While trying to select a MOSFET for a project, I see a lot of them incorporate a schottky diode. What is the purpose of this?

 

Look at Figure 7, and the paragraph before it:

http://www.irf.com/technical-info/appnotes/an-967.pdf

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How would this affect using one of these in a simple switching application a reverse polarity protection circuit?

 

Wouldn't you just use a diode in a reverse polarity protection circuit?

Are you sure you aren't referring to the parasitic diode that's always present in a MOSFET? Or are you actually referring to a diode they integrated into the MOSFET on purpose?

Your last sentence didn't seem to make too much sense though. Try writing two sentences.

 

Yeah...I was cutting and pasting and it didnt work out. A lot of the data sheets that I've seen of MOSFETs have a diode or schottky diode in the symbol . Im planning on using a mosfets as a simple 12V switch, and also as reverse polarity protection (a regular diode will drop about .7 V, but with a mosfet, there would be less. (see figure D)

http://www.elecdesign.com/Files/29/9945/Figure_01.gif

Sorry about the poor previous post.

 

THose are the intrinsic parasitic diodes that come with every MOSFET. They don't purposely put it in. It just ends up there always. There is no real reason people "put it in". It's supposed to be a very powerful, but very slow diode. I guess for some reason they have less voltage drop than REGULAR diodes so you can use this to your advantange. I'm pretty sure if you don't look for regular diodes you will find diodes with less voltage drop than the MOSFET intrinsic diode. It's not hard to find MOSFET intrinsic diodes with much higher voltage drops than discrete diodes- just hunt around on IRF's datasheets for a while.

 

The secret in polarity protection circuit using MOSFET is to exchange the original purpose of DRAIN and SOURCE pin so that the polarity of the instrinsic diode is now the right way round.

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L.Chung

 

I've tried that and it works very well too. With other control circuitry MOSFETs make very good bridge rectifiers as well.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

I'm getting tired of having electrolytical caps blowing up in my face when building my guitar tube boosters vintage design. It is hard to know right away what pole I need and by then I am sitting there with fluff in my face and a hearing problem.

The pedal has great sound but I don't want to sell any of them until I figure this out.

Could anyone explain the figure discussed and draw a schematic that I can understand without an education.

I am using 24VDC powersupply with several volts and mA to spare. I get 12VDC and 18VDC of regulated voltage to use on the tube. I could use a Schottky diode I guess. But what kind? And how to connect it?

Summary:
Using 12 and 18 VDC voltage regulators
Electrolytical Capacitor 100uF (are running out of stock if this continues)
Power Supply: 24VDC 1A
Tube: need 12 and 18VDC, at least 150mA
Schottky Diode: Goes where??? What kind? Will it protect circuit during wrong polarity?



I attached a picture with my working configuration without polarity protection and zip file of me playing guitar with the pedal for your listening pleasure. I am a total n00b at guitar too. =)

Attached Thumbnails

 

Attached Files

tube preamp2.zip (4.61 MB, 9 views)

 

The higher voltage drop is actually preferred in back-emf protection diodes. By requiring the voltage to be higher, you're reducing the current the diode must handle. The voltage in the inductor will ramp up until it starts to conduct, whereby the rest of the energy is delivered as current.

Be careful when using the diodes that are built into mosfets. The mosfet has to dissipate the heat for the diode as well, and you have to take this into account in your heat calcs.

Never use the body diode for a protection diode. This diode is part of the mosfet structure and is "in use" while the mosfet is conducting. If it is trying to handle a reverse current when you turn the mosfet on, you can get some spectacular results ending with buying more mosfets.

 

Your schematic of a vacuum tube circuit makes no sense whatsoever.

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Uncle $crooge

 

Please explain. You're saying that reverse recovery energy is somehow going to break something when the channel is turned on?

It would seem to me that the extremely popular "synchronous rectification" process is operating in this mode.

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de KI6RWX

 

Very true. For example, when used as the "flywheel" device in a synchronous forward converter, the body diode (which has the same current rating as the FET) can sometimes "turn on" before the MOSFET. In such cases a schottky is often placed in parallel to prevent it from conducting... not because anything bad will happen, just that its more efficient not to use it. Sometimes the MOSFET's are run in the third quadrant and it doesn't matter if the synchronous circuitry is working or not, the supply will still function because of the body diode, but efficiency will take a big hit when running that way.

 

As mosfets are used heavily in high power amplifier circuits, the diodes are in circuit to protect against eddy currents seen as speakers are seriously inductive loads. On switch off you can get high return currents that unless the diodes are there, will fry naff transistors like tip's and darlington pairs etc.

Similar in motor control circuitry a diode is placed in parallel with the load to eliminate these same currents backing up into circuit when the devices switch off and the loads are still effectively generating.

They merely protect semiconductive components as fuses and switchgear are just not practical or effective at the typically low levels transistors operate at.

 

The point I wanted to make is the difference between the body diode, which is the intrinsic diode that is formed as part of the mosfet structure, and a true reverse protection diode which is sometimes embedded into a mosfet, or just used externally.