I'm trying to reverse calculate the resistors used in a slope compensation network on a UC3843N current mode PWM controller (i need to change the current limit). Does anyone know of any material that explains it.
I've read some app notes from unitrode and texas, but they refer to different topologies to the one i've got. It's being used in a boost configuration.
Can anyone point me in the right direction? If further information is needed just let me know.
Thanks
Andrew
Slope compensation only applies to DC-DC converters like the booster and fly-back where the energy goes to the output when the transistor is off. It is only needed when the current in the inductor is continuous. I am assuming that both of these are the case in your topology.
The first step is to understand the DC-DC converter as it is supposed to work. The duty cycle of the transistor is usually determined by the ratio of the voltages on the inductor. Work out what the duty cycle is intended to be.
Now work out what the current in the inductor is doing and what the ramp on the current sense resistor looks like. It really helps to make a little sketch of the inductor current and current sense voltage. The sketch should be several cycles long. It doesn't need to be at all accurate just contain the idea of the shape.
Consider the point on the current sense ramp where the transistor is just about to turn off. This point should be in a cycle near the right hand edge of your sketch. (Point-1)
Go back from this point one full cycle and assume that the transistor stays on a little extra "dX" time. Figure out how much extra current ends up in the inductor.
Since things are sort of linear, this current decays with a L/R time constant while the transistor is off and then a different one while the transistor is on. You can figure out how much extra current ("AdX") is flowing at the instant of consideration. Often it is almost right to just say "it doesn't decay at all" and just carry on with that current. We want the amount of current when you get to Point-1.
=46rom the amount of current extra ("AdX") you can find out how much earlier this turn off would be shifted ("BAdX"). The extra current makes a little extra voltage in the current sense resistor. You divide by the slope of the sense resistor waveform to get back to time.
The minimum slope compensation is just enough that the shift on this cycle ("BAdX") is less than the shift you assumed to cause it ("dX"). A good practical limit is that it should be under half the amount.
A schematic showing the network feeding the CS pin might help. Current sensing and slope comp are cooked up and physically combined in many different ways, not all of them for published effects.
Flyback and boost will use have the same slope requirements, as they are the same basic topology. Some TI articles covering common isolated flyback will be: sluu096 (IET CCM) slup072 slup078 (CET DCM)
RL
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