🚀 go-pugleaf

Fred Bloggs wrote:
> 
>> So I would have thought that this would be a simple problem but the 
>> more information I get the more I get confused......
>>
>>
>> I want to PWM a whole lot of LEDs. The current consumption of the bank 
>> of LEDs is in the order of 15 Amps, at 12V.
>>
>> For reasons I won't go into, my only option is to use a high side mosfet.
>>
>> I have chosen an N-channel MOSFET (IRL3803) mainly because of its huge 
>> current capability and very low on resistance of 7 milli ohms.
>>
>> I am hoping to use PWM at a reasonably high frequency, say 20khz or 
>> higher, and I want to use an efficient MOSFET driver IC that is 
>> designed for the job.
>>
>> I understand that to switch an N channel mosfet on the high side of 
>> the supply line, I have to drive the gate with 10V **more** than the 
>> supply voltage. This is correct yes?
>>
>> This is where the question comes in. I am looking at the option of 
>> using the IRS2001 to switch the high side fet. But looking at the 
>> datasheet, I am confused. How can the  IRS2001 generate a voltage 
>> greater than VCC without actually being connected to the high side 
>> voltage????? There is no talk of a charge pump in the description and 
>> in any case it does not know what potential the fet is connected to..
>>
>> Maybe it is not suitable for my application as the IRS2001 seems to be 
>> used for switch mode power supplies? I can't make sense of the first 
>> diagram on the datasheet.
>>
>> here is a link to the datasheet:
>>
>> http://www.irf.com/product-info/datasheets/data/irs2001spbf.pdf
>>
>>
>>
>> OK so if the above IC is not suitable, I have been looking at the MAX621.
>>
>> http://datasheets.maxim-ic.com/en/ds/MAX620-MAX621.pdf
>>
>> This one looks to be more straight forward. But I'm not sure how this 
>> IC will go with 20Khz switching. There are many **low side** mosfet 
>> drivers that are in the 2 to 7 amp range to ensure that the mosfets 
>> are hard on or hard off within a few nS. This one doesn't look like it 
>> will do anywhere near that switching current.
>>
>>
>> Any other ideas for a high side N channel driver IC that packs a punch?
>>
> 
> 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: 
> http://powerelectronics.com/mag/710PET24.pdf and put more effort into 
> your planning.
> 
> 

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