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SG3524 buck converter


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  • 4 weeks later...

First of all, where're connection of your load or smoothing inductor?

Sound like u r reversing both output transistor.
both of them are NPN transistor, they need higher voltage  at Collector and lower voltage at Emitter to switching.

any mean, U must have some pull up transistor at Collector (pin 12&13) to forward bias them.


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This topic was around before but disappeared for some reason...

OK... one more time, the 3525 is an "old" PWM. There are better ones out there for buck converters. The last schematic that was posted was also a little different. For starters, pins 11 & 14 should be going to the driver and pins 12 & 13 want to go to some fixed voltage (Vref maybe) NOT the R2, MOSFET drain node. This is a switching node... voltage goes from +15V to gnd here. You have pins 6 & 7 shorted by R6.

Maybe a "general" explanation as to how these things work is in order...

This is a voltage mode controller, it has an oscillator internally which generates a ramp waveform signal whose frequency is set by Rt and Ct . An "error" voltage is fed into pin 2 (here you have the error amp pin 9 connected to pin 1... unity gain configuration a.k.a. buffer. This signal is what controles the PWM action... higher voltage, bigger duty cycle, lower voltage, smaller duty cycle. The leading edge of the clock starts the whole cycle, at which time the "output is enabled... then a comparator monitors the ramp amplitude compared to the error voltage on pin 2... when the ramp reaches the error voltage,  the output turns off and the "ramp" finishes it's cycle and the whole thing starts over again. The key here is to understand that as the error voltage is changed, it is directly propotional to the duty cycle. You make the error voltage a little higher, it take longer for the ramp to get to the error voltage level so the output stays on longer. Make the error voltage smaller, and the ramp voltage "gets there" sooner, so the output shuts off sooner and the duty cycle is reduced... there you pretty much have it ... PWM action.

Look at your circuit again... why would you connect the voltage divider that "simulates" your error voltage to the Rt and Ct pins... ground would be a better choice. You don't need the current sense circuitry to start playing with this chip. Tie pin 4 & 5 to ground. Pin 4 (- input) has a  -200mV offset so that should keep the current limit comparator output high and out of the circuit for now. Once you have the rest of the test circuit running you can introduce the current limit function and see how it can "limit" the duty cycle.

Good luck!!

Oh... one more thing, you need a pull down resistor for the driver input... otherwise it's floating in space when it's supposed to be off.

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