Ideal Diode

How to drop the diode dropout voltage

For some but not particularly important reason I had the imagination of saving half a watt of electrical power. I've got a low voltage DC power distribution network installed at my home. This network provides power to a handful of always-on devices like the internet router, my desktop data display, a NAS box and so on. The average power consumption of these devices is about 45 Watts.

The intention of the DC distribution network is to save electric power: One central power supply unit provides an intermediate DC voltage (here about 20 Volts). Some local high-efficiency DC/DC converters provide individual power to all the devices (5V, 12V). The total efficiency (from 230V AC line to DC input of the devices) is somewhat better than using their wall warts.

This provides a few (about 3 ... 5) watts power saving, assumed the system is running off the AC line. Additionally my system achieves a huge amount of power saving by providing power off a solar panel. As long as there is enough sunshine, the solar panel fully supplies the devices, and no AC power is required.

If there isn't full sunshine, the solar panel provides part of the required power.

Both sources (solar panel and AC powered supply) and an additional battery must be combined. Until now I've used some shottky diodes to do that. The drawback of these diodes is their forward voltage drop (about 0.3 ... 0.5V). So the diodes dissipate total about 1W, just for beeing there and beeing necessary to decouple the power sources.

Using P-Channel MOSFETs and some active circuitry, I've made a triple "ideal diode". This circuit is able to combine the power sources while maintaining about 30mV...40mV voltage drop, resulting in 0.1W dissipation.

The circuit

Schematic diagram of the triple ideal diode

A short explanation of the circuit:

The P-CH power MOSFET T1 is "reverse" connected from the input to the output. As long as there is no gate drive, it acts like a normal silicon diode.
R1 and D2 provide the power supply for U1. U1 senses the voltage drop across T1. It is offset by 20mV through R7.
As long as the input voltage (T1 drain) is lower than the output voltage (T1 source), U1s output voltage stays at 0V related to T1s source. The ideal diode is in the blocking state.
If the input voltage gets higher than the output voltage (more than about 20mV, set by R7), U1 outputs a negative gate drive voltage to T1, which in turn makes T1 conducting. U1 regulates T1s drain voltage 20mV above the source, resulting in 20mV voltage drop across the now conducting "ideal diode". If the current through T1 is large enough, the Rds_on of T1 causes more voltage drop, and U1s output is saturated at its negative supply.

Using the components as shown in the schematic, this diode should be able to operate with input voltages from 12V to 25V, and output current up to 5A.

Some electroporn pictures of the prototype

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