help regarding H-bridge

[abhishek564]

hey everyone...I need some help ASAP regarding my h bride..
I needed a bridge which would deliver about 10-15A current @ 12 - 14v DC
so I made this circuit..
with reference to the schematic -

I got these readings when I connected-

1. Vcc 18v dc (3.5 A) -- voltage across the motor leads was around 12 - 13 V DC (1.0 A current)

2. Vcc 12v DC (25 A) (car battery) -- voltage across motor leads was 9 - 10V (0.5 A)

3. vcc 24 v (25A) (2 car batteries)-- voltgage across motor leads 20v (1.0A)

4. vcc 36 v (25A)(3 car batteries) --- voltage across motor leads (30v) (2.0 A)

the input to the bridge is a pwm signal taken from a hobby servo..

When I connect a 12 v battery @ 25A to the motor directly ,it draws 1A current and 6A under load...

but when I do the same via the H bridge , the motor does not even move..

but if it connected to a 18v supply @ 3.5A it works ..but does not provide enough torque...

and as I know torque is directly proportional to current...


so my question is.. how do I generate 10-15 A current from a 12v dc source ?
do I need to change the transistor...because I think the npn transistors are causing a lot to loss...
if I use MOSFETs (2pmos & 2nmos )in the same schematic will it work ?

so please help me someone...I've wasted days figuring this out...and have to submit a project within the next few days which requires this bridge...

hope to hear from someone soon...



 

[ericgibbs]

hi,
The TIP35xC have a gain of only 10 at high currents, so for a 10A collector current the Base current must be 1Amp and for 15A Ic , , Ib => 1.5A

The Base drive current to the TIP's is far too low, you need to add intermediate drive transistors between the logic and the TIP's.

 

[abhishek564]

hi,
The TIP35xC have a gain of only 10 at high currents, so for a 10A collector current the Base current must be 1Amp and for 15A Ic , , Ib => 1.5A

The Base drive current to the TIP's is far too low, you need to add intermediate drive transistors between the logic and the TIP's.

firstly thank you for your speedy reply..

can you suggested which intermediary should I add between the logic and Transistors..

or can I use transistors which have a higher gain (eg- tip122 and tip127- gain of 1000 at 3A).. and make 2 or 3 h-bridges and connect them in parallel..
or what about MOSFET..do they have higher gain ?

 

[ericgibbs]

firstly thank you for your speedy reply..

can you suggested which intermediary should I add between the logic and Transistors..

or can I use transistors which have a higher gain (eg- tip122 and tip127- gain of 1000 at 3A).. and make 2 or 3 h-bridges and connect them in parallel..
or what about MOSFET..do they have higher gain ?

hi,
For that current requirement ie: 12V at 15A I would recommend that you use Power MOSFET's

Darlington transistors will have a too high Vce sat for efficient use at 12V

 

[abhishek564]

hi,
For that current requirement ie: 12V at 15A I would recommend that you use Power MOSFET's

Darlington transistors will have a too high Vce sat for efficient use at 12V

so will the same logic circuit work if I replace the TIPs with the mosfets..

 

[ericgibbs]

so will the same logic circuit work if I replace the TIPs with the mosfets..

You have not stated the driving Logic levels.

If you are using 5V Logic, you will need to increase the MOSFET Gate drive voltage levels to at least 12V.

 

[abhishek564]

You have not stated the driving Logic levels.

If you are using 5V Logic, you will need to increase the MOSFET Gate drive voltage levels to at least 12V.

yes ..the logic level is at 5v.. but how do I increase the logic level to 12v without damaging the servo circuit(which runs at 5v)..

 

[4pyros]

If you look on line there are many setups using MOSFETs.
Look Here; **broken link removed**

 

[abhishek564]

If you look on line there are many setups using MOSFETs.
Look Here; **broken link removed**

I've already looked at that circuit...the problem with that circuit is that it has 4 inputs.. whereas I require only 2..thanks anyways

 

[4pyros]

the problem with that circuit is that it has 4 inputs.. whereas I require only 2

Just tie the inputs together. Power A and D at the same time or C and B at the same time.

 

[abhishek564]

You have not stated the driving Logic levels.

If you are using 5V Logic, you will need to increase the MOSFET Gate drive voltage levels to at least 12V.

I increased the logic voltage to 12v @ 1A from 5v...
and replaced the 2N2222s with TIP122 ...
I am still getting around 1amp current on the motor leads...instead of 10amp...(in the original circuit)

 

[ericgibbs]

I increased the logic voltage to 12v @ 1A from 5v...
and replaced the 2N2222s with TIP122 ...
I am still getting around 1amp current on the motor leads...instead of 10amp...(in the original circuit)

As I said earlier the TIP's require at least 1Amp Base current for a collector current of 10A.

Do the following test with a voltmeter, while the motor is running at 1A, measure the voltage across the motor and also the voltage drop across each TIP35/36, lets know what you measure.?

 

[audioguru]

The 1k base resistors for the TIP35 and TIP36 transistors limit their base current tro only 10mA to 18mA, way too low.

 

[abhishek564]

As I said earlier the TIP's require at least 1Amp Base current for a collector current of 10A.

Do the following test with a voltmeter, while the motor is running at 1A, measure the voltage across the motor and also the voltage drop across each TIP35/36, lets know what you measure.?

okay so I did the test..
results..
vcc 18.5v @ 3.5A
motor voltage 14v @ 0.9A
TIP 36s readings --- across Q1 (BC=0.6 , CE=0.09 , BE=0.7)
Q2 (BC=0.61 , CE=0.09 , BE=0.7)

TIP 35s readings --- across Q3 (BC=2.1 , CE=2.9 , BE=0.67)
Q4 (BC=1.7 , CE=2.33 , BE=0.66)

Q1,Q2 are high level
Q3,Q4 are low level in my schematic

 

[audioguru]

Your readings show that the TIP35 is barely turned on with a VCE voltage loss of 2.9V because its base current is too low. Its base current will be less when the supply voltage is less then its voltage loss will be much higher when its base current is too low.
You are extremely lucky that the TIP36 has a voltage loss of only 0.09V. Its base current should also be much higher in case you are not lucky.

 

[abhishek564]

Your readings show that the TIP35 is barely turned on with a VCE voltage loss of 2.9V because its base current is too low. Its base current will be less when the supply voltage is less then its voltage loss will be much higher when its base current is too low.
You are extremely lucky that the TIP36 has a voltage loss of only 0.09V. Its base current should also be much higher in case you are not lucky.

Thanks for the reply..
do you think ,if I use MOSFETs which are specifically designed for logic level input at 5v would function properly , if I replace the TIPs
with mosfets in the same schematic ?
mosfets like -- https://www.electro-tech-online.com/custompdfs/2011/10/RFP50N05L.pdf

also can you suggest a few mosfets which have a logic level gate drive...

 

[audioguru]

When Mosfets are used then the operation of the circuit must be known. Is it used for continuous full power forward and reverse or is it also used for PWM motor speed control?
What IC supplies the logic signals?
When these questions are answered then a circuit can be designed to use logic level Mosfets, not the circuit (with too low base current) designed to use transistors.

 

[abhishek564]

When Mosfets are used then the operation of the circuit must be known. Is it used for continuous full power forward and reverse or is it also used for PWM motor speed control?
What IC supplies the logic signals?
When these questions are answered then a circuit can be designed to use logic level Mosfets, not the circuit (with too low base current) designed to use transistors.

I'm now planning to make the bridge with MOSFETs..
the input will be pwm signals from a servo circuit..(0-5v)
do you think the attached circuit will work..??
also I'm planning to use TC4426 instead of 4428..in the circuit..because I think 4428 is used incorrectly in the circuit (which I found on the internet..)
and MOSFET IRF540(nmos) and IRF9540(pmos)

so please tell me if this will work..

 

[audioguru]

The Mosfets will burn up trying to slew the 1uF capacitors. The capacitors also might overheat.
You want the Mosfets to switch as fast as is possible so remove the 1uF capacitors.

Mosfets usually oscillate at a very high frequency if they don't have 10 ohms to 47 ohms in series with the gate at the gate pin.

 

[abhishek564]

another thing..
should I use a higher voltage source to drive the TC4426 ..say 15v..than the 12v source for the bridge..
in that case what will happen?
please advice.

..I did a quick test using just a single Nmos-IRF540 (as a switch)

input was 0-5v pwm to driver IC and motor as load..the voltage across the motor was 11.5v..

i.e. 0.5v drop. compared to the voltage source..

so if I increase the voltage to the driver IC , will the voltage across the motor also increase. .