Limitations of the Ideal Diode
An ideal diode is a one-way street for current that allows current flow in the forward direction and blocks it in the reverse direction with zero resistance and no voltage drop. However, a real diode is not as perfect and has many limitations that make its use more difficult.
The IRLML2244 is a simple ideal diode controller that monitors the power supply voltage to provide a digital power-good signal or overvoltage protection. Let’s take a closer look at its specifications and see how it compares to a schottky diode.
Characteristics
An ideal diode has infinite current when forward-biased and no reverse voltage drop. It also has no cut-off current and no saturation current. In contrast, a practical diode has a small but negligible reverse saturation current.
The i-v characteristic of an ideal diode is a curve that plots the relationship between current and voltage across the device. In the i-v diagram of an ideal diode, a positive value on the X-axis represents the forward voltage and a negative value on the Y-axis represents the reverse voltage. The device is considered to be forward biased when the voltage on the Y-axis is greater than the forward voltage on the X-axis.
A key feature of an ideal diode is that it does not perform the phenomenon known as reverse breakdown voltage, which causes a diode to fail and begin conducting ideal diode heavy currents. Instead, an ideal diode offers infinite resistance and prevents current flow irrespective of the magnitude of the reverse voltage.
An ideal diode can be used in a variety of applications, including as a simple forward voltage protection circuit for battery-operated devices, a diode OR for high-reliability redundant power supplies and overvoltage protection on power inputs. Maxim’s MAX40200 is an example of a complete ideal diode device that operates from 1.5V to 5.5V and handles up to 1A. It’s available in a tiny 0.73mm square 4-bump WLP or SOT23-5 package and is thermally self-protecting.
Applications
The ideal diode concept serves as an essential foundation to understand and predict semiconductor behavior. While oversimplified, it provides an effective model to explain how diodes control current flow in circuits and devices. This understanding allows engineers to build on the concept for more advanced simulations and designs. Real diodes possess a number of complex features, such as forward voltage drop, reverse saturation current, and junction capacitance, which are not factored into the ideal diode model.
The perfect behavior of an ideal diode is exhibited by its linear I-V (current-voltage) characteristics. A vertical line indicates current flow under forward bias, while a horizontal line signifies no current flow under reverse bias. Additionally, an ideal diode exhibits no cut-off or saturation current.
These characteristics make an ideal diode a valuable component for a wide variety of applications, including amplifiers, power management, and circuit protection. In many cases, an ideal diode can replace a series-resistor in amplifiers to lower the overall system cost and improve reliability.
An ideal diode can also be used in backup supplies, where it will protect the battery from damage by blocking the high-priority source. To ensure the MOSFET body diode is always conducting, a low-leakage capacitor can be charged from the battery to keep the voltage on the diode above the threshold required to start conduction.
Materials
An ideal diode uses semiconductor materials with a structure that allows current to flow in only one direction. This feature allows for devices like solar chargers to harness energy from the sun to power electrical equipment and charge batteries for use later.
When forward-biased, an ideal diode offers zero resistance and passes infinite current. It also doesn’t have a reverse breakdown voltage, which is the point at which conventional diodes break down and conduct large amounts of leakage current when reverse-biased.
However, despite these characteristics, an ideal diode isn’t practical to produce because the junction barrier of a semiconductor device needs a threshold voltage in order to be overcome by current. Also, an ideal diode cannot offer infinite polarity reversal because the holes and electrons in the semiconductor would recombine in an unbalanced manner.
In spite of these drawbacks, an ideal diode is still a useful tool to ideal diode manufacturer have in the circuitry of electronic devices. Straightforward experiments such as the Diode Circuit demonstrate a diode’s primary functionality of converting Alternating Current into Direct Current. Creating a Half-Wave Rectifier Circuit showcases a diode’s ability to convert Alternating Current into Direct Current by periodically reversing the flow of current. Lastly, contriving a Zener Diode as a Voltage Regulator demonstrates a diode’s role in maintaining a stable output voltage despite changes in input voltage or load conditions.
Processes
While the ideal diode is a convenient model for simplifying circuit analysis, real-world manufacturing processes introduce some limitations. Understanding these limitations helps you to better apply the principles of diode physics to real-world applications.
The ideal diode consists of two types of semiconductor material with a PN junction at their interface. When a voltage is applied, the holes in the p-type semiconductor and the electrons in the n-type semiconductor are forced to cross over this junction. This results in a unidirectional current flow, which can help power electrical devices and charge batteries for later use. The ideal diode also has infinite resistance to reverse voltage, effectively blocking all currents in the opposite direction.
In reality, this PN junction has some intrinsic voltage drop (as seen in the piecewise linear I-V characteristic), as well as a small leakage current when reversed biased (known as the Shockley Diode effect). These effects should be taken into account when designing circuits using real diodes.
To avoid these effects, our ideal diodes consist of back-to-back MOSFETs with low on-state resistance and matched gate threshold voltage to minimize parasitic currents. This greatly reduces power dissipation, which translates into lower thermal management costs and improved reliability for your application. In addition, our EDI-based method of controlling the MOSFET source-to-drain voltage eliminates comparator-based techniques that allow DC reverse current or oscillate between on and off during supply switchover. This allows you to achieve the high-side supply isolation and voltage drop control required in many high-speed applications.