buck converter PWM CC - analog clocked PWM adjustable

493

04:27:38

Decided to roll my own PWM adjustable SMPS BCCC (Buck Converter Constant Current mode) for an LED project I've been having troubles getting to work trying to hack CV mode SMPS modules. This is about as basic as it can get without losing regulation. 
You may get better results and maybe cheaper if using a comparator and one NPN transistor to drive the MOSFET gate.

A few caveats if trying to build a physical version: 
It's likely to be less than 80% efficient. (I don't mind).

Proper design techniques and component selection should be used with a >= 2x safety factor to ensure longevity. Keep all high current traces short and wide as possible.

There is an effective "soft-start" function to limit inrush current. This soft start feature is a function of your PWM filter network. (upper right quadrant). It provides a fairly steady Vref to the comparator. 

I've modified the MOSFET to act more like a real high power MOSFET with 3x more width. The logic AND gate is required to regulate the width of the pulse to the MOSFET. 

published 7 years ago

kiani

7 years ago

The two transistors on the output of opamp,, what happens if the base of top one is low and the bottom one swiches on, don't they need some kind of synchronisation!?

rinks

7 years ago

The two transistors form a basic "AND" gate. So only when the clock signal turns on the lower one AND the op-amp turns on the upper one is the gate of the MOSFET to be connected to GND turning it on fast to reduce switching losses (heat) caused by turning on/off slowly. A clock signal is needed to prevent the op-amp partially turning on the MOSFET finding equilibrium and overheating the MOSFET. The fast clock at the bottom ensures instability in the system and helps make sure the MOSFET switches fast and never partially on.

kiani

7 years ago

Thanx. For that.

hurz

7 years ago

softstart part helps to overcome CFS in everycircuit but can be done by a ground connection. The random gamble AND gate is not needed at all. While the feedback voltage is better taken from separate voltage divider so your hand is free to change the load. the PMOS is not state of the art, but anyway will work if you take a four times highe gate capacitance into account for a real design in comparision to an NMOSFET. http://everycircuit.com/circuit/5943667257507840

PrathikP

7 years ago

Why are your feedback resistors in the milliohms?

rinks

6 years ago

PrathikP - the 50 mOhm is an actual sense resistor I'm using. The 200 mOhm is the load.

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