“The editor usually checks and calculates the drain current IDI_DID of the Power device MOSFET during the design process of the motor controller is an important task. Here is a brief description of some of my own derivation processes, mainly for the motor controller matched by a certain type of car motor. The power device is N-MOS, the output waveform of the AC terminal is a sine wave, and the modulation ratio m = 1 m= 1m=1.

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The editor usually checks and calculates the drain current IDI_DID of the power device MOSFET during the design process of the motor controller is an important task. Here is a brief description of some of my own derivation processes, mainly for the motor controller matched by a certain type of car motor. The power device is N-MOS, the output waveform of the AC terminal is a sine wave, and the modulation ratio m = 1 m= 1m=1. The variables appearing in the following are as shown in the title picture.

If there are errors and ambiguities in this article, readers are urged to criticize and correct.

1. AC output terminal line current IL I_LIL:

For a specific phase, such as U phase, the effective value IL I_LIL of the AC output current is actually provided by the MOS connected in series, so the value is equal to the corresponding MOS drain D input and source S output The drain current ID I_DID has:

IL = ID I_L=I_DIL=IDI D I_DID, drain current (effective value);

Since the three phases are symmetrical, so are the expressions for the V and W phases. For the sake of brevity, the subscript indicating the U phase is not marked here. If there is no special explanation below, the analysis is also for the U phase.

2. AC output terminal line voltage UL U_LUL:

Since the AC terminal is three-phase, it should be considered to distinguish between phase voltage and line voltage. Among them, the peak value of the AC line voltage UL U_LUL at point U relative to point V is equal to the DC bus voltage UDC U_{DC}UDC. For SVPWM mode, the effective value of the line voltage UL U_LUL is 1 / of the DC bus voltage UDC U_{DC}UDC 2 1/sqrt{2}1/2, namely:

UL = 1 2 ⋅ UDC U_L=frac{1}{sqrt{2}}cdot U_{DC}UL=21⋅UDCU DC U_{DC}UDC, DC bus voltage;

3. The phase current I p I_pIp at the motor end:

The motor adopts star connection, the A-phase winding of the motor and the U-phase of the motor controller are connected in series, so the phase current I p I_pIp of the motor A phase is equal to the line current IL I_LIL of the U phase of the motor controller:

I p = IL I_p=I_LIp=ILI L I_LIL, output terminal line current;

4. The phase voltage U p U_pUp at the motor end:

The motor adopts the star connection method, and the phase voltage of the motor A phase is equal to 1/3 1/sqrt{3}1/3 of the motor controller line voltage, namely:

U p = 1 3 ⋅ UL U_p=frac{1}{sqrt{3}}cdot U_LUp=31⋅ULU L U_LUL, output terminal line voltage;

5. Active power P PP:

Consider the A phase winding of the motor. Its active power is equal to the product of the phase voltage U p U_pUp applied to it, the phase current I p I_pIp passing through it, and the current power factor cos φ cosvarphicosφ. Then consider that there are three identical windings. So the active power P PP of the entire motor is:

P = 3 ⋅ cos φ ⋅ U p ⋅ I p P=3cdot cosvarphicdot U_{p}cdot I_pP=3⋅cosφ⋅Up⋅Ip Substituting the expressions of phase voltage U p U_pUp and phase current I p I_pIp obtained above, Obtain the expression of active power P PP about the line voltage UL U_LUL and line current IL I_LIL of the motor controller:

P = 3 ⋅ cos φ ⋅ UL ⋅ ILP=sqrt{3}cdot cosvarphicdot U_{L}cdot I_LP=3⋅cosφ⋅UL⋅IL and then substituting the expressions of line voltage UL U_LUL and line current IL I_LIL to obtain the active power P PP Regarding the expression of DC bus voltage UDC U_{DC}UDC and drain current ID I_DID:

P = 3 2 ⋅ cos φ ⋅ UDC ⋅ IDP=frac{sqrt{3}}{sqrt{2}}cdot cosvarphicdot U_{DC}cdot I_DP=23⋅cosφ⋅UDC⋅ID In the above equations,

cos φ cosvarphicosφ, power factor;

U p U_pUp, motor terminal phase voltage;

I p I_pIp, motor terminal phase current;

UL U_LUL, AC output terminal line voltage;

IL I_LIL, AC output terminal line current;

UDC U_{DC}UDC, DC bus voltage;

ID I_DID, drain current;

6. Drain current ID I_DID:

From the expression of active power P PP, the drain current ID I_DID can be inverted:

ID = 2 3 ⋅ cos φ ⋅ PUDC I_D=frac{sqrt{2}}{sqrt{3}cdot cosvarphi}cdot frac{P}{U_{DC}} ID=3⋅cosφ2⋅UDCPP PP, active power;

cos φ cosvarphicosφ, power factor;

UDC U_{DC}UDC, DC bus voltage;

IDC I_{DC}IDC, DC bus current;

It should be noted that the drain current ID I_DID here is a single half bridge arm, in this case, it is the drain current provided by the single MOS shown in the title picture. But if the half bridge arm is connected in parallel by N MOSs, ID I_DID should be the sum of the drain currents of the N MOSs.

7. Peak drain current ID m I_{Dm}IDm:

ID I_DID is the effective value of the drain current. Since the output waveform is a sine wave, the peak value of the drain current is 2 sqrt{2}2 times the effective value of the drain current, namely:

ID m = 2 ⋅ ID I_{Dm}=sqrt{2}⋅I_DIDm=2⋅ID assumes that the bridge arm uses N MOS in parallel. Then you can use the allowable value of continuous drain current provided in the specification to check ID / N I_D/NID/N and ID m / N I_{Dm}/NIDm/N, and use the allowable value of pulsed drain current to calibrate Core ID m I_{Dm}IDm.

Note here that the total drain current peak value ID m I_{Dm}IDm of N MOS is compared with the allowable value of pulse drain current of a single MOS. This is considering that there are differences between the MOSs and the turn-on time is not the same, so there must be a moment when only one MOS is turned on. At this time, the MOS that is turned on in advance bears all the current that should be shared by the N MOSs, that is, ID m I_{Dm}IDm.

Of course, the check is not simply to be less than the allowable value even if it is qualified, but also to consider a sufficient safety factor. This safety factor will have different values due to factors such as different raw materials, processes, scenes, and customers (fog), so I won’t expand on it here.

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