# High power PNP question

Discussion in 'Electronic Basics' started by kostas, Jan 5, 2005.

1. ### kostasGuest

Hello

I want to design a PWM to control the duty cycle of o big fan (80watt).
I have seen a nice circuit at

http://www.cpemma.co.uk/pwm.html

The nomad's with the kick start function. The maximum current i want to
control is 6.6 Amber and in order for the kick start to work i should
use a PNP transistor, correct? Can i use a N channel MOSFET like irf540?
Do i have too use any special driver after 555 and before MOSFET or PNP?

2. ### John PopelishGuest

If you are trying to build something like Nomad's 555 circuit, yes, it
uses a PNP transistor, but the design is capable of nothing like a 6
ampere output. If you use a good, high current PNP transistor, say a
TIP36 or D45H. But this circuit will not provide enough base drive to
switch one of these with a 6 ampere collector current. to get good
saturation (less than .3 volts collector to emitter) one of these may
need a base current between 1/20th and 1/10th of the collector current
(.3 to .6 amperes). The design shown has a 470 ohm resistor limiting
the base current. After you discount the 1 volt base voltage and the
1.5 volt loss through the 555, this leaves 10 volts across that
resistor for a base drive of 10/470=.021 amperes. This resistor would
have to be lowered to about 20 ohms to get the needed base drive, and
then the 555 would overheat.

So the output capability of the 555 has to be increased. Adding a
second PNP (say, a 3 amp rated TIP32) with its base connected directly
to the 555, its collector connected directly to the negative rail, and
a 15 ohm 5 watt resistor between its emitter and the big PNP's base,
you get the needed base drive. Then you need to put a 10 ohm resistor
between the base and emitter of the big PNP to suck the stored charge
out of it when the 555 output goes high, to have a clean turn off.

All these designs are also missing the diode that should be across the
motor, cathode up. Without this, the high voltage pulse you get when
you interrupt the motor current is likely to damage the switching
transistor. For a 6 ampere motor, this diode should be fast (turns
off fast when reverse bias is applied by the transistor) and rated for
at least 3 amperes, average. Candidates would be the SR503 (30 volt 5
ampere schottky) and 80SQ045 (8 ampere, 45 volt).

If you want to simplify the circuit quite a bit, replace the entire
PNP and resistor network with a big P-channel mosfet (gate directly
connected to the 555). Something like IRF5305 (0.06 ohms on
resistance, 55 volt).

3. ### kostas_Guest

You have been very detailed, thank you. If i put a P-channel mosfet the
kick start function won't work, right? I think without that i'm gone
have problem starting the fan because it's very loud and i'm gonna work
it with small duty cycle. If i put an N channel won't the kick start
work (and i think i will not have to add a second transistor, i can
driver it directly from 555, or i'm wrong?)

4. ### John PopelishGuest

I don't understand what you mean by 'kick start'. Either the PNP or
P-channel switch will allow the motor to see zero to almost 12 volts,
average. If the PWM frequency is high enough, the vibration should be
acceptably low.
I see no functional difference between N and P channel switches from
the standpoint of motor operation. One switches the positive side of
the motor and one switches the negative side. The motor current does
not know the difference.

5. ### Robert MonsenGuest

The 'kickstart' thing is done by keeping reset low for a few seconds,
which powers the fan during that time. Using a mosfet like JP suggested
doesn't affect that at all

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

6. ### kostas_Guest

With the kick start i mean the 555 is a in reset mode for about 2 second
in order for the fan to take 12V directly for the first spin and then
then 555 comes in the game. If the 555 is in reset condition the output
will be 0 so i need a PNP in order to drive the fan for 2 second when
the 555 is off. If i put an NPN then the kick start won't work. So i
think i can put an irf540 n channel mosfet to do the job. I don't know
exactly how mosfets work. Some explanation and a link for additional info?

7. ### John PopelishGuest

I see.
An N-channel mosfet and an NPN transistor do approximately the same
thing. Both turn on when the input pin (base for the NPN and gate for
the mosfet) is more positive than the common pin (emitter for the NPN
and source for the mosfet). The difference is that there is a diode
junction in parallel with the NPN base-emitter, but the mosfet gate to
source is open circuit (insulated). so to keep the NPN on, you need
to supply that diode junction with a current, while no current is
needed to keep the mosfet on. It does take a lot more voltage (most
of the 12 volts) to turn the mosfet completely on. If you reverse
voltages, the same applies to PNP and P-channel mosfets.

Since the reset signal pulls the 555 output low, that would turn on
either a PNP or P-channel output. If you wanted the same function
with either an NPN or N-channel output, you would have to invert the
555 output, using either a PNP small signal transistor or a small
P-channel mosfet or a cmos inverter (CD4049 CD4069 or some inverting
gate).