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# UCC2808A-2: Trouble with PWM duty cycles

Part Number: UCC2808A-2

Hi Dear Support!

I'm trying to run the device from WEBENCH designer, here's the link:

The problem is UCC2808A-2 is not reacting for changes of Vin and (or) Vout. I have tested circuit with full range of Vin (14-50 VDC) and the PWM signal delivered to MOSFET's gate are always +/- 47% DC. Could you help me with diagnosis which feedback loop is not working? I don't understand how the IC is setting DC, by current on FB pin? Could explain me like for a newbie wich loop is setting DC and how?
• Oskar,

Thanks for sending along and your interest in TI here. I've assigned to an applications engineer to assist. Please expect a reply by end of day Monday Dallas time.

Regards,

John

• Hi,

Do you mean you tested your circuit on bench after you built your board?

• Hi,

I have built the circuit from the link and now I'm testing it. Output voltage at 31V Vin od about 27 V and decreasing with any load, like for 100mA output current it is 17.3 V. Shapes od signals Vds and Vgs are fine, but i noticed the %DC are always at max lvl (about 47-49% DC) for both IC's outputs

• Hi,

What is your transformer turns ratios?

• From:

Vct = 1,05*Vin = 1,05* 50 = 52,5 (53)

N = Vct/Vout = 53/24 = 2,208 and that's my transformer turn ratio. Did I calculate it correct?

• Hi,

You need to add the duty cycle value into the calculation.

Let’s use the Datasheet page 12 circuit diagram.

Np1/Ns1 = (Vin / (Vout+Vf) x duty x 2

Fill in Vin with your minimum input voltage, Vout your maximum output voltage, Vf your secondary diode voltage 1V, duty < 48% suggest initial 40 %.

For example, Vin min = 30 V,  Vout = 24 V, duty = 40%, then Np1/Np2 = 30 x 80% / (24+1) =  0.96.