Improving efficiency and energy conservation are of primary importancein today's home-appliance applications. For this reason, brushless DCmotors have beenwidely used in home appliances.
Another reason these devices have gained broad application is thatthey use an electrical rather than mechanical commutator, and are thusconsidered more reliable.
Specifically, designers favor the three-phase brushless DC motor dueto its efficiency of up to 95 percent and its small size relative to agiven delivered power.
A standard t hree-phasepower stage is used to drive a common three-phase brushless motor. Thispower 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 controlproducessix stator fluxvectors that must be changed at a given rotor position. A Hall-effectsensor scans the rotor positions. To provide a six-step current tothe motor, the power state uses six power that can switch in differentspecific sequences.
|Figure1: A standard three-phase power stage is used to drive a commonthree-phase brushless motor.|
One popular switching mode providing a six-step current is shown in Figure 1, above. Q1, Q3 and Q5switch at high frequency (HF) while Q2, Q4 and Q6 switch at lowfrequency (LF). The powering stage occurs when an LF MOSFET is in anON-state and an HF MOSFET is switching as follows:
Step 1: The power stagepowers two phases simultaneously while the third phase is not powered.Suppose that L1 and L2 are the powered phase and L3 is not. In thiscondition, Q1 and Q4 are in an ON state and the current flows throughQ1, L1, L2 and Q4.
Step 2: MOSFET Q1 is turnedoff. The inductance cannot interrupt the current abruptly; it generatesan extra voltage until the body diode D2 is directly biased, allowingthe freewheeling current to flow. The freewheeling current path is D2,L1, L2 and Q4.
Step 3: MOSFET Q1 is turnedon and so the intrinsic body diode D2 is abruptly reverse-biased. Nowthe total current on Q1 is the sum of the operating current (as in Step1) and the recovery current of diode D2.
|Figure2: The cross section of a MOSFET device shows the intrinsic diodebetween body and drain.|
The cross section of a MOSFET device in Figure 2 above shows the intrinsicdiode between body and drain. As noted, during step 2, the currentfreewheels into the body-drain diode (D2), which is forward biased,thus, minority carriers are injected in both the n- epi and p bodyregions of the device.
When MOSFET Q1 is turned on, the D2 diode is reverse-biased and itsminority carriers in the n- region are swept toward the p+ body regionand vice versa.
This rapid displacement results in a significant current flowingthrough the diode, from n- epi to the p+ region, which is from thedrain to the source.
The L1 inductance presents a high impedance against the currentspike, which flows through Q2 and Q1. Q1 now presents an additionalcurrent spike, increasing its switching losses during turn-on.
|Figure3: When the diode in a fast-recover MOSFET is reverse-biased after itsconduction, the reverse current spike Irrm is smaller and the timeneeded to complete the recover is shorter.|
As shown in Figure 3, above, a MOSFETwith a fast-recover Diode, can improve the body diode performancein these particular applications. When the diode in is reverse-biasedafter its conduction, the reverse current spike Irrm is smaller and thetime needed to complete the recover is shorter.
A standard MOSFET was tested against an STD5NK53ZD fast recoverdevice (Figure 4, below ). Whenthe latter was placed in Q2 (LF), switching losses during turn-onoperation of the Q1 MOSFET (HF) were reduced by 65 percent.
|Figures4: A standard MOSFET (a) was tested against a fast-recover MOSFET. Whenthe latter was placed in Q2 (b), switching losses during turn-on of Q1MOSFET were reduced by 65 percent.|
Using this device led to improvements in efficiency and thermalbehavior (the case temperature is reduced from 60°C to 50°C infree air with no heat sink). For this reason, in this topology wherethe intrinsic body diode of the MOSFET is used as freewheeling diode,the use of a MOSFET with fast-recover diode is more suitable.
Stefano Finocchiaro and AntoninoGaito are application engineers with the Power MOSFET Division ofSTMicroelectronics. To read a PDFversion of this article goto “ Fast-diodeMOSFETs in current commutation.”