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High Power Relay H Bridge

talha_tahir

New Member
Hello.
I have created a schematic for a high power dual h bridge utilizing relays and IRF540 Mosfet but i cant seem to calculate the values for the gate resistor to the mosfet plus i would appreciate if anyone could point out any mistakes i made in the design. The schematic is attached below in the zip. The design is for two motors which needs to be PWM'ed , with operating current of about 1.5A No load and Stall current of 14.5 A. This is one side of the h bridge for the full schematic open the zip.
final h bridge.PNG
 

Attachments

  • final h bridge .zip
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Last edited:

JimB

Super Moderator
Most Helpful Member
Welcome to ETO.

Your schematic, as drawn, is a very bad example and very difficult to read.
Many tangled wires, unnecessary bends, and just plain difficult to read.

A good schematic should be easy to read and show what the circuit is doing without having to untangle a spider web of lines.

I suggest that you redraw the schematic and re-post it if you want anyone to analyse it for errors.

To help you re draw it, I have done some basic editing to make parts of it easier to read.
I suggest you redraw the whole schematic in a similar style.

My edits:
Edited bad schematic.png



JimB
 

sagor1

Active Member
Just a comment, with a 5V supply, and triggering through an optoisolator, you will not have enough voltage to turn on the IRF540. It needs almost 10V on the gate to be fully on.
The logic level 540 might work (IRL540 series). At a 4V gate, its RDSon it 0.11 ohms.

Since the MOSFET is a voltage triggered device, you probably don't need a gate resistor, it does not draw any significant current.
 

crutschow

Well-Known Member
Most Helpful Member
As JimB noted, your schematic is quite difficult to read.
Never use more jogs in the schematic wires than absolutely necessary. which means all the lines should be as straight as possible.
Look at this example from your schematic, which could be replaced by the much simplified connections below.
1648305367159.png

1648305648171.png

Move and orient the components as necessary to achieve that goal.
Some jogs are necessary, but just about every wire in your schematic has several jogs, making it look like a puzzle maze.

Once you clean up the schematic, we can then read it and render an opinion on any problems it may have.
 

talha_tahir

New Member
Thanks for telling me that. I have changed it like JimB said. The problem I used to have was that when i had not grounded the 2n2222 BJT with the 10k resistor and trial tested the circuit it went up in flames. i then read some where that i have the saturate the BJT to that it has the least resistance and doesnt heat up, How can i do that.
Welcome to ETO.

Your schematic, as drawn, is a very bad example and very difficult to read.
Many tangled wires, unnecessary bends, and just plain difficult to read.

A good schematic should be easy to read and show what the circuit is doing without having to untangle a spider web of lines.

I suggest that you redraw the schematic and re-post it if you want anyone to analyse it for errors.

To help you re draw it, I have done some basic editing to make parts of it easier to read.
I suggest you redraw the whole schematic in a similar style.

My edits:
View attachment 136341


JimB
This is the updated circuit
hbridge neat.PNG
 
Last edited:

talha_tahir

New Member
Just a comment, with a 5V supply, and triggering through an optoisolator, you will not have enough voltage to turn on the IRF540. It needs almost 10V on the gate to be fully on.
The logic level 540 might work (IRL540 series). At a 4V gate, its RDSon it 0.11 ohms.

Since the MOSFET is a voltage triggered device, you probably don't need a gate resistor, it does not draw any significant current.
Thanks for telling me that it seems to have gotten past me when i looked up the datasheet for it. I also wanted to ask you said that mosfet gate does'nt draw current but the gate of the mosfet when uncharged acts as a capacitor which will draw a large amount of current so in order to limit that i need a resistor in that place. I wanted to know if you had any idea how should i choose the resistance
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The gate resistor should just limit the charge or discharge peak current to whatever the drive circuit can safely provide.

If it's only occasional on/off switching, it's generally not critical as components have time to cool between switching events.
That becomes a very different situation if you will be using PWM, you then need a proper high-current gate driver or buffer circuit of some sort.
 

talha_tahir

New Member
The gate resistor should just limit the charge or discharge peak current to whatever the drive circuit can safely provide.

If it's only occasional on/off switching, it's generally not critical as components have time to cool between switching events.
That becomes a very different situation if you will be using PWM, you then need a proper high-current gate driver or buffer circuit of some sort.
I would be using this mosfet for PWM as it will be used to sink the current to ground after passing through the motors. So could you refer to me about i should go about making a high current gate driver or a buffer circuit
 

Pommie

Well-Known Member
Most Helpful Member
What are you trying to do? Your optos are wired as emitter followers which will reduce the base voltage on the 2N2222. The opto LEDs are also rated at 50mA continuous, assuming 5V in (you don't say) your LEDs are only getting ~20mA.
Is the relay supposed to be on when the input is high?

Mike.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
So could you refer to me about i should go about making a high current gate driver or a buffer circuit
Example:

Data on that:

Or, the simplest buffer is just a PNP and an NPN transistor, both arranged as emitter followers with bases and emitters connected, something like a crude Class B audio amp. With suitable devices the gate drive can be increased by 50, 100 times or more from a simple logic-level signal, but you lose out on the voltage "swing" at the gate as each transistor drops about 0.6V before conducting, so 1.2V less out than in.


Also note that you need to be careful in selection of the opto isolator for PWM, as many common types are rather slow turning off and may not work well or not at all at high frequencies.

You can get ones with built-in logic to clean up the signal levels, and specified for transferring high speed signals, eg.

Or even with medium current drivers included, that can work directly with smaller power FETs
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Here's an example I've been using recently, while it's a highside driver the principle is the same - without the npn/pnp drivers the FET was getting far too hot. The 2N7000 is to invert the logic for the P-channel FET, and also to provide level shifting from 3.3V from the PIC to 12V for the FET power.

FET_Driver.png
 

talha_tahir

New Member
What are you trying to do? Your optos are wired as emitter followers which will reduce the base voltage on the 2N2222. The opto LEDs are also rated at 50mA continuous, assuming 5V in (you don't say) your LEDs are only getting ~20mA.
Is the relay supposed to be on when the input is high?

Mike.
,The optos will be used to switch on the relays and mosfet when the input is high, they would be powered by 5v
 

talha_tahir

New Member
n

That is a 2n2222 npn BJT not a mosfet and thanks for the gate drive links
 

talha_tahir

New Member
Example:

Data on that:

Or, the simplest buffer is just a PNP and an NPN transistor, both arranged as emitter followers with bases and emitters connected, something like a crude Class B audio amp. With suitable devices the gate drive can be increased by 50, 100 times or more from a simple logic-level signal, but you lose out on the voltage "swing" at the gate as each transistor drops about 0.6V before conducting, so 1.2V less out than in.


Also note that you need to be careful in selection of the opto isolator for PWM, as many common types are rather slow turning off and may not work well or not at all at high frequencies.

You can get ones with built-in logic to clean up the signal levels, and specified for transferring high speed signals, eg.

Or even with medium current drivers included, that can work directly with smaller power FETs
I much appreciate your help. As for the PWM i would be giving it a signal from a Arduino Mega which works on a operating frequency of 490Hz if i remember correctly so would it be slow enough that the opto PC817 won't have trouble matching its speed
 

rjenkinsgb

Well-Known Member
Most Helpful Member
operating frequency of 490Hz if i remember correctly so would it be slow enough that the opto PC817 won't have trouble matching its speed
OK, that is quite a fast opto, so it should be OK as long as the load resistance is not too high; 1K would be idea, it gives a reasonably fast recovery & makes it usable with full output up to around 5KHz, according to the data.

Note that you also need "flywheel diodes" to prevent back EMF spikes from the motor causing damage.
The simplest setup with an H bridge config is to use four diodes in a bridge rectifier style configuration, with the AC connections to the load (motor) and DC to the incoming supply positive and negative/ground.

Use fast recovery or schottky diodes, and make sure you have a decent size capacitor across the incoming supply, for energy recovery. Power will be regenerated from the motor if you reduce the drive duty cycle faster than the more decelerates - active braking.

(You could use a single diode between FET drain and positive power, but that would not provide protection to the relay contacts or from spikes going more negative than ground; the four diode bridge covers everything).
 

talha_tahir

New Member
OK, that is quite a fast opto, so it should be OK as long as the load resistance is not too high; 1K would be idea, it gives a reasonably fast recovery & makes it usable with full output up to around 5KHz, according to the data.

Note that you also need "flywheel diodes" to prevent back EMF spikes from the motor causing damage.
The simplest setup with an H bridge config is to use four diodes in a bridge rectifier style configuration, with the AC connections to the load (motor) and DC to the incoming supply positive and negative/ground.

Use fast recovery or schottky diodes, and make sure you have a decent size capacitor across the incoming supply, for energy recovery. Power will be regenerated from the motor if you reduce the drive duty cycle faster than the more decelerates - active braking.

(You could use a single diode between FET drain and positive power, but that would not provide protection to the relay contacts or from spikes going more negative than ground; the four diode bridge covers everything).
Thank you so much you dont have many times i have thought of ways to reduce the emf spike from the motor but you solved it so easily, i had calcuated and RC snubber , TVS , every nook and cranny of the internet in order to reduce the voltage spike but your idea solved for that i am very thankful. Also I wanted to ask the load resistance for the optocoupler you are referring to is the one connected on the input side right that goes to the led. Also i wanted to ask what type and value of capacitor should i use , Either electrolytic or ceramic.
 

danadak

Well-Known Member
Most Helpful Member
Also select bypass caps with care, as you can see from their ESR curves -

1648420331845.png



OS-CON caps are polymer tants.


Regards, Dana.
 

danadak

Well-Known Member
Most Helpful Member
Thanks for catching that Pommie, I spaced out on that one. Of course the
ESR is reflected in the minimum Z value, but it was wrong to consider the
ESR is a f(w) (to a first order approximation). Obviously we are seeing ESL
and ESC and ESR.

Kind of interesting to see, for several technologies, the "inverse Q, my term" is pretty flat
over 2 or more decades, in prior chart.

But all effects have ESR as a f(w) as follows ....

1648428630459.png


Electrode loss skin effect I believe.


Regards, Dana.
 
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