The second port controller and DC/DC converter are connected on the SVCC, before the load switch and before

the current sense resistor. Using this configuration, both the load switch and the current sense act only on the

main TypeC output. The GPIO0 pin is driven by a logic signal providing information about the connection of

the second port: depending on its logic level, the ST-ONEMP chooses two different sets of PDOs and APDOs

providing either full or reduced power on the main TypeC port to implement a power sharing scheme. The GPIO1

is used to keep the DC/DC converter off during startup until the output voltage is stable.

8.2

External component selection

8.2.1

Symbols used in this section

Ipk peak magnetizing current

Lp primary side inductance

Lk primary side leakage inductance

Vi converter input voltage

Vo converter output voltage

n transformer turns ratio

Tsw switching period

fsw switching frequency

Po converter output power

D duty cycle

Cph phase (switching) node total capacitance (typically, switch Coss + transformer input capacitance)

Cclamp clamp capacitor value

VpriPk peak primary side voltage

fsamp control loop sampling frequency

Cout output capacitor value

Vdrop voltage drop during step load transient

Istep load transient current step

8.2.2

Transformer design

8.2.2.1

Turns ratio

The ideal operating condition for a flyback is with duty cycle around 50%. The turns ratio is anyway limited by the

voltage capability of the primary side switches.

As a first approximation, the turns ratio can be selected considering the maximum input and output voltage and

selecting the highest value for which the voltage stress on the primary side switches is acceptable.

The voltage stress can be calculated asVpriPk=Vi+nVo+Ipk LkCclamp

(1)

The last term in the equation can be ignored for a first approximation, then considered after component selection.

Typical turns ratio value for a wide input range, 20V output USB-PD application is around 6.

8.2.2.2

Primary side inductance – switching frequency

The ST-ONEMP Non complementary active clamp operates in a similar way as a quasi-resonant flyback, so

as a first approximation the relation between the switching frequency and the primary side inductance can be

calculated as

Lp= VinVo

Vi−nVo 2 12fswPo

(2)

For a more accurate calculation the clamping time and the deadtimes must be considered, especially for high

frequency converters. The following equation can be used to calculate the switching frequency when the system

operates in ZVS:

ST-ONEMP

External component selection

DS13989 - Rev 2

page 16/39