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Enhanced lineup of power MOSFETs with medium to high voltage withstand and improved performance by optimization of device structure
Power supplies such as AC-DC converters (switching power supplies) and DC-DC converters are installed in almost all electronic devices. The power MOSFET plays an extremely important role in the power supply. The characteristics of the power MOSFET used, such as the output voltage accuracy of the power supply, conversion efficiency, heat generation, and EMI noise change significantly depending on the power MOSFET used. What kind of power MOSFET should you choose? It would not be an exaggeration to say that this will greatly affect the future of power supply design.
In addition, since the power MOSFET is mounted on many electronic devices, the market scale is extremely large. That's why various semiconductor manufacturers around the world are bringing power MOSFETs to market, so there is a wide range of products to choose from.
We took some time to hear about medium to high voltage MOSFET's from Mr. Masataka Tsuji of the Power Management Application Engineering Department, Discrete Semiconductor Division at Toshiba Device & Storage, the Japanese manufacturing “leader” in the power MOSFET market, and asked about the the newest products, what to be careful about with power MOSFET's, optimal applications, etc.
(Interviewer: Katsumi Yamashita = Technical Journalist)
What is the voltage range for medium and high voltage withstand?
Tsuji: Our medium to high voltage withstand is in the range of 400-900V. Mainly for AC-DC converters (switching power supplies) and power factor correction (PFC: Power Factor Correction) circuits that support universal input (85V to 264V AC). Applications are diverse, with small output capacities such as chargers and AC adapters for portable electronic devices. If the output power is relatively large, it is used in servers and AC-DC converters (switching power supplies) in wireless communication base stations (Fig. 1).
What products are currently being launched by Toshiba Device & Storage in the medium and high voltage power MOSFET market?
What are the characteristics of each product series?
Tsuji: Regarding the performance of the π-MOS series, the switching speed is not very fast, and many products have a higher on-resistance lineup than the DTMOS series. However, the fact that the switching speed is not so fast is not a disadvantage. Because of the low EMI noise, it is said that this is a power MOSFET that is easy for engineers to use.
The DTMOS series, on the other hand, has a characteristic of extremely low on-resistance. If the on-resistance is low, the power loss of the AC-DC converter can be kept low. That means that it is easy to increase the conversion efficiency. Suitable for applications where high conversion efficiency is important. The switching speed is fast. For this reason, the advantage is that high-frequency switching is possible, but the disadvantage is that EMI noise is easily generated.
The structure of the divice is different
Why are the characteristics of DTMOS series and π-MOS series are so different?
Tsuji: The device structure of power MOSFETs are completely different. The DTMOS series employs a super-junction structure and the π-MOS series employs a planar structure. The planar structure has a so called double-diffusion structure, which is the basic structure of an old power MOSFET. Super-junction structures are relatively new.
Although the trench structure is widely known in power MOSFETs, a trench structure can only be advantageous up to a breakdown voltage of about 200 V at most. For this reason, with a withstand voltage range of 400 to 900 V, either the super-junction structure or the planar structure is selected.
Why can adopting the super-junction structure greatly reduce the on-resistance?
Tsuji: There is a reason for the device structure. See Figure 3. In the super-junction structure, a p-type Si layer is included in the vertical direction of the device, and a drift layer through which electrons move is formed of an n-type Si layer. On the other hand, in the planar type, the drift layer is an n-type Si layer. Generally, when comparing an n-type Si layer with an n-type Si layer, the carrier concentration of the n-type Si layer is higher. The resistance component of the drift layer is reduced accordingly. This is the reason why the on-resistance can be greatly reduced by adopting the super-junction structure.
Super-junction power MOSFETs have already been commercialized by competitors. How are your products different from those competitors?
Tsuji: There is no significant difference between our products and competitors' products in the super-junction structure itself. However, our products outperform in terms of performance. Specifically, in the Figure of Merit (FOM), which is obtained from the product of the On-Resistance (Ron) and the charge amount between the gate and drain (Qgd).
The latest product of the DTMOS series, the “DTMOSVI series”, has reduced FOM by about 40% compared to the previous generation “DTMOSIV-H” (Figure 4). Compared to competitor products, FOM is about 30% lower. If the FOM is low, the conversion efficiency of the AC-DC converter (switching power supply) can be increased accordingly (Figure 5).
What is the reason for reducing FOM despite the fact that there is no difference in the super-junction structure itself?
Tsuji: In the cross-sectional structure of the device shown in Fig. 2, the FOM was reduced by optimizing the MOS structure at the top to reduce the amount of charge between the gate and drain.
Reduce noise while maintaining efficiency
What are the features of the latest products in the π-MOS series?
How much conversion efficiency can be obtained?
Tsuji: When the latest product π-MOSIX series "TK750A60F" was applied to an AC adapter for notebook PCs with a 65W output, there was a conversion efficiency of about 87.5 to 89.5% with an output power range of 15 to 60W ( Figure 7). This is almost the same as when using the "TK10A60D" of the previous generation "π-MOSVII series".
Is that degree of conversion efficiency an industry trend for 65W output AC adapters?
Tsuji: If there is a 90% peak efficiency, that's good. Therefore, if the π-MOSIX series is used, you can get conversion efficiency at almost the same level as the industry trend. Regarding AC adapters for notebook PCs, it seems it is more important to reduce costs than to increase conversion efficiency.
On the other hand, if the switching power supply for industrial equipment uses a super junction power MOSFET, the conversion efficiency is targeted at 95% or more. To start, since the output capacity is large, even with a loss of 1%, the running cost (electricity fee) increases, the amount of heat generated also increases and heat countermeasures become difficult.
In the π-MOSIX series, what changes were made to the previous generation products to reduce EMI noise?
Tsuji: The basic structure is the same as the previous generation π-MOSVII series. However, like the DTMOSVI series described above, the MOS structure located at the top of the device has been improved. As a result, characteristics such as feedback capacitance can be optimized and noise can be reduced while maintaining conversion efficiency. When we measured the radiated noise (EMI) of a 65W AC adapter for notebook PCs, we found that using the newest product reduced the EMI level by about 5dB over a wide frequency band (Figure 7).
Are there any other characteristics that improved other than EMI noise?
Tsuji: As can be said for the entire π-MOS series, the advantage is high avalanche resistance. In general, as power MOSFETs become newer, the process technology becomes finer and as a result, there is a reduction in avalanche withstand capability. However, the π-MOSIX series guarantees the same avalanche ruggedness as the previous generation π-MOSVII series by changing the device structure. This is another characteristic that improved.
Competition to acquire the biggest share in the medium to high voltage market.
What is Toshiba's share in the medium and high voltage power MOSFET market?
Tsuji: Toshiba MOSFETs are in the top group of overall market share in the world, for both the planar structure and the super-junction type structure and various power supply manufacturers in Japan and overseas have adopted Toshiba MOSFETs.
What are the reasons behind the timing for the release of the π-MOSIX series?
Tsuji: Toshiba has not released a new-generation planar MOSFET with a withstand voltage of 600V since the previous generation of the π-MOSVII series was released about 11 years ago in 2008. However, in general-purpose power supplies for consumer use, the demand for lower-capacity and lower-cost power supplies has led to a large demand for planar-type power MOSFETs, and the market scale is still large. As a result, we have launched the π-MOSIX series with further improved device characteristics.
Please tell us specifically what DTMOSVI and π-MOSIX series products are on the market?
Tsuji: I would like to tell you about 7 products of the DTMOSVI series and 10 products of the π-MOSIX series (mass-produced products as of May 2019) (Fig. 8). The DTMOSVI series has a withstand voltage of 650V, and the π-MOSIX series has a withstand voltage of 600V, and both are n-channel products.
The DTMOSVI series has three TO-247 packages, three TO-247-4L packages, and one TO-220SIS. For example, the lowest on-resistance is "TK040N65Z" of the TO-247 package and "TK040Z65Z" of the TO-247-4L package. In each case, the on-resistance is as low as 0.04Ω (maximum value), and the drain current is 57A.
All 10 products of the π-MOSIX series are packaged in TO-220SIS (full mold type) and all 10 products have different on-resistance and drain current. The range of the on-resistance is between 0.37 and 4.1Ω, and the range of the drain current is between 2 and 15A.