Power Technologies: Raising the Stakes
Melissa Lum from Linear Technology, now part of Analog Devices, explains the benefits of using a low quiescent current diode controller with reverse input protection for automotive power applications.
In automotive systems, blocking diodes are commonly used to tackle various issues. Specifically, a series diode—positioned between the power supply and the load—protects the connected module from negative input voltage, which can occur if the battery is accidentally connected in reverse during replacement or jump-starting.
This series diode also helps electronic components handle cold crank conditions when the battery voltage drops to 4V. It does so by preventing reverse current and stabilizing the load voltage with reservoir capacitors. Additionally, when multiple batteries or backup power supplies are present, diodes enable redundancy and ensure a smooth power down by connecting the various supplies in an OR configuration.
Diodes are straightforward to use but come with the downside of significant power dissipation due to their forward voltage drop, which can be a problem in both low voltage and high current setups. Even Schottky diodes, which have a lower forward voltage drop, still cause a loss of at least 500mV in supply headroom. This can be a significant issue in 12V automotive systems, especially during cold crank situations when the supply can dip to as low as 4V.
The fixed voltage drop of 400 to 700mV for diodes, regardless of their current rating, leads to considerable power dissipation in the 1 to 2A range, particularly for surface-mount applications. For applications exceeding 5A, the power dissipation necessitates complex thermal designs or expensive heatsinks to keep temperatures within safe operating limits.
The heat generated by the engine further complicates thermal design. Large heatsinks hinder the ability to increase a car’s electronic content or extend its driving range by adding weight. Automotive circuit designers need a more efficient solution.
A better approach is to replace diodes with power MOSFET switches. The MOSFET is configured so that its built-in body diode points in the same direction as the replaced diode. During forward conduction, the MOSFET is switched on, creating a low-loss path through the MOSFET channel, bypassing the body diode. When the current reverses, the MOSFET turns off, and the body diode blocks the current, maintaining diode-like behavior.
This setup can reduce the forward voltage drop and power dissipation by up to ten times compared to traditional p-n or Schottky diodes. An ideal diode controller can drive n-channel MOSFETs in various power supply applications, including reverse blocking, OR-ing, and hold-up. MOSFETs with extremely low RDS(on) values, such as 1mΩ, are readily available, enabling these ideal diodes to handle currents over 50A with significantly lower voltage and power losses than any conventional diode arrangement.
For instance, consider a low quiescent current controller operating in the range of 4 to 80V. The 4V lower limit is crucial for cold crank conditions where diode drops are unacceptable, while the 80V upper limit ensures the controller can handle high voltage transients in 12V, 24V, and 48V automotive systems. This also protects downstream electronics from reverse inputs down to -40V, which can happen if battery terminals are connected incorrectly.
When using a battery, minimizing discharge current is critical in normal operation and even more so when the load is off. This can be achieved with an operating quiescent current of 155µA, which can be reduced to 14µA in shutdown mode. Although the MOSFET is off in shutdown mode, its body diode can still conduct forward current. Some scenarios require controlling the load or managing power delivery, and this low quiescent current controller provides an efficient solution to those needs.
