What is an Ideal Diode?
An ideal diode is one that works like a perfect conductor when forward voltage is applied & offers infinite resistance when reverse voltage is applied. Its characteristics mainly depend on its two biasing conditions such as Forward & Reverse.
TI has a full line of ideal diode controller ICs that eliminate the need for external FETs and provide front-end input protection against reverse battery conditions and micro-shorts. These controllers also help minimize power dissipation and voltage drop.
Working
Ideal diodes are designed to work like a perfect conductor when it is forward biased and like an ideal insulator when reverse biased. Conventional diodes have a threshold voltage beyond which the junctions start to break down and conduct current but even though they are small, they have internal resistance and so the amount of current they can carry is finite. Hence they require a heat sink to handle high currents.
But an ideal diode has no internal resistance and therefore it is able to conduct current infinitely when forward voltage is applied above the threshold voltage. This is what gives it the green-curve in the figure above.
The same logic works when it is reverse biased. It is able to inhibit the flow of current through it no matter how much reverse voltage is applied. Hence it behaves like DC converter an ideal insulator as shown in the red-curve above.
These properties make it an excellent replacement for conventional diodes in battery protection circuits. The low forward drop of ideal diodes also reduces voltage losses in external MOSFETs and lowers power dissipation. Furthermore, the fast reverse recovery of ideal diodes prevents huge reverse current from flowing in to the input of the chip during supply line disturbance and micro-short conditions. The reverse recovery time depends on the speed of the internal reverse comparator and the gate pulldown current of the external MOSFET.
Characteristics
A diode is a two terminal device that permits current flow in one direction and blocks it in the other. It acts like an automatic switch. It includes a p-type semiconductor and an n-type semiconductor connected to two electrical terminals which are called the anode and cathode.
The circuit symbol of an ideal diode consists of a triangle against a line and is used to represent the device as a short or open circuit when it is forward-biased. When it is reverse-biased, the device behaves as a perfect insulator with infinite resistance. Conventional diodes have a junction barrier that they must overcome, and they need a threshold voltage before they can conduct current. Ideally, they would have zero internal resistance in both directions of operation.
Conventional diodes also suffer from other characteristics that limit their use. They tend to generate a small amount of leakage current in the reverse direction, and their reverse voltage drop is very high when they are not clamped. This makes them less desirable for applications that require a low voltage drop across the diode. They are also prone to damage from excessive reverse current flow. This is because the diode will start to deteriorate due to the reverse current, which will eventually cause it to give way. This is called the breakdown voltage.
Equation
The key function of a diode is to control the direction of current-flow. If there is a voltage applied to the n- and p- terminals of an ideal diode, it will conduct forward electric current and block the FPGA Modules current trying to flow in the reverse direction. They’re like the one-way valves of electronics.
It is possible to calculate an ideal diode using a couple of basic equations. First, the Thevenin equivalent is used to determine the diode resistance. Next, the Shockley diode equation is used to determine the diode voltage.
If you want to get more technical, the equation for an ideal diode is: where I0 is the dark saturation current, e is the (exponential) ideality factor, and T is the absolute temperature in Kelvin. Changing the dark saturation current will change the turn on voltage of the diode as well as the shape of its characteristic curve.
Conventional diodes have a threshold voltage where they start conducting a finite amount of current when the correct polarity of voltage is applied to them. They also have a breakdown voltage where they stop conducting when voltage is applied to them with the incorrect polarity. An ideal diode does not have a breakdown voltage and will always conduct a small amount of current when applied with the correct polarity.
Differences
The ideal diode is a semiconductor switch that operates as an open circuit when reverse-biased and as a short when forward-biased. It should offer infinite resistance to the flow of charge carriers when reverse-biased and should completely inhibit current flow from p to n, when forward-biased.
When the forward voltage VF is applied to the ideal diode, it should conduct infinitely current from the positive terminal (known as anode) to the negative terminal (known as cathode). This model does not account for voltage drop that occurs across the depletion region and junction barrier, but it is a useful starting point when designing circuits that utilize diodes.
Other diode models provide more detailed information about how a real-world diode behaves in different conditions. For example, a silicon-based conventional diode has a variable voltage drop that varies with temperature. This variation is called the PN-junction slope or forward voltage-drop characteristic, and it allows us to predict how a real-world diode will perform under various conditions.
For applications requiring high-side current blocking, ideal diodes such as the LM74700-Q1 Low IQ Reverse Battery Protection Ideal Diode Controller reduce power dissipation by driving a MOSFET with very low on-state resistance. This results in less thermal management needs and improved reliability. Moreover, this approach ensures the MOSFET does not drain the high-side rail, thereby providing the desired reverse current block feature.