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