H bridge!!

[zeeshan]

Hey ppl! i want to have some good reads on h bridge..preferably made by bjts..ill like if someone can send me a proteus etc schematic for starting and some guidelines..i am making my first robot which has to do the task in the given link...hoping to HAVE SOME HELP ASAP!

NERC 2012 - Arena

[newInRobotics]

Hi there 

There are many good reads here 

But in all seriousness, what is rated voltage and current of the motors You want to use?

[zeeshan]

i will be using 12v supply and my motors have no load current of about 120mA and a stall current of 3.22A at this voltage..

 

Hi there 

There are many good reads here 

But in all seriousness, what is rated voltage and current of the motors You want to use?

[newInRobotics]

Hi again,

Something like TIP42C for the high side and BDX53C for low side, plus smaller NPN transistors to drive high side transistors' bases should do.

Using Bipolar Transistors As Switches article should help You with calculations for base resistor values.

Would You mind me asking why do You prefer BJTs? Working with MOSFETs is much easier and more efficient.

[zeeshan]

Hi again,

Something like TIP42C for the high side and BDX53C for low side, plus smaller NPN transistors to drive high side transistors' bases should do.

Using Bipolar Transistors As Switches article should help You with calculations for base resistor values.

Would You mind me asking why do You prefer BJTs? Working with MOSFETs is much easier and more efficient.

i m surprised that ive heard exactly the opposite here in my little town...i heard bjts was simpler...so can u guide me a bit on mosfets?

[zeeshan]

Thanx for the article though...im on it!! 

[newInRobotics]

With MOSFETs You apply only threshold voltage to Gate and that's it, no Base resistors required.

[zeeshan]

Can u gimme a schematic design with either MOSFETS or BJTs?

With MOSFETs You apply only threshold voltage to Gate and that's it, no Base resistors required.

[newInRobotics]

Have a look at How to drive pMOS and nMOS H-bridge? thread. In the circuit attached top N MOSFETs (the ones that drive P MOSFETs) should have pull down resistors as well (like the ones in the lower side of the bridge). 1k resistor in the middles should be raplaced by a motor.

[Soeren]

With MOSFETs You apply only threshold voltage to Gate and that's it, no Base resistors required.

You need to apply more than just the threshold voltage to get proper switching, but you're right that no base resistors are needed, just like BJT's don't need gate resistors

Another thing to note is, that MOSFET's are much easier to kill by ESD than The rugged BJT's, so for a beginner, it makes good sense to use the latter.

[newInRobotics]

You need to apply more than just the threshold voltage to get proper switching, [...]

Well, yes, my mistake not to mention it 

just like BJT's don't need gate resistors

That's not what I said  I said that MOSFETs do not require Gate resistor as opposed to BJT which do require Base resistor 

Another thing to note is, that MOSFET's are much easier to kill by ESD than The rugged BJT's, [...]

Do You need to shield MOSFETs in any special way?

[zeeshan]

With MOSFETs You apply only threshold voltage to Gate and that's it, no Base resistors required.

You need to apply more than just the threshold voltage to get proper switching, but you're right that no base resistors are needed, just like BJT's don't need gate resistors

Another thing to note is, that MOSFET's are much easier to kill by ESD than The rugged BJT's, so for a beginner, it makes good sense to use the latter.

so what else do we need other than the 5V from the mcu pin? i mean can u elaborate wats cum between the mcu pin and the gate of the mosfet.

[newInRobotics]

You should use MOSFETs with V_GS(TH) < V_uC/2 (it's not a rule, better to use I_D(ON) vs V_GS chart in datasheet to see at what Gate voltage there is least resistance (r_DS(ON))) in order to get well above threshold level to make sure that MOSFET is in saturation mode.

Example:

Have a look at RFP12N10L Datasheet.

V_GS(TH) is 2V (threshold voltage at which MOSFET starts to switch ON (but is NOT fully ON);

find "I_D(ON), ON-STATE DRAIN CURRENT (A) vs V_GS, GATE TO SOURCE VOLTAGE (V)" chart (on page 3) and analyse it;

what it shows is that current through MOSFET is highest when V_GS is @ ~4.5V, meaning that MOSFET is fully saturated, hence lowest resistance (r_DS(ON)), hence least dissipated heat.

[Soeren]

Hi,

That's not what I said  I said that MOSFETs do not require Gate resistor [...]

This isn't entirely correct either. In a switching configuration (like with eg. PWM), you either need a small resistor (10-100 Ohm, it depends...) or a small inductor in the gate. If your switching is close to DC, you can usually leave it out as there's not much chance for oscillations.

Do You need to shield MOSFETs in any special way?

The ESD sensitivity is mainly a problem until it's in a circuit and they have grown better over time in that respect. It's because they have a very high impedance, so static charge will easily be able to get magnitudes over what it can safely handle.
When they're mounted in a PCB, circuit impedance will usually keep it intact.

Back in the eighties, a lot of MOSFETs popped just from handling and some even came with a thin shunting wire that you cut after installation.

I had to make a gizmo for shorting the pins to the housing of some TO-3 devices back then, as the company I worked for had the annoying problem that a few of them died from mounting, even with ESD safe soldering equipment. It wasn't that many, but even a few here and there means large costs when they have to be identified and reworked.

In short, keep your MOSFETs in conductive foam, wrapped in tin foil or similar and when you grab them, be sure you're not charged (don't walk over synthetic carpets with them either).

A really good eye opener is to make a simple setup - an N.ch. MOSFET with a loudspeaker in the drain to eg. +5V and with the gate bend out (you could even mount 5..10cm of wire to the gate, but not connected otherwise. This will allow you to test how much ESD is generated from different materials, by rubbing them in the vicinity of the gate wire - and you'll never throw your MOSFETs in a plastic bag or even a paper bag after that, perhaps not even in the pink "antistatic" bags.
Mind you, that you may kill the MOSFET very quickly this way, so do it to one that you're prepared to sacrifice - although its death will likely serve to save quite a few other innocent brothers from bad handling.

[zeeshan]

You should use MOSFETs with V_GS(TH) < V_uC/2 (it's not a rule, better to use I_D(ON) vs V_GS chart in datasheet to see at what Gate voltage there is least resistance (r_DS(ON))) in order to get well above threshold level to make sure that MOSFET is in saturation mode.

Example:

Have a look at RFP12N10L Datasheet.

V_GS(TH) is 2V (threshold voltage at which MOSFET starts to switch ON (but is NOT fully ON);

find "I_D(ON), ON-STATE DRAIN CURRENT (A) vs V_GS, GATE TO SOURCE VOLTAGE (V)" chart (on page 3) and analyse it;

what it shows is that current through MOSFET is highest when V_GS is @ ~4.5V, meaning that MOSFET is fully saturated, hence lowest resistance (r_DS(ON)), hence least dissipated heat.

since im just a beginner i wish to use the FET u gave in example...tell me the equivalent pmos to be used with this...assuming that this will work perfectly with a pin of mcu..

and yes wat do u say about mosfet drivers such as the irf3205?

[Soeren]

Hi,

Have a look at RFP12N10L Datasheet.

V_GS(TH) is 2V (threshold voltage at which MOSFET starts to switch ON (but is NOT fully ON);

V_GS(TH) is the point where it barely conducts. It's nowhere near fully on and it has got a long way to go to get there (see below).

find "I_D(ON), ON-STATE DRAIN CURRENT (A) vs V_GS, GATE TO SOURCE VOLTAGE (V)" chart (on page 3) and analyse it;

what it shows is that current through MOSFET is highest when V_GS is @ ~4.5V, meaning that MOSFET is fully saturated, hence lowest resistance (r_DS(ON)), hence least dissipated heat.[/i]

The transfer characteristics (fig. 4) is not the best way to judge a device, as it says nothing about charge.

Look at eg. this FDP8874 datasheet instead, as the RFP12N10L have no voltage/charge scales on the gate charge waveform (fig. 13) and it's this graph that's the most revealing. It's fig. 14 in the FDP8874 datasheet and this gives a crisp picture of the gate charge.
This graph has the added benefit (compared to most others) to include the effect of different currents as well.

Starting from 0,0, to a little over 2V (the 1A curve) at around 7..8nC (nano-Coulomb) the charge begins, but not much happens. Then it reaches the very critical Miller Plateau, which takes current (the more the better) to get through. At the point where the curve starts rising again, the device is ON, as hard as it can be at the given gate voltage. Raising the gate voltage (if possible) lowers R_DSON further.

In short, always look at the gate charge curve when you want to judge a MOSFET, as it's the voltage at which the Miller Plateau resides that gives the minimum ON gate voltage (at that voltage, not much current flow, so go for at least a bit higher) and as you can see form this particular curve, for 1A you need to pump around 15..16nC into the gate to charge it fully.

And this is also a message to those that think, that the description of MOSFETs as being merely voltage controlled, means they can skimp on the current requirement.
No current - no cookies

[Soeren]

Hi,

[...] wat do u say about mosfet drivers such as the irf3205?

The IRF3205 is a MOSFET, not a MOSFET driver (which is something you use to drive the gate of a MOSFET).
The IRF3205 won't work at 5V, the FDP 8874 I just linked to will, but it would probably be gross overkill, which don't just mean it's more expensive, but also that the gate capacitance is higher, than on a more appropriate device - and the lower the gate cap. the faster you can switch it.

To be able to find the best MOSFETs for your application, we need to know the max. current of your motors and at what voltage you will use them.

[zeeshan]

Hi,

[...] wat do u say about mosfet drivers such as the irf3205?

The IRF3205 is a MOSFET, not a MOSFET driver (which is something you use to drive the gate of a MOSFET).
The IRF3205 won't work at 5V, the FDP 8874 I just linked to will, but it would probably be gross overkill, which don't just mean it's more expensive, but also that the gate capacitance is higher, than on a more appropriate device - and the lower the gate cap. the faster you can switch it.

To be able to find the best MOSFETs for your application, we need to know the max. current of your motors and at what voltage you will use them.

i wish to use it at 12V and max current at this voltage is 3.22A...........so??

[Soeren]

hi,

i wish to use it at 12V and max current at this voltage is 3.22A...........so??

So.. Something like FQP30N06L (datasheet) would be more than fine for te N-ch. devices and for the P-ch. devices (which btw. doesn't have to be logic level, as they have 12V to work on) I'd recommend FQP27P06  (datasheet).

They're very conservative choices, as they're rated for quite a bit more current, even when working hot. I'd recommend a simple driver, or using more than one output pin on the controller to switch them as fast as possible (combined input capacitance is around 2.0-2.5 nF, where the IRF device you found is more than 3 times the FQP30N0L).

The P-ch. MOSFET should be driven via an NPN (bipolar) transistor, like eg. BC337.