UCC28910: Modify EVM for 12V output

I will look at this and reply to you with steps to modify the board for 12V/300mA output.

I used the design tool and calculated the changes needed. The transformer needs to change as turns ratios and Lp are all very different to the 12V output converter. I will provide the calculation sheet as well as marked up schematics to you through email.

The design sheet and the correction markups have been sent out through email.

The design sheet and markups of changes for 12Vout have been sent out through the email.

Hong,

I might be able to get away with 265VAC max input voltage. 

Is there any way the schematic can be modified to use the same transformer and obtain 12v out?   Perhaps a voltage divider on the VDD input if the aux winding is providing excessive voltage to VDD?  I'm not exactly sure what's preventing it's use, technically.

It is highly desirable to use the same transformer as we will be using it in another device (volume pricing and it's on hand and in stock at our factory), and we know it passes conducted emissions per CE standards.

Thank you,

Eric Wood 

The existing transformer turns ratio and magnetizing inductance are too much away from so a new transformer is needed to meet the specs 12Vout across the input voltage range and the load range.

Can you please specify what you want to evaluate so I can look at what condition 12Vout can be ok? For example, can it be ok to have 12Vout at a very narrowed input and < 0.3A load, etc?

I tried to see if 12Vout can be obtained with the existing transformer. It is found that the magnetizing inductance is too low to achieve reasonable output current at 12Vout.

- The existing turns ratio can be ok when Vin(min) > 140Vac.
- The peak current is increased to the maximum UCC28910 can do.
- The desired inductance will be 5mH while existing is 1.1mH.
o If using 1.1mH, it will require switching frequency to reach 412kHz to output full power.
- But UCC28910 can only achieve about 105kHz or so.
- That means the very best result for the existing transformer is to achieve 12Vout at < 16.8mA load current, for Vin(ac) > 140Vac, and for fsw about 105kHz.

If this sounds ok for your evaluation, I will look at other parameter changes. Please let me know.

Hong,

What are the total transformer changes needed to attain the below ratings.  I'd like to continue the conversation with Wurth Electronics.

Vin: 90VAC to 300VAC

Vout: 12V

Iout: 300mA minimum

Thank you,

Eric

The below is obtained based on the UCC28910 design calculation tool for the specified converter using UCC28910.

(1). Converter terminal specifications:

Vin: 90 Vac to 300 Vac

Vout: 12 V

Iout: 300 mA (note: if the Iout > 300 mA, please provide a number so I can re-run the calculation so the below is on the assumption Iout = 300 mA)

(2). Transformer Specifications (Calculation Result):

Primary Inductance:                                Lp =	1.61	mH
Primary to Secondary Turns Ratio:      Nps =	6.54
Primary to Auxiliary Turns Ratio:          Npa =	4.48
Peak Primary Current:                       Ip(pk) =	0.25	A
Primary RMS Current:                     Ip(rms) =	100	mA
Peak Secondary Current:                   Is(pk) =	1.65	A
Secondary RMS Current:                 Is(rms) =	655	mA
Maximum Switching Frequency:            fsw =	105	kHz

Primary Inductance:        Lp =	1.6	mH
Primary to Secondary Turns Ratio:      Nps =	6.54
Primary to Auxiliary Turns Ratio:      Npa =	4.48
Peak Primary Current:   Ip(pk) =	0.25	A
Primary RMS Current: Ip(rms) =	90	mA
Peak Secondary Current:   Is(pk) =	1.635	A
Secondary RMS Current: Is(rms) =	403.2	mA
Maximum Switching Frequency:      fsw =	100	kHz

Hong,

Wurth Electronics suggested the use of p/n: 750312597.  Could you please verify if this is suitable for the application? Does the 300VAC max input voltage present any concern?  The UCC28910 datasheet suggests 265VAC max, but the web tools suggest 300VAC max.

Thank you,

Eric

750312597.pdf

The transformer, 750312597, can make the board output 12V and 0.3A with additional changes, based on the UCC28910 calculation sheet:

- Bulk capacitor value increase to > 22uF

- Design full load current =  0.46A, or 5.52W, while only run load up to 0.3A. At 105kHz, the load current is estimated as 0.302A.

If the above sounds good, I will provide all remaining changes based on the UCC28910 calculation sheet.