Part Number: UCC28740
I'm currently designing a 4.2amp 5v usb charging circuit for a customer. I used web bench for the initial design. It gave me a circuit very similar to the UCC28740evm-525, with the exception of some input and output filter values. Current I have not been able to get my circuit to hold any current on the output. I believe the root. I was hoping to get some advice on the proper tranformer configuration (turns, core, ratios, etc.) to hopefully get my circuit up and running.
Originally I tried a custom transformer with no success. This transformer also did not hold more than 100mA. Then tried an off the shelf transformer that was designed specifically for the UCC28740 (WE part number: 750313823). This transformer also did not hold more than 100mA. I have attached the datasheet for the WE part number: 750313823.
I have attached my schematic . please let me know if anything looks incorrect or concerning.
Thanks in advance for your help.
Kevin pcb619 schematic.pdf
I reviewed the schematic and functionally it looks correct. You had mentioned that Wurth had designed a transformer specifically for the UCC28740 750313823. Did they give a recommendation of the input and output voltage range and deliverable output power?
There is an excel design tool that you can use to check the transformer turns ratio and primary magnetizing inductance for your design. It can be found at the following link. Please note to function correctly the primary leakage inductance should be less than 3% of the magnetizing inductance. http://www.ti.com/lit/zip/sluc487
Wurth has manufactured transformer’s for the UCC28740 family of devices that use the auxiliary to secondary turn’s ratio for OVP protection. This should work for the input and output requirements they designed for. You should be able to get at least this rated output voltage and power for your design. If you cannot the issue might be layout. The data sheet gives some layout recommendations that might help in section 10 with an example layout in Figure 28.
You had mentioned UCC28740EVM-525 which is an 85V to 265V RMS input to a 5V 10W output. You might want to get a hold of one of these EVMs to evaluate. It will give an example of what the critical signal should look like (CS, Aux, VDD, FB); as well as, an example of a proven layout.
I have found one of the top reasons why the UCC287XX family of devices has issues starting up and regulating is due to the auxiliary winding signal being too noisy. In the Device Functional Mode section of the data sheet on the top of the page down to figure 14 gives guidelines to what the auxiliary signal should look like. If this signal is too noisy it can trigger an output over voltage protection (OVP).
I would recommend trying to figure out why the design is not regulating. Is the converter shutting down to a fault or a noisy signal? I would start by evaluating the CS, Aux, VDD, and feedback signals of your design with an oscilloscope.
In reply to Mike O':
If you were interested in doing a dual 5V 2.4A (4.8 A) offline USB charger, there is a proven reference design PMP10885 using the UCC28740. The following link will get you to the reference designs schematic, BOM and test data. http://www.ti.com/tool/PMP10885?keyMatch=PMP10885&tisearch=Search-EN-Everything
I have included the Gerber files if you are interested in perusing a dual output design.
Thank you for your response. So I was able to get my circuit running with the transformer stated above. In fact I was able to get it to run at the full rated current (just over 2 amps). The inductance of the above transformer was approximately 367uh. I tried a new transformer with a higher inductance (also higher output current) and my output is now sagging after just a few hundred milliamps applied at the output. Do you have any suggestions on what I should be checking to resolve the issue? Is there any waveform captures I could upload that my help exemplify what is going wrong?
Thanks for all your help so far.
In reply to Kevin Knapp:
Great thanks Mike.
One more quick question. When the calculator states a specific inductance, say 350uh, is that the inductance at the typical 10khz or the maximum operating frequency?
You select the maximum frequency in your design by selecting the primary magnetizing inductance of your transformer. This should be around 60 to 80 KHz.
You should be able to calculate the gap (lg) of the transformer with the following equation. Ae is the core cross sectional area Np is primary turns u0 is 4*3.14*10^-7Wb/A*m. Lpm is primary magnetizing inductance.
Even though you can go through this calculation it might be easier to due trial an error on the gab when you are building the transformer.
Thanks for all of the help so far. So I was able to successful get my circuit to run at 4.5A 5v, which is great. Now I am having trouble with the UCC28740 going into thermal shutdown. I know that I can use a Mosfet with a lower gate charge to reduce the burden on the IC, and have one coming in tomorrow to try. Is there anything else I could be doing to the circuit to reduce the heat on the UCC chip?
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