Ideal Diode Mosfet

Description:

Practical Diode
Ideal Diode Mosfet For Sale
Ideal Diode Mosfet Cross Reference
Ideal Diode Mosfet Controller

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.

Ideal Diode controller ICs are available from some manufacturers, for example Linear Technology and Maxim produce a small range of these that work with or incorporate a power MOSFET, sensing the voltage across the.
Current Regulator Diode Breakout Device. DbreakVV: Voltage-Variable Capacitance Diode Breakout Device. DbreakZ: Zener Diode Breakout Device. IDEALNCH: Ideal N-Channel JFET (AA Enabled) IDEALOPAMP: Ideal Operational Amplifier (AA Enabled) IDEALPCH: Ideal.

Performance:

The MOSFET (Ideal, Switching) block models the ideal switching behavior of an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET).

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.

Specifications:

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 2500V_AC, 1 minute
Thermal resistance junction to metal case (R_θJC) 0.21°C/W () *
Thermal resistance junction to ambient (R_θJA) ~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

Connecting a battery backwards to an electronic circuit can rapidly do a lot of damage — current will flood through (and destroy) many integrated circuits when powered up the wrong way, and electrolytic capacitors have a famous tendency to explode. For this reason, it’s common to use a blocking diode in a circuit to provide reverse polarity protection:

If the battery is connected correctly, as shown, current flows through the diode to the circuit, and the circuit operates normally. If the battery is reversed, the battery tries to pull current through the diode the wrong way, and the diode refuses to conduct — protecting the load from damage.

The diode can also be placed on the low rail, as shown below. This is completely equivalent to the circuit shown above for battery-powered applications. However, it may make less sense in circuit provided positive DC power by an external supply, where having a consistent ground can be important. On the flip side, a circuit that operates using negative DC power (which is much rarer) would be much better off with the circuit below for the same reason.

This blocking diode approach works great for many applications. However, when the diode is conducting, there is a voltage drop across it (typically around 0.7V for silicon diodes, 0.2V for Schottky diodes) which means that the load sees a bit less voltage across it. This is a particularly major problem for low voltage applications, where a 0.3V drop can represent almost 10% of a 3.3V system’s power, wasted at the very first component. For higher power applications, a fair bit of power can be wasted as heat in the diode as well.

An alternative solution: MOSFETs

The wonderful thing about MOSFETs is that they can be designed to have incredibly low voltage drops, which translates into less waste heat and more voltage for the load to operate. You can routinely get MOSFETs with resistances of 20 milliohms and below, which translates to allowing 5 amps to pass with a drop of only 0.1V, less than any diode.

You can’t just use them as a direct drop-in replacement, though, because you have to drive the gate of the MOSFET somehow. Here’s the trick: you can just use the other terminal of the battery for this:

So, how does this work? I’m going to define the voltage of the bottom net, to be ground, the voltage of the battery will be 9V, and the theshold gate voltage of the FET will be –4V. You can see a diode drawn as part fo the symbol for the MOSFET — that’s known as the body diode. Before the battery is connected, and will both be zero as well.

At the instant that the battery is connected, will rise to 9V. Before the body diode begins to conduct, will remain at zero as well. This means that will be zero, so the FET will still be switched off.

It turns out that this design actually relies on the body diode to work, at least briefly. Current will flow through the body diode to the load, raising to 7V or so (body diodes don’t tend to have the best forward voltage drops). This brings to –7V, which goes well beyond the threshold voltage and will turn the FET on. At this point, will rise to 9V (minus the small voltage drop across the FET).

Practical Diode

When reverse biased, the body diode will be reverse biased, and will therefore not conduct. will be somewhere between 0V and +9V, that is to say, somewhere between off and very off. So the FET performs its blocking duties admirably.

An N-channel FET can be used on the bottom rail instead, like so:

The same comments apply as for the aforementioned blocking diode on the bottom rail. There’s one additional advantage here, though: N-channel FETs tend to have better performance characteristics than P-channel FETs (although these days, both are remarkably excellent).

What’s the catch?

There are a number of reasons why this MOSFET circuit is not always a suitable replacement for a normal blocking diode:

Some blocking diodes are used in applications where current from a generator is used to charge two separate batteries. If one battery ends up with a higher voltage than another, the blocking diodes prevent electricity from flowing out of the higher voltage battery, back onto the generator leads, and into the other battery. The circuit above will completely fail at this job, because will be across the battery leads, meaning the the FETs will just permanently be switched on at all times. This is a circuit to prevent the load from becoming reverse biased, not to prevent current escaping the load the wrong way.
It might not always be easy to physically connect the gate of the MOSFET to the opposite rail, especially with a circuit laid out with a normal diode in mind.
MOSFETs have a few more maximum limits to check and look after than diode, owing mostly to the fact the a MOSFET has an extra leg. This isn’t a practical disadvantage, just something to be careful about.

Is it safe to pass current through the MOSFET in the non-conventional direction?

It might seem unusual to pass current up through the MOSFET, especially since the curves in datasheets don’t seem to cover this region. However, operation in this so-called third quadrant is routinely used in buck converters, where a MOSFET is used to replace the reverse recovery diode (source). The reasons for replacing the reverse recovery diode with a MOSFET are exactly the same as for replacing blocking diodes — it’s to avoid energy wasted due to diode voltage drop.

MOSFET selection