PWM output for mosfet

[simayonis]

Hi guys, basicaly my PWM is giving an output of 3V and 0V using a 555timer... my gate of PMOS needs 16V to switch off and at < 10V to switch on... i need a solution to boost the OUTPut voltage of the PWM so i can get +19V and +9V..

ive tried op amp.. but the output of my PWM changes from square to triangle..thanks in advance guys

 

[dknguyen]

Why wouldn't you just run the 555 timer at 12-15V? 555 timers are able to work at much higher voltages than many other ICs.

But as for the triangle wave, it's because op-amps have limited slew rates. Your op amp has too low a slew rate. BUt why wouldn't you just do what I said above?

 

[MikeMl]

A LM555 or NE555 will not operate properly on 3V. Why dont you increase the supply voltage to the 555 circuit to 12V to 15V, and then tie the Positive terminal of that supply to the Source of the PFET. The output of the 555 will be "negative" logic, i.e., the PFET is on when the output pin of the 555 is "low".

Another question: Why are you using an PFET switching the high-side of your load rather than using an NFET and switching the low-side?

 

[simayonis]

im using ts555cn... welll i know wad u r trying to say... but when i build it on the verobard.. and increased the voltage of the supply Vcc the output voltage didnt increase much... 16V supply gives around 6V... btw i dont under wads high side and low side.... could you explain hehe?

And also the reason y i changed to PMOS was that the voltage i need on the gate using NMOS would be very high... and i need to connect the source of NMOS to ground using that topology... we'lll im attaching my whole circuit...hope to get some reply soon

works with simulation.. but on veroboard... not quite

Actually I have another current mirror the control the duty cycle but its not included here...

 

[MikeMl]

What frequency/duty cycle is the 555 running at?

How much current can the CMOS version of the 555 source and sink?

I'm guessing that the CMOS 555 can not charge/discharge the gate and Miller capacitance of the PFET at the rate you are running it. The NPN version (LM555) can source/sink 200mA. Yours, maybe 8mA.

 

[simayonis]

well my duty cycle would be controlled later on using a PIC controller.. the duty cycle of the 555timer should vary from 40~70%.... so i can adjust the output of my buck.. er... im not sure about the current.. well the output shows +3V and +0V which is insufficienct to on/off my mosfet... my PMOS theoretically and from simulations needs at least 16V to turn it off..

ive tried from simulation using an OPAMP to try boosting it.. the square wave becomes something like a square but not entirely a square.... any ideas how can i improve the outpuit of op amp? so i can get the desired as input? think i was using LT353 op amp.... and i dont really understand the data sheet.. "not rail to rail" meaning what?

thanks guys

 

[indulis]

Why is D6 there? GND pins, except in a few cases, should be tied to GND.

 

[simayonis]

well, its because i use it to set my voltage so teh ripple is at +19V and +10V... the zener is +10V...... instead of +19V and 0V

 

[audioguru]

Like I said on the other website, the Cmos 555 does not have enough output current to quickly charge and discharge the high gate capacitance of the Mosfet. use an ordinary 555 instead.
The lousy old LM324 opamp that you used has terrible performance above only 2kHz.
Your 50kHz frequency is far too high for your parts.

The zener diode raises the "0V" pin of the 555 to +10V so that the gate of the P-channel Mosfet can be driven from +10V when it is turned on to +16V when it is turned off.

 

[simayonis]

wow audioguru ur everywhere ! lol awesome...... anyways ordinary 555? can u give me some recommendation? and for the opamp as well..

 

[audioguru]

I chat about electronic circuits on many websites. I am a retired old geezer so I have all the time in the world. Every day is a Saturday or a holiday.

Nearly every semiconductor manufacturer makes an "ordinary" 555 that has a design that is about 40 years old. Its max output current is 200mA. If you use one then an opamp amplifier is not needed.
A Cmos 555 is newer and has an output current of only 10mA high when its power supply voltage is high. 10ma is not enough current to quickly charge or discharge the high gate capacitance of a Mosfet so high frequencies are a low level triangle waveform that do not work.

 

[simayonis]

i really envy u... u know so much....well people like me should be grateful that there are people like u around u that knows that much.... well unfortunately i dont have time to order another 555timer... thats all my uni provides me with... but the good thing is i found that they have got a LF355 op amp i read the datasheet it says it has got a high slew rate and i tried simulating it... the graph is acceptable ... okok i think i should post @ the other forum... think u shud reply me there... haha

 

[lodivar]

need the replacement of 40to3go/power mosfet

need the replacment of 40to3gp / power mosfet

 

[indulis]

...so teh ripple is at +19V and +10V...

What ripple?

The internal resistor divider of the 555 is referenced to the GND pin (at 10V) but your timing components are GND referenced.

Given that configuration, I'm surprised you got...

...basicaly my PWM is giving an output of 3V and 0V using a 555timer...