Mosfet H-bridge Motor Driver

[john_909]

I am designing a motor driver for 12vdc geared motor with mosfets. I need to amplify the digital output from microcontroller pic18f, tell me which transistors would be best. Moreover i need to know the working of mosfet drivers for driving the mo. Attached is the schematic in which i am using IRF530 & IRF9530 mosfets. I need to know if there is anything missing in the circuitry other than transistors.

 

[doraemon]

Hello!

You may be interested in buying an integrated bridge.
If you search Digikey, for examples, there are 3000 types of H bridge circuits
ranging from 2mA to 165 A and up to 40V. I guess there is one for you.

An example at random: https://www.vishay.com/doc?70590

Dora.

 

[enjunear]

I am designing a motor driver for 12vdc geared motor with mosfets. I need to amplify the digital output from microcontroller pic18f, tell me which transistors would be best. Moreover i need to know the working of mosfet drivers for driving the mo. Attached is the schematic in which i am using IRF530 & IRF9530 mosfets. I need to know if there is anything missing in the circuitry other than transistors.

MOSFETs, especially power FETs, have a large gate capacitance due to the large channel (for large current handling). When your circuit transitions from a low voltage to a high voltage on the gate of the FET to turn it on, it needs to push a lot of electrons into the gate to fill up the gate capacitance. As that internal gate capacitance gets filled up with charge, the voltage on the gate pin rises. If you have a microcontroller, chances are that it can't source (or sink) a lot of current. Current is the flow of electrons, so you can't push (or pull) a lot of electrons into (or out of) the gate capacitance very quickly. What will happen is that you'll slowly turn the FETs on and off, getting more of a voltage ramp than a nice clean on/off switching action. This transition state is where the part will dissipate a lot of power (the linear region, not in cutoff or saturation as a switch should act).

In order to allow low-current devices like logic gates and microcontrollers to control power MOSFETs, someone got smart and built little current amplifiers and called them "mosfet drivers". They take very little current at their input, and can source/sink a LOT of current at their output. This allows the FET drivers to pump lots of electrons into, or out of, a power MOSFETs gate... which quickly fills or empties the inherent capacitance. If the charge changes quickly on the internal capacitance, the voltage also changes quickly. This tells us that the MOSFET turns on and off very quickly, which means your FETs (which are supposed to act like on/off switches), spend very little time in the linear region (now dissipating very little power), and spend nearly all of their time in cutoff ("off") or saturation ("on").

Hope that helps you see what FET drivers do for you.

 

[john_909]

Thanks for replying and sorry for late reply I have designed the mosfet H-bridge motor driver using above diagram. I am getting correct output voltage from drains. The problem is when i connect the output to motor terminals, the voltage drops to zero. I am using 12V 4.5A battery and applying 6 volts 0.5A at gates. Can you help me with this?

 

[alexxx]

I need to know if there is anything missing in the circuitry other than transistors.

Don't you need motor current measurement? You could accomplish that with a resistor.

I am getting correct output voltage from drains. The problem is when i connect the output to motor terminals, the voltage drops to zero.

Could you please share those drain voltage values for each FET?

 

[john_909]

I am getting 12V on drains...

 

[adnan_merter]

i strongly advice you to have L298 H-bridge IC, it makes everything better and easier . it can supply power up to 40v,2A

 

[alexxx]

I am getting 12V on drains...

Since no driving circuit is provided for the FETs and taking under consideration that a battery is used, a shorted motor is the obvious think to say.
But again, if this was the case you would have at least one burned FET.
With the information provided, only assumptions could be made.
You can use use lamps instead of motor for your tests, but keep in mind that the temperature will rise a lot faster, so be careful not to fry anything.
Finally I agree with enjunear that a driver with strong sink and source cabability is needed for the FETs, like a push pull.

 

[enjunear]

Thanks for replying and sorry for late reply I have designed the mosfet H-bridge motor driver using above diagram. I am getting correct output voltage from drains. The problem is when i connect the output to motor terminals, the voltage drops to zero. I am using 12V 4.5A battery and applying 6 volts 0.5A at gates. Can you help me with this?

Something sounds wrong there. If you are using MOSFETs, then your gate current should be zero (once the gate capacitance has been charged/discharged, which only takes a few mA for a very short duration). If you are supplying a continuous current into the gate, then your FET is broken (you burned through the gate oxide layer, which gives you a high resistance at that pin). If that's the case, replace the FET. Be very careful to not touch the leads, which keeps ESD potential down. A small static discharge may not arc through the air, but a few hundred volts is more than enough to arc through the oxide layer and kill your FET.

Additionally, you should add pull-up and pull-down resistors to your FETs. All FETs should have a resistor installed from Gate to Source, so the N-channel gets it's gate pulled low (cut-off) when nothing is connected to it, and the P-channel gets pulled up to the voltage supply rail (cut-off, also).

 

[babatundeawe]

i will also enjoin u to use the L298 hbridge driver it makes the work easier and more compatible to ttl output that u have frm your micro. just do a google search of L298 hbridge driver. then u get more of the circuit.

also interfacing wit fets is also very easy if u need more power to your motor.

 

[john_909]

Actually i am going in a competition and in that it is recommended to design motor driver with discrete components. I am using this motor https://www.pololu.com/catalog/product/1103. I have tested motor with L293D and L298, it is working. I have also tested it with BJTs H-bridge driver and it gives the same behavior as it does with the FETs. Plz Hep me.

 

[alexan_e]

Add a couple of TC4424 or TC4427 mosfet drivers and it should work fine.

You haven't posted your schematic so no one has any idea of the mcu connection to the bridge.

 

[john_909]

i am testing motor driver without mcu and using the above posted circuit. i can't use mosfet drivers as they are ICs.

 

[alexan_e]

You said it is recommended to design motor driver with discrete components but now you are saying that you are not allowed to use any IC and everything has to be discrete?

Anyway, your bridge is fine, maybe it would be a good idea to use pull up/down resistors to the gates to ensure that the mosfets are off unless you drive them on but we have no idea of how you are trying to drive it so unless you provide a more detailed schematic I don't see how we can help.
So far the only info you have provided is "applying 6 volts 0.5A at gates" but it is not enough, we need to see a schematic of how you do that.

 

[monudhiman123]

i am using h bridge mosfet driver circuit to drive motor.i am not getting sufficient current at the output.when apply 12v to the gate i get 7 volt at the output.what is the reason for this drop.i want to incresethis output voltage.i am using ir2110 ic.plz help me

 

[FvM]

The original post is showing a complementary H-bridge that won't work with IR2110. Please clarify which H-bridge circuit you are using with IR2110.