What to do when a reverse current is generated?
Back-to-back MOSFETs are available! When the MOSFET is turned off, it can block the current from both directions.
So how is the reverse current blocked?
There are generally two reasons for the generation of reverse current: the first is that when the power supply and the system are disconnected, the input voltage drops suddenly, leaving a higher voltage at the output, resulting in the generation of temporary reverse current.
The second is when the MOS transistor is used for load switching, the diode is forward biased, and the reverse current can flow in the opposite direction through the body diode, and the output voltage is greater than the input voltage, which will generate a reverse voltage, resulting in a linear increase in power dissipation at both ends of the body diode.
So what are the dangers of reverse current?
It can damage internal circuitry and power supplies, in addition to reverse current spikes that can damage cables and connectors.
How do I block a reverse current? There are three common ones:
- Diodes are used, but they cause a drop in the forward voltage, which increases the total power dissipation and reduces the power supply by 0.6V to 0.8V, resulting in a shorter battery life.
- The second type is the back-to-back MOSFET we mentioned at the beginning, which blocks the current in both directions when the MOS tube is turned off. It has a lower voltage drop than a diode, but requires more space.
3. Finally, the position of the MOS transistor allows the body diode to move from the input to the output, and when the MOS transistor is closed or opened, there will be no reverse current flow.
However, the disadvantage of this circuit is that it cannot be turned off. Because between the power supply and the load, the diode is always on.
So when do you need to block the reverse current?
1. When the power supply is multiplexed, when the 3.3V power switch is turned on, a 5V voltage is applied to the system, for the switch, using a simple FET solution, the reverse current can flow through the FET body diode even if the switch is disconnected.
2 or gate applications, similar to power multiplexing, except that the highest voltage always powers the system, rather than choosing a power supply to power the system.
3. Large capacitance, when the input power supply drops, the supercapacitor keeps the output, in order to protect the upstream power supply or components, it is necessary to set the reverse current protection.
4. The sudden loss of input power may produce reverse current, the capacitance of the output of the switch is greater than that of the input end, and the voltage attenuation of the output end is very slow, that is to say, the voltage drop speed of the output end of the switch is slower than that of the input end. The voltage on the switch output will be greater than the input, and the reverse current will flow through the switch.
Some of the above pictures and sources come from the Internet