AN2783 Datasheet, PDF (24/47 Pages) STMicroelectronics – PM8800 demonstration kit for standard and high power PoE

Power converter
The maximum duty cycle can be obtained as:
Equation 7
T_-----o_---n_{----m----a---x-}
Tsw
=
-₍--_V----_i--_n----_m----_i-_n---–-^N--_V----^â‹-_d--⁽-_s--^V--₎--^o-+---^u--_N-^t----+-_â‹---₍^V--_V---^d--_o--⁾--_u---_t---+-----_V-----_d---₎-
AN2783
10.2
Main switch current and current sensing
The current shape in the primary power switch is different when the flyback converter is
working in DCM or CCM. In DCM the shape is triangular with the current starting from zero,
while in CCM the shape is trapezoidal.
For CCM operations the peak current can be computed as:
Equation 8
Ipeak = Ipave + ^∆-----^I-^L- = ------------------^I--^o---^u----^t----------------- + ^V-----ⁱ--ⁿ---^m----^a---^x----^â‹---^D-----^m----^a---^x----^â‹---^T----^s---^w---
2 η ⋅ N ⋅ (1 – D_max)
2Lp
which can be expressed as:
Equation 9
Ipeak = -----------------------^P----^o----^u----^t---------------------- + ^V-----ⁱ--ⁿ---^m----^a---^x----^â‹---^D-----^m----^a---^x----^â‹---^T----^s---^w---
η ⋅ D_max ⋅ (Vin – Vds)
2Lp
The RMS current can be:
Equation 10
Iprms =
D_max ⋅
Ipea
k²
–
⎛
⎜
I
peak
⋅
-^∆---^I--^L-
+
∆_----I_--L---²_{-⎟⎞}
⎝
2 3_⎠
The above formula can be used to calculate the right current sense resistor, taking into
account that the first level of OCP is for PM8800 at 500 mV:
Equation 11
Rcs = -----⁵---⁰----⁰---^m------^V-------
1.3 ⋅ Ipeak
The associated power dissipation on the sense resistor is:
Equation 12
Pcs = Rcs ⋅ Iprms²
10.3
Main switch power dissipation
The power dissipation on the main power MOSFET is the sum of two terms:
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