MOSFET for reverse polarity protection

[POTL]

Hello everybody.
Once again I decided to take an interest in protection from reverse polarity.
In classic pedals we have 2 popular protection options.
1) 1N4001 at which we lose tension
2) 1N4148 which works like a kamikaze
Both options are popular, but have the disadvantages described above.
Tell me more about the p-channel mosfet.
I understand correctly that it does not burn like 1N4148 and does not make voltage drop like 1N4001?
There are many variants of these transistors, what characteristics should I look at?



I looked at the Walrus boards and saw the components in the body of the sot-223.
Other elements capable of protecting the circuitry I did not see on the board.
What do you think - is this a mosfet?
If so, I have 2 questions:
1) What are the variants of schemes?
2) What parameters are important for a transistor?

[bean]

This will tell you everything you need to know.
http://geofex.com/Article_Folders/mosswitch/mosswitch.htm

[POTL]

This will tell you everything you need to know.
http://geofex.com/Article_Folders/mosswitch/mosswitch.htm

Hello, thank you.
A bit offtop when waiting for diy / shop project updates on your site?
For a long time no new projects are visible.

[POTL]

I read the article, but I'm not sure, except for Rds you have to look at other characteristics?
For example volts and amperes or something else?

The Internet offers various variations of the scheme, in the article geofex it is said only about the addition of a resistor between the ground and the gate.

[R.G.]

When trying to pick a device for a power switch, the first two things you look at are, as you guessed, voltage capability and current capability. Presumably if you're trying to put a power switch into a pedal circuit, you already know what power supply voltage the pedal works on, and can make a good estimate, or even just measure, the current it takes.

The MOSFET has to be rated for enough voltage and current to do the job.For powering a pedal, that means a voltage of greater than 9V (over a decade ago when that was written) or maybe greater than 18V for even the high range of voltage in some pedals today. As to current, it's unusual for a pedal to pull more than 100ma, unless it's one of the current-sucking digital power hogs. Those are a special case.

So a suitable MOSFET would need to be able to withstand maybe 20-25V and carry greater than 100ma, except in special case. It is unusual for single MOSFETs to ...not... be able to do that. The smallest TO-92 MOSFET at the time I wrote that was capable of 200ma and 30V. So it didn'e make much sense to dig into it too deeply. Today, you might have to worry a bit more than that. But you have the answers easily available.

You pick a MOSFET with a voltage rating bigger than the power siupply voltage.
You pick a MOSFET with a current rating bigger than the current used by the pedal.

The gate resistor is indeterminate. A MOSFET gate is a piece of high purity glass twenty volts thick. There is ... zero ... current going through it, and you only need enough current to the gate to charge the gate capacitance, and that only determines the speed it switches, not whether it switches. So pretty much any resistor will do, from 100 ohms up to a meg or so. It's generally a good idea to use biggish ones for a variety of subtle reasons, so good ranges of resistor are maybe 100K to 1M. And do not forget to protect the gate from puncturing with a zener from gate to source.

[ElectricDruid]

How about option (3)?

3) 1N5817 Schottky in series which doesn't drop much voltage at reasonable currents (less than 0.5V even at 1A according to the datasheet)

https://www.diodes.com/assets/Datasheets/ds23001.pdf

I've been round and round this question, but I think I've finally settled on something that for me is "good enough". Still willing to hear why I'm wrong and there are better options though.

[POTL]

How about option (3)?

3) 1N5817 Schottky in series which doesn't drop much voltage at reasonable currents (less than 0.5V even at 1A according to the datasheet)

https://www.diodes.com/assets/Datasheets/ds23001.pdf

I've been round and round this question, but I think I've finally settled on something that for me is "good enough". Still willing to hear why I'm wrong and there are better options though.

0.5 volts is usually insignificant.
However, I noticed that my projects are very sensitive to voltage changes, even by 0.5 volts, so I'm currently using the classic version with the parallel diode 1N4001.
MOSFet should give the best from both worlds.
In my country, 1N5817 costs 6 cents, and the appropriate mosfet is 10-14 cents, which does not make any difference.
In addition, as I understand it when the mosfet passes the current it has its resistance RDS which can replace me with a series resistor in the filtering power supply, since I use resistors 0805 I will be able to reject resistors 100 ohms in the amount of 1206, I like it)

[R.G.]

Schottky diodes are indeed usually good enough. The one issue with Schottky diodes is that they have low reverse withstanding voltage, sometimes as low as 20V, but more commonly 30V. Some are even better today. That didn't use to be a problem in a 9Vdc world, but budding beginning genius effects users happily stacking 12V and 18V power supplies and who knows what, you may lose a few to over voltage.

The cost of a MOSFET was an issue when I first wrote that. They were US$0.50 to US$1.00 at the time. At $0.10 each, cost is no longer an issue. And it does require a MOSFET, a resistor, and a US$0.10 zener to do the MOSFET version. A single diode, even a single Schottky, will come in US$0.20 or so cheaper.

@POTL: I think you're hung up on the wrong issue with rds. MOSFETs do have an rds, but it is a Bad Idea to count on it as a resistor for filtering your power supply. MOSFET rds is often tiny, down in the fraction of an ohm, even for the small SOT2 devices. To make it act like a bigger resistance, you have to tinker delicately with the Vgs voltage on the MOSFET in its active region. This will require special circuits to do right, and every MOSFET will have to be adjusted, because the tolerances on the exact Vgs needed are quite large. So if you want switching, use a MOSFET and turn it fully on. If you want a resistor as a filter, use a resistor.

[POTL]

When trying to pick a device for a power switch, the first two things you look at are, as you guessed, voltage capability and current capability. Presumably if you're trying to put a power switch into a pedal circuit, you already know what power supply voltage the pedal works on, and can make a good estimate, or even just measure, the current it takes.

The MOSFET has to be rated for enough voltage and current to do the job.For powering a pedal, that means a voltage of greater than 9V (over a decade ago when that was written) or maybe greater than 18V for even the high range of voltage in some pedals today. As to current, it's unusual for a pedal to pull more than 100ma, unless it's one of the current-sucking digital power hogs. Those are a special case.

So a suitable MOSFET would need to be able to withstand maybe 20-25V and carry greater than 100ma, except in special case. It is unusual for single MOSFETs to ...not... be able to do that. The smallest TO-92 MOSFET at the time I wrote that was capable of 200ma and 30V. So it didn'e make much sense to dig into it too deeply. Today, you might have to worry a bit more than that. But you have the answers easily available.

You pick a MOSFET with a voltage rating bigger than the power siupply voltage.
You pick a MOSFET with a current rating bigger than the current used by the pedal.

The gate resistor is indeterminate. A MOSFET gate is a piece of high purity glass twenty volts thick. There is ... zero ... current going through it, and you only need enough current to the gate to charge the gate capacitance, and that only determines the speed it switches, not whether it switches. So pretty much any resistor will do, from 100 ohms up to a meg or so. It's generally a good idea to use biggish ones for a variety of subtle reasons, so good ranges of resistor are maybe 100K to 1M. And do not forget to protect the gate from puncturing with a zener from gate to source.

Let's put all the points on i.
1) Do I understand correctly that VDSS (Drain-Source Voltage) should be greater than the voltage required to operate the pedal? for example, the pedal consumes 9 volts, and the VDSS should be 10V or more?
2) Is the ID (Drain Current) greater than the current required to operate the pedal?
For example, the pedal consumes 50 mA, the ID should be greater than 50 mA?
3) RDS (on) - resistance when the transistor passes current, so? It turns out if RDS (on) 1 ohm, then the resistance will be 1 ohm, with the CORRECT POLARITY?
For example, in a circuit with a parallel diode 1N4001, I use a 100 ohm resistor in the power filter (and as a fuse), it turns out that I can drop the resistor 100 ohms, since the resistance of 1 ohm from the transistor will participate in the filtering power supply?
If I understood everything correctly, then this is a list of cheap and suitable transistors, if I'm wrong, please correct me, if right, this list can be useful.

1) IR IRLML2246TRPBF
VDSS (Drain-Source Voltage) -20V
ID (Drain Current) -2.6 A
RDS (on) 0,135 Ohm
https://www.infineon.com/dgdl/irlml2246pbf.pdf?fileId=5546d462533600a401535664c91f25f2

2) IR IRLML9303TRPBF
VDSS (Drain-Source Voltage) -30V
ID (Drain Current) -2.3 A
RDS (on) 0,165 Ohm
https://www.infineon.com/dgdl/irlml9303pbf.pdf?fileId=5546d462533600a401535668ef1d2642

3) Alpha&Omega AO3401A it looks perfect
VDSS (Drain-Source Voltage) -30V
ID (Drain Current) -4 A
RDS (on) 0,05 Ohm
http://www.aosmd.com/pdfs/datasheet/AO3401A.pdf

4) IR IRLML6402TRPBF
VDSS (Drain-Source Voltage) -20V
ID (Drain Current) -3,78 A
RDS (on) 0,065 Ohm
https://www.infineon.com/dgdl/irlml6402pbf.pdf?fileId=5546d462533600a401535668d5c2263c

5) NXP PMV65XP.215
VDSS (Drain-Source Voltage) -20V
ID (Drain Current) -3,9 A
RDS (on) 0,076 Ohm
https://assets.nexperia.com/documents/data-sheet/PMV65XP.pdf

6) IR IRLML5203TRPBF
VDSS (Drain-Source Voltage) -30V
ID (Drain Current) -3 A
RDS (on) 0,1 Ohm
https://www.infineon.com/dgdl/irlml5203pbf.pdf?fileId=5546d462533600a40153566868da261d

[POTL]

Schottky diodes are indeed usually good enough. The one issue with Schottky diodes is that they have low reverse withstanding voltage, sometimes as low as 20V, but more commonly 30V. Some are even better today. That didn't use to be a problem in a 9Vdc world, but budding beginning genius effects users happily stacking 12V and 18V power supplies and who knows what, you may lose a few to over voltage.

The cost of a MOSFET was an issue when I first wrote that. They were US$0.50 to US$1.00 at the time. At $0.10 each, cost is no longer an issue. And it does require a MOSFET, a resistor, and a US$0.10 zener to do the MOSFET version. A single diode, even a single Schottky, will come in US$0.20 or so cheaper.

@POTL: I think you're hung up on the wrong issue with rds. MOSFETs do have an rds, but it is a Bad Idea to count on it as a resistor for filtering your power supply. MOSFET rds is often tiny, down in the fraction of an ohm, even for the small SOT2 devices. To make it act like a bigger resistance, you have to tinker delicately with the Vgs voltage on the MOSFET in its active region. This will require special circuits to do right, and every MOSFET will have to be adjusted, because the tolerances on the exact Vgs needed are quite large. So if you want switching, use a MOSFET and turn it fully on. If you want a resistor as a filter, use a resistor.

It's a pity that RDS will not replace me with a resistor. I wanted to "kill two birds with one stone"
On the other hand, is the 100 ohm resistor in the power filter important? I've often seen circuits where this resistor (or a resistor with a different rating) is not at all.

[R.G.]

It does seem confusing, doesn't it?

Your circuits need clean, pure DC  - like your body really does need clean, pure water, no funny stuff carried along in the water you drink. Just like with your body, the water you can get easily, from a pipe or a well, is sometimes just a little polluted. So you have to do your own filtering to make it good enough.

Notice that "good enough" depends on how fussy your circuit is. Some circuits don't care much about pure DC. They have a high "power supply rejection ratio", that being the ratio of how much of the power supply noise gets through to the output. Opamps often have quite high power supply rejection ratios, sometimes huge - 80 to 120db of rejection is common. That 1/10000 to 1/ 1,000,000 of the noise on the power supply getting through the opamp. The other end of the spectrum is single ended amplifiers, like a single bipolar transistor, JFET or MOSFET with only a resistor to the power supply. These circuits have a zero db rejection - yep, whatever is on the power supply rides right through, unattenuated, or only inconsequentially attenuated. For circuits with bias directly from the power supply without bypassing on the bias circuit, rejection is even worse, because noise on bias supplies is amplified by the circuits it feeds - including those otherwise immune opamps.

So the amount of filtering you need in your circuit depends on (1) how clean your DC supply is; batteries are pretty clean, excepting for a rising amount of noise as they get exhausted; and (2) how much your circuit can "ignore" power supply noise.

Notice the use of the word "your" in that sentence. If your pedal is intended only to run from batteries, you probably don't need much power supply filtering, although an electro cap from + to - can be nicer for your circuit as the battery wears out. If you use a cheap, poorly made "9Vdc" switching power supply, you need a lot, as these can be quite noisy unless they are designed for, manufactured for, and tested for performance with pedals. Your circuit may also need special power supply filtering for oscillation or RF control issues. Almost every opamp data sheet specifies a ceramic capacitor of from 0.01uF to 0.1uF right at the pins of the opamp to suppress possible self oscillation.

A 100 ohm resistor and a 22uF? 47uF? 100uF? 1000uF? capacitor filter low frequency noise on power supplies, but don't do so well on high frequency noise, oscillation or radio pick up. Just like in audio where bass is very different from treble, power supply hum is very different from hiss or radio frequency pickup. About all the series-resistor-plus-electro-cap has going for it is that it almost never really hurts. But it may not help either.

You're right at the edge of needing to learn more about power supplies, filters, and self oscillation. It would be great if one circuit did it all. But that's not the case.

[POTL]

It does seem confusing, doesn't it?

Your circuits need clean, pure DC  - like your body really does need clean, pure water, no funny stuff carried along in the water you drink. Just like with your body, the water you can get easily, from a pipe or a well, is sometimes just a little polluted. So you have to do your own filtering to make it good enough.

Notice that "good enough" depends on how fussy your circuit is. Some circuits don't care much about pure DC. They have a high "power supply rejection ratio", that being the ratio of how much of the power supply noise gets through to the output. Opamps often have quite high power supply rejection ratios, sometimes huge - 80 to 120db of rejection is common. That 1/10000 to 1/ 1,000,000 of the noise on the power supply getting through the opamp. The other end of the spectrum is single ended amplifiers, like a single bipolar transistor, JFET or MOSFET with only a resistor to the power supply. These circuits have a zero db rejection - yep, whatever is on the power supply rides right through, unattenuated, or only inconsequentially attenuated. For circuits with bias directly from the power supply without bypassing on the bias circuit, rejection is even worse, because noise on bias supplies is amplified by the circuits it feeds - including those otherwise immune opamps.

So the amount of filtering you need in your circuit depends on (1) how clean your DC supply is; batteries are pretty clean, excepting for a rising amount of noise as they get exhausted; and (2) how much your circuit can "ignore" power supply noise.

Notice the use of the word "your" in that sentence. If your pedal is intended only to run from batteries, you probably don't need much power supply filtering, although an electro cap from + to - can be nicer for your circuit as the battery wears out. If you use a cheap, poorly made "9Vdc" switching power supply, you need a lot, as these can be quite noisy unless they are designed for, manufactured for, and tested for performance with pedals. Your circuit may also need special power supply filtering for oscillation or RF control issues. Almost every opamp data sheet specifies a ceramic capacitor of from 0.01uF to 0.1uF right at the pins of the opamp to suppress possible self oscillation.

A 100 ohm resistor and a 22uF? 47uF? 100uF? 1000uF? capacitor filter low frequency noise on power supplies, but don't do so well on high frequency noise, oscillation or radio pick up. Just like in audio where bass is very different from treble, power supply hum is very different from hiss or radio frequency pickup. About all the series-resistor-plus-electro-cap has going for it is that it almost never really hurts. But it may not help either.

You're right at the edge of needing to learn more about power supplies, filters, and self oscillation. It would be great if one circuit did it all. But that's not the case.

Good.
I've been building various pedals for several months, such as boost / fuzz / overdrive / distortion.
I tried various variations: op-amp / BJT (Silicone / Germanium) / JFet / MOSFet, CMOS.
With many different devices, I chose JFET and Germanium as the most pleasant elements of amplification (for me).
After I tried the variation of power supply and noticed that the voltage drop made the sound less pleasant (the difference was noticeable in a direct comparison, but still it is). In the future, I will try the schemes with great excitement, but at the moment I use 9 volts of power.
The battery power is not interesting to me, so I feed my circuits from a quality adapter.
I noticed that JHS in its pedals in the power filter section uses a 10 ohm resistor
Top picture JHS Moonshine R1 10 ohm
The lower picture of JHS Double Barrell R20 10 ohm



If I install this mosfet can i cancel the resistor 10-100 ohm?
Because the RDS of this transistor is 10 ohm?
1) Fairchild BSS84
VDSS (Drain-Source Voltage) -50V
ID (Drain Current) -130 mA
RDS (on) 10 Ohm
http://www.onsemi.com/pub/Collateral/BSS84-D.PDF

[Transmogrifox]

f I install this mosfet can i cancel the resistor 10-100 ohm?
Because the RDS of this transistor is 10 ohm?
1) Fairchild BSS84
VDSS (Drain-Source Voltage) -50V
ID (Drain Current) -130 mA
RDS (on) 10 Ohm
http://www.onsemi.com/pub/Collateral/BSS84-D.PDF

10 ohms is 10 ohms.  It's true this is nonlinear in a MOSFET, but not so extremely that you can't assume it roughly 10 ohms for filtering purposes.

Pay attention to the safe operating area (SOA plot), and if you're more technically minded and can do thermal transient simulations, then the transient thermal response can help you.

Now consider this:
You have a capacitor bank (say several hundred uF if you want 10 ohms to yield significant 120 Hz attenuation).

When you first switch on the pedal, your FET sees 9V/10 ohms = 900 mA.

Looking at the SOA curve, there is no amount of time, no condition in which 900 mA is ok for the BSS84 -- not even for 100 us.  After turning on the power a few times you will probably damage the FET.  Sometimes these fail short drain-source so you may not notice it is damaged until you accidentally do reverse battery connection and realize it doesn't give any reverse battery protection.

It is highly probable you will need a lower RdsON FET with an external current-limiting resistor or go up a package size to a SOT-223.  These have a solder tab with considerably more thermal mass so most of them are likely to handle the short-term pulse currents during startup.

If you really want well-regulated power then put a SEPIC or buck-boost converter on the front end and then you could take anything between 3V and 30V on the input and be ok.  For these small <10ma pedals you could use a SOT-23 N-CH FET (2N7002), 0603 chip bead inductors and 0402 sized capacitor.  Then you can use a Schottky or even a Si diode on the front end for reverse polarity protection because you would then be able to handle about anything on the input and still keep the output clean. The controller IC's are generally pretty small and inexpensive.

The output capacitance filter would then be a small bank of MLCC caps. If you keep the feedback loop in the converter stable and with good phase margin then you won't have audible noise coming from the SMPS.

The down-side is if you are selling these then you have to comply with RF emissions regulations.  Anything with fast edges (like an SMPS) becomes unattractive from a regulatory point of view.