What is an Ideal Diode?
An ideal diode has zero resistance when forward-biased, allowing current to flow. But when reverse-biased, it blocks current completely.
Diodes are a critical part of semiconductor physics, and understanding them can help you understand and predict their behaviour in a multitude of applications.
Creating a simple Ideal Diode circuit demonstrates the diode’s primary functionality of controlling unidirectional current flow. A more advanced ideal diode controller demonstrates how adding an additional MOSFET to the system can provide more controllability functions.
No Voltage Drop or Voltage Loss
As its name suggests, an ideal diode is a theoretical component that perfectly conducts current in one direction and blocks it completely in the other. As a result, it has no internal resistance and zero voltage drop. This makes it a great tool for learning about basic diode theory and semiconductor physics, but it oversimplifies the actual behavior of real-world diodes.
A conventional diode has a small internal resistance that resists the flow of electric current through it. Consequently, the I-V characteristics curve for a diode has a threshold voltage above which it begins to conduct current. Moreover, a diode only fully conducts when the applied voltage is more than its threshold voltage.
An ideal diode, however, has no threshold voltage and is instantly conductive when any forward voltage is applied. Moreover, it is completely resistant to reverse voltage and will not conduct any current in the reverse direction no matter how much reverse voltage is applied.
This characteristic of an ideal diode, also known as its ideality factor, is a key part of the Shockley Diode Equation ideal diode and the Ideal Diode Model. As such, it shapes how we understand and predict diode behaviour and how we use them in a myriad of electronic circuits.
No Breakdown Voltage
Conventional diodes have a threshold voltage that must be exceeded for current to flow through them. This is typically 0.7 Volts for a silicon diode. An ideal diode, on the other hand, does not have a breakdown voltage. The ideal diode behaves like a perfect conductor when it is forward biased and acts as an ideal insulator when it is reverse biased.
This characteristic allows for very fast switching, as compared to conventional diodes. It also makes ideal diodes much more reliable as a power switch, since they will not fail due to a breakdown voltage or leakage current.
While real diodes do have some internal resistance, this is often not significant enough to cause problems in most applications. The ideal diode, however, has zero resistance when it is reverse biased. This is because the diode is acting as Passive component manufacturers a perfect insulator, not allowing any current to flow from anode to cathode.
This makes the ideal diode an excellent choice for high-speed switching circuits that require fast rectification. TI’s ideal diode controllers, such as the LM7480-Q1 and LM7472x-Q1, are ideal for front-end input protection in automotive power supplies, reverse current blocking, and protection during supply micro-shorts. They feature low shutdown current, gate drive and isolation voltages to ensure reliable operation in harsh environments. Check out TI’s Reverse Battery Protection Ideal Diode Controller Application Report for more information about the advantages of using these devices for automotive power management.
No Leakage Current
Diodes have a very special property, in that they block current flowing the reverse direction. This stops energy being wasted, a process called leakage current. This makes them very useful for preventing electric shocks and protecting delicate components from damage.
If we imagine a one-way street for electric current to pass through, an ideal diode would allow it to freely flow in the forward direction and completely block it from moving in the reverse direction. This is the essence of a diode’s primary functionality, and arguably the most important function that they perform in our electronic circuits.
In practice, a perfect diode doesn’t exist. However, the concept of an ideal diode is useful for simplifying our understanding of how a real diode works, and the underlying principles that make it tick. It also allows us to compare and contrast this theoretical model with its real-world counterpart, ensuring a full appreciation of the diode’s capabilities.
In short, an ideal diode has zero resistance in the forward direction and infinite resistance in the reverse direction, meaning it never experiences a voltage drop when conducting. It also switches instantly from the conducting state to the non-conducting state without delay, and its characteristics are not affected by temperature changes. Considering these properties, an ideal diode can be used to model the behavior of a real diode in various electrical circuits.
Perfect Conductivity
The ideal diode has perfect conductivity, which means that it will conduct current instantaneously when the forward voltage is applied. This characteristic distinguishes it from real diodes that have a region of conduction and a region of non-conduction.
In fact, the perfect diode has zero resistance in the forward direction and infinite resistance in the reverse direction. This makes it a very efficient device for converting Alternating Current into Direct Current and maintaining a constant output voltage regardless of input or load conditions.
Conventional diodes do not have this feature and will only conduct a limited amount of current when the reverse voltage is applied. This is because the diode junctions will break down and give way to the reverse voltage until a certain point is reached, which is known as the breakdown voltage.
If you could build a perfect diode that would have perfect conductivity, it could act as a very efficient rectifier for powering a DC motor and turning Johnson noise into useful work. This is a very desirable feature and anyone who can devise such a device should be richly rewarded. However, this is probably impossible due to the Second Law of Thermodynamics and other practical considerations. Nonetheless, some devices like the TI LM74610 come close to the perfect diode. These chips are excellent for powering DC motors and can also protect them from back-currents caused by inductive loads.
