This article describes the Utility Vehicle Manufacturer brushless motor controller thermal design. Works, including the introduction of the controller, MOSFET power loss calculations, thermal analysis of the model, the calculation of steady-state temperature, thermal conductivity material selection, thermal simulation.
1 Introduction

As the power MOSFET has a drive current, switching speed, etc., have been widely used in electric vehicle controller. However, if properly designed and used, will often damage the MOSFET, and once damaged MOSFET's drain-source short-circuit, the wafer is usually burned very severely, most users can not be accurately analyzed the causes of damage caused by MOSFET. So in the design phase, the reliability of the MOSFET design is of crucial importance.

The MOSFET is usually damage modes include: over-current, over voltage, avalanche breakdown, beyond the safe working areas. But these are ultimately causes damage to the wafer temperature is too high because of damage, so the controller in the design, thermal design is very important. The MOSFET junction temperature must be calculated to ensure that in the course of MOSFET junction temperature does not exceed its maximum allowable value.

2 Introduction to brushless motor controller

Because brushless power golf trolley with high torque, long life, low noise, etc., in various fields has been widely applied, its working principle has been widely known to everyone, not go into details here. Domestic electric vehicle motor controllers usually work for the three-phase six-step, the power level diagram shown in Figure 1, where Q1, Q2 for the A-phase on the tube and down tube; Q3, Q4 for the B-phase on the tube and down tube; Q5, Q6 for the C-phase on the tube and down tube. MOSFET full use AOT430. MOSFET operating in twenty-two conduction mode, turn the order Q1Q4 → Q1Q6 → Q3Q6 → Q3Q2 → Q5Q2 → Q5Q4 → Q1Q4, the controller output by adjusting the PWM pulse width on the bridge to achieve, PWM frequency is generally set to 18KHz or more. When the motor and controller in a certain phase of work (assuming B phase C phase on the tube under the control of Q3 and Q6), in each PWM cycle has two states:

State 1: Q3 and Q6 turns on, current I1 through Q3, the motor coil L, Q6, the current sense resistor Rs into the ground.

State 2: Q3 off, Q6 turns on, current I2 flowing through the motor coil L, Q6, Q4,
This state is called freewheeling state. In state 2, if the Q4 ON, the controller is called synchronous rectification. If Q4 off, I2 flow through the body diode of Q4, called non-synchronous rectification work.

The current flowing through the motor coil L and called I1 and I2 of the phase current controller, the current flowing through current sense resistor Rs, the average line current I1 as the controller, the controller than the phase current line current controller to be large.

3-power computing

Control the MOSFET power dissipation to increase as the motor load increases, when the motor stall, the controller of the MOSFET maximum loss (assuming the controller for full output). For analysis convenience, we assume that when the B-phase motor stall on the pipe work in PWM mode, C phase has been turned down tube, B phase under the control of synchronous rectification work (see Figure 1). Motor stall when the waveform shown in Figure 2 - Figure 5. Power loss is calculated as follows:

3.1 B-phase power loss on the tube:

3.1.1 B-phase on the tube opening loss (t1-t2), shown in Figure 2;

   
3.1.2 B-phase turn-off losses on the tube (t3-t4), shown in Figure 3;

   
3.1.3 B relative to the conduction losses (t5-t6), shown in Figure 4;

   
The total loss on the B-phase tube:

Phs (Bphase) = Phs (turn on) + Phs (turn off) + Phs (on) = 5.1 +3.75 +7.5 = 16.35W

3.2 B-phase power loss under control:

3.2.1 B-phase flow under the pipe continued loss (t7-t8), shown in Figure 5;

PLS (Bphase) = PLS (freewheel) = I2 × Rds (on) × (1-D) = 402 × 0.015 × (1-20/64) = 16.5 W

3.3 C phase under the control power loss

Because the C phase has been under the control conduction, so power loss is calculated as follows:

PLS (Cphase) = PLS (on) = I2 × Rds (on) = 402 × 0.015 = 24 W

The total loss of power management controller is:

Ptatal = PHS (Bphase) + PLS (Bphase) + PLS (Cphase) = 16.35 +16.5 +24 = 56.85

4 thermal model

Figure 5 shows a typical installation of TO-220 structure and thermal model. Thermal resistance and resistance similar, so we can Rth (ja) looked at a few small resistors in series, which has the following formula:

Rth (ja) = Rth (jc) + Rth (ch) + Rth (ha)

Of which:
Rth (jc) --- junction surface thermal resistance of the MOSFET
Rth (ch) --- MOSFET surface to the heat sink thermal resistance
Rth (ha) --- heat sink to ambient thermal resistance (the way with the installation of the radiator) usually from junction to heat sink is carried by conduction from the radiator to the environment through conduction and convection. Rth (jc) is determined by the device, so a system, if the MOSFET has been determined, in order to obtain a smaller thermal resistance we can choose a better MOSFET thermal conductivity material and the well installed in the radiator.

5 the calculation of steady-state temperature rise

Data sheet from AOT430 we can obtain the following parameters:

Tjmax = 175 ℃ Rth (jc) max = 0.56 ℃ / W

5.1 motor, the MOSFET junction temperature to calculate the surface (because the motor is running, the top tube and down tube only one-third of the time working, so the average power should be divided by 3):

5.1.1 node on the pipe to the surface of the steady-state temperature rise of the power control

   
5.1.2 under the control node to control the surface of the steady-state temperature rise of power

   
5.2 motor stall, the MOSFET junction temperature to calculate the surface

5.2.1 B-phase power on the control node to control the surface of the steady-state temperature

Tjc = Tj-Tc = Phs × Rth (jc) = 16.35 × 0.56 = 9.2 ℃

5.2.2 B-phase power under the control node to control the surface of the steady-state temperature

Tjc = Tj-Tc = Pls × Rth (jc) = 16.5 × 0.56 = 9.24 ℃

5.2.3 C with lower tube junction to the surface of the steady-state temperature rise of the power control

Tjc = Tj-Tc = PLS (Cphase) × Rth (jc) = 24 × 0.56 = 13.44 ℃

We can see from the above calculation, when the controller in the motor stall has been conducting the MOSFET (under control) the maximum temperature rise, should be key consideration in the design of motor stall when the MOSFET temperature rise.

6 Select the appropriate thermal material

Figure 7 SilPad series of thermal conductivity of the TO-220 package material thermal conductivity changes with the pressure curve. 6.1 thermal conductivity material SilPad-400, the pressure of 200psi, its thermal resistance Rth (ch) is 4.64 ℃ / W.

Is: Tch = Tc-Th = PLS × Rth (ch) = 24 × 4.64 = 111 ℃

6.2 thermal conductivity material SilPad-900S, the pressure of 200psi, its thermal resistance Rth (ch) is 2.25 ℃ / W.

Is: Tch = Tc-Th = PLS × Rth (ch) = 24 × 2.25 = 54 ℃

Shows the thermal conductivity of different materials a great impact on the temperature, in order to reduce the MOSFET's junction temperature rise, we can choose a better thermal conductivity material to obtain good thermal conductivity, so as to achieve our design goals.

In order to make the controller more reliable, we will usually control the MOSFET surface temperature below 100 ℃, which is in use because there will be other high-energy pulse occurs, for example, the motor phase short circuit, load abruptly and so on.

7. Thermal simulation:

Since in practice it is difficult to determine the thermal resistance of heat sink surface to the environment, in order to carry out fully by calculating the thermal design is more difficult to get, so we can make use of thermal simulation software to simulate the design in order to achieve our goal.

Simulation conditions: Ptotal = 56.85W, Ta = 45 ℃, the controller heat sink size: 70mm × 110mm × 30mm, natural air-cooled, MOSFET installation shown in Figure 8. 7.1 motor thermal simulation run-time controller

Figure 9 shows that, under the control of the temperature rise was significantly higher than on the tube temperature .7.2 motor stall controller thermal simulation

Figure 10 shows, when the stall has been the hottest on-the down tube, the temperature is close to 150 ℃. Figure 11 shows, in stall 100 seconds after the MOSFET temperature rise has not been stable, if the block has been transferred, will burn MOSFET. Therefore, if the use of simulation in the radiator size, we can not have stall, must take appropriate protective measures. We can use gap protection method, that is, when the motor stall, the stall for some time, protection for some time, so that the MOSFET temperature does not exceed the maximum junction temperature. Figure 12 shows the stall 1.5s, 1.5s transient temperature protection schematic, the figure shows, this approach can effectively protect the MOSFET. Conclusion: the thermal design of the controller in the product design stage is very important, we must calculate the power, the thermal model, thermal simulation to calculate the temperature rise at the same time should be considered in the design of the harshest environments, Eventually, through practical tests to verify the correctness of our thermal design.