Home |
Search |
Today's Posts |
|
Electronic Schematics (alt.binaries.schematics.electronic) A place to show and share your electronics schematic drawings. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
Posted to alt.binaries.schematics.electronic
|
|||
|
|||
Interfacing a power mosfet to a pic running on 3 volts - mosfet drivers.gif (0/1)
I wanted to use a power mosfet with a gate threshold of 5 volts in a 3
volt pic circuit. 3 volt nominal - the pic only sees about 2.7 volts so the output drive is rather low. Plan A was to just use the pwm output of the pic to toggle a voltage doubler or tripler. Doesn't work, the 4 diode drops in the doubler eat up 2.4 volts and the output is only ~3 volts (the pic eats another 1.2 or so) Plan B is to use a blocking oscillator and step up the voltage. Works very well. While the oscillator only requires about 5 milliwatts, the low voltage of the output pin won't drive it directly, so a switch to ground is called for. Running the pic on 4.5 volts will drive the blocking oscillator directly from the pic without the transistor. If the low side switch is left in, to allow 3-4.5 volt operation (the two forward biased 1N4148's have to be replaced with one 1N4148 and one reverse bias zener to prevent the mosfet from staying in its linear range when it should be "off.") I wound a 15 turn center tapped coil on a tiny ferrite core for the blocking oscillator transformer - the open circuit output under no load is on the order of 80 volts (and may eat the 2N3904 - so it needs a load). The red LED and Zener diode regulate the voltage to a more reasonable 10 VDC gate drive - and indicate the circuit is working. Another, isolated 15 turn winding on the oscillator, would give an isolated 10 volt power supply and allow the use of an N channel mosfet as a high side switch. Plan C was a return to the pwm approach, and just use it to switch an NPN transistor and use inductor flyback pulses to provide a high voltage power source. The pwm drive is 10 KHZ at 50% duty cycle. For 3-4.5 volt operation the diode/zener is necessary to prevent the mosfet from staying biased on when the drive signal isn't present. Works well and uses fewer parts. I originally used the blocking oscillator toroid for the inductor - that worked. Any inductor in the 1-3 mhy range works, but a 3 mhy gives a better output voltage margin as the batteries deplete. -- ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
basic power mosfet question | Electronics Repair | |||
Desoldering a power MOSFET | Electronics Repair | |||
power mosfet source | Electronics Repair | |||
mosfet power amp transistor replacement and debugging | Electronics Repair | |||
Amp's power MOSFET died. Can I just remove it? | Electronics Repair |