Have a look at this MOSFET circuit and comment

[D n T]

I need someone who knows, to have a look at this because I want it to work and its affecting my logic.
I am using four of the same MOSFETs to drive a motor in an H bridge, the MOSFETS are driven of an L293D. Most tell me that I should use two P type and two N type but according to others, that is not always the case.
The MOSFET is a P60N06 controlling 24 volts. I was considering running a pair in parallel rather than just having individuals (so 8 in total)
By the way there is a minimal lag in the program so that there one side of the motor is positve and the ther is negative or in an instant they might both be negative, but never both positive because apparently thats no a good thing
The L293D seems to be either on or off and changes state quickly, so minimal heat, is this the case.
I tried to work out a way to fit the suppresion diode, but it has to work in both directions.

By the way,power in from the top, out bottom.
The part you can't see is that the voltage to the MOSFETS ( not the gate charge) is Pulse Modulated to contol motor speed, so the H bridge MOSFETs don't need to pulse, they just need to act like an L293D on steriods.
I want to run 24 volts up to perhaps max 5 amps ( I know stalled or starting it will be higher for a little bit), the motors are 250 watt geared motors and if they go above 5 amps then thats no where near normal.

Enough rambling, can you have a look, have a comment, have no one around to ask or I would.
There is a box of virtual chocolate bars in it hippy, but sharing is manners my mum said.

Thank you.

 

[Jaguarjoe]

It will not work with D2 and D8 in the circuit. When Q1/Q4 conduct all of the current will flow through D8 instead of through your motor. When Q2/Q3 conduct all of the current will pass through D2.
In order to turn on an N-channel mosfet the gate must be some voltage higher than the source. I don't see that happening with Q1 and Q3.

 

[Andrew Cowan]

As Jj points out, a N channel FET requires a positive voltage to turn on. P channel FETs require a negative voltage to turn on, so these are normally used to switch the positive line.

Note that for non logic level FETs, the voltage needed to fully turn them on is 10v above the source voltage (N channel), or 10v below (P channel).

A

 

[Dippy]

Errors as pointed out above. Why try and make something simple SO complicated?

"that is not always the case"
- true, N Chans have several advantages as high-sided drivers.
The problem is that you have to know what you are doing.

"the voltage to the MOSFETS ( not the gate charge) is Pulse Modulated to contol motor speed, so the H bridge MOSFETs don't need to pulse, they just need to act like an L293D on steriods."
- eh? (Jaw approaching ground).

The basic problem is the usual confusion with gate voltages.
Vgs...
The Gate Voltage with-respect-to the Source.
Now you know that check the relative voltages in your circuit.

Apologies if I get this wrong, but I would suggest you have a word with your technical teacher who can give you a one-to-one on this subject.
It's so difficult to describe all this stuff in writing without 20 paragraphs of text which won't get read .

 

[fernando_g]

Why are you adding D1, D5, D6, D7?
They are redundant, and cause a 5% voltage drop.

Also depending on the speed you are switching your PWM, dead-time control to prevent shoot thru between the upper and lower Mosfets is a must.

 

[BeanieBots]

It's just SO wrong on SO many accounts.
Many good tips above, mine is do a google on H-Bridges and FET driving.
(or have a good browse of this forum)
It's such a common circuit and there are so many designs out there. Why try your own?

"the voltage to the MOSFETS ( not the gate charge) is Pulse Modulated to contol motor speed, so the H bridge MOSFETs don't need to pulse, they just need to act like an L293D on steriods."

Exactly, by PULSING.

 

[pyrogaz]

I want to run 24 volts up to perhaps max 5 amps ( I know stalled or starting it will be higher for a little bit), the motors are 250 watt geared motors and if they go above 5 amps then thats no where near normal.

The nominally 24v 250w motors we use in the Greenpower races typically draw 100A at start up and it doesn't take much of a load to hit 40-50A under "normal" conditions.

 

[BeanieBots]

Good point pyrogaz.
I couldn't find any data for "P60N06" but they need to handle at least 200A or they'll just vapourise at startup unless there is some very clever current limit / slow startup circuitry / software ramp (with feedback) being implemented.

 

[Andrew Cowan]

40A continous max (25 degrees)
160A absolute max
Rds 0.011 ohms

A

 

[BeanieBots]

Border line then.
With ~30W ea dissipation @ 50A they'll need some meaty heatsinks too.

 

[Andrew Cowan]

Imax 25 dgrs: 40A
Imax 100 dgrs: 28A
Vds max: 24v
Vds max (spike): 30v

DnT - you're running very close to the specs! With 50A, you'll need to keep it under 25 degrees in order to keep it happy - hard to get rid of 30W at the same time!

A

 

[Dippy]

Where did you get your Vds figures from Andrew?
I've seen a couple fo D/s and can't find that - probably different makes.

But I agree, the MOSFET doen's look man enough, esepcially when device resistance increases with Tj.

So, summary. Unsuitable bridge design and unsutiable devices.

D n T , if you are determined to make one I would suggest you look at other peoples' designs for Power H bridge and/or look around at more suitable MOSFETs and how to drive them properly. I'm really not sure if you have grasped the principles. Basically, mate, it's back to the drawing board and the semiconductor text-books for you

 

[Andrew Cowan]

Dippy - here's the datasheet I was looking at. Made by ST.

DnT - looking at your circuit again, were you planning to run the L239D at 24v? If so, watch the FETs maximum gate voltage - it's 18v max.

A

 

[dougyy]

For those of us still trying to learn about mosfets..... Motors draw varying amperages so which do you use to pick your mosfet? Quick example, normal load 6.5A, load at clutch slip 14.5A. But the kicker for many of us, how does one come up with inrush amps?

 

[moxhamj]

If this is a Greenpower race, then (just guessing here), would that not imply that most of the time you are going forward, and don't really need reverse? (even it it is a robot, the following still would be useful)

So - instead of an H bridge, with all the complications of high side switching and all the scary fault conditions as you ramp up the switching speed and don't turn the gates on and off fast enough, how about a single low side mosfet and a DPDT relay to reverse direction?

If I wanted to build this circuit quickly, I'd drop in a mosfet I know (and there are lots of good choices out there, especially the ones in the picaxe manual), but I'd use a BUK555 because I've got a pile of them, direct 5V drive on the gate from a picaxe, up to 60 amps switching power, DPDT reversing relay driven by a BC547 off another picaxe pin, and I'd have something working in a few hours. I'd also run the PWM as slow as I can get away with to avoid gate capacitance issues.

Then again, there are commercial off the shelf units that do all this nowdays.

Re Inrush Amps, well if you really want to keep it simple, you could drop in a series power resistor with a relay, then short that resistor out after a few seconds when you have left the starting line. Or - perhaps more elegantly, you keep the PWM pulses short at startup. Ok, you won't burn rubber at the starting grid, but that probably isn't very productive anyway. And so a picaxe could provide software for a 'soft start' regardless of the lead foot of the impatient driver!

 

[pyrogaz]

D n T doesn't mention the usage in his original thread, it was me that mentioned Greenpower because our motors are also rated at 250W and I was simply pointing out that momentary current draw can be many times higher than your anticipated steady state current.

 

[moxhamj]

Yes, true, we don't know the purpose. Greenpower? Buggy? Robot Wars?

It does affect things in that if there is rapid change in movement, those peak currents are going to be high. Just thinking aloud, if a peak current is n Amps from stationary to moving forward, will it draw even more than n Amps if you are moving backwards and then suddenly switch to moving forwards? I think the answer might be yes, and if so, it affects the calculations for the mosfet in that you would want to over rate things more.

 

[D n T]

Oh relay

I think I will go back to relays for direction control.
I still have work out a method to PWM my supply.
Its not fo green power or the like, I buy my controllers for that( which gets up my nose because they handle 24 volts 250+ watts and only use two P60N06 type MOSFETs). They get a bit warm though at 40 + amps for a few seconds at a time.
I would like to get back to my technical teacher for some help, except for the fact that I AM the teacher, thankfully the course I teach is only basic and this is not for the course its for me.
I want to find out who teaches the teachers, hell the electronics units I did at uni were based on project handbooks that were out of date in 180, and/or gates were new... enough said.
I would rather not hear " those who can do, those who can't teach".
I think I need to go down to the local tech and enrol in a night school course
Text books? suggest some, I am in the process of getting the art of electronics.
To all those involved, thanks

 

[Andrew Cowan]

Dr A - you're right there. The current spike is due to the voltage drop across the motor coils. As the rpm picks up, an emf is generated by the coil revolving in the magnetic coil. The higher the rpm, the higher the voltage generated, so the less current is needed. When moving backwards, a negative emf would be generated, so even more current is needed.

DnT - 35% of my electronics knowledge comes from this forum (20% of that from dippy, 20% from hippy). 55% from experimenting and reading on the net, and 10% from being taught (GCSE electronic products & AS and A level systems and control).

A

 

[D n T]

Hippy and Dippy, experimenting and 10% being taght

Andrew, I wish I had systems and control when i went to school, what text books did you use? Advanced Design and technology third edition?
Hippy and Dippy certainly assist many a lot.
Experimenting aka the kid method, play with it until it breaks, then get another on and try something else, been there done that, reminds me of an african radio company post that someone put on here. They would get a radio that worked and take awy parts until it didn;t work then they would put that part back and take away others until it didn't work. Then, using less parts they could build acheaper radio?
Anyway, time for a beverage
thanks again all

 

[Andrew Cowan]

I don't tend to do much texbook learning, but I have copies of Advance Design and Technology (second edition) and 'Creative Design and Technology' (by M. Jordan).

A

 

[BeanieBots]

Fully agree with Doc's comments in post #15 but be a little careful with the inrush current bit.
Reducing the PWM duty will reduce the average current but the peeks seen by the FET remain the same. The FET needs to be rated for these peeks. That does NOT mean the peek value in the datasheet for the FET. The FET CONTINUOUS rating must be able to cope with those peeks. The datasheet value for peek is the point at which it gives up. That value can easily be reach (for fractions of a uS) while it charges up cable capacitance and motor supression caps etc.

@DnT.

They would get a radio that worked and take awy parts until it didn;t work then they would put that part back and take away others until it didn't work. Then, using less parts they could build acheaper radio?

That's what the Chinese do and look at the unreliable cheap rubbish they put out
Those 'extra' components are put in there for a reason

Take a switchmode power supply for example.
PWM source, big FET, inductor, diode and a couple of caps with a bit of feedback maybe from an op-amp. Job done with about a dozen parts. Run it up in pSpice or something similar and you can demonstrate it working. You could probably even get it to work if you built one.
Now pop the case off your PC's power supply and do a component count.
Trust me, they're all there for a reason.

 

[fernando_g]

reminds me of an african radio company post that someone put on here. They would get a radio that worked and take awy parts until it didn;t work then they would put that part back and take away others until it didn't work. Then, using less parts they could build acheaper radio?

No this was actually done in the US in the 1950's by Earl William "Madman" Muntz, and the technique was called "Muntzing".

If you want to find out more, check the following Wikipedia link, and scroll down to the paragraph "Muntz TV". It is actually quite fascinating reading...

http://en.wikipedia.org/wiki/Madman_Muntz

But back to your original problem, Google is your best friend (unless you live in China). Google H-bridge drivers. Many semiconductor companies build chips to properly drive high powered Mosfets with lots and lots of protection schemes, from shoot-thru prevention, overcurrent protection, and anti-saturation detection. You may also want to add a fan to your heatsink (Picaxe controlled, of course).