Electronics > Projects, Designs, and Technical Stuff

DC Lighting with MOSFETs help

<< < (3/3)

am2pgs:
I can't help thinking that may be your load is inductive. The negative spikes then get coupled to the gate through capacitance. an easy way to deal with that is to put a diode across the load.

sile:
Yes, a 22 Ohm resistor between the TLC59108F and the gate on the mosfet brings the negative voltages to around -2V.  So, that does seem to help quite a bit.  I guess I only need to pull down Vgs to only -3V for my P-Channel mosfet (AOD-417).  If i used a 470ohm pull up resistor and 470ohm resistor between the gate and driver that will pull my gate voltage to 6V, giving a vgs of -6, which should be good.  Although Rds(on) won't be quite as good as a -12V Vgs.  But, I think I will take that for higher reliability.

Once I get my AOD-417, i'll test with that.

The load I am testing with is a 12V 6W light bulb.  

Amp2gs:
To answer your questions about the schematic, all the power stuff is at the top right.

Vin, will be connected to a car battery expected to be around 12-14V
V+ is just Vin ran through a fuse, so all the current for all 8 mosfets is fused
V12 is V+ going through a 1ohm current limit resister and has a 16V zener.  This is to protect the LED driver and why I use V12 as my gate pull up.  So, under normal circumstances V+ is connected to v12 via a 1ohm resistor. 

CafeLogic:
That whole set up would be a whole lot easier if you used N-Mosfets on the low side.

Zero999:
The schematic with the P-channels won't work because they won't turn off. The output from the PWM controller will be 5V when high which so Vgs will be 7V which is enough to turn the MOSFET on.

You need to use an NPN transistor to pull the gate down.

What's the PWM frequency? Using pull-up resistors will slow down the off time considerably.

You can use bootstrapping to drive high-side N-channel transistors but it's a pain to implement using discrete components and won't work up to 100% duty cycle. Here's an example but it suffers from the same weakness as using an NPN pull-down.

Navigation

[0] Message Index

[*] Previous page