MOSFET switching a load on a buck module really fast a good idea? - Page 1
Forum section: electronics / beginners

author: skillz21
author_rank: Frequent Contributor
timestamp: July 28, 2019, 01:23:47 pm
content: I want to use an EPS8266 to control 2 MOSFETs. The first MOSFET will be a switch to control the input power to a buck converter module. For the second one, I will be using analogWrite() to PWM control a 10W LED (the load of the module). I also wanted to manually speed up the function so the PWM was at a higher frequency, removing the possibility of any faint flickering. But then I realised that doing this may create, or worsen, a problem.I want to know whether this setup would work. I think the first MOSFET should be fine, but I wasn't sure about the second one, the one that controls the brightness of the LED. Would switching the load on and off (through the use of analogWrite()) at such a fast speed affect the voltage of the buck converter? Would the voltage regulation of the module be fast enough to keep up with the switching of the MOSFET? If the buck converter is not fast enough to keep up with the switching of the microcontroller, that would make the voltage output of the buck converter unsteady, right? If this is the case, what sort of effect could it have on the LED?

author: Seekonk
author_rank: Super Contributor
timestamp: July 28, 2019, 03:19:29 pm
content: You can save yourself some trouble, just control the buck converter. Typical buck converter has a sense voltage of about 1.25V. PWM thru a resistor to a filter capacitor and then another resistor AND A DIODE to fake the converter into thinking the output voltage is higher than desired. As this is driving a LED, below a certain voltage the LED will just shut off. Only a minor adjustment to the output voltage is needed and resistances can be quite high. Pure DC and no flashing in video work. Since you have an arduino, you can monitor current and control for temperature

author: Kasper
author_rank: Frequent Contributor
timestamp: July 28, 2019, 05:35:35 pm
content: Try searching for LED drivers. Digikey has a whole category just for this.Look for one with 2 inputs:enable: so you don't need input switch. Bucks have a high side switch in series with input, no need for you to add an external one.dimming: so you can control the output current. Digikey has a filterable category for this so you can choose how you want to control the dimming.Nice thing about LED drivers is thier output is usually current controlled instead of normal bucks that are voltage controlled.

author: skillz21
author_rank: Frequent Contributor
timestamp: July 28, 2019, 10:30:40 pm
content: The thing is, I've already ordered the buck converter and all the parts. Adding PWM control for the LED brightness was an afterthought. The module I bought wasthis. I went with the cheapest one I could find with current limiting since 10W isn't a whole lot of power.Is there any possibility of doing my idea of PWMing the 10W LED on this board's output? Or what @Seekonk said with directly PWMing something on the board? If so, anyone got a link to a tutorial or a guide that I could have a look at to learn a bit more about the process?

author: KL27x
author_rank: Super Contributor
timestamp: July 29, 2019, 12:47:39 am
content: You can do it just like you want to.Yeah, you can probably put your buck into oscillation if you hit the right frequency. If so, change it.But be aware that the higher your switching frequency the lower your efficiency. While switching, the FET will drop voltage and dissipate some heat. The faster you try to go, the more time the FET will spend in this zone. You may have to choose FET carefully and use a gate switching circuit or IC to reach desired pwm frequency.

author: skillz21
author_rank: Frequent Contributor
timestamp: July 29, 2019, 01:35:09 am
content: Am I able to directly wire into the buck circuit and somehow control it between 0v (or the lowest possible) and the maximum voltage, which I would set with the onboard potentiometer?

author: Seekonk
author_rank: Super Contributor
timestamp: July 29, 2019, 02:39:51 pm
content: The 2596 shutdown pin #5 far right is quite fast. I use it to turn these modules into MPPT solar controllers. Just lift that lead from the trace and feed your PWM signal directly into that pin. Then just set the maximum voltage from the pot and let the micro adjust it down. High turns it off. Regardless, you should learn to do this trick.

author: skillz21
author_rank: Frequent Contributor
timestamp: July 29, 2019, 11:29:28 pm
content: I had a look at the datasheet yesterday, and I did notice the ON/OFF pin, although I didn't think of PWMing it. If I directly used a PWM signal on that pin, I could just ditch the first MOSFET altogether... right? Since when the pin is in the OFF state, it will basically switch the whole circuit off...?

author: skillz21
author_rank: Frequent Contributor
timestamp: July 30, 2019, 01:54:21 am
content: Also, just a thought, if I were to go with my initial solution of putting a MOSFET in beetween the buck converter and the LED, would I just be able to put a large capacitor on the output to avoid the converter output from swinging around too much?

author: Seekonk
author_rank: Super Contributor
timestamp: July 30, 2019, 12:50:08 pm
content: All these modules have just enough capacitance to keep them stable and extra capacitors should be added. That would be a good idea. If you go with the ON/OFF pin, you have to do that also the capacitor is needed to average the lower frequency of the PWM.