Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

Buy Site Supporter Role (remove some ads) | LPF Donations

Links below open in new window

FrozenGate by Avery

FREE DIY open source BOOST driver!!! Tested & working!!

Last edited:





More dumb questions...

What about a value between 10uF and 22uF if I can find one?

Also, in regards to the voltage. Are we trying to get headroom over the Vin or Vf? So is 16v about three times Vf, or four times Vin?

Which is more important, lots of voltage headroom or increasing the capacitannce? Obviously capacitance is important based on what I am seeing, but where is the tipping point?

Would you go lower than 16v to get more uF?

Thanks for all of the good info...
 
In this case, I would even go to 10V capacitors for a higher capacitance. When will you ever drive more than 10V of LDs?
 
OK, here is another electronics 101 question...

The Schottky diode:

What is important in its specs?

For instance, this is the diode I am told to use:

PMEG1020EH,115 NXP Semiconductors Schottky (Diodes & Rectifiers)

This one is less expensive but has higher voltage... but some of the other specs are different.

PMEG2020EH,115 NXP Semiconductors Schottky (Diodes & Rectifiers)

The 1020 (recommended) has

Peak Reverse Voltage: 10 V
Forward Continuous Current: 2 A
Forward Voltage Drop: 0.46 V
Maximum Reverse Leakage Current: 3000 uA

The 2020 has

Peak Reverse Voltage: 20 V
Forward Continuous Current: 2 A
Forward Voltage Drop: 0.525 V
Maximum Reverse Leakage Current: 200 uA

What are the important characteristics of the Schottky diode?

I also wonder the same thing about the MOSFET.
 
Ah. Schottky diodes are like normal diodes, except they have almost no response time, i.e. they can turn on/off really quickly. These are used in DC-DC converters because the diode that is needed to prevent reverse bias flow of current needs to be able to react at the frequency that the converter operates at. In the case of these, the converter is operating at nearly 1MHz, so you need a fast response time.

However, you also want something with a low voltage drop. The higher the voltage drop, the more voltage is lost when current is forward biased - think of it as the Vf of the Schottky diode.

Finally, there is the current specs. You need the Schottky diode to be capable of handling your PEAK current through it, which in the case of this driver, should be something around 2-2.5A or so.

So the most important aspects of the Schottky are the current specs (for AVERAGE current handling your necessary PEAK current - to give you enough overhead) and the Vf - the lower the Vf, the better!
 
Wolfman29 said:
Ah. Schottky diodes are like normal diodes, except they have almost no response time, i.e. they can turn on/off really quickly. These are used in DC-DC converters because the diode that is needed to prevent reverse bias flow of current needs to be able to react at the frequency that the converter operates at. In the case of these, the converter is operating at nearly 1MHz, so you need a fast response time.

However, you also want something with a low voltage drop. The higher the voltage drop, the more voltage is lost when current is forward biased - think of it as the Vf of the Schottky diode.

Finally, there is the current specs. You need the Schottky diode to be capable of handling your PEAK current through it, which in the case of this driver, should be something around 2-2.5A or so.

So the most important aspects of the Schottky are the current specs (for AVERAGE current handling your necessary PEAK current - to give you enough overhead) and the Vf - the lower the Vf, the better!
Click to expand...

Thank you, that explained a LOT for me in a very small amount of space!

If you have time, I have a couple more questions.

What does the MOSFET do and what are the important specs on it?

What are the caps doing? Just smoothing things out? Why three and why all the same specs - just because basically we want the most capacitance in the smallest amount of space we can?

One more thing - what about the inductor? Anything special there? Any areas that could be improved or minimum specs that have to be watched out for?

You have helped me more in the last couple of posts than you can imagine! I know that spoon-feeding is annoying, but you are helping me get a grasp of the driver so much better and I think that it will help me "learn to fish" rather than just feeding me a fish for a day - so to speak. +1 when I can!
 
No problem. I like to think that my E&M physics course I took recently this year, along with all of the independent learning regarding DC-DC converters (for the design of the BlitzBuck), has made me a pretty good source of information when it comes to these on the forums.

So first - the MOSFET.

A MOSFET is essentially a big transistor, which has some special properties. Whereas with normal transistors, you have the Base, collector, and emitter, a MOSFET as a source, drain, and gate. On a normal BJT or whatever, whatever current flows into the base amplifies the current flowing through the collector and emitter junctions. Basically, it's used as an adjustable resistor - less resistance when the base has more current flowing into it.

A MOSFET, on the other hand, while it does act like this in some cases (particularly, in the linear region of the MOSFET - I am pretty sure that's how linear regulators work), after a certain point (the threshold voltage of the MOSFET) it basically becomes a constant "short" between the drain and the source. This makes it act like an electronic switch - apply a voltage above a certain point to the MOSFET's gate, and it opens. Of course, there is some internal resistance to MOSFETs, but it's really low - usually around 50mOhms.

Now, then, the important specs of a MOSFET are the current handling capability, the Vthreshold, the internal resistance, the gate charge, and the power dissipation ability. The current handling capability is pretty straight-forward - you need it to be able to handle the current you are pumping through it. Typically, they have "pulse" operations and "CW" operations. I typically make sure my MOSFET could handle my max current in CW. Then there is the internal resistance. This directly relates to power dissipation and voltage drop. Obviously, you want a low internal resistance so that less voltage, and therefore power, is lost when the current passes through the MOSFET. Then there is the gate charge - that dictates how fast it switches on/off and how much current is necessary for it. Typically, a lower gate charge is better, because that implies it switches on and off faster and needs less current to switch on/off. Finally, the power rating - MOSFETS typically dissipate a lot of power, especially if they are driving 5+A. So you need to make sure that the current through them squared times the internal resistance is less than the power dissipation capabilities of the MOSFET.

On more thing - MOSFETs can be used as reverse protection, because they require a certain polarity of input to the gate to open the switch, depending on which type of MOSFET it is. I always get this part mixed up, but I believe NPN MOSFETs require a positive voltage at the gate to allow flow of ground, and PNP MOSFETs require a negative voltage (or ground) to allow flow of positive voltage. But I would have to look that up.

Next up is the capacitors.

Whereas yes, they are just smoothing things out, they are also providing "boosts" of current when the source isn't providing it. For instance - boost drivers and buck drivers require LARGE current pulses of input current from the battery. That's what the input capacitor is for. Batteries can't typically discharge that much current that quickly, so with a ceramic capacitors EXTREMELY low equivalent series resistance (ESR), it can provide the short pulses necessary by taking the charge that the battery gives it during down-time and releasing it when the battery needs help. The same is true for the output capacitor: when the inductor is not putting out current (inductors oscillate in the current that they put out), the output capacitor can help by releasing current stored inside to balance out drive current. Then, obviously, the inductor has to output more current than the drive current in order to charge the capacitor AND drive the circuit. For the BenBoost, there are three identical ones because those just happen to work. Further, the larger capacitance means the more smooth output, so that's why we want the highest capacitance possible. But also note that, even if your capacitance is large, if your ESR is too large (typical in electrolytic capacitors), then you won't get smoothing no matter how large your ESR is.

Finally, there are simple smoothing capacitors, used in linear regulators - they just take in any excess current not necessary for regulation and emit it when it drops out of regulation - keeps it stable.

Then there are inductors! We learned about the operation of those in Physics, because it isn't well described anywhere else.

The thing about inductors, and why they are used in bucks and boosts is that they HATE a change in current. Inductors will do whatever they can to resist the change in current. So, for example, if you have a current flowing through them and suddenly cut it off, it will induce an electromotive force in it (called EMF, equivalent of voltage) that will drive current in the same direction in which it was initially flowing before current was cut off. This is the basis of boost converters: they pump a lot of current through the inductor and then cut it off and once they cut it off, the induced EMF in the inductor can be MUCH larger than the input voltage - it will do whatever it can to make sure that the current stays the same. But, so that voltage doesn't flow the wrong way, the schottky diode is there - so current only flows forward. And guess what is used to do the switching? A MOSFET ;)

The induced voltage in an inductor (that's where the word inductor comes from!) is proportional to the change in magnetic flux through the inductor, which is proportional to the change in current. So the faster you switch off your inductor, the higher the induced voltage is.

Regarding improvements - it's really usually all in the datasheets. There are typically equations to help you calculate what the inductance for any given application is. BUT, you need the saturation/overheat current to be ABOVE whatever you expect the peak current through your inductor is. The peak current is going to be the peak current that your battery puts out, so you should always have the inductor's current capability as higher than the switch current of the MOSFET doing the switching, whether internal or external.

Any more questions? I love this stuff!
 
You should be teaching this stuff! I laughed out loud when you said, "I like to think that my E&M physics course I took... has made me a pretty good source of information when it comes to these on the forums."

I took that course too! Around 1986-1987. So I forgot some of it. :D

Seriously, that was a huge help. I'm not going to go design a driver tonight, but I am so much better informed and I am understanding the driver circuit and the components so much better.

Really, I owe you one!

I'll give you a break for now, but I will have more questions later.
 
Haha. Well, it helps to be young, and it's all fresh in my mind - just took my final on Friday! Well, I suppose it helps to have a basic understanding of Maxwell's equations too (I'm a physics/math double major).

And no problem. I'd much rather write about this stuff than do a comparative analysis between Pascal and Borges (procrastinating on papers is fun!). The key to driver design is not just finding the right ICs, but being able to read the datasheets well. That, and being good at PCB layout ;)
 
Wolfman29 said:
Haha. Well, it helps to be young, and it's all fresh in my mind - just took my final on Friday! Well, I suppose it helps to have a basic understanding of Maxwell's equations too (I'm a physics/math double major).

And no problem. I'd much rather write about this stuff than do a comparative analysis between Pascal and Borges (procrastinating on papers is fun!). The key to driver design is not just finding the right ICs, but being able to read the datasheets well. That, and being good at PCB layout ;)
Click to expand...

I did an Engineering Physics major which was more or less a double major and a minor in Math. I hated Math in high school, but Calculus and particularly Differential Equations were fascinating! Also loved working in n-space with imaginary numbers and modeling dimensions that exist in mathematics but don't make much sense to our - well, senses!

Upon graduation, I completely pissed off my father - who worked for NASA at KSC at the time and turned down offers there to be a radio dj. You can imagine his disappointment. It took him several years to get over it.

For quite some time I figured that I had the engineering degree to fall back on, but I'm pretty sure that isn't an option now. :D
 
OK,

I think that I found a good deal on a good capacitor. Someone let me know if there is a problem with this one...

CAPACITOR; CERAMIC; CAP 22UF; TOL+-20%; SMD; X7R; FLEXIBLE TERMINATION; VOL-RTG 16V $0.16 (Each)

Unless I am missing something, it has the best of ALL THREE characteristics...

Price, Capacitance and Voltage in an 0805 package. Not sure about the flexible termination, although that sounds like a bonus too!

Capacitance : 22 μF
Case Size : 0805
Dielectric Characteristic : X7R
Material, Element : Ceramic
Special Features : Flexible Terminations
Termination : SMT
Tolerance : ±20 %
Voltage, Rating : 16 VDC

Temperature Characteristic: Designated by capacitance change over temperature range — X7R: ±15%; –55°C to +125°C.

Significantly Reduces Catastrophic and Potentially Costly Failure Due to Board Flex
Greatly Reduces the Likelyhood of a Low-IR or Short-Circuit Condition in a Board Flex Situation
Channels Flex Stress to the End Terminations (Flexible Termination)
Allows for Up to 5 mm of Board Flex Capability (Flexible Termination)

The only catch I see is that there is a limited quantity at this price. And RHD won't like it because it has good temperature specs! :eg:

Any thoughts?
 
Last edited:
those are good ones and the special price is sweet (although it won't last long get as many as you can :D)
 
foulmist said:
those are good ones and the special price is sweet (although it won't last long get as many as you can :D)
Click to expand...

I bought a few. ;)

I would think these would be good for builders here. Maybe I should post in the deals section.
 
OK So I figured out my problem with the dual 859ma benboosts parallell. It was my leads from the battery. This saik build has been a nigtmare.. lol but I got it even the Green Wrapped Sony cells run theses at 2030ma So I spent 400$ on batteries for nothing.. It is true the Ultrafires still wont work but I knew the 10A continuous discharge 26650 should have. So I looked for other possible problems and got a little flash in my head when I saw some pics of a couple drivers I recently baught the custom leads on a PSU got me thinking that they arent getting the A they need.. so I doubled up some silicone wire made new leads and it's awesome 2x 859's are yielding 2.03A. Well I'm happy! Took me 3 wks to figure it out... lol

I'm still working on these I havent got my PID yet for the oven but when I do I will show it to yall..
 
You must log in or register to reply here.





Back
Top