Dual Supply - N channel FET switch
The problem that most people run into when using N channel MOSFETs with dual
supplies is that the MOSFET used to turn on and off the positive power supply
voltage, Vcc, will not work. This is the MOSFET which sits between the motor
and Vcc.
The reason it does not work, is that a MOSFET is a voltage controlled device
(transconductance). A voltage on the input (gate-source) controls a current on
the output (drain-source). The voltage on the gate must be above the threshold
voltage (4.5 to 7 volts) of the MOSFET in order for the MOSFET to turn on.
The problems with N channel MOSFETs
Here lies the problem. The MOSFET's drain lead is connected to Vcc. The goal
is to turn the MOSFET on such that the top of the motor is at Vcc potential.
This means that the MOSFET's source lead will be raised to Vcc's potential. BUT
we need the gate to be 4.5 to 7V higher than the source to keep the MOSFET
turned on and we only have Vcc to work from! We need a gate voltage that is
higher than Vcc by 4.5 to 7 V!
Voltage Doublers!
A higher voltage can be accomplished by using a voltage doubler circuit to
generate a voltage that is larger than Vcc. The chip that I selected is the
Intersil 7662 negative voltage source which can be used as a voltage doubler
also. It is easy to use, small (8 pin DIP) and inexpensive. The ICL7662
provides
about 5 mA of output which is more than sufficient to drive the MOSFETs. It is
good for an input Vcc up to +18V or so while its sister chip the ICL7660 is
only
good for a Vcc up to +10V. This results in a maximum of +33.6V and +18.4V
output
respectively.
How does it do that?
The components C2, D1, D2 and C3 of Figure 2 make up the voltage doubling
circuit. Basically, the ICL7662 voltage doubler is an oscillator that keeps
switching pin 2 from ground to Vcc and back again (See Figure 2). To start the
cycle, pin 2 is internally switched to ground. This charges up C2 by the
following path: Vcc to D1 to C2 to pin 2. C2 charges up to Vcc minus the
voltage
drop across D1 which is roughly 0.7V or so. Across C2 there will be (Vcc-0.7)
Volts
When ICL7662 switches again, pin 2 goes to Vcc but C2 has a full charge on
it (Vcc - 0.7V). This means that the anode of D2 is now Vcc from pin 2 PLUS the
charge of C2. The voltage on C2 is elevated above Vcc!
Diode D2 dumps the charge into C3. C3 now has a charge that is equal to Vcc
from pin 2 plus the charge on C2 MINUS the voltage drop across D2 (0.7V or so).
C3 has Vcc + Vcc-0.7V - 0.7V on it which is 2xVcc - 1.4V.
The voltage loss across the diodes can be minimized by using germanium
(0.3V) diodes or schottkey (0.25) diodes. The frequency of operation of the
ICL7662 is 10 kHz at room temperature.
Bonus - Negative voltage source!
In addition to being a voltage
doubler, the ICL7662 is a negative voltage source (actually that's its main
purpose - we're just not using it right ;-) . If you don't need a low-power
negative voltage source, you can leave capacitor C4 and diode D3 off the
circuit.
The capacitors C1 and C4 and diode D3 make up the negative voltage source.
Basically, the ICL7662 voltage doubler is an oscillator that keeps switching
pin
2 from ground to Vcc and back again and at the same time switches pin 4 from
ground to pin 5 (See Figure 2). To start the cycle, pin 2 is internally
switched to Vcc and pin 4 is switched to ground. This charges up C1 to Vcc with
the positive side of the charge connected to pin 2 (very important).
Next, pin 2 is internally connected to ground and pin 4 is internally
connected to pin 5. The positive charge on C1 is connected to ground (not
shorted - only connected). The capacitor has been electronically turned upside
down! This places the negative plate of C1 at pin 4 which results in a negative
Vcc onto pin 5.
Pin 5 is connected to diode D3 which dumps the negative Vcc into C4. The
charge on C4 is -Vcc minus the voltage drop across diode D3 which results in
-(Vcc-0.7V) across C4.
Figure 1 (Rev
01)- (10k) is a schematic of the dual supply N channel FET switch discussed
here. Figure 2- (2.6k) is a schematic of a simple
inexpensive voltage doubler circuit. Both are GIF files that can be opened by
any Web Browser.
IMPORTANT:
MOSFETs are extremely static sensitive but more important is that if the
Gate is left open (no connection), the MOSFET can self- destruct. The Gate is a
very high impedance device (10+ megohms) and noise can trigger the MOSFET.
Resistors R4 & R9 of Figure 1 have been added specifically to stop the
MOSFET from self destructing. It is very important to install these resistors
FIRST before installing the MOSFETs. You will find that after these resistors
are installed that the MOSFETs are quite stable devices. The resistors
pull-down
the Gates and turn off the MOSFETs, not to mention add some static
protection.
Back EMF protection
D1 & D2 route back EMF from the motor back to the power supply. Some
MOSFETs (actually most) have these diodes built-in, so they may not be
necessary.
Control Transistors
Q1 & Q4 are NPN transistors that invert the control signals to Q2 and Q5
respectively. Q2 & Q5 are PNP transistors that control Q3 and Q6.
Looking at Q1 and Q2 operation only, we can see that a High at A will turn
on Q1. This allows current to flow from Q2's base. This turns on Q2 and raises
the Gate voltage of Q3 to +24V. Q3's gate is +12V greater than its source. Q3
turns on, +12V is connected to the motor's positive lead. R3 is provided to
reduce the high input current of Q3 gate-source capacitance.
When point A goes low, Q1 turns off, which turns off Q2 and Q3's gate is
pulled to ground by R4. Q3 turns off.
When point B goes High, Q4 turns on and current is allowed to flow from Q5's
base. Q5 turns on and raises the Gate voltage of Q6 from -12V to ground
potential by the voltage divider consisting of R8 and R9. Q6 turns on, -12V is
connected to the motor's positive lead.
STOP Mode
When A=0 and B=0, the motor is stopped. R4 and R9 pull down the Gates of Q3
and
Q6 respectively and turn off the MOSFETs.
REVERSE Mode
When A=0 and B=1 (+5V), the motor is in reverse. Q1 is turned off, Q2 is
turned
off and Q3 is turned off due to R4.
Q4 is turned on by the voltage at B. Q4's collector pulls Q5s Base to
ground. This turns on Q5 which raises Q6's gate to +0V (due to the voltage
divider of R8/R9). This turns on Q6. The +ve side of the motor is connected to
-12V. The current path for the motor is from ground to tthe motor's -ve
contact
to motor's +ve contact to Q6 to -12V.
FORWARD Mode
When A=1 and B=0, the motor is in forward. Q4 is turned off, Q5 is turned off
and Q6 is turned off due to R9.
Q1 is turned on due to the voltage at A and Q1's collector goes to ground.
This turns on Q2. Q2's collector raises the Q3's gate to +24V which turns on
Q3.
Q3 raises the motor's +ve side to +12V. The current path for the motor is
from
+12V to Q3 to the motor's +ve contact to the motor's -ve contact to ground.
NOT ALLOWED Mode (or fuse test
mode)
If A=1 and B=1 then both MOSFETs turn on which shorts out the power supply and
lets the smoke out of the components - Not recommended.
Miscellaneous Info
The tricolor LED allows you to test the circuit without connecting the motor.
The LED will be green for one direction and red for the other. Handy test.
Motors make a lot of electrical noise from the brushes when running and
huge electrical spikes when stopping, starting and especially changing
direction. C1 and C2 try to suppress the noise spikes. Negative spikes are
shorted to either ground or the power supply by D1 to D4. Z1 tries to clip the
positive spikes.
MOSFETs turn on very fast, if you have problems with noise, you may want to
put 0.1 uF capacitors in parallel with R4 and R9 to slow the turn-on time. This
will reduce the EMF generated by the motors. If the rise time is too slow the
MOSFETs may heat up excessively!
Try to keep the motor supply separate from the logic supply if possible or
go to extreme filtering techniques using coils, diodes and capacitors to
filter out the motor noise.
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Disclaimer: The circuits described here are for reference purposes only. There is no guarantee in any way shape or form that they will work for your specific application. Use them at your own risk.
Please send any corrections, suggestions or errors that you may have
caught to me.
Copyright Eugene Blanchard Jan 2007
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