4 transistors vs. 6 transistors h-bridges

[hypersnapper]

I've seen a lot of h-bridges one of them is 4 transistors h-bridge which is composed of 2 pnp transistors and 2 npn transistors while 6 transistors h-bridge is composed of 2 pnp and 4 npn transistors. Which do you think is really better?

 

[DemonicSandwich]

Depends on the design. You can make both do the same thing.

Usually if the control circuitry is powered from a different voltage level than that powering the motor, it will use 6 transistors. Using the two extra NPNs to turn on the PNP power transistors

Also note that you absolutely need reversed diodes across the power transistors of an h-bridge.
Without them the transistors will quickly be destroyed.

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As for the examples you have uploaded:

The 6 transistor example only allows forward, backward, and coast. While it uses more parts than the 4 transistor example, it's simpler to control using two inputs.

4 transistor example is a "Full Control" h-bridge. It allow forward and backward movement like the 6 transistor h-bridge but also allows braking. However it also requires 4 inputs.

 

[MikeMl]

The ones with only four have huge shoot-through current spikes unless you use four port pins to control them.

 

[audioguru]

The 6-transistors circuit is missing 6 important resistors.
The transistors connected to the motor have a very low max current rating.
The base resistor values are far too high.

 

[hypersnapper]

The ones with only four have huge shoot-through current spikes unless you use four port pins to control them.

what do you mean by shoot through current spikes?

The 6-transistors circuit is missing 6 important resistors.
The transistors connected to the motor have a very low max current rating.
The base resistor values are far too high.

why 6 important ressitors?


Thanks for replying

 

[audioguru]

"Shoot through" is a very high current caused by one side turning on before the other side has enough time to turn off so the transistors short-circuit the power supply for a moment. Usually a pause called "dead time" is added to prevent it.

Your extremely simple circuit is missing two resistors that limit current and 4 resistors that quickly turn the transistors off.

 

[hypersnapper]

Your extremely simple circuit is missing two resistors that limit current and 4 resistors that quickly turn the transistors off.

Are the four 1k ohms resistors used to quickly turn the transistor off? how do they do it? sorry im just beginner.

How did you calculate the 33ohms for the base resistor of the TIP transistors?

 

[audioguru]

Are the four 1k ohms resistors used to quickly turn the transistor off? how do they do it? sorry im just beginner.

Yes.
Thje 1k resistors quickly discharge the base-emitter and wiring capacitances.

How did you calculate the 33ohms for the base resistor of the TIP transistors?

My example has a 3A load.
I used a 300mA base current for the output transistors that is 1/10th the collector current as shown on the datasheet for a saturated transistor so their base current is 300mA. Then it shows their base emitter voltage drop and the Vce saturated voltage drop of the driver transistors.
The total voltage drop driving the output transistors is about 2.1V so the 33 ohm current-limiting resistors have a voltage drop of 12V - 2.1V= 9.9V. Their current is 300mA so their value must be 9.9V/300mA= 33 ohms. The calculations are simply a child's arithmatic.

 

[hypersnapper]

Yes.
Thje 1k resistors quickly discharge the base-emitter and wiring capacitances.

are the 1k resistors fixed value everytime I use this kind of circuit?

I used a 300mA base current for the output transistors that is 1/10th the collector current as shown on the datasheet for a saturated transistor so their base current is 300mA.

sorry i dont get it. i don't see 1/10th collector current rule on the datasheet. please tell me what transistor you are stating.

 

[audioguru]

I used 1k resistors to turn of the power transistors because they are big with a fairly high capacitance. If I use small transistors then I use 10k or 100k ohms.

Every transistor datasheet shows the DC current gain (hFE) with a certain collector to emitter voltage so that the transistor is a linear amplifier and is not saturated. The Vce voltage listed for a BC337 transistor is 1V and is 5V or 10V for other transistors.
But in an H-bridge you want the transistors to be a saturated switchand turned on hard so every datasheet lists the max collector-emitter saturation voltage when the base current is 1/10th the collector current. Some very high gain European transistors list the base current as 1/20th the collector current even if their hFE is as high as 800.

 

[hypersnapper]

Thanks, you're referring to Hfe sorry.. I almost got it.

The total voltage drop driving the output transistors is about 2.1V so the 33 ohm current-limiting resistors have a voltage drop of 12V - 2.1V= 9.9V. Their current is 300mA so their value must be 9.9V/300mA= 33 ohms.

What do you mean the output transistor here? Is it the BC337 or the TIP42?

 

[audioguru]

What do you mean the output transistor here? Is it the BC337 or the TIP42?

It is obvious that the TIP41 and TIP42 transistors are the output transistors.
But even the BC337 driver transistors must saturate in an H-bridge so that all the transistors saturate and make good switches.