Control design for PWM converter

  • #51
Baluncore
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The comparator and saw wave oscillator can use a Texas Instruments LMV7219 which is a 7-ns 5Volt Comparator with Rail-to-Rail Output. $2 each.
You only need a single op-amp.
Attached is the schematic showing comparator, saw wave generator and a higher current gate driver.
 

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  • #52
@Baluncore
Thank you so much, I must admit it has not been an easy affair.
Please I still count on you to complete this task. I have simulated your design but the LED current still stay high "closed to 4A" the reason why is not working. Please can you help?
I have tried many values for the resistor on the integrator, i have also reduced the reference voltage.

Here attached is the design and the simulation

Best regards
Hubert
 

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  • #53
Baluncore
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Maybe you have the wrong control loop polarity ?
Follow the closed loop, is this sequence correct ?
If LED current rises.
INA210 Vout rises.
LM334s are disconnected.
Current flows into U73, C1-c, so V(U73)out falls.
U74 voltages rises.
Vgate rises.
MOSFET turns on.
D5 conducts.
LED current rises.

Does that appear to be a positive feedback loop ?
You need negative feedback to regulate so reverse the input connections to U74.
Does the circuit now work ?


If you need to further diagnose the bug you may need to isolate where it is happening in the simulation. I would take the output from the INA210 and feed it along with the 2.5Vref directly to the final comparator, with comparator polarity correct for negative feedback.
 
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  • #54
Baluncore
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Note: You are using a comparator with built-in hysteresis as an op-amp integrator. That will not work. You need to use an op-amp there.
 
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  • #55
@Baluncore,
thank you so much. I will look at it. Have a great weekend

Best regards
Hubert
 
  • #56
Baluncore
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For the integrating op-amp you might consider an AD8615 which is a precision, 20 MHz, CMOS, Rail-to-Rail Input/Output Operational Amplifier. It has low bias current, (less than 1nA), and will run on a single 5V supply. Digikey sell them for about US$3.
 
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  • #57
@Baluncore
Thank you so much. The circuit is perfectly what I wish to have. I will then proceed to design the PCB.
sharing a grass of beer would have been a nice idea.
Once more thank you.

Best wishes
Hubert
 

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