Choosing values for Bipolar Totem-pole MOSFET driver

Hi all,

I am trying to drive a 300mA LEDs with arduino. I am using this driver circuit as shown in the attached picture. How do i choose the values for all the component/MOSFET/BJT as i really have no idea.

How do i decide which MOSFET is suitable for the LED, what parameter should i look for in the MOSFET?
Same goes for the BJT, bypass capacitor and resistor.

Regards,
Dyllan

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I am using this driver circuit as shown in the attached picture.

Why?
There is no current limiting in that circuit so you will fry your LED.

You need a constant current supply, like this one:-

You have to be careful when you make a totem pole circuit.
You have to match the characteristics of the NPN an PNP transistors, especially their gain.
If your value for R is small or zero you can destroy the totem.

Why have you selected this kind of drive circuit?

Indeed.

I'm wondering why he wouldn't just use an AMC7135 (300-350ma fixed current, 25 cents a pop. They say you need to put 0.1uf caps on both sides of them, but I have had zero problems running them without. I'd maybe put the cap between output and ground. Control and PWM it by using an N-channel MOSFET to drive the ground pin of the AMC7135. These automatically lower the current as the chip heats up.

For a beefier, single chip option, the AMC7140 is adjustable up to 700ma by using an external current setting resistor. This chip also has an Output Enable pin, so you don't even need an external mosfet. These are nearly a buck a pop, but you can drive big long series strings with them, they're rated at up to 50v. These have an overtemperature shutoff

Both AMC's are cheap on ebay.

There also exist 2 terminal components that use a "self biasing transistor" and only require two connections (the AMC71xx's require a separate power pin as well). Digikey has these.

These all dump the extra power as heat, so they get hot. Be sure you have a heatsink on them for continuous duty. Those dirt cheapo half-inch-square heatsinks on ebay, and some thermal epoxy, are perfect.

For a 300ma driver, the AMC is the cheapest easiest (? Maybe not due to it's size)

But the constant current circuit built using a sense resistor is an easy project to build and no waiting around for specialized parts.

All standard available parts from your local electronic supplier …

Grumpy_Mike:
Why?
There is no current limiting in that circuit so you will fry your LED.

You need a constant current supply, like this one:-

Sorry but i am not very good at this, so what do you mean by current limiting? Can this circuit support fast switching? Lets say at a frequency of 1M bits/s? can the transistor handle it?

LarryD:
You have to be careful when you make a totem pole circuit.
You have to match the characteristics of the NPN an PNP transistors, especially their gain.
If your value for R is small or zero you can destroy the totem.

Why have you selected this kind of drive circuit?

I chosen this circuit as i found in on the web http://www.ti.com/lit/ml/slup169/slup169.pdf - Design And Application Guide For High Speed MOSFET Gate Drive Circuits. And this paper state that it is the most popular and cost effective drive circuit. Maybe you can give me more advice on this? Thanks!

DrAzzy:
Indeed.

I'm wondering why he wouldn't just use an AMC7135 (300-350ma fixed current, 25 cents a pop. They say you need to put 0.1uf caps on both sides of them, but I have had zero problems running them without. I'd maybe put the cap between output and ground. Control and PWM it by using an N-channel MOSFET to drive the ground pin of the AMC7135. These automatically lower the current as the chip heats up.

For a beefier, single chip option, the AMC7140 is adjustable up to 700ma by using an external current setting resistor. This chip also has an Output Enable pin, so you don't even need an external mosfet. These are nearly a buck a pop, but you can drive big long series strings with them, they're rated at up to 50v. These have an overtemperature shutoff

Both AMC's are cheap on ebay.

There also exist 2 terminal components that use a "self biasing transistor" and only require two connections (the AMC71xx's require a separate power pin as well). Digikey has these.

These all dump the extra power as heat, so they get hot. Be sure you have a heatsink on them for continuous duty. Those dirt cheapo half-inch-square heatsinks on ebay, and some thermal epoxy, are perfect.

Actually i would like to built one circuit myself without much waiting.

cjdelphi:
For a 300ma driver, the AMC is the cheapest easiest (? Maybe not due to it's size)

But the constant current circuit built using a sense resistor is an easy project to build and no waiting around for specialized parts.

But where does your signal that is going in connect to?

1MHz PWM is ludicrous, don't even think about it unless you really need it, in which
case you'll have to think about GaN FETs and drivers and specialised circuit layout
with surface mount parts and a lot of attention to RFI.

The constant current circuit can be PWMd, at a sensible rate (depends on the cutoffs
frequencies of the various devices, but start at 1kHz unless you have a reason not to.

If you want faster PWM (20kHz or suchlike) then you should use a MOSFET driver
chip to drive the MOSFET (obvious really!). MIC4422 is my favorite, but complete overkill for a 300mA load.

If you want to modulate the light really fast then I'd go for a current-limiting resistor so
to get the maximum switching speed you can (but expect switching losses in the MOSFET),
and a proper gate drive is mandatory - but again here you have a tiny load that a simple
switching transistor could manage.

so what do you mean by current limiting

See:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Can this circuit support fast switching? Lets say at a frequency of 1M bits/s? can the transistor handle it?

As MarkT said, don't be silly.

MarkT:
If you want faster PWM (20kHz or suchlike) then you should use a MOSFET driver
chip to drive the MOSFET (obvious really!). MIC4422 is my favorite, but complete overkill for a 300mA load.

If you want to modulate the light really fast then I’d go for a current-limiting resistor so
to get the maximum switching speed you can (but expect switching losses in the MOSFET),
and a proper gate drive is mandatory - but again here you have a tiny load that a simple
switching transistor could manage.

Sorry but i really have no idea, could you explain to me how do you go around choosing which MOSFET driver to use? What parameters do you look for? I have found this circuit on a website. But i have no idea how he choose the parameters. He used a 10W LED,and its forward voltage is around 7V when turned ON the LED draws approximately 0.5 A.

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Did you not read that link?
There is nothing to control the current unless it is in the LED itself.
Yes you can find all sorts of rubbish on the net.
What is a 6.8E? And wtf is a 1M resistor doing across the gate, nothing much I suspect.

So with such a poor circuit the values are drawn out of a hat, nothing to calculate here.