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# Tag Info

27

They cost more, have higher reverse leakage current, and are physically larger according to a quick search. Of course they're much faster though :) Looks like in a same size comparison they can't dissipate as much power as a typical power diode. Also with larger currents you lose that Vfw advantage. Oh and wiki says they normally have lower reverse ...

23

Oli's answer is good on the mechanics of what happens : Without the diode, as increasing base current turns the transistor on harder, the transistor Vce falls below Vbe, until the transistor saturates at Vce=0.2 or even 0.05V. And with the diode present, as Vce falls below about 0.45V ( 0.7V minus the diode 0.25V forward voltage) the diode will start ...

20

What happens is: As the base voltage rises, the transistor begins to turn on and it's collector voltage drops (assuming it has a collector resistor or similar current limiting element) Normally a typical bipolar transistors saturation voltage is around 200mV or less. When the collector voltage, Vce drops below Vbe - Vschottky though, the schottky starts ...

15

Far from a comprehensive list: Schottky diodes of a comparable rating are generally more expensive than PN silicon diodes. I've seen price differences of 20% - 200% depending on the rating. Schottky diodes have a lower maximum reverse voltage rating than is possible with PN diodes.

14

You need to understand three things in order to understand a Schottky transistor. In an ordinary BJT, the saturation state collector-emitter voltage falls below or equal to 0.2 V. In an ordinary BJT, the base-emitter voltage must be equal to or above 0.8 V. The saturation state voltage drop of a Schottky diode is around 0.3 V. Now, if we ...

13

The bigger problem you're likely to run into is operation under forward bias conditions. Schottky diodes still have a voltage drop under forward bias, say 0.25V. That means at 100mA, you're dissipating 25mW of power. Better than a standard silicon diode, but not great especially for a battery constrained device. A better way to get reverse bias protection ...

11

In general, single diodes in 3-lead SM packages have no connection on the third pin. This is a fairly common practice in the industry. I don't have a solid answer as to 'why' they do this, but will share a few possibilities for debate: You cannot install a single diode in a 3-lead package with reverse polarity because of the asymmetry of the package; if ...

11

It's a dual diode. There are two diodes in one package. You can parallel them if you want (connect 1 and 2) but there is no matching characteristics given, so you can't really depend on the rating to be much better, and the leakage will definitely double. The characteristics are for each device, however they are (obviously) tightly thermally coupled so ...

10

For essentially the same reason that schottkys have low forward drop, they have large reverse currents. From the diode equation: $I_f = I_s\cdot e^{\frac{-qV_f}{kT}}, V_f = \frac{kT} {q}\cdot \ln\frac{I_f}{I_s}$ -- having a large Is term is what makes Vf small. However, the reverse leakage current is also equal to the Is value. From their structure, ...

9

It's called a Schottky transistor. Basically, it's a transistor that incorporates a Schottky diode junction that keeps the transistor from saturating, speeding it up in switching (logic) applications.

9

I think you are on the right track, but the 12 in the part number is a red herring, IMO. Probably a date-code. After a bit of detective work, the most likely candidate I found was this ESAC63-004 common cathode Schottky diode array: From what you say, it could be the rectifier of the output of the power supply, as can be seen in an image of a DELL ...

8

Your schematic seems to have a bug. If you apply +5V from USB, then D1 is reverse-biased, and μC wouldn't get +5V. Here's what I can propose instead. Of course, this is just a rough diagram, which shows only power distribution. FTDI chip is bus powered only always. When +12V source is not present, the gates of Q17 and Q18 are pulled low and the +5V rail ...

8

Here's one that may sound a bit strange, but is important in some uses: low forward voltage drop. Sometimes it is useful to distribute the heat dissipation among the components in a device. Take for example the traditional linear voltage source: You have a transformer, a full wave rectifier, large capacitor and a voltage regulator plus some smaller ...

6

Schottky diodes typically have much higher reverse leakage currents than traditional diodes.

6

If you run a diode near its rated current, you don't get much advantage, but that has little to do with the process involved. Instead, the limit in such cases is the power dissipated by the package, and for the same current level that implies the forward voltage will be about the same. So the Schottky diode will be wimpier (for the same current) and will ...

6

This circuit is not good. The very important thing to understand is that it is using an opto-isolator to give complete electrical isolation between the mains on the left and the logic on the right. There must be no connection between the two. So what's wrong? The return path is connected to earth. This should never be done. It should be connected to the ...

6

Yes, you should really use a diode with fast reverse recovery. A Schottky is a obvious choice at this low voltage. Schottkys have effectively instant reverse recovery for your purpose. The lower forward voltage of the Schottky will also cause less backwards EMF on the inductance during the off time, making the overall system more efficient. I see there ...

5

Silicon schottkys can be found at 250 Volt easily but at 250V there is a VERY limited selection.Manufacturers via their sales reps state that they can not make them above 250 V.There is the problem of increased reverse leakage current that can upset some circuits AND cause thermal runaway at elevated temps below Tjmax at voltages below Vrmax .This runaway ...

5

If the common is connected to a low-impedance node you can leave it floating. If not, I'd short it out lest it act as an accidental antenna/detector. I'd also suggest using the single out of the pair that would be present if you bought the one-diode version of the 3-pin package (if possible), just for future compatibility. IOW if you're using the BAT54C or ...

5

I don't see any useful functionality for that part. In the Jameco schematic the IRF46 MOSFET used will handle very large transient currents through the body diode, which is directly in parallel. It also has a reasonably healthy 28A/11mJ avalanche rating. A reverse connection of the load will put all the source voltage (minus a diode drop) across the sense ...

5

Schottky diodes do not have reverse recovery time. Recovery from what? In a normal p-n junction diode, there is a charge carrier depletion region, and so the correct polarity electric field applied (the voltage drop) is actually switching it from non-conducting to conducting. If that field is removed, or applied in the opposite polarity, it is switched off ...

5

What is the reasoning behind using the NPN-transistor? The NPN is configured as an emitter follower and basically this means if you set a certain voltage on the base (using the low power zener), the voltage on the emitter will be between 0.5V and 1V lower on the emitter across a large range of load resistances. In other words it's a unity gain voltage ...

5

Using the schematic option produces ugly results, but I hope the following diagram will help. simulate this circuit – Schematic created using CircuitLab It shows a bolt passing through a single layer of material, with a sholder washer on each side, and a nut, all in cross-section. When the nut is threaded onto the bolt, the washers are held in place.

5

The equivalent is very similar, just mirrored:

5

Here's the relevant part of the system diagram from the datasheet you linked: A Schottky diode typically has a forward voltage drop of 0.2 to 0.3 V. So if 5 V is applied at "V+", then only about 4.8 V is available to supply the Richtek switching converter or any other circuits that might be connected to that same circuit node. The reason it's called a "...

5

Why is the MOSFET source connected to load and drain to battery? As shown in the schematic, the MOSFET has a body diode connected from drain to source. If the MOSFET were attached the other way around, USB power could flow through D1 and the body diode into the battery, charging it faster than is safe. (Note that when is MOSFET is on, it allows current to ...

5

(1) In the limit as Vin approaches Vout the dropout voltage across a good LDO will be well under 1 volt. The series input diode adds 0.6 - 1.0 V depending on diode and current. In limiting cases the impact of the series diode can be significant. (2) When power is 'turned off' it's usual for the system to want power turned off :-). Turning it off "more or ...

5

Yes, that would work fine, but you can make this even more simple: simulate this circuit – Schematic created using CircuitLab Zener diodes behave more or less as a "normal" diode when in forward mode so we can use that to clamp negative voltages. When using a 12V, 400 mW zener diode the maximum current through the zener diode can be 400mW / 12 V = ...

4

Reverse leakage current would only apply in the opposite polarity case. This is current that is leaking through the semiconductor material when the diode is in the off condition because it is incorrectly biased. Thus in your application, it is not an issue. However, you are right to be wary of the forward voltage drop. Only you can decide what an acceptable ...

4

I'd say that a Schottky is an excellent choice as it's forward voltage is less than what the ESD (Si) diodes on your semiconductors probably have. You are then diverting this mistake, and it's current away form more delicate parts into something that is beefier/ more robust. Reverse leakage can be an issue with battery powered devices a Schottky's do have ...

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