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Old 06-02-2011, 02:16 AM #1
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Default How boost converters work

Just a little bored and felt like edumacating people...

Non-isolated boost converters, such as those used in a boost laser driver use an inductor to bump up the voltage. An inductor is a coil of wire --a device that resists change in current. If you have current flowing through an inductor and the current wants to change, the inductor attempts to prevent this the only way it can; by changing the voltage in the circuit.

See this image: http://www.ecnmag.com/uploadedImages...00bFigure1.jpg

In a boost converter, a switch (typically a mosfet) will create a current through the inductor for a short time period. The mosfet then switches off and the current no longer flows. The inductor does not like this and will attempt to keep current flowing by boosting the voltage (turning its stored magnetic energy into electrical energy). However because the mosfet is open there is nowhere for the current to go: except through the diode.

The new electricity (at a higher voltage yet lower current) flows through this diode, typically a fast one such as a schottky, and out into a capacitor: a device that resists change in voltage. This capacitor will store the energy that the inductor discharges.

With the inductor almost fully discharged the mosfet switches on again and repeats the process (tens of thousands of times a second), and the voltage on the capacitor will reach a certain level determined by the % time (duty cycle) that the mosfet was on. A higher duty cycle will create a higher voltage, at the expense of current.

While all this is happening the laser diode uses the electricity stored in the capacitor. Typically boost converters are 80 to 95% efficient.

>gu'bai<



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Old 06-02-2011, 02:44 AM #2
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Default Re: How boost converters work

Well thank you for that pearl of knowledge and your first post too
and BTW welcome to the forum
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Old 06-02-2011, 02:47 AM #3
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Default Re: How boost converters work

Good info and something I have wondered about even if I haven't been bothered to look it up.
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Old 06-02-2011, 02:50 AM #4
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Default Re: How boost converters work

Good day Gren,


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Old 06-02-2011, 01:53 PM #5
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Default Re: How boost converters work

Yup... this is one of the ways. There 3 ways known to man to practically increase voltage.

1. Converting your DC to AC and then putting it through a transformer and then converting it back to DC.

2. Switch caps from being wired in series to parallel. The basics of a voltage doubler.

3. Inductor + cap +switching. The basics of a switching power supply.

Either way, its not a very hard feet to accomplish and you are getting massively robbed if your paying 20+ Usd for a pcb that can do this.
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Old 06-02-2011, 05:37 PM #6
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Default Re: How boost converters work

Espscially when the surface mount parts themselves cost pennies. Some ICS contain an entire boost converter in one package, inductor and all.
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Old 06-03-2011, 10:24 AM #7
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Default Re: How boost converters work

Since it costs only pennies PLEASE make a FlexDrive clone and sell it to us for cheap:
-constant and adjustable current up to 1.5A
-overheat and reverse voltage protection.
-buck/boost
-tiny footprint
....
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Old 06-03-2011, 05:49 PM #8
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Default Re: How boost converters work

Well... thats all do-ible but its all a matter of efficiency. I've been working with electronics for along time but i'm new to the laser business. I don't know the flex drive specs. In order to develop one i'd need a flex drive in order exceed or replicate the design and/or i'd need a highly accurate efficiency plot. In other words, for what DC voltage in + current in do you get DC voltage out + current out. Batteries only have a limited current potential so that is the key factor. I can boost 9V battery to 10KV rather easily, I just wouldn't get very much current out. Or easier yet, is there a specific design your looking for? Say, if I use this battery "X" I want to get at least this "x" much voltage out with this much "x" current. I'd be a fun project work on. Let me know if your interested.
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Old 06-03-2011, 06:02 PM #9
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Default Re: How boost converters work

By all means, please knock yourself out.
I'll just drop a few links for you here....

Manual:http://laserpointerforums.com/attach..._v5_manual.pdf
The original thread:Lava Micro FlexDrive Driver (lavadrive2) fits pens
Here you can buy one: For Sale V5 flex drives

Contest (feeler): 2-months, to design a NEW community boost driver!
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Last edited by anselm; 06-03-2011 at 06:08 PM.
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Old 06-03-2011, 06:16 PM #10
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Default Re: How boost converters work

Seems like we may have another circuit designer here! WOO!
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Old 06-12-2011, 08:55 PM #11
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Default Re: How boost converters work

Why the subscription doesn't work I don't even...

Yeah I could design one, but I have no access to a pick and place machine so I couldn't make them.
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Old 06-13-2011, 12:02 AM #12
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Default Re: How boost converters work

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Originally Posted by grenadier View Post
Why the subscription doesn't work I don't even...

Yeah I could design one, but I have no access to a pick and place machine so I couldn't make them.
Then why don't you design a functionally working (better than FlexDrive)
circuit in large scale firsts just for testing and trouble shooting....
Then worry about reducing the size once the design works....

Jerry
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Old 06-13-2011, 12:29 AM #13
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Default Re: How boost converters work

Can't right now for 2 reasons:

* SMPS controllers that are small enough to be used in a micro driver are rarely made in DIP or SOIC form, so I can't really prototype with them without getting somebody to breakout the pins.
* I'm working on a bunch of projects at the moment.
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Old 06-13-2011, 04:48 AM #14
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Default Re: How boost converters work

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Originally Posted by grenadier View Post
Just a little bored and felt like edumacating people...

Non-isolated boost converters, such as those used in a boost laser driver use an inductor to bump up the voltage. An inductor is a coil of wire --a device that resists change in current. If you have current flowing through an inductor and the current wants to change, the inductor attempts to prevent this the only way it can; by changing the voltage in the circuit.

See this image: http://www.ecnmag.com/uploadedImages...00bFigure1.jpg

In a boost converter, a switch (typically a mosfet) will create a current through the inductor for a short time period. The mosfet then switches off and the current no longer flows. The inductor does not like this and will attempt to keep current flowing by boosting the voltage (turning its stored magnetic energy into electrical energy). However because the mosfet is open there is nowhere for the current to go: except through the diode.

The new electricity (at a higher voltage yet lower current) flows through this diode, typically a fast one such as a schottky, and out into a capacitor: a device that resists change in voltage. This capacitor will store the energy that the inductor discharges.

With the inductor almost fully discharged the mosfet switches on again and repeats the process (tens of thousands of times a second), and the voltage on the capacitor will reach a certain level determined by the % time (duty cycle) that the mosfet was on. A higher duty cycle will create a higher voltage, at the expense of current.

While all this is happening the laser diode uses the electricity stored in the capacitor. Typically boost converters are 80 to 95% efficient.

>gu'bai<
Thanks for this. So if the cap can keep a particular voltage, how then does one take this voltage and make it into a function of some constant current?\

Or.. how do you make it a constant current source instead of a constant voltage source?
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Old 06-13-2011, 04:56 AM #15
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Default Re: How boost converters work

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Thanks for this. So if the cap can keep a particular voltage, how then does one take this voltage and make it into a function of some constant current?\

Or.. how do you make it a constant current source instead of a constant voltage source?
I think i'd sooner shoot myself in the foot than try to derive the equations for a switching power supply. Datasheets will give you approximations for ripple calculations but these are rough non theoretical equations based on product results. Also, its not constant current or voltage, there will be a ripple based upon your capacitance. Also depends on the ESR of your caps and a few other complicated thingeys. Even if you make a PCB you won't get the datasheet results unless your pcb coper plates are of proper sizes and your trace widths are the proper size and length!
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Old 06-13-2011, 04:41 PM #16
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Default Re: How boost converters work

Quote:
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Thanks for this. So if the cap can keep a particular voltage, how then does one take this voltage and make it into a function of some constant current?\

Or.. how do you make it a constant current source instead of a constant voltage source?
The cap doesn't keep anything at a certain voltage, it only filters the switching noise. To keep the voltage constant you'd have to measure the voltage at the output and adjust the switching of the inductor. Typically this is done by using some sort of interrupter that turns the switching circuit off when the voltage climbs too high. When the capacitor drains and the voltage starts to fall the interrupter kicks out and the switching circuit engages.

"I think i'd sooner shoot myself in the foot than try to derive the equations for a switching power supply"

That's why people use potentiometers.
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