Hi,
I have made an N-Channel MOSFET(IRFZ44) chopper to control a 12V permanent magnet motor that is taking about 8A when loaded . The microcontroller(89s52) generates the PWM signal. The PWM voltage has been applied across the gate source terminals of the MOSFET. A maximum volatge of 6V is being applied to the gate-source terminal. What I want to know is that, whether should I connect the microcontroller output to the gate of the MOSFET using a resistance? Or is it safe to connect directly? Or will I need a MOSFET driver? I could not find anything in the datasheet of IRFZ44 about maximum gate current. If I need to connect the resistance...what should be its value? I have attached a schematic of my circuit. Please help!!!

 

The MOSFET is a voltage controlled device so no gate current to speak of so no series gate resistor like a bipolar transistor would need in its base.

You may need a driver, though, as the output voltage of the 8051 is flirting with the turn-on voltage Vgs(th). spec'd as needing as much as 4V to start turning on. Also see Fig 3 of the datasheet. I am puzzled as to how you are getting 6V out of a 5V 89S52.

You should have a large resistor between gate and source to hold the device off when the processor is in reset.

 

Ok...I meant 5V...sorry... I get the part about putting a large resistor between gate and source. Can this circuit be implemented on a perf-board? What I find strange is that the mosfet terminals are very close to each other. So wouldn't the terminals draw sparks due to the high current when I solder it to the board?(Or maybe the voltage is too low for that to happen?) Thanks a lot for helping out.
Also, IRFZ44 is rated for 49A i guess. So would I need a heatsink for the mosfet?(Since its only taking 8A)

 

Also please do tell if MIC5021 driver would do the job or not?(the DC source is a 11.1V LiPo battery)

 

Oh yeah...and also should the rating of the diode that i am using in parallel be 8A? Sorry for bothering you with so many queries...

 

Also please do tell if MIC5021 driver would do the job or not?(the DC source is a 11.1V LiPo battery)

It should.

The low voltages you are using would not cause an arc. Current is not an issue for starting an arc. Current would affect how spectacular a short between the pins could be however.

Also, IRFZ44 is rated for 49A i guess. So would I need a heatsink for the mosfet?(Since its only taking 8A)

Whether you need a heatsink depends entirely on the power being dissipated by the device when its on. Power at the junction in WATTS is given by I^2/Ron. You need to 1) compute the power then 2) apply the THERMAL RESISTANCE ratings given in the datasheet. Junction to Ambient is 62 degC / WATT. So for 1 watt power with no heatsink, the junction will be 62degC + ambient temp. 2 watts 2*62degC = 124 degC above ambient and so on. So it gets hot fast without a heatsink. Heatsinks are also rated at degC/watt. Thermal resistance adds so the junction temp with a heatsink is now (1.0 + 0.5 + heatsink rating (in degC/watt)) * watts dissapated (from the datasheet again). That will tell you how big the heatsink needs to be. The datasheet has charts covering junction temp vs. various operating points.

Oh yeah...and also should the rating of the diode that i am using in parallel be 8A?

Not necessarily since the diode only conducts when the current is turned off and then only temporarily as the stored magnetic field collapses. You would need to know what the peak current is and for how long and then see what the diode's specs say. Offhand, I would say 8A is OK but you should make sure.

 

Did you mean I^2 * R(on)? then the power dissipated is comming to around 1.6 watts...and do you mean that I can use a diode of a smaller current rating maybe? or should it match the peak current?

 

And also do tell what the reverse blocking voltage rating of the diode should be(I've heard that voltage spikes can cause 10 times the supply voltage to be applied across the motor. Is that true?).

 

Did you mean I^2 * R(on)? then the power dissipated is comming to around 1.6 watts...and do you mean that I can use a diode of a smaller current rating maybe? or should it match the peak current?

So at 1.6W without a heatsink you can expect a junction temp of 25degC (ambient) + 1.6W*62degC/W = 124.2degC Tj. This is below Tj(max) so the device should be safe. The tab will be very hot though and touching it will cause burns. For that reason I would heat sink it anyway or at least bolt it to a metal chassis (with proper insulation as required).

The MOSFET has protection built in that should be enough. The specs and Fig. 12a describe what the limits are and how to test to see if you are within them.

(I've heard that voltage spikes can cause 10 times the supply voltage to be applied across the motor. Is that true?)

Absolutely true and frequently more. How much depends on the particular coil but its not unusual at all to see several hundred volts from a 12V relay. Keep in mind though that a clamp diode across a coil only has to block the normal voltage of the coil - its back-biased in normal operation. When the power is removed from the coil, the magnetic field around the windings collapses generating a voltage/current spike of reverse polarity. That will cause the diode to conduct and dissipate the generated power locally in the coil (as heat). So the diode has to have a voltage rating at least that of the coil and a surge current rating sufficient to handle the current generated by the collapsing field. The current can be quite large but for a very short time. A typical rectifier diode i.e. 1N4001 is sufficient for many applications. This Wikipedia entry covers the topic pretty well.

BTW: if you are using a cheapo meter, be aware that many do not have very high input protection ratings - I've seen as low as 500V. When you measure voltage on an unclamped coil when it turns off, the resulting voltage spike can and has zapped them.

 

Well yeah...this is exactly where I've read about the fact... So I guess as the inductor dissipates...the energy freewheels through the diode. So the current should also decrease when the supply is off. So shouldn't the peak current always be less than 8A?(Keeping in mind that my starting current is low, as I'm gradually increasing the voltage from zero). And does that mean the forward voltage rating of my diode should equal the voltage spike across the inductor?

And as for the ambient temperature...I live in India...So its going to be way hotter than just 25°C ...so I guess as you've said...I should better use a heat-sink. BTW...I also have another mosfet(P140NF55) that is much more expensive. But due to its low Rds(on)...wouldn't it be better suited for this job?
And if you were in my place...what would you prefer to use? A vero board, a perf-board? Or make a PCB? Because I'm finding it difficult to make long connections on a perfboard without the use of a wire. I just want to make the circuit without the use of wires(for the components mounted on the board).(I don't have the tools required to make a PCB )
(I'm actually trying to drive four motors...so four choppers...and the circuit should be a bit bulky I guess)

 

Well yeah...this is exactly where I've read about the fact... So I guess as the inductor dissipates...the energy freewheels through the diode. So the current should also decrease when the supply is off. So shouldn't the peak current always be less than 8A?(Keeping in mind that my starting current is low, as I'm gradually increasing the voltage from zero). And does that mean the forward voltage rating of my diode should equal the voltage spike across the inductor?

And as for the ambient temperature...I live in India...So its going to be way hotter than just 25°C ...so I guess as you've said...I should better use a heat-sink. BTW...I also have another mosfet(P140NF55) that is much more expensive. But due to its low Rds(on)...wouldn't it be better suited for this job?
And if you were in my place...what would you prefer to use? A vero board, a perf-board? Or make a PCB? Because I'm finding it difficult to make long connections on a perfboard without the use of a wire. I just want to make the circuit without the use of wires(for the components mounted on the board).(I don't have the tools required to make a PCB )
(I'm actually trying to drive four motors...so four choppers...and the circuit should be a bit bulky I guess)

Diode: Its the reverse voltage spec and forward current spec you are interested in. Reverse voltage (PIV) because your diode is normally reversed biased i.e. when the motor is running. Anything rated above the motor voltage is OK so a 12V diode would work. Since diodes are cheap and its always a good idea to rate higher than you need, a diode rated at 50V PIV or more would be fine.

Keep in mind that unlike a relay coil, the fact that the motor is spinning in the same direction when the power is off (between PWM pulses) means that the resultant voltage on the terminals is in the same direction and the diode will still be back biased under normal conditions. Pulsing the motor with a stalled rotor (as in startup) will generate the negative spikes but I doubt that it would exceed the 50A surge rating of a 1N400x. If in doubt, use that 8A diode and it should be fine. The best way of course is to actually calculate then verify by measurement what the transient conditions are and handle it from there. One way to decide is to think about how many you are going to build. Just one on perfboard? Use the bigger stuff. Its not worth lots of testing for a one-off project. Going into production? The difference in cost over many units makes it worth specifying parts closer to save cost. This would involve more testing time to make sure that what you have is robust enough.

Your MOSFET should be fine with a heatsink. Something with a rating of 20degC/WATT or better (like 10degC/WATT) would be prudent. If you can bolt it to a strip of aluminum or the chassis/box/etc. it would be better than nothing.

As far as wiring.. that's up to you. Perfboard or veroboard would do it. If you have some longer runs consider stripboard. I personally don't like any of it any more and rather than stare through magnifying goggles at breadboards (my eyes are getting bad) I just lay out a PCB on Eagle and send it out for any but the smallest circuits. A few guys on this board have used some cheap Chinese PCB suppliers with good results. Search on printed circuit sources here and you should get some good info. Places like ExpressPCB and some Chinese houses have free layout software that you can use to generate boards that you buy from them. You also can get free evaluation versions of Eagle, DipTrace and likely others that will generate PCB gerber files that you can use to order boards from anywhere. The free versions are usually limited in the number of holes or board size but you might find them useful. One big reason that I went to doing even prototypes on PCBs is that its way easier to track changes and you can smoke one and have spares at hand.

Sounds like its time to wire one up and give it a try, yes?

 

Yup... I'll give it a try...all is good if it works...otherwise I will post the new circuit here again and bother you with my queries...

BTW: This is really an experimental endeavor as a student of Electrical and Electronics Engineering. As you can see, I'm really a novice. And also, I haven't worked with PCBs and the softwares that you have mentioned. But since you prefer them, I would like to give it a try. But can PCBs handle currents of 32A?(8A * 4 motors)
Lets hope for the best. Thanks a lot for helping out...
​

 

Yup... I'll give it a try...all is good if it works...otherwise I will post the new circuit here again and bother you with my queries...

BTW: This is really an experimental endeavor as a student of Electrical and Electronics Engineering. As you can see, I'm really a novice. And also, I haven't worked with PCBs and the softwares that you have mentioned. But since you prefer them, I would like to give it a try. But can PCBs handle currents of 32A?(8A * 4 motors)
Lets hope for the best. Thanks a lot for helping out...
​

Sounds like a plan. Sure. PCBs can handle the current if the traces are wide enough/thick enough. Route the 8A circuits individually. Google 'PCB trace width calculator' for help. But choose a construction method that suits you, your skill level and budget rather than what some old codger in Texas says...

Build one channel first and make it work like you want. Then duplicate it.

Good luck!

 

Well...my skill level and knowledge is so low...that I couldn't even decide as to where I should begin . But I'll give this a try with the knowledge you have given me. Nonetheless, this subject fascinates me to a great degree. But, unfortunately I have my semester exams going... So I will report my progress after two weeks...

 

I use a TC4420 for driving mosfet gates from a microcontroller.
The diode could be a 1n4004.

 

I am not getting a MIC5021 anywhere in Kolkata... Neither will I get a TC4420...Some people in Kolkata have suggested that I should look for Fairchild Semiconductors, International rectifiers etc. Will IR2184 do the job? If not, please suggest some drivers manufactured by International rectifiers...
(I'm down with high fever and cough btw )

 

So I finally went to the market and got myself a IR2111 mosfet driver. But I'm really confused about the driver connections after going through the datasheet. All I want is to drive the motor in a single direction. Is this correct? (referring to the attachment) Should I keep the LO pin open or ground it? I'm really confused about the Vb and Vs pins. Please help!!!

 

So I finally went to the market and got myself a IR2111 mosfet driver. But I'm really confused about the driver connections after going through the datasheet. All I want is to drive the motor in a single direction. Is this correct? (referring to the attachment) Should I keep the LO pin open or ground it? I'm really confused about the Vb and Vs pins. Please help!!!

Glad you are feeling better...
The IR2111 is a half-bridge driver which means that it has low side and a high side driver.

The low side driver is similar to your original schematic i.e. the motor is in the drain circuit and the MOSFET turns on to connect the switched motor line to ground. The voltages that the MOSFET sees are referred to ground and are small.

The high side driver switches the power side i.e. the drain is connected directly to the power source. This means that when the motor is running, the source pin of the MOSFET is at a high voltage (compare with source pin voltage of the low side driver - its essentially 0V.) The high side driver incorporates floating, level shifting circuitry to do this to provide gate voltage for the high side MOSFET. VB and VS are voltage inputs that provide not-at-ground reference points so that the high side gate control circuitry can apply the proper gate drive, referred to the MOSFET source pin, as it has to be.

VB - is the high side high voltage source, 12V in your case. Connect to +12V and the MOSFET drain.
VS- is the high side low voltage reference, the top of the motor in your case and the MOSFET source.
GND - connect the other end of the motor to ground.
HO- is connects to the MOSFET gate.
Vcc/GND is the logic power (and low side MOSFET source (which you are not using).
LO- is not connected since you don't have a low side MOSFET to drive.

Unfortunately for you, the IR2111 is not really 8051 compatible. It wants Vcc at least 10V with logic '1' > 6.4V, preferably 15Vcc, 9.5V logic '1'. You might be able to use it by using Vcc =12V and a level shifter on the logic input.

I'm not sure what to recommend since you have limited sources but whatever you look at, read the datasheet first, at least the Electrical Characteristics for logic and supply levels to see if you are in the right place. You don't need a half-bridge, just a single ended, low side driver. Google will return lots of hits including these two which have good discussions of driving MOSFETS like you want including some discrete circuit examples.

Fairchild
Vishay


EDIT: Looking at the IR stuff, how about an IR2121? Its a low side driver, compatible with 12V gate drive but with TTL logic level control. AN1014 (and the IR site in general) has good application info.
Good luck.

 

Damn! I checked the maximum ratings when I was buying that! The datasheet said Vin= 0 to Vcc(600V), but it was actually referring to LOGIC '0' i guess. Ok...so I know now that I need to look at the Electrical characteristics. IR2121 looks perfect for the job...but I don't think I will get it here. Lets hope for the best. I will try to look for similar gate drivers if I don't get this one.
Both the links were very informative. I didn't understand everything, but I certainly know a bit more about MOSFET drivers now...

 

I certainly know a bit more about MOSFET drivers now...

Me too! Let us know how you are getting along..