5V to 12V High Current Switch

[Armando Maldonado]

I am using a MSP430 to switch a high powered LED. MSP430 output pin can source 5ma at 5V. I want to switch a 12V source to a LED that will be driving 5A and it will be switched like a strobe very quickly. I need some help with choosing the right transistor or MOSFET in order to accomplish this.

I believe I need a 2 stage transistor switch circuit or something. Does anyone have any suggestions on component selection as well as a circuit schematic?

Thank you!

 

[ronsimpson]

If the MSP430 has a 5V supply it should be able to drive a "logic level" MOSFET directly.
I would add a 100k ohm resistor from gate to source. So that when the micro's I/O pin is in 3-state it will be pulled low.
Choose a 25 volt part (or more).
Do not use a 5A part. Go for 10A.

 

[alec_t]

it will be switched like a strobe very quickly.

How quickly? 5mA may not be able to charge the FET gate capacitance quickly enough. You might need a push-pull buffer to provide more gate current.

 

[ronsimpson]

witched like a strobe very quickly.

5mA may not be able to charge the FET gate capacitance quickly enough.

"Quickly" is not defined well. It might be 3 times in a second. I think 5mA is just fine.
If you are doing PWM at 100khz then no, not enough drive current.

 

[Armando Maldonado]

I am doing a pulses of 15us, so very quickly. I'm making a stroboscopic light to capture images of something that is spinning at 10,000rpms. The light is working but with such a low duty cycle to get a crisp image the light is pretty dim.

10,000rpms = 166.667Hz = 0.00599sec period
I want a degree or less of movement to capture the image so
0.00599sec / 360deg = 0.000016 sec per degree = 15usec flash duration

I have used a single MOSFET and I can even do ZERO resistance in series with the LED without burning it out but its still too dim. Any ideas to make it brighter?

How do I get more gate current?

 

[ericgibbs]

hi,
Do you have a datasheet for the LED or a web link.?
It may be a 'slow' LED

E

 

[Armando Maldonado]

**broken link removed**

I cant find its rise and fall times but I need something pretty bright.

 

[ericgibbs]

hi,
I could not see any data regarding switching speeds, its been my experience when this type of information is not published, the LED is not designed for high speed operation.
Consider using say 3 or 4 faster LED's in a light source cluster.

E

 

[Armando Maldonado]

I'll look for some other LEDs to use that show the response times in the datasheet. Thanks for the input!

 

[Armando Maldonado]

I just talked to Cree and the xpg2 has a rise and fall time of 10 nano seconds.

So the LED is fast enough i think i just need something else to do with the MOSFET. More gate current?

 

[ronsimpson]

I agree that those LEDs are fast!!

We could build a gate driver out of transistors but.....
They make MOSFET gate drivers. (do not get "high side" driver)
Most of those will do 1 to 5 amps. I switch MOSFETS at 200khz 50% and pump 2A for 20nS. Remember the current is only there are the edges. Because you are charging capacitors. There are 8 pin IC that are good. Watch out you need capacitors on the supply pins.
..........................
p.s.
You should be measuring the LED current with a current probe on a scope. Putting voltage on LEDs does not equate to a known current. Really a constant current source, but who wants to do it right.

 

[audioguru]

How do we dim an LED? We use Pulse width Modulation to make short duration pulses. Pulses less than 30ms appear to be dimmed even when the current is full blast.

 

[Armando Maldonado]

ronsimpson

I have never used a gate driver.

So i just get a MOSFET gate driver and put it before the MOSFET? It basically turns my MSP430 5V 5ma signal into a higher current to the switching MOSFET?

I understand we normally use a PWM in order to dim LEDs but Im wondering if i can even over driver the LED like 5A because the average current will still be low since i have a ridiculously low duty cycle.

 

[ronsimpson]

a ridiculously low duty cycle

There should be a duty cycle verses current graph. OK I can't get the PDF to load...... From my bad memory...
100%=1watt. 10% does not =1watt but 0.5watt. (some thing like that) (depends on time)

You are not regulating current, so it is dangerous to drive the LED. If you are over driving then you better get a good handle on the current.

 

[Armando Maldonado]

Im still going to regulate the current through the LED with a appropriately sized resistor in series. Ill just have to find DC vs current chart like you said.

Why not get a high side driver?

 

[ChrisP58]

Armando, all of the comments above are based on using the mosfet as a Low Side Switch. This means that the source terminal of the mosfet is connected to ground, and the drain terminal of the mosfet connects to the cathode of the LED. If you're got the mosfet on the positive side of the LED, you won't get very much voltage to the LED without going to a more complex circuit.

PS. Make sure that you have some type of current limiting component or circuit in series with the LED or you'll burn it up.

 

[ronsimpson]

Why not get a high side driver?

A low side driver assumes the SOURCE is on ground and the load is on supply.
A high side driver assumes the DRAIN is on supply and the load is on ground.

Here is a picture of a high and low side driver in one IC.
**broken link removed**

 

[ericgibbs]

I just talked to Cree and the xpg2 has a rise and fall time of 10 nano seconds.

So the LED is fast enough i think i just need something else to do with the MOSFET. More gate current?

hi
Perhaps you could suggest to Cree that they add the switching times to their datasheet as its an important parameter, save us wasting our time.
E

 

[spec]

I am using a MSP430 to switch a high powered LED. MSP430 output pin can source 5ma at 5V. I want to switch a 12V source to a LED that will be driving 5A and it will be switched like a strobe very quickly. I need some help with choosing the right transistor or MOSFET in order to accomplish this.

I believe I need a 2 stage transistor switch circuit or something. Does anyone have any suggestions on component selection as well as a circuit schematic?

Thank you!

Hy AM,

Here is an outline circuit to drive your LED fast.

It is only an idea at the moment and has not been designed or analyzed in detail

It uses the same principal as already suggested, but the MOSFET driver is a cheap and common LM555 timer chip.

If you are interested in this approach just say and I will do a practical circuit.

spec

THIS SCHEMATIC IS ONLY FOR A CONCEPT. SEE POST #22 FOR A PRACTICAL CIRCUIT

​

ERRATA
(1) Ground pin of LM555 shoulod be connected to 0V

 

[ericgibbs]

spec,
Check pin 8.
Also the required switching voltage levels for the 555 Trig pin #2
E