Optimizing Adapter Power Supply Design

Jul 1, 2010 12:00 PM
Jason Sun, Fairchild Semiconductor, China

Where:

Irms = RMS current of the inductor

Bmax = Maximum magnetic flux density

Ko = Effective coefficient of the inductive window

In this example, we selected a magnetic core of RM10, which has an Ae value of 98mm2, and an Aw value of 69.5mm2.

Calculate the minimum turns:

(See eq. 6)

Calculate the auxiliary winding turns:

(See eq. 7)

(See eq. 8)

The final PFC inductance parameters are shownin Figure 3.

In reference to the maximum current and voltage of the MOSFET to be used in this example, we selected Fairchild's SupreMOS™: FCPF16N60NT, 16A, 600V, RDS(ON) = 0.17Ω, ID=10A (Tac=100°C). The power loss of this MOSFET can be calculated as:

(See eq. 22)

For the rectifier, we selected the FDH08H60S. The power loss of this rectifier can be calculated as:

DC-DC TRANSFORMER DESIGN

In reference to the maximum input voltage (400V) and the output voltage (19V) of the DC-DC, we assume a 650V rated voltage for the MOSFET at the primary side and a 100V rated voltage for the rectifier at the secondary side. We can calculate the VQ_max and VD_max as shown below:

In this example, the voltage headroom values (K) for the MOSFET and the rectifier are set to 0.9. If larger values are required, set them to 0.8 or other proper values. It should be noted that different K values means that different voltage specifications will be required for the MOSFET and rectifier.

Determine reflect voltage (VRO) and max. duty cycle (Don_max):

This example uses a quasi-resonant topology, so we have to assume a fall time (Tf =1µs) for the MOSFET, To reduce the turn-on voltage of the MOSFET as much as possible, hence the loss and EMI.

Calculate the inductance, maximum current, RMS current:

(See eq. 27 on page 34)

(See eq. 29 on page 34)

.Calculate the AP to select the magnetic core

(See eq. 30 on page 34)

Based on the AP value calculations, we were able to select a magnetic core of the PQ3225 type, which has an Ae value of 114mm2 and an Aw value of 161mm2.

Calculate the minimum turns of the primary winding:

Determine the turns ratio (n) and the secondary winding turns:

The turns of the primary winding can be recalculated from n and Ns as:

Np=Ns x n = 30Ts > Np_min

Figure 4 shows the final transformer parameters.

Choose the primary MOSFET and secondary rectifier. In reference to the maximum current of the MOSFET that is calculated above for the primary MOSFET, we selected Fairchild's UniFET™ series: FDPF15N65, RDS (ON) = 0.44Ω, ID = 9.5 (Tac=100°C).

For the secondary side rectifier:

(See eq. 37)

To enhance efficiency of the total system. we selected Fairchild's synchronous rectifier controller, the FSR510 (Figure 5, which integrates a synchronous rectifier and a synchronous MOSFET (RDS(ON) = 9mΩ, Vf = 100V), needs only four external components, all while delivering a good performance. The circuit diagram of the Final design is shown in Figure 5.

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