What is the H bridge?

  Because the circuit is named after the letter H, it usually contains four independently controlled switching elements. For example, the following figure shows four MOSFET switching elements Q1, Q2, Q3 and Q4.

They are usually used to drive loads with high current, such as motors.

There is a DC motor M in the middle of the H bridge circuit.

D1, D2, D3 and D4 are free-wheeling diodes of MOS-FET;

Switch state

  Taking the control of a DC motor as an example, several switching states of H-bridge are briefly introduced, in which forward rotation and reverse rotation are artificially prescribed directions, which can be divided according to the actual situation in actual projects.

positive rotation

Usually, the H-bridge is used to drive an inductive load. Here we drive a DC motor:

Open Q1 and Q4

Turn off Q2 and Q3.

At this time, it is assumed that the motor is rotating forward, and the current flows through Q1, M and Q4 in turn, as shown by the red line in the following figure.

reverse

   The other state is that the motor reverses, and the states of the four switch components are as follows:

Turn off Q1 and Q4

Open Q2 and Q3.

At this time, the motor reverses, and the current flows through Q2, M and Q3 in turn, as shown by the red line in the following figure.

regulate

    If you want to speed the DC motor, one of the schemes is:

Turn off Q2 and Q3.

Turn on Q1 and Q4, and input 50% duty cycle PWM waveform to it.

In this way, the effect of reducing the rotational speed is achieved. If the rotational speed needs to be increased, the duty cycle of the input PWM is set to 100%, and the current direction is shown by the red line in the following figure.

Stop state

    Here, for example, the motor switches from forward rotation to stop.Q1 and Q4 are turned on at the right time. At this time, if Q1 and Q4 are turned off, the inside of the DC motor can be equivalent to an inductor, that is, an inductive load, and the current will not suddenly change, so the current will continue to flow in the original direction. At this time, we hope that the current in the motor can quickly decay.There are two ways.The first type:Turn off Q1 and Q4. At this time, the current will still flow through the reverse freewheeling diode. At this time, turn on Q1 and Q3 briefly to achieve the purpose of quickly attenuating the current. The current direction is shown by the red line in the figure below.The second type:When preparing, turn off Q1 and turn on Q2. At this time, the current will not decay quickly, and the current will circulate among Q2, M and Q4, and the electric energy will be consumed through the internal resistance of MOS-FET.

Supplement-Another H-bridge circuit

    In this paper, there is an H-bridge with four N-type MOS transistors, and another H-bridge with two N-type and two P-type MOS transistors. The following figure is the H-bridge circuit. It consists of two P-type field effect transistors Q1 and Q2 and two N-type field effect transistors Q3 and Q4, and the four field effect transistors on the bridge arm are equivalent to four switches.Compared with the H-bridge circuits of the four N-type MOS transistors mentioned above, one advantage of this circuit is that no matter what the state of the control arm is (it is never allowed to float), the H-bridge will not appear "common conduction" (short circuit).Principle of MOS transistor switching circuitThe p-type MOS transistor is turned on when the gate is at low level and turned off when the gate is at high level.The n-type MOS transistor is turned on when the gate is at high level and turned off when the gate is at low level.

positive rotation

    FET is a voltage-controlled element, and the current passing through the gate is almost "zero".Because of this feature, when the control arm 1 is set to a high level (U=VCC) and the control arm 2 is set to a low level (U=0) after the above circuit is connected, Q1 and Q4 are turned off and Q2 and Q3 are turned on.At this time, the left end of the motor is at a low level and the right end is at a high level, so the current flows in the direction of the arrow, and the motor is set to rotate normally at this time.

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