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UC2855A: Seeking control IC for boost converter with zero voltage transition (ZVT) with auxiliary switch, for PFC (line current shaping) application

Fabio BARONE
Prodigy 50 points
Part Number: UC2855A
Other Parts Discussed in Thread: UC2855B, UCC28180, UCC28064A, UCC28051, UCD3138

Tool/software:

Dear Sir/Madam,

we are designing a PFC input stage for a power converter.  High efficiency, low power loss, and low EMI (low dv/dt, low di/dt) are all important requirements.  We have identified a suitable power topology for this stage: Zero-Voltage-Transition (ZVT) boost, operating in CCM for most of the AC cycle for mid to high load power. 

This topology requires the following components:
1. Two (2) controlled power switches: the main boost switch, and an auxiliary switch to control the current in the auxiliary path that sets up ZVT/ZCT for the main power switch (and the main power diode).
2. Passive and non-controlled components for the auxiliary current path: inductors, capacitors, diodes.
3. A control IC that can provide all the control and protection functions, and the gate drive to both the main switch, and the auxiliary switch.

The UC2855A (and UC2855B) seems to be a good candidate for item 3.  However, it has been around a long time (pre-2000), and its pricing would suggest that it is nearing end of life. 

So, my question is:  Are there any alternative products that provide the same functionality as the UC2855A or UC2855B, in particular:
(1) the ability to shape the line current of a CCM PFC stage, and
(2) implement ZVT?

Separate Issue: your website, and its silly recommendations for "alternative devices".

On a separate issue: some of the recommendations for "alternative devices" that I have seen generated by your website in recent times have been less than satisfactory.  I have simply been ignoring most of these, but this last instance has triggered me.  Without any prompting from me, the webpage for the UC2855A suggests an alternative; that alternative is UCC28180.   Then when I clicked the button: "Compare alternates", the results are ever more disappointing: nowhere is it mentioned that the ZVT driver present in the UC2855A is **not** replicated in the alternate product.   This was a complete waste of my time.

Here is the link to the webpage, and below (or attached) is the screenshot of the alternate part suggestion.

www.ti.com/.../UC2855BDW

Here is the link and image of the comparison page:

https://www.ti.com/compare-products/?id=360000&type=GPT&mode=alternate-opn&partList=UC2855BDW,UCC28180DR&sticky=true

I just tried to insert two images to this post; the first seems to have worked (I can see it in my post), but the second just appears as tiny square - not sure if it worked.  

  • 0
    TI__Genius 12795 points

    Fabio,

    Dissatisfaction is acknowledged and we will improve our website. 

    I am not aware of other PFC controllers in our latest portfolio that offer ZVT feature, other than digital controllers. I assume you want an analog controller.

    Can you please share more details of your system so we can see if there are other options?

    Best,

    Ning

  • 0 in reply to Ning Tan
    Prodigy 50 points

    Hello Mr Tan,

    In regard to sharing more of the system: This is for an industrial PSU, 24Vdc output, very small form factor, high operating temperature so thermal loading is a major concern, hence we are seeking all possible ways to reduce power loses.  Main power stages are PFC stage for connecting to the AC supply, and an isolation stage which we are implementing as an LLC with synchronous rectifiers on the secondary side. 

    So we need to control the power switches for:
    1. The AC line current shaper (PFC, boost, CCM).
    2. The LLC primary switches.
    3  The LLC secondary SRs.
    Isolation voltage is high, hence we want minimal interaction between the primary and secondary of the isolation stage.  

    I would prefer separate controllers for each of the above stages.  We may consider a combo controller for the PFC + LLC primary switches, however, since we've identified that we need soft-switching for the PFC stage, the combo ICs with that capability are limited.  

    I would prefer an "analog" controller for each stage (which is why I was looking at the UC2855 for the boost PFC), but if digital is the only option, then we will consider it.  What digital options would you suggest as a good starting point?

  • 0 in reply to Fabio BARONE
    TI__Genius 12795 points

    What is the system output power and input voltage, as well as efficiency target and ambient temperature?

  • 0 in reply to Ning Tan
    Prodigy 50 points

    Input power voltage:

    AC 88VAC to 276V
    DC 88VDC to 288V
    Power: 90W
    Ambient operating temperature: 85C
    Cooling: convection (no fan).

  • 0 in reply to Fabio BARONE
    TI__Genius 12795 points

    Can you clarify why there is a DC input? DC input doesn't need power factor correction. 

    What is the AC freq?

    Will you consider using GaN FET to improve the PFC efficiency?

    For 80W application, digital controller solution may not be cost-effective. In general, the hot spots in a AC/DC stage are diode bridge, transformer and secondary rectification stage. I understand you want to reduce the power loss at every stage, but I'd address other low hanging fruits first before PFC stage.

    In the meantime, we will discuss internally and get back to you on the PFC controller recommendation later.

  • 0 in reply to Ning Tan
    Prodigy 50 points

    AC (50 & 60Hz) and DC input operation is required - this is PSU intended for worldwide use.

    We may consider SiC and GaN, and yes, we are aware of the "low hanging fruit".

    However, what we seek urgent assistance with is selection of suitable controller ICs, as stated in my first post. 

    Thanks.

  • 0 in reply to Fabio BARONE
    TI__Mastermind 40580 points

    Hello Fabio, 

    Unfortunately, TI does not have any analog PFC controller that implements ZVT other than the UC2855A/B.   
    I don't know why it seems to be priced to discourage use, except that it is designed in an old process and uses an expensive lead frame and package.
    Maybe it is to accelerate end-of-life status as you suggest.  

    For a 90-W power level, some efficiency improvement may be had using Transition-Mode PFC (aka, BM, CrCM, CrM) where the boost diode can be a standard recovery part with low VF because it turns off with zero current.   
    I suggest to consider UCC28051 or UCC28064A.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 50 points

    Thanks Ulrich,

    Seems a digital controller is the only option available from TI.  Could you please suggest a couple of options for us to consider, that are low-cost, currently active parts with at least, say, 5 years of proven reliable service in the market to date.  

  • +1 in reply to Fabio BARONE
    TI__Mastermind 40580 points

    Hello Fabio, 

    Digital controllers are covered by another group which is better suited to make recommendations for you. 

    Please start an new E2E thread using "UCD3138" as the candidate controller reference and ask for help on ZVT PFC.
    I will close out this inquiry into analog control. 

    Regards,
    Ulrich