應用於三相轉換器拓樸中的快速二極體MOSFET

Fast-diode MOSFETs in current commutation

關鍵字： 直流馬達 功率級 家電設備

提高效率和節能是家電應用中的首要問題。三相無刷直流馬達憑藉著高效率與小尺寸優勢，已被廣泛應用於家電設備及多種其他裝置中。此外，由於採用電子換向器取代了機械換向裝置，三相無刷直流馬達也進一步提高了可靠性。

Improving efficiency and energy conservation are of primary importance in today’s home-appliance applications. For this reason, brushless DC motors have been widely used in home appliances. Another reason these devices have gained broad application is that they use an electrical rather than mechanical commutator, and are thus considered more reliable. Specifically, designers favor the three-phase brushless DC motor due to its efficiency of up to 95 percent and its small size relative to a given delivered power.

標準的三相功率級(power stage)被用來驅動一個三相無刷直流馬達，如圖1所示。功率級會產生一個電場，為了使馬達良好運作，這個電場必須保持與轉子磁場之間的角度接近 90°。六步序列控制可產生6個定子磁場向量，這些向量必須在一個指定的轉子位置下改變。霍爾效應感測器可掃描轉子的位置。為了向轉子提供6個步進電流， 功率級採用了6個可依不同特定序列切換的功率MOSFET。以下將解釋一種可提供6個步進電流的常用切換模式。

A standard three-phase power stage is used to drive a common three-phase brushless motor. This power stage creates a field that, in order for the motor work to well, must maintain an angle of close to 90° with the flux rotor. Six-step sequence control produces six stator flux vectors that must be changed at a given rotor position.

A Hall-effect sensor scans the rotor positions. To provide a six-step current to the motor, the power state uses six power that can switch in different specific sequences. One popular switching mode providing a six-step current

is shown in Figure 1.

MOSFET Q1、Q3和Q5會在高頻(HF)切換；而Q2、Q4和Q6則會在低頻(LF)切換。當一個低頻MOSFET處於開狀態，且一個高頻MOSFET處於切換狀態時，就會產生一個功率級。

Q1, Q3 and Q5 switch at high frequency (HF) while Q2, Q4 and Q6 switch at low frequency (LF). The powering stage occurs when an LF MOSFET is in an ON-state and an HF MOSFET is switching.

步驟1：功率級同時為兩個相位供電，而第三個相位則未供電。假設供電相位為L1、L2，L3未供電。在這種情況下，MOSFET Q1和Q2處於導通狀態，電流會流經Q1、L1、L2和Q4。

Step 1: The power stage powers two phases simultaneously while the third phase is not powered. Suppose that L1

and L2 are the powered phase and L3 is not. In this condition, Q1 and Q4 are in an ON state and the current flows through Q1, L1, L2 and Q4.

步驟2：MOSFET Q1切斷。因為電感不能突然中斷電流，它會產生額外電壓，直到體二極體D2被直接偏置，並允許續流電流流過。續流電流的路徑為D2、L1、L2和Q4。

Step 2: MOSFET Q1 is turned off. The inductance cannot interrupt the current abruptly; it generates an extra voltage until the body diode D2 is directly biased, allowing the freewheeling current to flow. The freewheeling current path is D2, L1, L2 and Q4.

步驟3：Q1打開，體二極體D2突然反偏置。Q1上的總電流為供電電流(如步驟1)與二極體D2上的恢復電流之和。

Step 3: MOSFET Q1 is turned on and so the intrinsic body diode D2 is abruptly reverse-biased. Now the total current on Q1 is the sum of the operating current (as in Step 1) and the recovery current of diode D2.

圖1：三相轉換器拓樸結構圖。

Figure 1: A standard three-phase power stage is used to drive a common three-phase brushless motor.

圖2：MOSFET元件剖面圖，電流流經內部體二極體。

Figure 2: The cross section of a MOSFET device shows the intrinsic diode between body and drain.

圖2顯示了MOSFET元件剖面圖，可看到其中的體-漏二極體。在步驟2，電流流入到體-漏二極體D2(見圖1)，該二極體被正向偏置，少數載子注 入到二極體的N型磊晶和P型區。當MOSFET Q1導通時，二極體D2被反向偏置，N區的少數載子進入P+體區，反之亦然。這種快速轉移導致大量的電流流經二極體，從N型磊晶區到P+區，即從漏極到源 極。電感L1對於流經Q2和Q1的尖峰電流表現出高阻抗。Q1表現出額外的電流尖峰，增加了在導通期間的開關損耗。圖4a描述了MOSFET的導通過程。

The cross section of a MOSFET device shows the intrinsic diode between body and drain. As noted, during step 2, the current freewheels into the body-drain diode (D2), which is forwardbiased, thus, minority carriers are

injected in both the n- epi and p body regions of the device. When MOSFET Q1 is turned on, the D2 diode is reverse-biased and its minority carriers in the n- region are swept toward the p+ body region and vice versa.

This rapid displacement results in a significant current flowing through the diode, from n- epi to the p+ region, which is from the drain to the source. The L1 induc-tance presents a high impedance against the current spike, which flows through Q2 and Q1. Q1 now presents an additional current spike, increasing its switching losses during turn-on.

為改善在這些特殊應用中體二極體的性能，研發人員已開發出具有快速體二極體恢復特性的MOSFET。當二極體導通後，會被反向偏置，反向恢復峰值電流I_rrm較小，完成恢復所需的時間更短(見圖3)。

A MOSFET with a fast-recover diode, such as STMicroelectronics’ STD5NK53ZD (SuperFREDmesh series), can improve the body diode performance in these particular applications. When the diode in is reverse-biased after its conduction, the reverse current spike Irrm is smaller and the time needed to complete the recover is shorter.

圖3：具有快速體二極體恢復特性的MOSFET，反向恢復峰值電流較小，恢復時間縮短。

Figure 3: When the diode in a fast-recover MOSFET is reverse-biased after its conduction, the reverse current spike Irrm is smaller and the time needed to complete the recover is shorter.

為瞭解標準MOSFET和快速恢復MOSFET的差異，我們對這兩種元件進行了測試比較。ST推出的 STD5NK52ZD (SuperFREDmesh系列) 放置在Q2 (LF) 中，如圖4b所示。在Q1 MOSFET(HF)的導通工作期間，開關損耗降低了65%。採用STD5NK52ZD時，效率和熱性能均獲得很大提升 (在不採用散熱器的自由流動空氣環 境下，殼溫從60℃降低到50℃)。在這種拓樸結構中，MOSFET內部的體二極體可作為續流二極體，採用具有快速體二極體恢復特性MOSFET更為合 適。

A standard MOSFET was tested against the STD5NK53ZD (Figure 4). When the latter was placed in Q2 (LF), switching losses during turn-on operation of the Q1 MOSFET (HF) were reduced by 65 percent. Using this device led to improvements in efficiency and thermal behavior (the case temperature is reduced from 60°C to 50°C in free air with no heat sink). For this reason, in this topology where the intrinsic body diode of the MOSFET is used as freewheeling diode, the use of a MOSFET with fast-recover diode is more suitable.

圖4：a) Q2採用標準MOSFET的開狀態作業；b) Q2採用ST公司STD5NK52ZD MOSFET的開狀態作業。

Figures 4: A standard MOSFET (a) was tested against a fast-recover MOSFET. When the latter was placed in Q2 (b), switching losses during turn-on of Q1 MOSFET

were reduced by 65 percent.

SuperFREDmesh技術彌補了現有的FDmesh技術，具有降低導通電阻，齊納閘保護以及非常高的dv/dt性能，並採用了快速體-漏恢復 二極體。N通道520V、1.22歐姆、4.4A STD5NK52ZD元件可提供多種封裝，包括TO-220、DPAK、I2PAK和IPAK封裝。該元件為工程師設計開關應用提供了更大的靈活性。其他 優勢還包括非常高的dv/dt，經過100%雪崩測試，具有非常低的本徵電容器、良好的可重複製造性，以及改良的ESD性能。此外，與其他可選模組解決方 案相較，使用分離式解決方案還能在PCB上靈活定位元件，實現空間的最佳化，並獲得有效的熱管理，這是一種具有成本效益的解決方案。

引用網址: http://www.eettaiwan.com/ART_8800468604_675763_TA_05d090d6.HTM

STP5NK52ZD: MOSFET N-CHAN 520V 4.4A 70W TO-220

其他名稱: STP5NK52ZD、STP5NK52ZD、497 5953 5 ND、49759535ND、497-5953-5