UCC28631 Vsense or Rp resistor selection

Ilya,

Unfortunately, the UCC2863x family of controllers were developed specifically for AC-DC Flyback power supplies, targetting the universal AC range of 90-264 Vrms.

The resistor network on the VSENSE pin, including Rp, is used to measure both output voltgae during the Flyback interval, and the bulk cap level during the primary FET on-time interval. the bulk cap level information is used internally for several functions, in particular the input AC voltage UV/brownout function. As shown in Table 6 of the datasheet, the AC on threshold is nominally 80 Vrms (i.e. 113 Vpk on the bulk cap), and the AC off threshold is 65 Vrms (92 Vpk on thr bulk cap). There is a 40 ms delay for the AC off threshold, to cope with short-duration AC dips and drops.

As a result, for a 55 Vdc input rail, this IC will never turn-on. This IC family is not suitable for low-voltage DC input.

Maybe there is another more suitable part, i can make some suggestions if you can give me details about your spec or application requirements.

 

Thanks,

Bernard

Bernard,

Thanks for your reply, I was under the impression that the UCC2863x family has both AC-DC (28630) and DC-DC chips (28631). Am I mistaken? (e.g. pg 14 of the datasheet shows the HV pin of the chip connected to the DC-side of a bridge).

If you could suggest a more suitable part, that would be great: I'm looking for a DC-DC flyback controller with primary side regulation, with an extended temperature range (-40-125C), to convert 55V-15V.

-Thank You

Ilya,

The UCC28631/2 devices are intended to allow DC-operation, but it must be high-voltage DC, effectively the same range as the equivalent AC input (90-264 Vrms * sqrt(2)). They were not intended for low voltage DC input operation.

These devices were offered for customers who do not require the X-cap discharge feature, since that feature requires an AC feed for the HV pin, which adds 2 dedicated diodes to (full-wave) feed the HV pin. The UCC28631/2 can save the cost of these diodes where X-cap discharge is not required by connecting HV pin to  the bulk cap.

 

To be able to suggest a more suitable controller, what power level do you need? Do you need output isolation? What is the input voltage range (55 V +/- ??)

 

LM5021 is a programmable-freq single-ended controller that could be considered.

LM5023 is a variable-freq QR Flyback controller.

UCC2870x family is a PSR controller that eliminate opto feedback, similar in many ways to the UCC2863x. But the input UV levels are adjustable by choice of upper resistor in the divider on the VS pin.

 

Thanks,

Bernard

 

Bernard,

So the UCC28631/2 device will not start up unless the HV pin sees >90V, correct?
Is there a way to get this device to operate at lower voltages, or is it necessary to choose one of the other chips you suggested?

-Thank you.

Ilya,
The UCC28631/2 devices must see a DC voltage greater than the line UV startup level - per table 6 in the datasheet, the ACon level is 80 Vrsm, ACoff level is 65 Vrms. Multiply by sqrt(2) to get the peak level or equivalent DC level. For DC, the on threshold is nominally 113 V, off level is nominally 92 V.
It's not possible to use these IC's at lower voltages than these levels, they were not designed for lower voltage applications.
I would recommend using the LM5021 (user-programmable fixed-freq) or LM5023 (variable-freq QR).
The UCC2870x devices could also be used - the input turn-on level can be programmed by adjusting the upper divider resistor on the VS pin. However, the input range (ratio of Vinmax to Vinmin) of these devicesd must be limited to about 4.5:1 ratio, to ensure that the VS pin negative current stays within the recommended max level of 1 mA.
I hope this helps answer your question?Bernard