AN901 Datasheet, PDF (20/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
4.3. Q1 Selection
The instant S1 opens, Q1’s drain voltage increases rapidly from nearly 0 V and settles to:
V_{ds 1 – D} = V_IN + nV_OUT + V_fD1 = 24 + 16.5 = 40.5V
Equation 54.
However, energy stored in L_lkg must be dissipated in the secondary which causes V_ds,(D) to spike a higher voltage.
Q1 must be able to tolerate this voltage spike between drain and source.
A N-channel MOSFET with 100 V rating was chosen to accommodate the expected voltage stress caused by L_lkg
4.4. D1 Selection
Equation 25 and 26 define the requirements for D1. Substituting into Equation 25,
I_{D1 AVE1 – D} = ^I--₁^L---^O-–---^A--_D--^D-- = ₀---¹-_.--₆- = 1.6A
Equation 55.
Diode current capacities are specified in rms. Assuming a linear current through D1, consider the translation of
average to rms:
Substituting into Equation 26:
I_{D1 RMS1 – D}
=
I _{D 1}
AVE
1
–

D_
---²--₃- _
=
1.84A
Equation 56.
V_{D1 REVD} = ^V---_n--^I-^N-- + V_OUT = ²--₃--⁴-- + 5 = 13V
Equation 57.
Equation 26 and 57 do not include the voltage spike due to the interaction of the diode capacitance and secondary
side leakage inductance, and as a result, a diode with a larger withstanding voltage is required in practice.
When selecting D1, diodes with low V_f are the preferred choice as it minimizes the associated power loss.
P_{D11 – D} = V_fD1  I_{D1 AVE1 – D}
Equation 58.
Several diodes were tested in the circuit. A 5 A, 50 V diode was chosen for its tolerance to high operating
temperatures at which diode leakage and package heat transfer characteristics affect overall performance and
efficiency.
4.5. External Regulator Circuit
For this design, an external regulator circuit was designed to work with the VREGA voltage reference to create a
regulated supply for VDDA. R14 was selected for a 950 μA sink current.
R14 = ^V-----^I-^N-----–----_I-^V-_R---^R----^E---^G----^A-- = ²--₀--⁴-_.--₀--–-₀---⁴-₀---^.9-⁸--₅--⁵- = 20.15k
Equation 59.
R14 was set to 19.6 kΩ and C14 to the recommended 0.1 μF. MMBT2222 was selected for Q1.
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Rev. 0.1