AMSA84
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Don't think of voltage mode and current mode as referring to what is being sensed or control on the output of the converter. The terms actually refer to what is effectively manipulated at the input of the converter.Hi guys,
First let me ask you guys a noob question: What does a Voltage-Mode controller? It controls the output voltage of the power-supply that we need? So there is no control for the current, right? And what about the Current-Mode controller? It controls the output voltage and the output current, right?
Now, I have read that in this post: https://www.edaboard.com/threads/56315/ that "In voltage-mode control ... output of the switching power supply is divided (usually using a voltage divider), is subtracted from a reference and is compensated using an error amplifier. Then the error voltage at the output of the error amplifier is compared to a sawtooth to generate the driving signal for the switching transistor. So voltage-mode control is a single loop control technique" and I agree with that. It make sense. But in this kind of operation we can't control the current, right?
Hi mtwieg, thanks for the reply.
I didn't understood "modulating the voltage applied to the converter input" and "current mode controller works by modulating current applied to the converter input". But the control scheme is not to set the output voltage and current to the desired value?
EDIT: Another question.
Looking to the UC3845 datasheet and application notes, we have a voltage feedback input. That voltage feedback is assured by, for example, a resistive divider that takes a sample from the output voltage. One interesting thing is that the non-inverting pin is connected to a voltage reference value of 2.5V. Now, imagine that we want an output voltage of 7VDC. What we need to do is calculate the values of the resistors in order to get out from that resistor divider 2.5V. So that 2.5V that we took from the voltage divider will have a correspondence to the 7VDC. If the voltage goes to 7.1VDC, the voltage at the Voltage Feedback Input will go up to 2.51V (suppose) creating a voltage error at the output of the Error amplifier, that will be applied to the current loop.
Am I right? Thinking right?
That's the basic theory. The feedback loop will always try to drive the circuit back to a steady-state condition (a target output voltage, in most cases). The current limiter is an additional function that can be tied into the basic switch-mode supply topologies.So, we are not controlling the current/imposing the current that we want. The controller only stabilize the current when there is load variations? The current limiter is another thing?
It doesn't appear that TI offers anything like that (it's a complex device, so the SPICE model would be pretty ugly). For learning the basic SMPS concepts and waveforms, I googled "smps spice model" and found this little website. It looks like it talks about the fundamental topologies and has SPICE files you can download. The more complex, integrated controllers like the UC3845 will act very similar, but will generally have a lot of the feedback and ancillary circuits pulled inside the package.BTW, there is any model of the UC3845 for PSPICE or something? So that I can study the waveforms?
Please consult the datasheet of your PWM controller. On the very first page, there is a block diagram. You can clearly see the zener diode (1V) at the second comparator. Also, in the application note, there is a section about dimensioning the sense resistor. As a matter of fact, there is an internal reference.Al3ko, and how the SR Latch knows that the peak currents has been reached? There is no reference current set, (like the voltage reference, with the voltage divider) unless the error output from the error amplifier. But how the SR Latch knows that, with the error voltage that comes from the E/A, has been reached the current peak value?
If you want to use a feedback control, you will have to implement a controller like P-, PI- oder PID. That is the compensator and stabilizes your feedback control. So yes, you will have to deal with that stuff in order make your closed loop control work. Though you will have to know about your SMPS transfer function.By the way, can you tell me if the compensation circuit is needed? (Between pin 1 and 2) Is a feedback circuit right? For what is that for? It's to implement a PI compensator? What are the benefits.
YesNow, when we sample the output voltage through the voltage divider, we have to design the resistors in order to get in the inverting input, the same value of the non-inverting input?
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