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UC2854B: UC2854B Current Error Amplifier

John_Fletcher
Prodigy 40 points
Part Number: UC2854B
Other Parts Discussed in Thread: UCC3818, UCC28070

CURRENT ERROR AMPLIFIER

This error amplifier configuration conventionally has a resistor from the inverting input to common, and a resistor from the multiplier output to a negative current sense voltage. The negative current sense voltage is from a current sense resistor or from a current transformer circuit such as in DN-39E (slua172) Figure 3.

Instead of this configuration, can the multiplier resistor be tied to common, and the inverting input connected to a positive current sense signal?

The data sheet says that it has a CMRR Common mode rejection range of -0.3V to 5.0V. Is this referring to the amplifier inputs? If so, could not a positive current sense signal in that range be applied?

With this not being a transconductance amplifier and having a short circuit current of 1.5 to 3.5A, it looks like there is no way to bypass this amplifier with an external one.

  • 0
    TI__Genius 10555 points

    Hello there,

    Regarding your first question, please take a look at figure 9 (p11) in this document: https://www.ti.com/lit/pdf/slua144. Is this the configuration you're looking for?

    On your question on CMRR, I'm honestly not sure this parameter is listed as CMRR. It is my interpretation that this is the limit of the signal that can be applied to the inputs, MOUT and ISENSE. So neither pin should go lower than -0.3V nor higher than 5V.  If that in itself poses an issue for your design, I will have to dig a bit further to understand exactly why this is spec'd as it is.

    On your last question I believe you're referring to the Voltage Amp (Isc typ = 1.5mA, max = 3.5mA).  If you want to incorporate an external EA, the only way to accommodate one is to connect the UC2854B amp into a x1 amp (where Rin = Rfb), and feed Rin from the external EA. Leave VAO alone, except for the Rfb to VSENSE. VAO is feeding into the multiplier so trying to go around it will cause trouble.

    Just as an aside, perhaps you are locked into this controller from working with and older design or other requirement, but if you're working on a new design, I would suggest you look at some of our more recent PFC controllers: https://www.ti.com/power-management/acdc-isolated-dcdc-switching-regulators/power-factor-correction-pfc-controllers/overview.html

    I hope this answered your questions.

    Ray

  • +1 in reply to Ray DiCecco
    Prodigy 40 points

    Hi Ray,

    Thank you very much for your answers.

    Yes, slua144 figure 9 is the configuration I am looking for. I am sorry I missed that. It has been years since I read that.

    Your interpretation of the -0.3V to 5V input range makes complete sense. The data sheet in the current amplifier table does refer to it as CMRR, but I will not worry about semantics.

    Configuring the UC2854B current amp into a unity gain amplifier by connecting pin 3 and 4 is a perfect and obvious solution. I should have thought of that because I have done that trick before elsewhere a long time ago.

    Thank you for your suggestions regarding other controllers. Years ago, I wrote the state space equations for the boost converter. I also wrote the equations for the average current mode controller in terms of the ramp and the switching frequency inductor current ramp function that appears at the output of the current error amplifier. I perturbed that equation and combined it with the boost converter equations. I have these equations incorporated into Mathcad files. Since this is the trailing edge version of current mode control, I am most comfortable with it. I have done designs with the leading edge modulated UCC3818 using the same equations and the feedback loops were stable. I should derive the equations for the leading edge version if I am constrained to use it again. Leading edge modulation introduces a phase delay exponential term that causes the phase to drop dramatically at higher frequencies, but it must happen at higher frequencies in this case.

    The thing that I like about the UC2854B and UCC3818 is the multiplier output limited by IMO<=2*IAC which gives a very nice current foldback when operating below the minimum input voltage, while maintaining power factor correction. I have had a PFC operating at 15VAC while still maintaining output regulation. For this reason, I have rejected using PFC controllers from other manufacturers.

    The last design I did was using the UCC28070. I had resisted using it on a previous design because of the stepped multiplier scheme and lack of the IMO<=2*IAC limit, but regretted the decision after. I worked out the mathematics of the stepped multiplier so that I could design with the UCC28070 for my last design and really appreciate a lot of features of it. It bugs me that it doesn't have the multiplier limit and I had to add an undervoltage inhibit circuit instead. That design was a bridgeless totem pole design and it worked well with my current sensing scheme.

    I have decided to use the UCC28070 on this design which is another bridgeless PFC of another topology. I am just trying to figure out whether to use a single stage or an interleaved design ( the design is 7.1kW).

  • 0 in reply to John_Fletcher
    Prodigy 40 points

    I forgot to click "This resolved my issue". You have fully answered my questions. Thank you very much.