Design Step Down dc-dc converter

[macmmt]

Hello,

I’m designing a step-down dc-dc converter. The input voltage is 7V and the output voltage is 2V. The control is done through PWM with an IC 555. The switching frequency is 300kHz.

I have tried to make it work, but it doesn’t. I would like, you help me checking it to find any error. Please let me know what is happening with this circuit. I have attached the circuit design.

Thank you very much for your help.

 

[ronsimpson]

Normally there is a capacitor from pin6 to ground.

 

[macmmt]

Sorry i uploaded the wrong image. I do generate the pulses with 555 but when I join the output to the MOSFET, the converter doesnt work.

 

[ronsimpson]

The MOSFET is N so it takes a positive voltage to turn it on. The gate needs to be 5 to 10 volts above the source. The 555 needs a higher supply voltage. Or change it to a P MOSFET and set the source on the positive supply.

 

[k7elp60]

Most SMPS that I have seen have voltage sense resistors on the output. This sense voltage to fedback to the PWM control to control the PWM to keep the voltage regulated.

 

[ronsimpson]

You could use a part made to do what you want. Example LT3470. The output voltage is set by R1 & R2. Your poor little 555 does not regulate the voltage.

 

[macmmt]

What I have to do, is to implent the circuit that is attached.
As the control circuit I'm using the 555. I have set a higher voltaje but it doesnt have any effect.

 

[ronsimpson]

Your first post has a N_MOSFET while your last post uses a P_MOSFET.

 

[macmmt]

I have been changing the MOSFET since it doesn't work and trying diferent ways.
Which Mosfet will you work with? and How would you implent the control circuit?

 

[ronsimpson]

OK…. Here is a circuit with both a P and an N MOSFET. I chose it so you can see how to use each. The N is sitting on ground. If pulls a node to ground when it’s gate is positive. The P is on the positive supply and pulls up when it’s gate is pulled negative. Note your last circuit is wrong the P MOSFET is backwards.

 

[macmmt]

Thanks I understood that part. Now the question is: If I use the N-Mosfet and the 555 in my ciurcit, would the converter work?? or should I include another element?

 

[ronsimpson]

source of Q1 is at ground when off. (-0.7 volts)
source of Q1 is at 7 volts when on. (6.9)
gate of q1 should be at 12 volts when on.
L1 is tooooo small 2.91uH is like a wire try 291uH or 2.9mH.
R3 is too large 10k---->220 ohms
V1 9 volts ---->12 volts.

 

[macmmt]

Sometimes it is needed to use optocouplers to separate the power circuit from the control circuit. Do I need them in this design?

 

[ronsimpson]

no opto
Pink line shows the currentin the inductor.
Blue line is gate voltage 12/0
Light blue is source voltage. 7/-0.7
Output about 4 volts at 50%

V1 is 7 volts.
V2 is what the 555 could be doing. 12 volts when on, 0 volts when off, delay of 1nS, raise time=1ns, fall time =1ns, ontime=5uS, frequency=1/10uS = 100khz.

 

[indulis]

For 7V in and 2V out and a N channel MOSFET's you need a ~28.57% duty cycle. With the 555 as wired, you can't get less than 50% on, so it won't work. If you add a diode across "R2" you can now have a faster "discharge" than "charge" time. If you like, the "CV" pin can be used for feedback, but the switching frequency will move around with line and load.