Puporse of schottky diode in a MOSFET

[sazerac99us]

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?

 

[Analog]

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:

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

 

[sazerac99us]

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

 

[dknguyen]

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.

 

[sazerac99us]

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)

**broken link removed**

Sorry about the poor previous post.

 

[dknguyen]

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.

 

[eblc1388]

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)

**broken link removed**

Sorry about the poor previous post.

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.

 

[Hero999]

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

 

[Rewind]

Voltage polarity protection

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. =)

 

[smanches]

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)

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.

 

[audioguru]

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

 

[mneary]

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.

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.

 

[indulis]

It would seem to me that the extremely popular "synchronous rectification" process...

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.

 

[DTaylor]

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.

 

[smanches]

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.

 

[mneary]

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.

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?

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.

I understand that the body diode ratings must be respected, but what is this reverse current problem? You're not referring to shoot-through on an H-Bridge due to timing problems?

 

[Mr RB]

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?
...

It shouldn't. If the body diode is conducting reverse current and the FET is then turned on it just gets greater conductivity (in either current direction).

If they use a schottky in parallel with the FET body diode it's just to decrease the dissipation as the schottky is faster and lower Vf and of course dissipation is also external to the FET package.

 

[RCinFLA]

Also called substrate diode or bulk diodes. They are not schottky. They are pretty fast recovery but have relatively high Rs so should not be relied upon for reverse current suppression.

Insulated gate bipolar transistors are putting fast diodes into their package that can be used for reverse current suppression.

Schottky diodes are limited to about 60 v reverse breakdown.

 

[smanches]

I still missed the point. How can activating a MOSFET while its body diode is in reverse recovery mode damage anything?

I understand that the body diode ratings must be respected, but what is this reverse current problem? You're not referring to shoot-through on an H-Bridge due to timing problems?

I think I was confused. I just reread everything again, and realize they were talking about h-bridges in particular. The extra reverse current being handled by one of the mosfets can cause the di/dt of it's complementary mosfet to increase too quickly, causing the parasitic BJT to turn on. This is what kills the mosfet.