Power Chopper

I have separated the Electronic Speed Controller into two parts.  The first part is the pulse width modulator (1st circuit), while the second part is the power chopper described here.  I have done this to keep the design modular and therefore more flexible.

The output of the PW Modulator is low voltage low current - it cannot be used to turn a powerful motor. The power chopper's function is to boost the levels of both the handled current and the voltage.  The design is intended to provide rotation of the motor in one direction only.  Reverse can be obtained by placing a cross-over relay between the Power Chopper and the motor.  The cross-over relay can be actuated either from another channel - requires a spare channel on the receiver and a switcher circuit; or by using a reversing pulse width modulator (2nd circuit).

The power chopper is basically a switch that connects one leg of the motor to ground thereby completing the battery circuit.  By periodically switching the motor in and out of the circuit, the speed can be finely adjusted.  This method of chopping the supply is by far much more effiecient than inserting a resistance to drop the voltage.  The advantages are well known.  The resistive method is both wasteful, since the power is dissipated as heat, as well as being unsatisfactory since the resistor also limits the current. This reduces torque and makes slow speed uneven. A chopper does not drop the voltage by dissipating it as heat, but buy varying the duty cycle (ratio of on-time to off-time), and it does not limit the current.

On the other hand for a power chopper, to be efficient, rquires that the switching element is fast and that it has low on-resistance.  The Mosfet transistor has been very popular in this application as it fullfils both requirements better then the bipolar transistor.  The circuit shown uses four BUZ11 mosfets, but other types can be used.

Another essential component of the power chopper is the freewheeling diode.  When the mosfets are on, the current flows through the motor. When the transistors are switched off, the current needs to continue to circulate.  If the current is not allowed to continue, then the voltage at the switch will rise until breakdown occurs. By placing a diode across the motor, the current is provided with a path of it to circulate.  This is the same reson you place a diode across a relay coil or other inductive elements.

The freewheeling diode is the simplest method to provide this function. Such a diode must (like the transistor) be both fast and low loss.  The loss in a diode is not resistive, but depends on a rather constant voltage drop.  A schotky diode is both fast and has very low forward drop. In the circuit an MBR2045 is used.  This is a dual device, and the diodes have been connected in parrallel to increase the current handling capability.  This is ok for a single pakage device as the diodes are (or should) be matched.  Other mothods can be used to provide the current path, such as using another set of mosfets, instead of the diode. This solution should be more efficient and more expandable (apart form reducing compontent type count) but is more complex.

A switching controller such as this, has got some very peculiar features which are not evident at first sight.  It acts more like a transformer (for those familiar with ac) than a voltage dropper.  This means for example that you can have a high voltage battery supplying low current, driving a motor that is taking a low average voltage but a large current! Very weird.

Many say that a mosfet is easy to drive because it is a voltage operated device and takes no current at the gate. This is not entirely correct. In a switching converter, the gate is being pulsed at a high rate, and the capacitance must be charged and discharged.  While the average currect consumed by the gate citcuit is very low, the transients are large.  For this reason it is imortant the provide a high current drive to the gate, otherwise the mosfet will be slow to turn on and off and waste too much energy during the switching period.   The driver circuit I prefer is based on the complementary npn-pnp transitor push pull pair.  This provides high speed because the transistors are not saturated, and it does not crowbar the supply. This stage is preceeded by the level shifter.  As its name implies the level shifter converts to 5V incoming pulses from the pulse width modulator to a high voltage required to drive the compementary pair.  In this way the gate drive can be at the main battery voltage say 12V and this ensures that the mosfets are fully on.  Driving the mosfet gates from 5V may either not turn them fully on and/or will make the switching slow. If the main battery is higher then 16V, it will be neccessary to use a separate supply for the driver circuit since the gate should not be given a voltage close to its 20V breakdown limit.    

The level shifter is provided by a capacitor across its base resistor. This has the effect of reducing the turn-off delay that is characteristic of this configuration. It is also important that the driver circuit is provided with a capacitor of about 100uF close to the output complementary pair.

The power chopper benefits by having also a number of paralleled capacitors across the main battery input connectors.  These capacitors serve to absorb the switching transients produced by battery lead inductance, especially if, like me, you have cables that are some 3 feet long. Otherwise this energy would be dissipated in the mosfets.  Furthermore when the mosfets are switched on, they will pull the battery supply point to ground, robbing current to the driver circuit, and in turn reducing the switching speed. This can be eliminated by putting an inductor in series with the motor, and/or using capacitors near the mosfets and near the driver circuit as mensioned above. 
  
For simplicity this chopper has not been provided with any form of protection circuitry, relying instead on the fuse to take care of abnormal conditions. 


Last edited on 1 June 2000