IGBT is more advantageous.

4. Thyristor and GTO are similar in structure, but the latter can achieve active shutdown. Introduce the differences in production processes.

A: It is difficult to make it clear in words that GTO is a ring structure between layers, so the degree of integration is better than that of ordinary thyristor, able to achieve Shutdown (For details, refer to the relevant literature ).

5. In large current applications, multi-tube parallel connections are sometimes required. Currently, you can choose between IGBT and MOS. Which can be used for Parallel Connection and which cannot? Why?

Answer: The MOs can be directly used in parallel, which is inconvenient for IGBT. Because the MOSFET is a negative temperature coefficient, the balance between the parallel tubes can be: T rise, RDS rise-I decrease. However, the positive temperature coefficient of IGBT is not good. Likewise, the dual-pole transistor cannot be simply connected in parallel. If parallel connection is required, a small resistor must be connected in series at pole E, which reduces the efficiency and is not practical.
In addition, the IGBT has latch problems. For more information, see the relevant literature.

6. In common dcdc converter, such as Buck converter or boost converter, the reverse recovery time of the diode has a great impact on energy loss. Two methods are provided to improve the loss. Tip: topology of the new device chicken.

Answer: solution 1: Replace the fast-recovery diode with SiC. After calculation, in many cases, although the SiC diode is relatively expensive, the overall product cost efficiency is higher, the reason is that the switch loss is small, and the heat sink is small...
Solution 2: replace synchronous boost and Buck converter. The upper arm tube also uses a MOSFET. This structure reduces the loss by using the feature of DS bidirectional conduction (seemingly only one resistance) during UGS voltage connection, especially in low-voltage drive circuits.
Solution 3: Let the pipe work in the discontinueous state.
The solution is dead, let the pipe work in the continueous state, but reduce the inductance, increase the ripple of the current, so that its minimum value is lower than 0 -- resonant pole, the disadvantage is that the overall loss of the mos increases.

All of the above are my summary. If there is any error, please forgive me.

Feeling: the major of this exam is not a semiconductor manufacturing process, but a general basic course. 50% of students must pass this test. I have observed the domestic forum for a long time and found that most of the topics discussed are more practical. However, I feel that if I do not have a real understanding of a system, but just float it on the surface, it is difficult to make the system work in an optimal situation. For example, how much ripple factor is optimal for system selection? Which product is used for electrolysis, and how long does it have in the "here" circuit? What is the lifetime of the MOs in this circuit? Indeed, these are very underlying things, but if our engineers do not master it and the design level stays at the "Approximate" stage, it is very difficult to compare the products with those outside China.

It is hoped that engineers engaged in technical work in China will learn more about the theoretical knowledge and understand why. The same is true for electronics and machinery manufacturing.