Introduction
Robust electronic systems often incorporate multiple power sources to enhance reliability and user convenience. When using different power sources to supply a device, it is essential to implement switching mechanisms to keep them isolated and prevent potential damage. One common method is to use multiple diodes in the power path. However, a more efficient and versatile solution is to utilize ideal diodes for this purpose. This article explores the advantages of ideal diodes and how they improve power management.
We’ll present two versions of ideal diodes in this article: one where the selection of input rail is independent of the voltage level, and a simpler one where the higher voltage always powers the system.
Challenges of Managing Multiple Voltage Sources
Many applications can operate using multiple voltage sources. Battery-powered devices, for example, often include an option to connect a plug-in power supply as an alternative to their primary energy source. Additionally, a wall wart AC-to-DC adapter, typically connected via a USB cable, is commonly used to supplement the main power supply.
Having multiple power sources not only enhances user convenience but also enables increased robustness through energy source redundancy. However, integrating different voltage sources adds complexity (requires higher effort) to circuit design. It is often necessary to ensure that one energy source does not flow backward into another energy source, as this can lead to potential damage.
Figure 1 illustrates a basic method for protecting unused voltage sources by incorporating diodes in the power path. This works reliably but has a key drawback - in such a setup, the power source with the highest voltage always supplies the load.
Moreover, diodes introduce a voltage drop ranging from 150 mV to 450 mV, in the power path, which generates high power dissipation especially at low voltages. This increased energy loss is particularly problematic for battery-operated devices, where efficiency is a priority.
To overcome the drawbacks mentioned earlier, ideal diodes offer an effective solution. The term ideal diode refers to components that use a switch (usually a MOSFET) instead of a diode.
In its switched-on state. They exhibit a significantly lower voltage drop which is determined by the actual current flowing through the switch and the MOSFET’s on-resistance (RDS(ON)).
Figure 2 illustrates a circuit utilizing two LT4422 devices as ideal diodes. These integrated circuits have a low voltage drop due to their low resistance of just 50 mΩ in the power path. Additionally, the ICs consume only 10 μA of power, keeping overall energy losses to a minimum. An LED can be added as an indicator for which voltage source is powering the load at any given time.
As a result, the design in Figure 2 serves as an improved replacement to the Figure 1 circuit, offering lower power dissipation along with extended features, such as the LED indicator.
However, one feature has remained the same in the circuit in Figure 2. —the voltage source with the higher voltage continues to supply the device. The ideal diode LT4422 includes an enable pin ((SHDN)̅), but the body diode of its internal MOSFET conducts when the input voltage exceeds the output voltage.
To prevent this, there is a derivative of the LT4422, the LT4423, which uses two MOSFETs in the power path back-to-back. These are arranged in such a way that the respective body diodes will not allow a current flow if the other MOSFET is not switched on at the same time.
Figure 3 illustrates a circuit design where the supply voltage can be freely selected to power the load, making its operation independent of the voltage level.
However, since two integrated MOSFETs are required, the resistance in the power path increases from 50 mΩ (LT4422) to 200 mΩ (LT4423) when switched on.
Finally, the version with two MOSFETs (LT4423) includes an integrated thermal shutdown feature. Unlike a standard diode, this ideal diode automatically turns off when the component temperature exceeds 160°C (typical), enhancing system reliability.
Ideal diodes not only help to allow different power supply options for a device, but they also enable greater robustness through implemented redundancy. In addition, ideal diodes offer features such as detection of the supply status with an LED and protection shutdown in the event of excessive temperatures.
Comparison of Power Switching Methods
| Method | Advantages | Disadvantages |
|---|---|---|
| Conventional Diodes (Schottky or standard diodes) | - Simple, low-cost solution | - Fixed voltage drop (150–450 mV) |
| - Prevents reverse current flow | - Significant power dissipation | |
| - Always uses the highest voltage source | ||
| Ideal Diodes (LT4422 - Single MOSFET) | - Lower power dissipation than conventional diodes | - Still relies on voltage level priority |
| - Efficient power selection | - Limited protection features | |
| - No significant voltage drop | ||
| Ideal Diodes (LT4423 - Dual MOSFETs) | - Independent power source selection | - Higher resistance (200 mΩ vs. 50 mΩ in LT4422) |
| - Integrated thermal shutdown at 160°C | - More complex design | |
| - Lower conduction losses compared to conventional diodes | - Slightly higher cost | |
| - Increased system robustness |
Conclusion
Ideal diodes are useful replacements for regular diodes to increase power efficiency in systems with multiple power sources. Besides the reduction of power losses, such ideal diodes also offer flexibility along with additional features. They are easy to use and simple to design with. This is especially true when devices with a high integration are used, such as the LT4422 and LT4423.
Applicable Part Numbers
EVAL-LT4422-AZ
505-EVAL-LT4422-AZ-ND
505-EVAL-LT4422-AZ
505-EVAL-LT4423-AZ-ND
EVAL-LT4423-AZ
505-EVAL-LT4423-AZ