Dear all:
I want to design a circuit dimmer with mc34063 and mcu
PWM from mcu will control the current output mc34063
but it not good.
Figure 1: circuit

Figure 2: 100% duty cycle

Figure 3: 50% duty cycle

Figure 4: 10% duty cycle

 

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Start a new thread when you wish to begin a new topic.
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A thread belongs to the original poster.
Start a new thread when you wish to begin a new topic.
Hijacking someone else's thread is not a polite thing to do.

but i can not creat a new thread!

 

A 1N400x series diode has a very slow reverse recovery time; it is not suited for use in a switch-mode power supply - it was meant for rectification of 50Hz and 60Hz (low frequency) power. Use a Schottky diode instead, like a 1N5819, or a ultrafast silicon rectifier.

Your R5/R6/C5/PIC network is changing the feedback path to CINV/pin 5, and C5 causes a delay in the feedback path response. If you pull the feedback line towards ground, the regulator will be running open-loop; putting out as much current as it possibly can. Instead, try pulling the feedback line (pin 5) higher than the 1.25v internal reference, but not higher than Vcc. Consider using an NPN transistor from the output of the PIC to turn on a PNP transistor connected to your 30v supply. You won't need much in the way of current to pull CINV higher than the reference voltage; 1mA would knock it out of the park (be more than sufficient).

You're using a rather large-value inductor; it takes a good while to charge it, and another good while to discharge it - you really want faster response. Select a lower value for the timing capacitor (Ct); you have 470nF which is HUGE. Try 1nF for starters. You will lose some efficiency due to switching losses, but the MC34063 isn't all that efficient anyway.

 

A 1N400x series diode has a very slow reverse recovery time; it is not suited for use in a switch-mode power supply - it was meant for rectification of 50Hz and 60Hz (low frequency) power. Use a Schottky diode instead, like a 1N5819, or a ultrafast silicon rectifier.

Your R5/R6/C5/PIC network is changing the feedback path to CINV/pin 5, and C5 causes a delay in the feedback path response. If you pull the feedback line towards ground, the regulator will be running open-loop; putting out as much current as it possibly can. Instead, try pulling the feedback line (pin 5) higher than the 1.25v internal reference, but not higher than Vcc. Consider using an NPN transistor from the output of the PIC to turn on a PNP transistor connected to your 30v supply. You won't need much in the way of current to pull CINV higher than the reference voltage; 1mA would knock it out of the park (be more than sufficient).

You're using a rather large-value inductor; it takes a good while to charge it, and another good while to discharge it - you really want faster response. Select a lower value for the timing capacitor (Ct); you have 470nF which is HUGE. Try 1nF for starters. You will lose some efficiency due to switching losses, but the MC34063 isn't all that efficient anyway.

Dear SgtWookie
Thank you for the ideal.
But i want dimmer led by current output.
if duty = 100% => current = 100% =440mA
if duty = 50% => current = 50 % =220mA
if duty = 10% => current = 10% = 50mA

So if i use an NPN transistor from the output of the PIC to turn on a PNP transistor connected to your 30v supply.
i can not change current output.

 

I can't respond any more this evening, I have had a long day and I am tired.
Tomorrow.

 

I can't respond any more this evening, I have had a long day and I am tired.
Tomorrow.

please help me!

 

Did you change the diode yet? You need a fast 1A diode like a 1N5819 and change C1 to 1nF (like SgtWookie already said).

You should also increase C5 a lot, try 10uF. You need to have a stable smooth DC voltage at point X.

The circuit should work OK after you do that. R2 will reduce the duty cycle once its voltage gets > 1.25v (on pin 5 of the 34063 IC).

Your PIC PWM and filter will cause an additional + DC bias on that pin, so will reduce the voltage needed on R2. That means it will reduce the 34063 PWM duty.

 

@viethung_tdh! Component/value vise, follow the suggested/typical circuit diagram given by the datasheet. Perhaps also using a digital controlled pot would be an idea in this application