High side driving N-channel Mosfet with PWM

For a gate capacitance that large ou might need more than the 20mA or so of peak gate charge current avaialble from the IRS part.

The IR2110 is a very commonly used part that is similar to the irs2001 in functionality. The high side gate bias voltage is essentially free due to the flying bootstrap capacitor supply.

The way it works is that when the low side fet is turned on the vs node is at ground potential (or 1 diode drop lower for a buck) and the VB node (bootstrap) is held to 1 diode drop below the 12V rail. This changes up the capacitor between Vb and Vs. When the high side is turned on, the high side circuit uses the capacitor as a floating power supply to supply the gate drive current.

Pretty easy to use and works great. You will need to generate your own PWM signals to feed this guy as it is only a driver.

Read up on the ap notes in the IRF website for more operational use and proper layout considerations for that chip. wirewrap is NOT recomended.

Why so high? If it's just for brightness control, you don't need that surely?

For your application, VCC will connect to "up to 200V" in the application schematic, on page #1. The drive supply VB is conventionally bootstrapped using the diode in the same drawing.

RL

f .

er,

if you strobe LEDs too slow, then it causes a strobe effect if there is any movement... if you scan your eye past some LED rear car lights you can get the strobe effect and it is very annoying... even if the LED is not moving, your eye is always moving

I don't know how fast you need to pulse to avoid this but a few hundred Hz is too slow. 5 kHz might be OK I don't know...

At 5 kHz you might get audio interference so that is a good reason to go to 20 kHz.

Mark

You're going about this all wrong, no one makes a dimmable LED supply like this, the main reason being your excessive ripple current resulting in unnecessary heating and exponential reduction in LED lifetime. Another good reason for not doing it this way is to avoid the fairly huge current spikes resulting from what will be a hefty equivalent load capacitance producing enough EMI to be detected in another galaxy. And you want to go to all this trouble to produce a crap design because it's the only thing you understand. Do some background reading:

and put more effort into your planning.

32 frames/Sec I believe is the minimum needed to prevent strobe flash in the human eye.. or some figure like that.

"

Somebody stuffed up Fig. 2, didn't they.

Fine for a stationary source you are staring at. But all you have to do is blink and you will see a trail of dots (especially if the LED is very much brighter than its surroundings) at 32 flashes per second, and a lot faster, too. If you haven't seen the effect, I suggest you try it.

--
Regards,

John Popelish

Can you explain why you can not just break this down into multiple units all centrally driven? say 15 units drawing 1amp each, or 30 units drawing

500mA?

It would be less convenient, but that is possible, and may end up being the way I go. There are motor controllers on the market that PWM high current motors. I thought it would be reasonably trivial to do seeing as MOSFETS come in high current versions, and seeing that LEDs don't have as much inductance as motors do.

I will go for the MAX621 and suck it and see. If the fet overheats, I will decrease the PWM frequency.

If it still overheats, I will break up the supply into groups of 3A loads.

No big deal either way.

I was thinking that would avoid the all eggs in one basket. if one unit fails, it only takes out that fraction.

The other point is the frequency and duty cycle might be such that instead of having to deal with an intermittent 15Amps, you could be dealing with a consistent 5amps say or less.

Depends on the nature of the "flash". Film looks okay because it moves quickly and smoothly between frames (at 24FPS). A light scene moving to a light scene without dark inbetween is a lot more pleasant than a really bright scene flashed only every so often. PWM LEDs will do that, because they're very firmly either ON or OFF, within microseconds I mean. You want to minimize the apparent movement of a PWM cycle with respect to the eye's movement, which might be...

Pffft... well lemme see here... I can glance 60 degrees in 0.1 second pretty easily, which is easily 10' travel at a distance of 8'. An average LED is 1/4" across, and you want to keep the "distance" between cycles below a diameter or so. The surface rate is around 100 FPS, or 1200 IPS, or 4800 LEDs/s. So the minimum duration is around 1/4800 LEDs/s = 1/4800Hz frequency. Which hey, that's pretty damn close to the earlier estimate in this thread of about 5kHz minimum. There ya go! Of course, this frequency can be lower for large LEDs, diffuse arrays, etc., or should be higher for really close, fast moving stuff (uh, passing pedestrians a few feet away at

60MPH?!?). And if nothing else, you can add a choke filter and recoil diode (aka buck converter) to smooth out the pulses and get perfectly level light output. Hey, you could use an iron-core choke to justify the switching loss savings of 400 Hz operation. :^)

Tim

-- Deep Fryer: A very philosophical monk. Website @

You can get P-channel MOSFETs in similar ratings. Maybe not quite so small Rds(on), but certainly in the amperage you need at that voltage. There are plenty of lower voltage P-channels (whereas the supply kind of dries up over 200V, sadly). Stick a zener on the gate to protect it from nasty automotive supply voltages and you can practically drive it from a naked

2N3904 collector sinking to ground.

Tim

-- Deep Fryer: A very philosophical monk. Website @

Aw, c'mon! Why not? This could turn out more interesting than the rest of the thread! :-)

Thanks, Rich

It shows up on scrolling marquees very dramatically - when it's scrolling, the letters look tilted, but I believe that's an artifact of the strobing. If you move your eyes to follow the letters, they square up.

Cheers! Rich

Probably because it is some stupid contraption to go on a car with only the chassis available as a -ve earth. Probably blow up on the first load dump it sees anyway :).

Not at all. Movies use 24fps, but open the shutter two or three times per frame to eliminate the flashing. TV interlaces 30Hz frames with 60Hz fields for similar reasons.

--
Keith