In circuit design, the full-bridge rectifier plays a crucial role. When four diodes in a bridge rectifier circuit are packaged together, they form a full-bridge circuit, which is essentially what we commonly refer to as an H-bridge circuit. This article will primarily introduce the working principle of an H-bridge motor drive, providing a comprehensive analysis from both counter-clockwise and clockwise perspectives.
Figure 1H Bridge motor drive circuit
Figure 1 shows a typical DC motor control circuit. The circuit is named "H-bridge drive circuit" because its shape resembles the letter H. Four transistors form the four vertical legs of the H, while the motor is the horizontal bar of the H. (Note: Figure 1 and the following two figures are only schematic diagrams, not complete circuit diagrams, and the transistor drive circuit is not shown.)
As shown in the diagram above, the H-bridge motor drive circuit includes four transistors and a motor. For the motor to operate, a pair of transistors on the diagonal must be turned on. Depending on the conduction of different transistor pairs, current may flow through the motor from left to right or from right to left, thus controlling the motor's direction of rotation.
For the motor to run, a pair of transistors on opposite sides must be turned on. For example, as shown in Figure 2, when transistors Q1 and Q4 are turned on, current flows from the positive terminal of the power supply through Q1 from left to right through the motor, and then back to the negative terminal of the power supply through Q4. As indicated by the current arrows in the figure, this current flow will drive the motor to rotate clockwise.
Figure 2 shows the H-bridge circuit driving the motor to rotate clockwise.
When transistors Q1 and Q4 are turned on, current flows through the motor from left to right, thereby driving the motor to rotate in a specific direction (the arrow around the motor indicates clockwise).
Figure 3 shows the H-bridge circuit driving the motor to rotate counterclockwise.
Figure 3 shows the case where another pair of transistors Q2 and Q3 are turned on, and the current will flow through the motor from right to left.
When transistors Q2 and Q3 are turned on, current will flow through the motor from right to left, thus driving the motor to rotate in the opposite direction (the arrow around the motor indicates the counterclockwise direction).
Enable control and direction logic
When driving a motor, it is crucial to ensure that the two transistors on the same side of the H-bridge do not conduct simultaneously. If transistors Q1 and Q2 conduct at the same time, the current will flow directly from the positive terminal through both transistors back to the negative terminal. In this case, there is no other load in the circuit besides the transistors, so the current in the circuit may reach its maximum value (this current is only limited by the power supply performance), potentially even burning out the transistors.
Figure 4 shows an H-bridge circuit with enable control and direction logic.
For the reasons mentioned above, in practical driving circuits, hardware circuits are usually used to conveniently control the switching of transistors. Figure 4 shows an improved circuit based on this consideration. It adds four AND gates and two NOT gates to the basic H-bridge circuit. The four AND gates are connected to the same "enable" signal, so this single signal can control the switching of the entire circuit. The two NOT gates, by providing a directional input, ensure that only one transistor on the same leg of the H-bridge is conducting at any given time. (As with the previous diagrams, Figure 4 is not a complete circuit diagram; in particular, directly connecting the AND gates and transistors in the figure will not work properly.)
Using the above method, the motor operation only requires three signals to control: two direction signals and one enable signal. If the DIR-L signal is 0, the DIR-R signal is 1, and the enable signal is 1, then transistors Q1 and Q4 are turned on, and current flows through the motor from left to right (as shown in Figure 5); if the DIR-L signal becomes 1 and the DIR-R signal becomes 0, then Q2 and Q3 will be turned on, and current will flow through the motor in the opposite direction.
Figure 5. Use of enable and direction signals
In practical use, it is very troublesome to make an H-bridge using discrete components. Fortunately, there are many pre-packaged H-bridge integrated circuits on the market now. They can be used by connecting the power supply, motor and control signals. They are very convenient and reliable to use within the rated voltage and current.
H-bridge circuits are frequently used in inverter and DC motor circuits. Here, we only explain the application principle of H-bridge circuits in DC motors. We hope that you can fully grasp the basic knowledge of full-bridge circuits, which will not only facilitate rapid design but also help you consolidate your fundamental knowledge.
