What are Break-Before Make (BBM) Switches?

The Break-Before-Make (BBM) is a switching configuration that breaks contact with one signal source before it makes contact with another, meaning that only one circuit can be completed at a time i.e there is a momentary time between switching where nothing is connected. These switches are also called “non-shorting switches.”  

Break-before-make (BBM) is a safety feature used in many analog switches and multiplexers. In BBM, when input control logic changes, there is no electrical connection between two multiplexed paths. This ensures isolation between two multiplexed paths and prevents signal distortion. Additionally, they are used in Load Switching ICs, which are serially connected between a power supply and load for falling back on a load wire or power supply. 

Fig. 1: Block Diagram of Break Before Make Switch  

Break-Before-Make time (tBBM) is the interval during circuit switching when both Circuit 1 and Circuit 2 are in an open state, as shown in Fig.1. This prevents the risk of short circuits by ensuring that both circuits are not closed simultaneously. The purpose of this mechanism is to maintain signal integrity and avoid potential electrical issues 

tBBM for Toshiba TC7PCI3212MT Switch  

The break-before-make switches are divided into two categories based on the delay timings between the switching: 

1. Open delayed

2. Open-in phase

In an open delayed transition, the transfer switch pauses between transitions from one power source to another. The delay lasts either a specific amount of time, or however long it takes the load voltage to drop below a pre-specified level, whereas, in an open in-phase transition, an automatic controller uses built-in intelligence to execute the transition at the exact moment it expects the normal and emergency power to be synchronised in phase, voltage, and frequency. They are typically completed in 150 ms or less.

Working of a Break-Before-Make Switch

Initially, circuit 1 is in a closed state, and circuit 2 is in open state. When the actuation force is applied to the switch, the circuit is momentarily broken and hence, both the contacts are opened. Therefore, it is also called a non-shorting switch because, unlike the Make-before-break switch, which shorts the circuit for a brief period of time during the switching event, it doesn’t short it. As the actuation force continues to be applied on the switch, the normally closed contact gets opened i.e. circuit 1 is in an open state, the normally open contact gets closed, i.e. circuit 2 is in the closed state), and the circuit is made.  

Applications of Break-Before-Make Switches 

Communication systems use these switches to change between different communication channels without any interference in the signals or cross-talk. Industrial automation uses this kind of switch to change from one mode of operation to another. In the aviation sector for instance, they have a role of changing from the redundant power source to the backup power and so on. In audio equipment, break-before-make switches are used to avoid short-circuiting and signal distortion while switching between inputs and outputs. Thus, it has great value in the prevention of momentary short circuits when used where even such a fraction is decisive. Due to their capability of disconnecting one circuit prior to creating another, they could be ideal for handling fragile instruments as well as safety-critical situations that require this type of control.