UCC3895: Using the UCC3895 below 50kHz

MOR DIOP

Prodigy 40 points

Part Number: UCC3895
Other Parts Discussed in Thread: UCC2895, UCC28950

Tool/software:

UCC3895/UCC2895 : Phase Shift Full Bridge

Hello,

I'm using the UCC3895 for a PSFB with high voltage ratio x30.

The frequency is arround 20kHz for the application. I've done the simulation with

PSpice for TI and it works well. However, in the datasheet, the minimum frequency of the IC is 50kHz.

Is it physically problematic if I use it at 20kHz.

In advance, many thanks for your return.

over 1 year ago

0 Ning Tan over 1 year ago

TI__Genius 12795 points

Where do you see the min freq spec? Why do you want to set 20kHz?

BTW, can you please UCC28950 instead?

0 MOR DIOP over 1 year ago in reply to Ning Tan

Prodigy 40 points

@Ning Tan thank you for your return.

UCC2895 is also ok. For this device, in "6.3 Recommended Operating Conditions" of the datasheet, it's mentioned that the minimum switching frequency is 50kHz and the maximum is 1000kHz. 20kHz is ok for many parameters considerations in our design. For 100V input, we have 3kV output. ratio of 30.

For the UCC28950 you recommend, is it a problem to use it at 20kHz ?

Thanks !

0 MOR DIOP over 1 year ago in reply to Ning Tan

Prodigy 40 points

@Ning Tan thank you for your return.

For the UCC28950 you recommend, is it a problem to use it at 20kHz ?

Thanks !

0 Ning Tan over 1 year ago in reply to MOR DIOP

TI__Genius 12795 points

Here is the section 6.3 in UCC2895 datasheet. I don't see min 50kHz is mentioned.

The 50kHz is mentioned in UCC28950 datasheet but I don't see why 20kHz is impossible. You may have to size your component value properly though.

0 MOR DIOP over 1 year ago in reply to Ning Tan

Prodigy 40 points

You're right ! in the datasheet of UCCX895 it's not mentioned explicitly.

However, at section 7.3.3 (CT Oscillator Timing Capacitor), it's noted that CT can range from 100 to 880pF.

At section 7.3.10 : RT range from 40kOhm to 120kOhm.

From my comprehension, does it mean that fosc_min is arround 90kHz and then Fswitch_min = 45kHz ?

Thank you again.

+1 Ning Tan over 1 year ago in reply to MOR DIOP

TI__Genius 12795 points

Based on the oscillator block below, if Rt is too small, the current mirror may not be linear anymore. Other than that, I don't see why you cannot set the freq lower.