UCC28701: What determines the UCC28701 Operating Frequency?

Qingxuan Ma

Prodigy 50 points

Part Number: UCC28701
Other Parts Discussed in Thread: TINA-TI

Hello,

I am using UCC28701 to design a flyback converter. And I am a little confused by the system operating frequency.

I see the following figure from the datasheet. Is VCL the same as the voltage sampled at VS pin?

If yes, the voltage at my VS pin reaches around 4V and based on the figure the fsw should be around 80kHz.

But the simulated frequency in TINA-TI Spice is much lower than what shows in the figure (only around 20kHz).

Could any one help clear my confusion? Will my hardware operating at 20kHz as the software shows?

The TINA-TI Spice file is also attached. Thanks.

UCC28701_Flyback.TSC

over 2 years ago

0 Claire Chang over 2 years ago

TI__Intellectual 2750 points

Hi Qingxuan, I am looking into this and will respond within 1-2 business days. Thanks for reaching out!

0 Qingxuan Ma over 2 years ago in reply to Claire Chang

Prodigy 50 points

Sure, thanks. My goal is to design a flyack with much higher switch frequency (>80kHz for example). Thanks.

0 Claire Chang over 2 years ago in reply to Qingxuan Ma

TI__Intellectual 2750 points

Hi Qingxuan,

V(CL) in this picture is the control law voltage inside the controller that is fixed. See 8.2 block diagram of datasheet (picture shown below)

If VS = 4V, it goes to the control law comparator and compares with reference voltage (4.05V+0.32V=4.37V), VS is lower than Vref and therefore control law should apply.

What is your primary inductance (Lp) value? I think something else may be wrong and effects the switching frequency. To check calculations, you can follow datasheet 9.2.2.7 for how to calculate Lp (equation 25).

Datasheet link; https://www.ti.com/lit/ds/symlink/ucc28701.pdf?ts=1675190286388&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28701

0 Qingxuan Ma over 2 years ago in reply to Claire Chang

Prodigy 50 points

My transformer is a commercial available one, which has a Lp of 460uH.

I've checked Rs1, Rs2, Rcs, Rlc, Rcbc values according to datasheet.

Would you please tell me what parameter affect the frequency most? My goal is to design a flyback using UCC28701 with high switch frequency. Thanks.

+1 Claire Chang over 2 years ago in reply to Qingxuan Ma

TI__Intellectual 2750 points

Hi Qingxuan,

Max switching frequency, output voltage/current, input voltage, efficiency are the first things we set before designing/calculating other passive components in the circuit. Not sure what your output voltage/current is but I think your Lp may be too large. You can try using the excel design calculator tool here (https://www.ti.com/product/UCC28701#design-tools-simulation). Input your desired output voltage/current, efficiency, max switching frequency, input voltage, and it will help you calculate the Lp and other values needed for your design.

Following an existing EVM for circuit reference may also help. For example here (UCC28700EVM-068, figure 2 schematic).

Hope this helps

Thanks.

0 Qingxuan Ma over 2 years ago in reply to Claire Chang

Prodigy 50 points

Hello Claire,

Sorry I was on another project last 2 weeks. Thanks, I will take a look at the tools you mentioned here.

Thanks for your help.

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UCC28701: What determines the UCC28701 Operating Frequency?