H bridge

I have a dilemma. How should I implement the H bridge?

Each has their own advantages and disadvantages. One choice is the use of the MOSFET over the BJT. I have chosen the MOSFET because it has zero current going through its gate, while the BJT current can be significant, contributing to complications.

I want a small voltage across the device when it has maximum current. In this region of operation, the BJT causes charge to build up. This charge takes time to dissipate. The MOSFET is much faster in this region of operation. Also, the minimum voltage for the BJT is much higher, and is independent of the base voltage. The minimum voltage for the MOSFET for a given current is much lower, and it does depend on the gate to source voltage. The equation for the MOSFET is much simpler to implement, and is readily adapted to many specifications. The transport model for the BJT has many more variables and equations, that could be beneficial when looking for a solution.

One final consideration is whether to use complementary p and n MOSFET transistors, or if I should only use n MOSFET transistors.

Using complementary transistors has some advantages, but they might also turn into disadvantages. A major disadvantage is each pair of kitty corner transistors now must have opposite highs and lows. Both types must change the voltage from digital 5 Volts to a voltage more suitable to turn the transistor on. Unless the gate voltage must be much higher than the voltage required to drive the desired current through the load, the highest voltage needed would be the power supply voltage.

If we chose to use 4 n MOSFETs, turning each transistor off would be made much simpler, since 0 Volts will do that. Turning the two upper, current source, transistors on would be more difficult, because now the gate voltage must be much higher than the voltage across the load.

This is only the tip of the ice berg. I am sure more challenges lay ahead.

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Electrical Design Engineer, Minneapolis, Minnesota