• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 1494 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UC3844: UC 3844 Application for 415V AC Input to 15V DC FLYBACK SMPS

Chetan Patel
Prodigy 130 points
Part Number: UC3844

Dear Sir,

We are using UC3844 PWM Control IC in our various products.

We are using 415V AC Input directly from Mains without Isolation Transformer and then convert 415V AC to DC and then using UC3844 PWM Control IC convert it to 15V DC output using Flyback Topology. We follow IEC-60950  and UL-60950 standard for our Products for Creepage and Clearance distance. Please find attached document for clearance distance document with explanation of terminology used in IEC-60950 and UL-60950 standard for clearance requirement for Primary circuits.

Query : As per shown in IEC-60950 and UL-60950 standard Clearance distance required for Primary circuits(Direct connected with Mains without Isolation Transformer) for 415V AC Input for basic insulation system is 1.3mm but for UC3844 IC if I mount in PCB than we get 1 mm distance between two IC Pad. So is OK to UC 3844 IC with 415V AC and Input with out using Isolation Transformer System to generate 15V output using flyback topology??.

Please answer in brief.

Regards,

Chetan Patel

Clearance and creepage_UL-60950_IEC-60950.pdf

  • 0
    TI__Mastermind 31580 points
    Hi Chetan,

    All the pins of the UC3844 are limited to 30V or less as described in the maximum ratings table in the datasheet so there is no issue with the pin to pin spacing on the IC from a safety perspective. The IC does not cross the isolation barrier between primary and secondary so pin to pin spacing are only relevant to functional isolation.

    The IC is primary side referenced component and adequate separation between the primary and secondary side components must be provided by the power supply design as outlined in the relevant safety standards. The flyback transformer forms part of the isolation barrier between primary and secondary and must be designed with the required standards in mind please work with your magnetics vendor to ensure the design is compliant.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 130 points

    Dear Peter,

    I understand your points regarding Primary and secondary isolation.

    My query is that as per shown in IEC-60950/UL-60950 standard clearance distance(which I already sent in previous post) between two pins shall be 1.3mm min. upto voltage upto 71V, but if I place UC3844 IC on PCB then after soldering pin to pin distance is less than 1mm so how to use IC in our Application. Please guide.

    Regards,

    Chetan Patel

  • 0 in reply to Chetan Patel
    TI__Mastermind 31580 points

    Hi Chetan,

    The pin to pin spacing only needs to meet functional isolation, F, in the table. The value of separation depending on the pollution degree which is also listed in the table. You need to work with a safety agency to determine environment that your design will operate in with regard to pollution degree and mains line transients then you know what portion of the table to use for creepage and clearance.

    The UC3844 is used in many isolated ac/dc power supply designs for a number of years now without issue.

    If you require assistance interpreting the EN60950 standard then you need to talk to a safety engineer from UL. I am happy to answer questions on TI PWM controllers.

    Regards

    Peter