PWM for buck converter

[DameDyslin]

The designing concept you are using is good. The pdf provided by the Microchip is very helpful to understand the designing concept of the Buck. I am not getting your Multisim simulation. Any error is there. Can you please update it?

[johnsorious]

The only problem I am having now is figuring how to build the smps. I have 18V from the pwm going into the gate of my mosfet but when I go to build it I don't know where I'm going to get the 18V. I was thinking about using a microcontroller but it only outputs 5v. I was thinking about using a gate driver but don't know how to select one. I would like to get it to simulate first to verify that it works before ordering a ton of parts to do trial and error.

[johnsold]

I have used the IRS2011 driver. It has a bootstrap circuit which can generate a boost voltage for the high side driver.

The model provided by IR is a behavioral model written for Pspice which uses many switches.  I could never get it to converge in my SPICE simulator. I spent a lot of time modifying the model to get something which would run and which modeled the data sheet behavior fairly well.  Even with simulation I still managed to smoke several of them in protoypes.  Second order effects related to ground currents can cause problems.

Lynn

[johnsorious]

Is there a way to replace the mosfet with a transistor instead and that would still be called a smps, correct? I would like to possibly still control it with a microntroller unless something else is easiest. Is there a downside to using a transistor versus a mosfet? Will it still output the consistent 5v dc that I need?

[johnsold]

The downside, if you want to call it that, to using a BJT in place of the MOSFET is that the drive is current rather than voltage.  For 2 A output current the base current will likely be in the range of ~100 mA.  Your microcontroller cannot handle that much current so an intermediate transistor will be required.  Also, BJTs in that current range tend to be slower than FETs.  If you need to slow the clock, then the filter components have to change also.

The consistency of the output voltage depends on the nature of the feedback (which is not in your present circuit). The feedback circuit needs to have sufficient range for whatever parameter it adjusts (such as duty cycle) to account for variations in input voltage, output current, and component variations.

Lynn

[johnsorious]

Is there an advantage of using a pmos versus an nmos or vice versa? Is there a specific way to select a gate driver? There's so many options to selecting a gate driver its a little overwhelming. Is there any ic's that do feedback so I don't have to build any external circuitry? Like if the output voltage is high the duty cycle will drop appropriately and if the voltage is low the duty cycle goes higher.

[johnsorious]

Would a pwm controller ic and a gate driver ic be what I would need to do the pwm for my smps? Would the pwm controller adjust so that my output is a constant 5v dc?

[johnsorious]

Would a pwm controller ic and a gate driver ic be what I would need to do the pwm for my smps? Would the pwm controller adjust so that my output is a constant 5v dc?