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UCC28070: UCC28070 Current Synthesiser and Current Amplifier

John_Fletcher
Prodigy 40 points
Part Number: UCC28070
Other Parts Discussed in Thread: UC3854A,

CURRENT SYNTHESIZER

The equation for the current synthesizer resistor R_SYN does not include the output voltage. Does the circuit get the output voltage information from the VSENSE pin? If so, will it adjust the slope with a variable output voltage? (That has got to be the case but I am just making sure.)

CURRENT AMPLIFIER AND MULTIPLIER OUTPUT

The current amplifier input common mode range is 0 to 3.6V. Is there any tolerance on those limits?

Is the input voltage range of the current sense inputs CSA and CSB the same as the current amplifier input common mode range?

On a previous bridgeless totem pole design with this IC, I had positive and negative current sense outputs. To deal with this I created a 1.8V virtual ground which split between the 0 to 3.6V input range. The current sense signals were referenced to this virtual ground and connected only  positive going ones to the inputs. The multiplier output resistor was connected to the virtual ground so that its output was referenced to this. Everything worked fine, but do you see any potential problems with this?

The only downside of this virtual ground scheme is that the current sense voltage swing is cut in half, and the corresponding signal to noise ratio. However, a 1.8V swing is still really good, especially for high power units where the current transformer sense resistor can dissipate significant power. Do you have any comments on that?

This is a really great PFC controller. I only wish that they implemented the multiplier output limit IMO = 2 x IAC limit as was done on the UC3854A/B.

  • 0
    TI__Genius 11350 points

    Hello "First_Name" ;)

    To your questions:

    CURRENT SYNTH:

    Per equation (12) in UCC28070 datasheet (page 19), Rsyn sets the internal synthesized down-slope as depicted by the red dashed line in Figure 19.
    Notice the equation contains all fixed constants.  There is no current information.  The slope is generated based on instantaneous VSENSE-VINAC applied to Rsyn for each switching cycle in the line cycle.  The resulting current in Rsyn is mirrored to discharge an internal capacitor to get the down-slope.

    CURRENT AMPLIFIER AND MULTIPLIER OUTPUT

    Regarding the 0V min and 3.6V max, the tolerances are already factored in, so those are the guaranteed values.

    The input range of the CSA and CSB are the same as the input common mode range.

    This is pretty clever using the 1.8V reference and splitting the signal. I don't see a direct issue, but the lower voltage (1.8 vs 3.6) provides half of the SNR but if your application can tolerate that, then no issue. Also, depending on how this was implemented, some of the signal from CSA may couple into CSB and vise-versa which may cause obvious issues. Something to keep in mind for next time.

    I hope I answered your questions.

    Regards,

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 40 points

    Hi Ray,

    Thank you very much for your answers.

    Can you please explain how some of the signal from CSA could couple into CSB and vise-versa? Are you referring to the case where the 1.8V reference is not solid with respect to common?

    In this case I decouple the 1.8V reference well with a capacitor close to the controller. Also the current transformers are tied only to the 1.8V reference where it is also decoupled at the current sense circuits. I need to be careful about common mode noise that might couple across the current transformers also.

    In the last project I only tested this with one channel.

  • 0 in reply to John_Fletcher
    TI__Genius 11350 points

    Hi John,

    Regarding the coupling, as you know, the CSx inputs are sensitive, so any unwanted perturbation on those nodes would detract from the controller performance.  For example, in the more common configuration with the CT's referenced to GND, we want the CT connection to return directly to the controller GND (pin 16), and not through the power ground plane so as to provide a solid and "noiseless" reference. I wasn't 100% sure how you connected these with the floating reference so I was (slightly) concerned that your reference may be "bouncy". So I guess CSA-CSB "coupling" isn't accurate, but simply CSA/CSB signal integrity. It sounds like you paid attention to your returns and were careful about noise, so no issue.

    Thanks again for your interest in the UCC28070. I'll make sure to forward your feedback on the IMO to the systems team for consideration on a future product.

    Regards,

    Ray

  • 0 in reply to Ray DiCecco
    Prodigy 40 points

    Hi Ray. Thank you for the reminder about grounding. I am always careful about grounding and will pay close attention to these inputs when I do the layout.

    Thank you very much for forwarding my IMO concern, because it is a great feature. If your company decides to do a control IC for bridgeless PFC converters, I could give them some ideas to think about.