H-bridge: black box or are details important?

-January 16, 2014

Engineers in all disciplines use electronics in their designs for sensing, actuation, and real-time control. Today there are few exceptions. A common component is an operational amplifier (op-amp), which most engineers treat as a black box containing many transistors and resistors.  Its performance is primarily determined by the components (e.g., resistors and capacitors) surrounding it as long as the op-amp has negative feedback and its limitations are not exceeded.

Another common electronic component is the H-bridge. Any engineer who has ever controlled a motor has most likely used the H-bridge, but perhaps treated it as a black box with no thought as to how it works and how it affects overall system performance. The H-bridge needs to be thoroughly understood for model-based design and optimum system performance.

The H-bridge, as shown in the diagram above, is named because of its configuration. It has four power devices (transistors, MOSFETs are a good selection, like the p-channel for the high side and n-channel for the low side in the diagram) with the load (usually brushed dc or stepper motor) at the center. The diodes are of the Schottky type with short turn-on delay. The four transistors can be turned on and off independently. If transistors 2 and 3 are turned on, the motor turns in one direction; turn transistors 1 and 4 on and the motor turns in the opposite direction. The transistors are usually controlled in a pulse-width modulated (PWM) fashion. When the transistor is on, it behaves like a small temperature-dependent resistor, the lower the value the better for heat dissipation.

When the transistor is completely off, it conducts no current. MOSFETs are voltage-driven devices; the gate forms a parasitic capacitor with the source, and this capacitance limits the speed at which the transistor can be turned on and off. In the transitional periods, the power dissipation due to switching is significant, especially when the switching frequency is higher than a few hundred Hz. The role of the diodes is often overlooked. While the bridge is on, two of the four transistors carry the current and the diodes have no role.

However, when the load is inductive, as with motors, and the bridge is commanded to turn the load off, the electromagnetic field associated with it will collapse and the stored energy will start to dissipate either through the bottom transistors and diodes or through the top transistors and diodes. The dissipated heat from the diodes can be of the same order of magnitude as the heat dissipation from the transistor switching.

The load (motor) is modeled as an inductor (Lm), resistance (Rm), and speed-dependent voltage (back emf) in series. The motor torque depends on the current flowing through this series combination.  There are two extremes. When the motor runs with no load, the current is low and the motor terminal voltage is close to the back-emf voltage. When the motor is stalled, the back-emf voltage is zero and the motor acts like an inductor.

The H-bridge can be driven in many different ways. In general, the on-time behavior is rather simple: turn on one high-side transistor and the opposite low-side transistor to allow current to flow through the motor. It is the off-time drive that makes the difference. Since transistors 1 and 2 (or 3 and 4) should never be turned on at the same time, there are only three different combinations for those two switches: transistor 1 conducts, or transistor 2 conducts, or neither conducts. There are many different drive modes. Andras Tantos has provided an excellent, detailed explanation here. I highly recommend it.  

The understanding of the internal behavior of the H-Bridge is of fundamental importance for motor control system design and also to mitigate electromagnetic effects while controlling inductive loads, which can affect the performance and behavior of surrounding circuits.

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