help for pwm to drive 4 volts @ 20 ampere

[mash.m]

hello,

i have build the follow circruit:

http://www.atx-netzteil.de/schaltungen-elektronikseite/pwm_mit_ne555_und_mosfet_schaltplan.GIF

i want to drive 20 seoul p4 led´s with 4 d-size mono nimh. i use the irl3803, but it will be get very hot, cause the 555 output and the operating voltage is to low to drive the hexfet full.

any ideas about a other hexfet that will be fully augustet with 3.xx volts?

thanks markus

 

[VidPro]

i hope you know how to read these things.

the one you have now
http://www.irf.com/product-info/datasheets/data/irl3803.pdf

one that has a claim of low voltage (mabey low voltage drop)
http://www.irf.com/product-info/datasheets/data/irf7822.pdf

the delay times for switching are much faster on this here second one.
the Vgs thing is the same for both ??

here is it all explained
http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm

then my head blew up

indications are that a good gate trigger voltage and speed, is needed on any of them to get to these low resistances. without speed, the mosfet will spend more time in high resistance, and therfore lots of heat.
but i KNOW there are lower voltage mosfet GATES, because the full switch is the key to low resistance, the lower the gate trigger voltage works , the better.
these both seem to be for 12V stuff even if they trigger much lower than that?
Mosfets can be parelleled so the small ones for very low voltage applications could potentially be triggered with the same trigger and parelleled for massive currents.

i donno, its all well beyond my skill, but i have had the same problem before., i cant read the data sheet, they dont speak english, some sort of tech gibberish

see here is another one
Optimised for high-current applications where a single control MOSFET is desired, the IRF6712S achieves a low gate charge (Qg) of 13nC and gate-to-drain charge (Qgd) of 4.4nC combined with low typical on-state resistance of 3.8 mOhm at 10VGS and 6.7 mOhm at 4.5VGS
so what good does that do? nothing, cause you dont even have the 4.5v let alone the 10v

they say we would need a "logic level" mosfet, which the max gate voltage is 5V (saturated) and the part number would start with an L.
also the minimum threshold value does not mean much. but in reality dont we need a level even lower than what is intended to be driven with 5V inputs.
and it seems that the max "Vgs" isnt required for full switching, indications are that its is the max before gate failure.

 

[JimmyM]

What you need is a N-channel mosfet driver like the MIC5014 (digikey $2.05) or similar. It has a built in charge pump to double the gate drive voltage over what is available as battery voltage. If you use 3-4V input you can drive the gate with 6-8 volts. That's enough to fully enhance the gate of a logic level MOSFET.
The IRF6635 has an Rds(on) of only 1.8 mOhm at a gate voltage (Vg-s) of 4.5V (1.3 mOhm at Vg-s = 10V). It's a non-standard package, but has great characteristics. In a "normal" package, the IRLR7843 has a Rds(on) of 3.9 mOhm @ Vg-s = 4.5V
20 Amps is quite a bit of current. You're going to have to parallel a couple of these to keep heat down. These can carry the current, but require heat sinking to carry a lot.
Personally, I'd try to use the IRF6635 on a 2oz copper board and use reflow soldring in a toaster oven or on an electric skillet. Don't laugh. There are DIY articles on the net on how to do just that.

 

[JimmyM]

hello,

i have build the follow circruit:

http://www.atx-netzteil.de/schaltungen-elektronikseite/pwm_mit_ne555_und_mosfet_schaltplan.GIF

i want to drive 20 seoul p4 led´s with 4 d-size mono nimh. i use the irl3803, but it will be get very hot, cause the 555 output and the operating voltage is to low to drive the hexfet full.

any ideas about a other hexfet that will be fully augustet with 3.xx volts?

thanks markus

Might I suggest using a comparator your circuit. It will allow a full 0 to 100% duty cycle adjustment.
Use the MIC1557 in place on your 555 (it does the same thing but in an SOT-23-5 package). What frequency are you going to use?

 

[mash.m]

hello,

thanks for your replays! i think the right way will be a mosfet-driver. i can´t find any micxxx type here in germany and i haven´t a comparation book! do you know a european type?
i just messured the gate voltage with my oszi. the maximum are 3.9volts, doubbled will be perfekt.

pwm frequency at maximum is 5khz.

at this moment i have 8 irl3803 parallel, it is ok so, but i little higher gate voltage will be better. i have stuck these 8 fet´s on the inside from my mag, so that they are cooled by the body.

many thnaks in advance, markus

 

[JimmyM]

hello,

thanks for your replays! i think the right way will be a mosfet-driver. i can´t find any micxxx type here in germany and i haven´t a comparation book! do you know a european type?
i just messured the gate voltage with my oszi. the maximum are 3.9volts, doubbled will be perfekt.

pwm frequency at maximum is 5khz.

at this moment i have 8 irl3803 parallel, it is ok so, but i little higher gate voltage will be better. i have stuck these 8 fet´s on the inside from my mag, so that they are cooled by the body.

many thnaks in advance, markus

I'd have to look around. You can't find an electronics supplier that delivers to Germany? You shouldn't need 8 of those. 2 or 3 IRLR7843s will do it.
Remember... a mosfet gate is like a little capacitor. It takes current to turn it on and off. If you're driving 8 of them, that's going to slow down your switching time, leaving the mosfet in its linear range longer. That creates heat. Also. all that heat is wasted energy. Use a switching frequency that's as low as possible without visual effects. The fewer times it switches, the less time the mosfets are in their linear range.
My PWM softstarter switches on in ~30 uS and off in 20 when driving 2 big D2PAK mosftes with large gate capacitance.

 

[mash.m]

I'd have to look around. You can't find an electronics supplier that delivers to Germany? You shouldn't need 8 of those. 2 or 3 IRLR7843s will do it.
Remember... a mosfet gate is like a little capacitor. It takes current to turn it on and off. If you're driving 8 of them, that's going to slow down your switching time, leaving the mosfet in its linear range longer. That creates heat. Also. all that heat is wasted energy. Use a switching frequency that's as low as possible without visual effects. The fewer times it switches, the less time the mosfets are in their linear range.
My PWM softstarter switches on in ~30 uS and off in 20 when driving 2 big D2PAK mosftes with large gate capacitance.

prefered i would like to order from germany. my problem is, that your part number is not listet at any supplier here in germany it is like the 2sc... in the us and bc... names in eu for transistors.

thanks for the tip with the switching frequency - it sounds logic!

markus