After realizing the l293d output won't suffice and the voltage drop of the l298 is quite high, I believe learning to construct a simple H-Bridge to suit would be beneficial. Before starting with tests, am I correct in assuming the voltage drop of the below circuit would be calculated by the data sheets Rds(On) of each mosfet in the high/low path multiplied by drain source current? If so, a theoretical n/p-channel pair with Rdson .4ohms with 2A would be a .8V drop. Round to 1V. Are there other obvious factors I am ignorant of that would affect voltage drop available to motor besides thermal characteristics? Thanks
You are correct about the voltage drops in the mosfets. The only other major factor would be your supply (Vdd) and its ability to source the current without dropping (e.g. if the wire between the bridge and supply is long and or small). The other thing to pay attention to when using the Rds(On) from the datasheet is the gate voltage (Vgs) that Rds(On) is specified at. If the Vgs is sufficiently different (i.e. the Vgs = 10V and you're going to drive it with 5V) then you should look at the curves to see roughly what Rds you'll get for that gate voltage.
Also, most MOSFETS have built in diodes (called a body diode) so adding the extra schottky diodes isn't usually necessary.
As you have the schematic now, you will have high switching losses. Things to consider are:
Gate driving. You do not want the mosfet to stay in linear region with high Rdson for a longer time than needed. Especially if you want to apply PWM on the gate.
You want to PWM the current, not the gate voltage. Use gate drivers, eg: MC33883.
The resistors around the gates in your current diagram will slow down the switch time.
back-EMF of the motor. When you turn off motor current, there will be current coming back out of the motor due the inductance and magnetic properties of the motor. When this current has nowhere to go, it will create a high voltage capable of destroying the mosfets.
To handle this you could rely on the internal body diode of the mosfet. But if you're dealing with a large motor, additional diodes might be required.
Note: mosfets fail shorted.
- Heat, the mosfets will become warm or hot. You're looking at 2A with 1V drop, that's 2 Watts. Don't forget the body diode current in the heat calculation. Make sure you do not fry the mosfets.
A benefit with using gate drivers is that they often have charge pumps allowing you to use only N-channels. Which have better Rdson == less heat.
Do not install the capacitor EMI, unless you install one additional choke at the output. With the capacitor installed, a lot of AC (PWM switching ) current will pass through it. The capacitor, you mention is for suppression of EMI due to brush commutated rotor current that is fed with DC voltage, already filtered and smooth.
However you can use the EMI capacitor, if you put a series inductor/choke which will block the AC current. A choke that at given PWM frequency has the reactance producing cca. 1%-2% voltage drop is enough.
Adding a little bit to the other answers...
If your motor draws 2A nominal, then it will draw a lot more when stuck/stalled/starting. It would not be unreasonable to expect 10A in a stall. Brushed DC motors have tons or torque but the price is high current current.
Now, this is trouble, as high-RdsON FETs will reduce your torque (by reducing the available current) but also dissipate lots of power on motor starts or stalls.
Since your PWM will be slow by modern switching converter standards (like 25 kHz, not 250k) you do not need state of the art switching speed, therefore you can use bigger (ie, slower, but less RdsON) FETs than in a much faster switching DCDC.
Since your voltage is low, you will easily find FETs with a RdsON below 10 mOhm, which will solve your problem.
Note that your NMOS/PMOS scheme requires proper drivers... and bulletproof dead time! Don't switch both FETs at the same time!
If you want to go all-NMOS, there are integrated bridge drivers which will drive your 4 NMOS, with onboard charge pump to generate the gate voltage, and... current limiting... always nice to have current limiting in case a screwdriver finds itself across the output terminals...
There are also beefier integrated H-Bridge chips.