A Schottky diode can prevent reverse currents, but this approach leads to significant power loss at higher forward currents in normal operation. A simple protection scheme based on series P-channel MOSFET shown in Figure 3 reduces this loss, but may not work well at low input voltages (for example, engine start) due to the device threshold voltage. A more efficient approach is to use an ideal diode controller, such as the LTC4376, which drives a series N-channel MOSFET that cuts off input voltages below ground. In normal operation, an ideal diode controller regulates the source to drain the voltage of the N-channel MOSFET to 30 mV or less, reducing forward voltage drop and power dissipation by more than an order of magnitude compared to a Schottky diode.
Superimposed Alternating Voltage
The superimposed alternating voltage test (ISO 16750-2: test 4.4) models the impact of the ac output of the vehicle’s alternator. As the name implies, a sinusoidal signal is superimposed on the battery rail with peak-to-peak amplitude of 1 V, 2 V, or 4 V, depending on severity level classification. For all severity levels, the maximum input voltage is 16 V. The frequency of the sinusoid is swept logarithmically from 50 Hz to 25 kHz and then back to 50 Hz over 120 seconds and repeated a total of five times.
This test causes large amplitude current and voltage swings in any connected filter network with a resonance below 25 kHz. It can also cause problems for switching regulators where loop bandwidth limitations make it a struggle to regulate through high frequency input signals. One solution is an intermediate rectifying element such as a power Schottky diode, but as with reverse voltage protection, this is a poor way to solve the problem.
An ideal diode controller will not work here like it did for reverse voltage protection because it cannot switch the N-channel MOSFET sufficiently fast to keep up with the input. The limiting factor is the gate pull-up strength, which is typically limited to 20 µA or so by an internal charge pump. While the ideal diode controller can quickly turn the MOSFET off, turn on is painfully slow, unsuitable for rectification of anything beyond very low frequencies.