0

I need to build a constant current driver for a LED with a forward voltage of 3.4V and 350mA maximum current. The driver will be controlled by a PWM signal from a 3.3V MCU.

Reading this post and doing the calculations using my system specs, I came up with the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

I had to choose a 12V power supply due to the high forward voltage of the LED, which resulted in a minimum power supply voltage of 6.5V. Therefore I couldn't use a 5V power supply.

However, I'm concerned about the power dissipation on the transistor. If my calculations are right, the power dissipated across it would be (neglecting base current and considering Vbe=0.7V):

enter image description here

And the BCP56 can only dissipate a maximum of 1.35W with properly sized mounting pad in the PCB.

First of all I would like to know if the my calculations are right and, in case they are, what would be a good solution.

The only two options I can think of are either picking a beefier transistor that can dissipate more power or reducing the power supply voltage, although I like the idea of using as 12V power supply since it's easier to find locally.

Furthermore, is BJT a good solution for this type of driver or changing for a MOSFET based driver a more suitable option?

Trevor_G
  • 46,364
  • 8
  • 68
  • 151
electronfriend
  • 3
  • 1
  • 2
  • I would recommend you use a Darlington power transistor, like the TIP series. Mosfets are recommended only for High power LEDs like 5-10W and above. – Kokachi Jan 31 '18 at 17:52
  • It sounds like you want the transistor either fully on or fully off, so I would use a FET. I find them simpler to understand and they have lower power loss at high current. For the circuit you have, I believe you want R1 on the collector, not the emitter, otherwise the current through the resistor will raise the emitter voltage, and change your base-emitter voltage. Most of the power will be dissipated by R1, not the transistor, that might be more obvious when you move R1. EDIT: I may have misunderstood your intention, are you trying to use the BJT to control current instead of a resistor? – pscheidler Jan 31 '18 at 17:53
  • Also, I highly recommend simulating all transistor circuits. LTSpice has good features and is free. – pscheidler Jan 31 '18 at 17:58
  • @pscheidler Yes, I'm trying to use the BJT to control the current and therefore R1 needs to be on the emitter side. I simulated it in LTSpice, and it seems that my power dissipation calculation is right. Just wanted to check if anyone can recommend a better circuit than mine. Thanks! – electronfriend Jan 31 '18 at 18:12
  • Do you prefer using MOSFETs? If so, Trevor's circuit is a very standard approach combining a small signal BJT with an NMOS. Another approach that is almost identical replaces the NMOS with another BJT. They both work just fine. So it really is a matter of what you can get and want to pay for, I think. There are other approaches, too, of course. In fact, I can think of at least 5 or 6 different basic approaches where all of them are about the same cost but have various advantages and disadvantages. – jonk Jan 31 '18 at 19:00
  • Can you use a much lower supply voltage? Because this is incredibly inefficient. You're using 4.2 Watts of power (12V * .35A), and only 1.2 Watts is going to the LED. The remaining 3 Watts is turned to heat. *Your circuit works better as heater than an LED driver :)* – Vince Patron Jan 31 '18 at 19:23
  • @VincePatron yes I'm aware it's extremely inefficient and it would be much better using a power supply of 7V. However, where I live it's hard to find power supplies with voltages other than the most 'common' ones (5V, 12V, 24V). – electronfriend Jan 31 '18 at 19:26
  • Consider using an LED driver IC, such as the [MIC4801](http://ww1.microchip.com/downloads/en/DeviceDoc/mic4801.pdf) – τεκ Feb 01 '18 at 01:27

2 Answers2

2

I'd be using 5V and a MOSFET here to limit the current required from the GPIO and coupling it in a typical current limiter as shown below.

R1 and Vbe or Q1 roughly set the current limit. If you need it more accurate than that you either need a pot in there or a more active circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

The gate threshold of the MOSFET needs to be under or close to 1V. Power lost to the MOSFET is about 300mW and R1 is 215mW.

Trevor_G
  • 46,364
  • 8
  • 68
  • 151
  • 1
    I've seen this circuit in other threads but I was reluctant of using it due to its higher component count since I intend to replicate the circuit to control several LEDs individually. However, after a second look I realised this approach is much more efficient than the one I posted and I would be able to use components with smaller footprints due to lower power dissipation. Thanks! – electronfriend Jan 31 '18 at 19:43
0

Your design wastes too much voltage on R1. Here is a design that only drops 0.6V across R1, allowing you to use a 5V supply, which means less power wasted in the transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

However this design is too dependent on the characteristics of the transistor. Better to use a more sophisticated circuit.

If your goal is to minimize power wasted, better to use a switching current regulator. You will find many sold as LED drivers.

Trevor_G
  • 46,364
  • 8
  • 68
  • 151
τεκ
  • 4,229
  • 1
  • 16
  • 17