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.