Need help for PWM circuit!!

[tkhao1986]

Hi guys,

Will like to seek your help. I have a 5V pwm controller/generator. This signal is output by a MOSFET. I will like to use this generator to execute PWM (15kHz) on a battery which has a high voltage of 40V. I believe i will need to connect this controller to probably another IGBT, which will be the main pwm switch for the battery. Can someone advise me on the extra resistor or any components i may require to make this whole circuit work? I am very very desperate!!!

 

[Marvo]

It wold help if you could give more details. A schematic would be nice including specs of the pwm unit .

 

[grantr37]

a pulse width modulator (pwm) frequency modulating unit generating a square wave on a MOSFET Driver ouput (Metal Oxide Semiconductor Field Effect Transistor) this is a standard 5 volt integrated circuit logic output and provides a 5 Volt peak digital signal.
I take it that you will be powering this modulator from a 5 volt supply, could you tell me the purpose that you need the PWM generator for when you want to modulate 40 Volts DC into a 15Khz variable pulse train? so that I know what you want to do with this high DC voltage, there are various factors that I need to look at including the equipment that this will be connected to, also any switching transistor will need to be a high power transistor and sufficiently heat-sunk, and there are different operating frequency thresholds for different transistors, also with increasing frequency there is increasing power transfer....if this (square)waveform is to drive further cicuits containing logic,or more transistors, there may need to be some additional capacitors and pull up resistors...


let me know what you are using this for and I can work something out for you from there...is it to control an electric motor's speed?

 

[grantr37]

A photograph of what you are using this for/with will be sufficient, dont need the schematics as you probably won't have them anyway...

 

[tkhao1986]

Hi guys, didnt expect the numerous replies i received. Thanks once again for the help. Attached is a picture which i hope can help. Do tell me if u require anything more

 

[grantr37]

I designed a circuit for you during the night on paper, I will put it on here as soon as I can using one of the cad packages....

 

[tkhao1986]

I designed a circuit for you during the night on paper, I will put it on here as soon as I can using one of the cad packages....

Hi grant, wow really really appreciate your big help. To elaborate more on the picture, u can first take the load to be a resistance load.

Someone has told me about using a mosfet driver and another mosfet to trigger the pwm for the 40V battery. Basically, the 5v pwm output is being input into the mosfet driver which will then generate the gate and emitter output for the power mosfet.

I have tried google about mosfet drivers and it seems like it only helps to improve the mosfet efficiency by outputting more current to the gate. Correct me if i am wrong. Will appreciate if the terms gonna be used be layman as i am still quite a noob. Haha.

 

[tkhao1986]

Hi anyone can help please?

 

[grantr37]

am already on it, I am going to have to scan/photograph the drawings as I could not find my CAD software disks, it will be on here within 24 hours

 

[tkhao1986]

No problem grant. Thanks for the help

 

[aptsys]

That circuit can't be correct. The LED on the 4N25 has no current limiting, so you'll immediately destroy it. The Opto-isolator/MOSFET combo appears to provide no useful function as the MOSFET will not conduct any current as there is no common ground between the two sides of the circuit, so you won't build up a field at the gate. Also any use of the MOSFET for high current switching purposes is negated by the phototransistor rated at 150mA in the opto-isolator.

The zener isn't really serving any useful function. The pot needs the other end connecting to Vdd. Switch debouncing can be done with a simple RC filter or in software.

You really need to boostrap the gate on the MOSFET on the 40V side to get the gate voltage higher than the supply rail as you're trying to switch the high side with a N-type MOSFET.

 

[rryles]

Are you taking the mickey, grantr37? That circuit's bonkers!

 

[grantr37]

I expect to see you help this person out

 

[aptsys]

There should be no common ground between the two circuits, as a common ground would result in damage

You either have to have a common ground between the two circuits, OR you must fully isolate the two circuits. If you need isolation, the switching voltage for the gate must be derived from the 40V side of the circuit, however in this instance there is absolutely no need for isolation.

you would be effectively connecting a 15KHz frequency at circa 40 volts right back to the PIC MICRO Integrated circuit chip and it would also feed-back into the first Transistor , and with the small delay in the feedback would cause interference with the characteristics of that first transistor, and it would propagate through the capacitor overheating and quickly destroying it as well, the PIC would follow shortly afterwards by a few seconds.

I won't even comment!

The LED on the 4N25 Opto isolator has an operating current of 60mA maximum and an inrush current of 3Amperes which would never be reached by the circuit in question, I/F at 5 volts LED supply for this IC is 5mA.

The LED on a 4N25 opto-isolator has a MAXIMUM operating current of 60mA, but the forward voltage is 1.5V MAXIMUM.

These are two separate circuits with the 5 volts Rail and the input and output of the Opto Isolator effectively being the 3 connection terminals to the other circuit so that the second circuit comprises the DC Supply/Battery, the Transistor , the Resistive Load and the Rectifier Diode (Battery Polarity Reversal prevention) in series forming a loop.

My other comments still stand. It cannot work.

I expect to see you help this person out

I'll draw up a schem tommorrow...

 

[tkhao1986]

Guys, sorry for the commotion caused. I guess we are all here to learn from one another, so please dont quarrel over any mistakes. I am already very appreciative that there's people out in this world willing to look and examine my problem.

Grant, did u remove your schematic diagram? I cant find it. It will be good for me to have a look so i know what u guys are commenting on.

Aptsys, thanks for the help.