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# UCC28781-Q1: Component purpose question

Part Number: UCC28781-Q1

Can R37, R38, R20 be replaced by a single 19.6k resistor?
What purpose do D11 and D12 serve? What consequences will occur if they were to be removed?

Thanks

• Hello Valentinas,

These components are arranged the way they are to prevent damage to the VS input of the UCC28781-Q1 when operating at 500Vdc input.

The VS pin of this device has an absolute maximum current rating of 2mA.  Pulling higher current out of the VS pin would degrade the controller over time and eventually it would fail.  When the primary MOSFET is on, the zener diode D12 clamps the voltage across R20 to 27V which limits Ivs to 27/15.4k = 1.75mA max.

Without this zener clamp, the maximum DC input voltage would be limited to 1.75mA*19.6k*6.6 = 226Vdc for the same max Ivs.
Diode D11 blocks positive current through zener D12 during the demagnetization phase of a cycle.
R37 & R38 make up the additional resistance needed in series with R20 to set the OVP level at VS during demag time.  They are split into two 0402-sized components in order to sustain the dissipation (with margin) and voltage stress when input voltage is 500Vdc and the AUX winding voltage is -76V during the FET on-time.

Regards,
Ulrich

• Thanks for a thorough explanation, as always.

"1.75mA*19.6k*6.6 = 226Vdc" - what is 6.6 here?

If the Vin is 30-85V, i suppose it would be fine to remove R37, R38 and change R20 into 19.6k instead?

Would you recommend to still keep D11 and D12?

Kindly,

Valentinas

• Hi Valentinas,

The 6.6 factor is the primary-to-auxiliary turns ratio.

Yes, for a low voltage input, a single resistor could be used for the function of Rvs1, and D11 and D12 would be unnecessary.
This is true for any situation where Ivs(max) can be kept < 2mA at the highest input voltage.

Note that Rvs1 value would probably be different than 19.6k.  That value is specific to the UCC287821 EVM design.

D11 and D12 are there for the sole purpose of protecting the VS input from excessive current in a high voltage situation.
Whether than are needed or not depends on the maximum input, the turns ratio and the targeted start voltage.
Different designs typically have a different value for each of these factors.

Regards,
Ulrich