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Using Steering Diodes for Circuit Protection

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Learn what a steering diode is.

  • Explore steering diode applications.

  • Pick up some design tips for steering diodes.

What would happen if you accidentally left the water faucet on and your sink was plugged?  Well, it is probably no surprise that you would soon have water spilling out everywhere since its typical path is impeded. Essentially, you would have a huge mess to deal with and would probably wonder why you allowed this to happen. 

In electronics,  this can happen with electric current too. However, instead of a wet floor, you’ll get a popping sound, sparks, and/or an acrid smell that soon follows. So, how can we ensure this does not happen with our electronic devices? The answer is a steering diode. 

What Is a Steering Diode?

5 steering diodes against a white background

A steering diode is configured to control the flow of current.

A steering diode isn’t a specific kind of a diode. Instead, the term describes how a diode is used to steer or direct current away from its usual path. To fully understand what a steering diode is, let’s quickly get back to diode basics. 

A diode is a semiconductor made up of a P-N junction. In a forward-biased configuration, a positive voltage is applied to the anode (P) and a negative voltage to the cathode (N). When the voltage applied exceeds the requisite threshold, which is usually 0.7V for silicon diodes, the current starts to flow.

A diode is said to be in reverse bias when the anode is supplied with a negative voltage and the cathode with positive. When reverse biased, no current is allowed to flow through and the voltage across the depletion region increases. When the reverse-biased voltage exceeds the breakdown voltage of the diode, the P-N junction is destroyed, thus enabling current to flow through the broken component.

The steering diode takes advantage of both forward-bias and reverse-bias behaviors. A steering diode is meant to create contingency paths for current in special scenarios. In normal circumstances, the steering diode is placed in reversed-bias. 

Steering Diode Applications

In electronics, you’ll find steering diodes widely used in protecting components against transient events like surges and ESD. These events involve a sudden rise in voltage and current that could damage pins and semiconductor ICs. Steering diodes are usually used to protect Ethernet, RS232/ RS485, and I/O ports. 

A schematic diagram of a steering diode in a simple circuit

A simple schematic showing a steering diode. 

The above image demonstrates a typical configuration of steering diodes for protecting a circuit from transient events. In normal circumstances, both D1 and D2 are reverse-biased. When a transient voltage occurs at the ground connector, D2 activates to channel it away from the circuit side.

Similarly, when the voltage on the connector rises to above Vcc, D1 is turned on to divert the excessive current away from the circuit. When the current is diverted to Vcc, there’s a risk that the current could flow and damage other components, which explains the placement of an additional Zener diode. The Zener diode would provide a safe path for excessive current to flow from Vcc to the ground. 

Design Tips with Steering Diode Arrays

Communication ports in an internet router protected by steering diodes

Steering diode arrays protect communication ports from surges and ESD.

Instead of placing a couple of diodes on every connector, you can opt for steering diode arrays, which are available as a single or compact component. It is space-saving and equally, if not more, efficient in protecting the circuit.

Be sure to choose steering diodes with a low capacitance when you’re protecting high-speed communication interfaces. Usually, steering diode arrays introduce less than 2 pf onto the communication trace. Having low capacitance is important for preventing signal distortion. 

Placement is also crucial in ensuring the effectiveness of steering diode arrays. The diodes should be placed between the connector and the component that is meant to be protected. Ideally, you’ll want to place the steering diode arrays close to the connector. Doing so minimizes the risk of excessive current flowing into the component before the diode starts to conduct. 

If you’re yet to add steering diodes into your designs, you can do so easily with an advanced PCB design and analysis software. You’re likely to find a prebuilt component from Allegro’s comprehensive component library, which saves you the time of creating one for yourself. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.