For Ig = 5mA gate drive (that should be enough head room for ATTiny).
f = 20kHz ==> ts =50us, even though we can simply use 4.4kHz with ATTiny as usual.
We get:
Qg = Ig * ts = ((5/1000)*(1/20000))*10^9 = 250nC
If we use less pessimistic values:
Ig = 20mA, f = 5kHz ==> ts = 200us
it goes up to:
Qg = ((20/1000)*(1/5000))*10^9 = 4000nC
…
Now I think of it, well we do cut the time even shorter when PWMing right? The ON state is really smaller, how much? 255 or 256 times? Or how many states is there to cut down the width of the power?
Then at 5kHz it would turn into 1.25MHz ==> ts = 800ns.
Qg = ((10/1000)*(1/1250000))*10^9 = 8nC
Ok, so yes it’s a good idea to drive this down.
“In my humble opinion for switching frequencies in the range of up to 20kHz it should be fine for ATTiny to drive any gate up to 250nC. Or even worse up to a couple thousands of nC.
While lower nC is certainly better for this application and faster opening gate means less heat, it’s not something to stress about.”
Or am I missing something? Does the Qg raise when there is low Vds/Vgs or low Id?
It seems to lower with lower Vgs.
Can’t find a place that would sell SIR424DP.
What bother me on the other hand is that not all MOSFETs have low enough Vgs, so in order to open them well and fully one needs too high voltage. That’s why I’m scouting here to find something that works from as low as 2.5V etc. and is safe to run at 3.0V.
Since the point is to eliminate voltage drop, low Rdson is welcome.
Here is the Fairchild mobile solutions guide which has plenty of low Vgs and low Q mosfets. Somehow I have a hard time believing they would provide the power you would need?
It’s a complete integrated circuit with a control loop in it to control the FET in it in order to limit the current going through the device. It’s not a bare component.
So it does have a FET! I don’t care if it’s a MOSFET or what’s the exact name of it. It does have to have some switching component in the circuitry. Yes it’s an IC otherwise.
If one can believe the datasheet, yes, it has an n-channel mosfet inside of it. It also has a band-gap voltage reference, a shunt resistor and a “Control Circuit” - Most likely a simple Op-Amp. Indirectly, it DOES have a Qg - You must supply all of the power into the VDD pin to power the Op-Amp, the band-gap reference and the Qg of the internal fet.
It’s a very small fet though - that’s why turning it on and off is ‘easy’ when compared to the multiple-amp-capable fets we’re talking about in this thread. There are exceptions to the rule, but IN GENERAL, bigger fets have higher Qg’s
Got IRF3704ZS, D2PAK, huge as an elephant, hardly usable on a 17mm board.
But, just now Vishay SIR424DP-T1-GE3 arrived! Awesome. It should fit on 17mm board, ATTiny13A, SIR424DP, and the few SMD resistors and capacitors needed.
Just need to make the board now or fit it somehow on the Nanjg105C.
Not yet done, been waiting for other parts. I do have 5 beauties named SiR424DP.
Checked the layout not so long ago and posted it in another thread.
Here you go:
Red marks with black outline are areas where one needs to remove traces.
I hope to touch up the gate from the other pad, put it as close as possible. There is a tiny leg outside the SiR424DP. Otherwise I might have to get copper tape, which is another option, make new bigger traces with a copper tape.
I’m not going to add anything between the SiR424DP and MCU. It will either work or not. But since this is the best small mosfet I could and PPTK could find it should be fine.
I just have to desolder one Nanjg driver and then reflow one side and then solder spring and capacitor on the other. Plus some other stuff if I will be making different mode selection.
Yes and probably $40 for shipping +customs.
In US, easy, probably very. Outside US? Not at all. I got mine from Vishay directly. Asked only for 5 which should be enough for me and my tinkering, hard to beat the price of free samples. Would buy them, if there was any place that wouldn’t charge me ten times more for shipping. No local stores have it.
And I don’t really need 1000 units reel to justify for the higher shipping costs.
I will try it soon, desoldered all chips yesterday and got some tools today. Will proceed to make one to test if it works well. Then I will make another one if the direct driver works well in my application.
That’s to optimistic.
The UPS customs handling fee alone is 40$ (FedEx ~20$ I think) plus 25$ shipping fee if your ordered value is below 90$ plus customs fee (here at least 19% EU import VAT)
So, yeah, ordering from overseas like DigiKey or Mouser is just insane. Farnell and RS Components don’t do business with private customers over here (anymore), but maybe in your country?
Farnell? Hmm, I knew people used to buy things there, those that do electronics. I thought it’s an Uk shop, so shopping there in EU should be fine. Except the prices and shipping costs XD
Yes I checked digikey and mouser and so on. The shipping costs go sky high.
It’s “only” 21% VAT here with an order over 22EUR ~= 29USD. Let alone that makes it not worth it.
