1. Testing A Mrf1508 Mosfet With Diode Checker
2. Body Diode
3. Mosfet Parasitic Diode
4. Mosfet With Diode Symbol
5. Plecs Mosfet With Diode
6. Ideal Diode Circuit Mosfet

1. Pulsed drain current (MOSFET) (t = 300 μs) IDM-15 Continuous source-drain diode current (MOSFET diode conduction) TC = 25 °C IS-4.5 a TA = 25 °C -1.6 b, c Average forward current (Schottky) IF 2 b Pulsed forward current (Schottky) IFM 3 Maximum power dissipation (MOSFET) TC = 25 °C PD 6.5 W TC = 70 °C 5 TA = 25 °C 1.9 b, c TA = 70 °C 1.2.
2. Si7703EDN, P-Channel 20-V (D-S) MOSFET With Schottky Diode.
3. This solution, however, creates a diode between the drain (cathode) and the source (anode) of the MOSFET, making it able to block current in only one direction. Body diodes may be utilized as freewheeling diodes for inductive loads in configurations such as H bridge or half bridge.
4. Yes, power MOSFETs have a parasitic diode called Body Diode. As a result of this diode, a single MOSFET can work only as a unidirectional switch. A single MOSFET can't switch-off the opposite direction, because the diode conducts independent of the gate. The body diode is usually fairly slow to turn on.

Continuous Source Current (MOSFET Diode Conduction)a IS - 1.05 - 0.75 Average Forward Current (Schottky) IF 0.5 Pulsed Foward Current (Schottky) IFM 7 Maximum Power Dissipation (MOSFET)a TA = 25 °C PD 1.15 0.83 W TA = 70 °C 0.73 0.53 Maximum Power Dissipation (Schottky)a TA = 25 °C 1.0 0.76.

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.

Testing A Mrf1508 Mosfet With Diode Checker

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).

Body 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

The figures below are for the P-FET design, N-FET design will be similar except with a few minus signs thrown around the place. This is just a rough guide, etc.

• The absolute maximum should be at least , where is the voltage of the power supply/battery. This is because in a situation when correct polarity rapidly switches to reverse polarity, you get , .
• The absolute maximum should be at least .
• Check the output characteristics to ensure that the FET is thoroughly on when . Roughly speaking, this corresponds to being greater than than (remembering, e.g., –2 is greater than –9).
• And of course, check that the FET can handle the current, the power dissipated, and the heat generated. And yes, these are three related but very different things that need to be checked independently (the last is calculated using thermal resistance, given in the datasheet).

Fin.

So there you have it. If anyone out there uses this circuit because of this page, I’d love to hear about it! And as always, I will attend to any questions left below.

Posted by Robert on February 22, 2013

https://www.rs20.net/w/2013/02/using-mosfets-as-blocking-diodes-reverse-polarity-protection/

by Crutschow

When you parallel a battery with another battery or other source, the batteries are often required to be back charged. This can be done with standard diodes, but that gives close to a half volt drop — even with Schottky diodes. This is especially problematic with low-voltage batteries, where that drop is a significant percentage of the battery voltage, noticeably reducing efficiency and battery life.

To minimize this forward drop you can configure a MOSFET as an ideal diode, which has a very low drop in the forward direction (equal to the current times the MOSFET’s ON resistance) while blocking the current in the reverse direction.

Below is the LTspice simulation of a simple ideal-diode MOSFET circuit. It uses inexpensive components consisting of a P-MOSFET (for use in the positive rail) with a dual PNP transistor and two resistors.

Q1 and Q2 form a current mirror circuit. The indicated values of R1 and R2 cause Q2 to be on and thus M1 off (Vgs ≈0V), when there is no voltage difference between the drain and source of M1. The mirror has a gain of ≈130 from the voltage difference between the two emitters to Q2’s collector voltage change.

In the forward direction (output voltage lower than the battery voltage) the current mirror becomes unbalanced due to the difference in emitter voltages, such as to turn Q2 off, which puts the P-MOSFET gate near ground potential, turning it on. This allows current to flow from the battery to the output (left to right) with a low drop. (MOSFETs conduct equally well in either direction when on.)

Mosfet Parasitic Diode

When the output voltage becomes slightly higher than the battery voltage, this voltage reversal across the MOSFET unbalances the current mirror in the opposite direction, causing Q2 to turn on. This causes the MOSFET gate voltage to rise, reducing Vgs [V(G,Out) in plot], which turns it off and prevents reverse current flow.

This can be seen in the simulation, as the current only goes out of the V1 battery when the V2 output voltage is lower than the battery voltage, and doesn’t flow in the reverse direction when the output voltage is greater than the battery voltage. The maximum voltage drop, when the battery is providing 2A current is ≈32mV with the MOSFET shown, demonstrating the near ideal diode operation.

Mosfet With Diode Symbol

The current mirror operation is very sensitive to any offset between the two transistor base-emitter voltages, which could possibly allow some current conduction in the reverse direction. It is thus recommended that a matched transistor pair be used, such as the DMMT3906W shown on the schematic (basically two 2N3906’s in one package), which have their Vbe matched to within 2mV max and are thermally connected.

(The simulation was done with 2N3906‘s which are perfectly matched in the simulation, unlike real life.) The DMMT3906W pair are quite inexpensive, selling for U$0.37 here, for example.

Plecs Mosfet With Diode

The P-MOSFET selected should have an on-resistance small enough to give a low voltage drop when conducting the maximum battery load current. If the battery voltage is less than 10V then a logic-level type MOSFET should be used which have gate-source threshold voltages (Vgsth) of less than 2V.

One of these circuits can be used at the output of each battery; however, many are in parallel.

You can read more articles by Electro-Tech-Online “Well-known” member, Crutschow, here.

Ideal Diode Circuit Mosfet