H-Bridge design - is p-channel/pnp high-side switching advantageous?

Circuit

Senior Member
I am proposing to control a simple D.C. very low current (circa 50mA) motor with an H-Bridge controlled by four outputs from a 14M2 chip - no PWM, just on/off and direction. I want the whole thing rather compact and therefore I don't want to use half of an L293D, but just a simple H-Bridge with four SOT23 MOSFETS. Now I have seen H-bridge circuits that use four N-channel MOSFETs and others that use two N-channel and two P-channel MOSFETs instead; the P-channel devices switching the high-side of the motor. I am now confused; I can only presume that four N-channel devices work so why do some designs use P-channel as well? The same seems to apply with simple NPN and PNP transistors; some designs just use NPN alone. Can the more knowledgeable help me with some advice/explanations as to the relative merits of these two approaches?
 

hippy

Technical Support
Staff member
Only guessing -- but does it depend on the motor supply voltage, whether higher than the gate switching voltage or not ?
 

rq3

Senior Member
I am proposing to control a simple D.C. very low current (circa 50mA) motor with an H-Bridge controlled by four outputs from a 14M2 chip - no PWM, just on/off and direction. I want the whole thing rather compact and therefore I don't want to use half of an L293D, but just a simple H-Bridge with four SOT23 MOSFETS. Now I have seen H-bridge circuits that use four N-channel MOSFETs and others that use two N-channel and two P-channel MOSFETs instead; the P-channel devices switching the high-side of the motor. I am now confused; I can only presume that four N-channel devices work so why do some designs use P-channel as well? The same seems to apply with simple NPN and PNP transistors; some designs just use NPN alone. Can the more knowledgeable help me with some advice/explanations as to the relative merits of these two approaches?
N channel mosfets require a gate voltage higher than the driven load voltage to be fully ON (simplistically).
P channel mosfets require a gate voltage lower than the driven load voltage to be fully ON (simplistically).

P channel mosfets have higher ON resistance, so tend to be less efficient as switches, but that may be offset by the simplicity of their gate drive (depending on the application).
 

techElder

Well-known member
Most bridge drives involve reversing the motor. Simple drives will turn an N-channel ON and the P-channel on the same side OFF with the same drive signal.
 

AllyCat

Senior Member
Hi,

To switch on an NPN or N-channel transistor needs a voltage higher than its emitter/source, about 0.6 volt for an NPN, probably several volts (>V threshold) for a FET. That's easy for the lower transistors in the H-bridge , but you either need a drive voltage higher than the normal supply rail, or must tolerate a voltage drop across the "top" switches in the H-bridge. The latter is what is done in the L293, as I posted in another thread recently:

L293d-outputs.pngL293d-darlingtons.png

Note that in the above case the lower voltage drop is also rather large, because the output stage is a Darlington configuration, so both switches have a "wasted" voltage drop due to their Vbe.

To allow the switches to have a low voltage drop (with a single supply rail) they need an "emitters (or sources) outwards" configuration, i.e. NPN/N-channel to the ground and PNP/P-channel to the supply rail. You may also need to ensure that the Positive side transistors have as "good" performance as the Negative side (in terms of saturation resistance, current gain or threshold voltage etc.).

But 50mA at 5 volts shouldn't be difficult, something like two pairs of BC327 + BC337 (or SMD equivalents), but of course need to be careful to avoid a "shoot through" current if both transistors are switched on at the same time.

Cheers, Alan.
 

westaust55

Moderator
If you don’t mind working with some SMD chips you can consider the LB1909MC
Which for a single motor use the output enable for motor off.

Alternatively consider the LV8548MC which has 2 inputs for control giving direction, braking and stop control.
These use DMOS technology for reduced voltage drop across the motor drive outputs and do not require a separate 5V supply.

For more on the topic see the thread I started 18 months ago here:
http://www.picaxeforum.co.uk/showthread.php?28938
 

Circuit

Senior Member
Many thanks for all the explanations - Especially to Alan for writing such a detailed reply at two o'clock in the morning!
Looking again at the sizing of all the components needed, I think that I will probably just use an L293D - the size differential is less than I thought it might be.
 

premelec

Senior Member
@Circuit - There are MOSFET drivers like Toshiba TB6612FNG which run better at low voltage and .5 ohm on resistance... Some on Ebay, Sparkfun etc...
 
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