Constant current in SMPS

[bogdan2011]

Greetings,
I'm building a variable lab power supply and I'm trying to understand how constant current / current feedback works with PWM.
I've been looking at danyk's lab SMPS project that uses UC3845 and a current transformer. According to the block diagram of the chip, the ISENSE pin is internally connected to a PWM comparator.
So how does vary the current by varying the duty cycle BUT keep the voltage constant?

 

[Silvio]

@Bogdan2011
In the chip used if it has a current sense comparator then to lower the current the voltage has to be reduced. There is no way to reduce the current and keeping the voltage constant for a certain load. If you make a simple assumption using Ohm's Law then it tell you directly.

Example
V=IR,,,,,, V/R=I Now we have a voltage of 10v and a resistance of 10 ohms the current is 1 Amp. Let us say that the voltage is doubled and now we have 20v with the same resistance of 10 ohms the current will double to 2 amps. If we need to maintain the same current of 1 Amp then the voltage has to be reduced to 10 volts otherwise it cannot be achieved. The PWM will reduce the pulse width until the current will stabilize to 1 Amp the resulting voltage has to be 10v to get the constant current of 1 Amp.

I hope that helps

Regards, Silvio

 

[bogdan2011]

@Bogdan2011
In the chip used if it has a current sense comparator then to lower the current the voltage has to be reduced. There is no way to reduce the current and keeping the voltage constant for a certain load. If you make a simple assumption using Ohm's Law then it tell you directly.

Example
V=IR,,,,,, V/R=I Now we have a voltage of 10v and a resistance of 10 ohms the current is 1 Amp. Let us say that the voltage is doubled and now we have 20v with the same resistance of 10 ohms the current will double to 2 amps. If we need to maintain the same current of 1 Amp then the voltage has to be reduced to 10 volts otherwise it cannot be achieved. The PWM will reduce the pulse width until the current will stabilize to 1 Amp the resulting voltage has to be 10v to get the constant current of 1 Amp.

I hope that helps

Regards, Silvio

So then the difference between the two is that in current mode, the current is more important in the equation (basically limiting the maximum current), while in voltage mode, the voltage is.
And obviously, internally the chip works differently.

Cheers, Bogdan.