Enhancing the Ideal Diode

Enhancing the Ideal Diode

In my last post, I introduced you to the concept of the ideal or perfect diode as well as the simplest form of how it could be implemented. Now I'll expand on that with several common additions to help protect the circuit.

Spike & Inductance Protection

The basic setup had no protection for bad voltage spikes or inductive kickback from the load. Generally this is accomplished by adding capacitors and diodes to both the input ant the outputs. Additionally, often the output diode (and sometimes the input diode) are TVS diodes that act like high current zener diodes that help kill positive spikes.

Normally, a large value capacitor with a low ESR rating is used on the output. Both the capacitors and diodes combine to help protect the MOSFET & IC from surges.

Reverse Input Voltage or AC Protection

Most ideal diode chips can't handle negative input voltages, so additional protection is needed if there is any chance that power may be hooked up backwards or is on AC input source is used.

By replacing the input diode with two back to back diodes, we can create a virtual ground for the IC chip, ensuring that it's GND can never be higher then Vin. Some people also add a resistor between the two diodes and the GND pin.

The above additions help make almost any ideal or perfect diode safer to use at very little cost or board space and should probably be considered mandatory except for special situations.

Keep in mind, when adding this to protect the circuit, the Voltage rating for the Q1 MOSFET must be increase because of the negative peak voltage on Vin compared to the positive voltage on Vout.

On/Off Control

Some IC chips allow for an On/Off input signal that can be used to disable the perfect diode. At a glance, this seems very useful, but there is another thing that you must take into account, the built in diode of the MOSFET's body! As mentioned earlier, the N-channel MOSFET has a built in diode that can't be ignored from Source to Drain, so when you turn it off, you still have a high current path through a real diode from Vin to Vout. If that isn't an issue, the following can be left out.

This can be solved by putting two MOSFET's back to back, so both MOSFET's get turned off & on, removing the diode from the circuit.

For IC chips without integrated support for double MOSFET's adding a couple resistors & a zener diode is the common solution.

R1 adds a slight delay to Q2 turning on (which is also why R2 is good to have) that makes the circuit run a little bit smoother, usually a very low value such as 10 ohm. ZD1 & R2 protect the Gate-Source voltage from getting too far out of range, protecting both MOSFET's from blowing. R2 is usually a high value resistor since there will still be some current bleeding through it and the body diode of Q1. ZD1 is usually 12V-15V range depending on the specifications of the MOSFET, and ZD1 & R2 can even be omitted  safely when working with voltages lower then the Gate-Source Vgs limit.

Some IC chips are designed with this in mind and include an additional pin to connect to the common Source line and don't require ZD1 & R2. Some designs also add a timing cap and resistor to the Q2 Gate to add a soft start feature.

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.