Half-bridge gate drive for off-the-shelf LED strips PWM dimming

[marcogmaia]

Is it feasible or recommended to dim the LEDs like this?
Or there is any easier, safer or cheaper way? I'd like to use 20+KHz PWM to avoid audible ripple.

 

[ZASto]

Why over complicating.
One Logic Level MOSFET as Low Side switch is enough. All my dimming of 12V LED strips are done this way.
My light above my desk consists of 5m white strips and consumes at full brightness ~72W

BTW you have wrong connection of LEDs in your schematic. They should be connected in SERIES not PARALLEL

 

[betwixt]

You are only sinking current so R14/Q10 serve no purpose and the 'VB' is never bootstrapped above the supply so the high side of the driver circuit is mostly wasted.

I would consider removing everything except Q9, connecting its gate to +12V through a 1K resistor and connecting an NPN bipolar transistor from the gate to ground. The signal would be inverted but you only need four components in total, Q9, the BJT, the 1K gate pull-up resistor and a resistor to limit the base current at the PWM input.

Brian.

[edit] ZASto beat me to it! Note that their solution requires a logic level MOSFET and 5V PWM drive, mine uses an extra transistor but can be driven with PWM down to about 1V logic level.

 

[marcogmaia]

thank you both, I think I'll go with your solution betwixt.
But I intended to source current, is it ok to sink up to 14 Amps? I read somewhere that it is better to cut Vcc than to cut GND?
And the transition time of the NMOS would be fast? Or it doesn't matter?

 

[KlausST]

Hi,

I read somewhere that it is better to cut Vcc than to cut GND?

Where did you read this?
In which regard "better"?
What context?

--> for the LEDs it makes no difference. They will never know.
--> It´s far easier to switch GND. In simplest case it´s just one MOSFET....

transition time:
* The faster: the less switching loss, = higher efficiency, less heating
* but: the more EMI noise. You spread HF. Maybe this causes problems with other electronic equippment nearby. All electronic devices need to comply with EMI/EMC regulations.

Klaus