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enter image description hereI am building a smps that brings 120 vac to 5v dc and have it all designed so far but am having trouble on what to do for the pwm on the buck converter portion. I was wondering if a pwm controller is something that I could use? How do they work exactly? Would need like 18V at 400khz at ~42% duty cycle to drive the gate of my nmos according to my simulations on multisim.

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  • \$\begingroup\$ You're just taking any controller which fits :) General usage PWM controller usually is more hard to understand and calculate (e.g. TL494), so many of the manufacturers (Power Integrations, TI, LT etc) does have specialized controllers for different topologies (flyback, push-pull etc) which is much more easier for understanding how it's works. Any controller works by one simple principle - it measures low side voltage, calculates the error and corrects it by recalculating duty cycle (trying to achieve zero error). \$\endgroup\$
    – Looongcat
    Commented Sep 11, 2015 at 18:36
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    \$\begingroup\$ If you haven't considered which PWM controller you want to use, I take issue with your assertion of having "it all designed". The power train is only one part of the complete solution. Also, you haven't fully described your intended application. For instance, if the 5V needs to be isolated from the 120VAC input (i.e. if it can ever be in contact with a person) you will need more than a buck converter. \$\endgroup\$ Commented Sep 11, 2015 at 19:11
  • \$\begingroup\$ I uploaded what I have designed for my smps so far. The buck converter seems to work with a pwm of 18v, 400khz, ~40% duty cycle driving the gate. When I go to build the smps in real life I don't know what to use for pwm to produce 18V like in my simulation. Will a pwm controller be something that will help do the pwm for my nmos? \$\endgroup\$
    – pc9460
    Commented Sep 12, 2015 at 1:14
  • \$\begingroup\$ Have it all designed so far? No, this is perhaps the first 10% of effort required to make it work. \$\endgroup\$
    – venny
    Commented Sep 12, 2015 at 12:33

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No a PWM controller is not a good fit for this application because it has not regulation capability, it has no feed back. In a true buck regulator, the regulator senses the output and adjust the PWM accordingly to make sure that the output is the correct voltage. This kind of regulation compensates the system across many different loads. If the PWM is fixed then it is only valid for a single load point.

A prebuilt buck regulator is the easiest solution for you. Add and inductor and some passives and your up and running. If you really want to roll your own there is a topology that uses two opamps and a triangle wave generator.

schematic

simulate this circuit – Schematic created using CircuitLab

These are not the correct values. I will leave it up to you to figure them out if you choose to use this setup.

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  • \$\begingroup\$ How come PWM controller to have no feedback? It controls the output variable based on its actual value. MC34063 has feedback, SG3525 has feedback, UC3845 has two. \$\endgroup\$
    – venny
    Commented Sep 12, 2015 at 12:29
  • \$\begingroup\$ What kind of op amps are you using? TLxxx I can't really tell. \$\endgroup\$
    – pc9460
    Commented Sep 12, 2015 at 15:23
  • \$\begingroup\$ @venny The three part numbers you referenced are prebuilt buck regulators. Going from the OPs diagram the PWM he is using has no feed back and may as well be a 555 timer. Same as you, I suggested using a prebuilt solution. \$\endgroup\$
    – vini_i
    Commented Sep 12, 2015 at 16:19
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    \$\begingroup\$ @pc9460 all of the components in the circuit are generic. The circuit is only for the reference of the general topology. As everyone has suggested so far, don't reinvent the wheel and use a prebuilt buck regulator. \$\endgroup\$
    – vini_i
    Commented Sep 12, 2015 at 16:22
  • \$\begingroup\$ I can't buy a prebuilt buck regulator, for my electronics project I have to build a custom one. \$\endgroup\$
    – pc9460
    Commented Sep 12, 2015 at 17:04

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