Ideal or Perfect Diodes

The semiconductor diode is a common device that lets current flow in one direction but not the other up to a rated voltage. The problem with them is they also drop voltage across the diode and the higher current the diode can handle, the greater the voltage drop tends top be or the greater the cooling that is needed. In high current situations and in places where efficiency is critical, these loses can be very bothersome!

Think about off-grid or battery power as examples, every single watt of power is important. A loss of 0.7V - 1V may not seem like much to most people, but when you start talking about 20A or even worse 50A combined with a limited power source, it can make a lot of difference. In low voltage situations, the % of loss is also higher. Think about 12V losing 1V is an 8% loss right there with one part.

The Ideal Diode Concept

Enter the concept of the ideal or perfect diode! An ideal or perfect diode is one that has no voltage drop across the diode. Of course the laws of physics don't allow for perfection, but we can at least improve on basic diodes using other circuits, the most common way is to intelligently use a MOSFET with a low Rds ON value and to turn it on when the current is flowing in the proper direction, and turning the MOSFET off to prevent reverse current.

Engineers have known about this concept for a long time, but many hobbyists don't realize how easy it can be to do or the benefits of using such devices in specific application. Using an ideal diode circuit results in lower voltage drop, lower wattage/cooling, and higher efficiency. While they are often more expensive, there definitely situations where the added cost is worth it.

If you look around, you'll find multiple IC chips specifically designed to control a MOSFET as an ideal diode or they have the MOSFET built in. There are a wide range of voltage ratings available, and some are designed for load balancing or redundant power from multiple sources (either by themselves or when used in parallel with other chips). Many chips for use with an external MOSFET even include a voltage boost circuit needed in order to use an N channel MOSFET as a high side driver, simplifying design.

Since I play with a wide range voltages, and a very good use for things like this can also involve high currents, I'll be unfocussed on easy to build circuits that use external MOSFET's capable of high currents.

A Simplified Ideal Diode Design

Without yet choosing any parts, lets talk about the basics of an ideal diode using an external MOSFET for control.

In it's simplest form, the perfect diode driver compares Vin to Vout and then drives the Gate as needed. When Vin is > Vout, the gate is driver high (using with a built in voltage booster) enough to make the MOSFET have a very low voltage drop. What the voltage drop is depends on the exact IC used as well as the current involved and RDSon of the MOSFET. At lower current the IC's optimal voltage drop is the main factor (in the multiple mV range usually). At higher currents once the MOSFET is fully on, then current and the natural resistance of the MOSFET creates a higher voltage drop them the IC would prefer.

When Vout is > Vin, the Gate shuts down the MOSFET and diode is off blocking any reverse voltage from getting through. The automated switching of the Gate combined with the low RDSon when the MOSFET is on is what makes this circuit act as a diode with very low voltage drops.

Please keep in mind, this is also the absolute simplest form, without any protection circuits and power comes the Vin and/or Vout, some chips do need additional power. Others have a minimum operating voltage before the ideal diode kicks in. Until the IC has enough power to kick in, the built in diode in the body of the MOSFET is being used with it's full voltage drop. Once the power reaches high enough to run the chip, those without dedicated power will start controlling the MOSFET, reducing the resistance and removing the built in diode from the circuit but shorting around it.

Next time we'll start adding additional circuitry needed for various types of protection.