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Dual Supply - N channel FET switch


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 August 2007

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