Producing Bode Plots for 65W Topswitch Flyback in DER243?...there's an added NPN in the error amplifier circuit

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Hi

We are calculating the Bode Plots for the 65W Offline DCM Flyback SMPS in DER-243 by Power Integrations……Please help to adjust the Modulator & Error amplifier transfer function? It needs modifying due to inclusion of an NPN (Q2) , as on page 7 of DER-243….

DER-243: 65W Flyback (Schematic on page 7)
https://ac-dc.power.com/design-support/reference-designs/design-examples/der-243-65-w-adapter/

The Modulator & Error Amplifier transfer function (without the NPN) is as on page 467 (equation 8.35) of the attached book page. (some of this expression is also defined on page 455).
(Attached Page 466 shows the same TL431 based schem as in DER-243, but unfortunately the component designators are different.)

Anyway, would you agree that Equn 8.35 on page 467 should be modified by simply multiplying it by the gain of the NPN? (ie, a number between 70 and 100) (albeit perhaps a bit less than 70 due to the emitter degeneration resistor , R25, seen on page 7 of DER-243)

AttachmentSize
Pages.zip1.48 MB

Hi,
The NPN transistor acts as a current amplifier and it has an effect on DC gain of a transfer function. Yes, you can add as a gain block for open loop gain transfer function. DER-243 has RC feedback on the amplifier, acts as an integrator adds phase boost. So you need to consider C16 into amplifier transfer function. C22 is a soft-finish capacitor, it doesn't effect your trasferfunction.