Backup power system sometimes require OR-ing of batteries. For example to get the system online while the main battery is discharged. When currents are low, this is done easily using a simple diode. However, when currents become more significant, power dissipation in a diode quickly increases to unacceptable levels because of the unavoidable forward voltage of the diode. Not to mention that this forward voltage wastes between 5 and 10 percent of the available power (from a 12V battery).

Diode with 16² cables

For this reason, this active 'ideal' diode block was designed. The diode is replaced with a high current mosfet. Without working voltage, this mosfet behaves as a regular diode. After applying voltage, the mosfet driver is activated. The driver enables the mosfet and regulates the forward voltage to 30mV (30 times less than a typical normal diode), or to the lowest voltage that can be archieved giving the current draw and the Rds resistance of the mosfet. This resistance causes a forward voltage of about 1mV per Amp.


To put this in context; the 16mm² wires and their terminals in the 60A test setup cause a voltage drop of 85 and 95mV respectively! The voltage drop accross the diode, measured on the terminal screws was 73mV.


  • Reverse voltage rating 40V
  • Current rating 600A
  • Package SOT-227 (footprint 25x38mm)
  • M4 screw terminals suitable for 4mm and 5mm ring terminals
  • Electrical isolation to metal base 2500VAC, 1 minute
  • Thermal resistance junction to metal case (RθJC) 0.21°C/W () *
  • Thermal resistance junction to ambient (RθJA) ~20°C/W () *
  • Maximum junction temperature 175°C
  • Minimum working voltage 8VDC
  • Drain-source resistance (Rds) 0.001Ω
  • Forward voltage (Vfw) ~30mV or 1.1mV/A, whichever is larger
  • Includes 3M thermal conductive pad
  • ~12cm (5") ground lead with serrated 4mm ring terminal

*) Practical thermal example: Diode with 195A current (resulting in 42W dissipation), screw mounted in the middle of a 50cm extruded aluminium 40x40mm profile without any thermal conductive materials, put flat on a bench. After thermal equilibrium at 20 degrees ambient, temperature of diode is 90°C and temperature of the alumiunium (in the middle) is 60°C. See thermal image. At such current, a normal diode would approach 300°C or more...

Special versions:

55V, 550A, 0.0013Ω, Vfw ~30mV or 1.4mV/A
75V, 480A, 0.0019Ω, Vfw ~30mV or 2.0mV/A


Item: Description: Status: Price: Action:
77-068-040Ideal diode 40V 600AIn stock€ 65 ex VAT
77-068-055Ideal diode 55V 550ASpec. order€ 70 ex VAT
77-068-075Ideal diode 75V 480AIn stock€ 65 ex VAT