The Field-effect Transistor Output Part of the Motor

There is a diode in reverse parallel between the source and drain inside the high-power field-effect transistor. When used as an H-bridge, it is equivalent to having four diodes in parallel at the output end to eliminate voltage spikes, so there is no external diode here. Parallel connection of a small capacitor (between out1 and out2) at the output end has certain benefits in reducing the peak voltage generated by the motor, but there is a side effect of generating peak current when using PWM, so the capacity should not be too large. This capacitor can be omitted when using low-power motors. If this capacitor is added, it is necessary to use a high voltage withstand capacitor, as ordinary ceramic capacitors may experience breakdown and short circuit faults.

A circuit composed of resistors, light-emitting diodes, and capacitors in parallel at the output end indicates the direction of rotation of the motor.

Performance indicators:

The power supply voltage is 15-30 V, and the maximum continuous output current is 5A per motor. It can reach 10A in a short time (10 seconds), and the PWM frequency can reach up to 30KHz (usually 1 to 10KHz). The circuit board contains four logically independent power amplification units with output terminals connected in pairs to form an H-bridge, which can be directly controlled by a microcontroller. Realize bidirectional rotation and speed regulation of the motor.

Layout and wiring of PCB:

High current circuits should be as short and thick as possible, and should avoid passing through vias as much as possible. If it is necessary to pass through vias, the vias should be made larger (>1mm) and a small circle of vias should be made on the solder pad. During welding, solder should be used to fill the vias, otherwise they may burn out. In addition, if a voltage regulator is used, the wire between the source of the field-effect transistor and the power and ground should be as short and thick as possible. Otherwise, during high currents, the voltage drop on this section of wire may be burned out by the positive bias voltage regulator and the conducting transistor. In the initial design, a 0.15 ohm resistor was connected between the source and ground of the NMOS transistor to detect current, which became the culprit for continuously burning the board. Of course, if we replace the voltage regulator with a resistor, this problem would not exist.

The PCB of the motor drive circuit requires special cooling technology to solve the power consumption problem. The thermal conductivity of printed circuit board (PCB) substrates (such as FR-4 epoxy resin glass) is poor. On the contrary, copper has excellent thermal conductivity. Therefore, from the perspective of thermal management, increasing the copper area in PCBs is an ideal solution. Thick copper foil (e.g. 2 ounces (68 microns thick) has better thermal conductivity than thinner copper foil. However, the cost of using thick copper foil is high and it is difficult to achieve precise geometric shapes.

Therefore, using 1 ounce (34 microns) of copper foil has become very common. The outer layer is usually used ½ Copper foil from one ounce to one ounce. The solid copper surface used in the inner layer of multi-layer circuit boards has good heat dissipation. However, since these copper surfaces are usually located in the center of the circuit board stack, heat will accumulate inside the circuit board. Increasing the copper area of the outer layer of the PCB and connecting or "sewing" it to the inner layer through many through holes helps to transfer heat to the outside of the inner layer.

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