Optimizing Adapter Power Supply Design

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

The test items include voltage regulation factor, voltage ripple, protection functions, inductor saturation status, maximum stress on main switch transistors and the temperature endurance margin of these transistors.

The last step is to test and verify the product's safety. This is done by carrying out tasks such as EMI (electromagnetic interference), thermal test (checking whether the maximum temperature of switch transistors, inductors, capacitors are within the specified range), safety testing such as lightning surge, Hi-pot, EFT, EDS, insulation resistance, leakage current test and finally we calculate the product's mean time between failures (MTBF).

DESIGN PROCEDURE

The above discussion presents a brief snapshot of the procedures involved in a power supply design. We will illustrate the discussion using a 120W adapter power supply design. The specifications of the power supply are listed in Table 1.

The circuit topology for this design could be flyback or half-bridge; if the budget is tight, flyback DC/DC topology would be a good choice.

As the design calls for a power factor correction (PFC) function, the design needs a PFC controller. A 120W power supply works well in inductance current critical conduction mode and there are many PFC controllers that support this mode. For this particular design challenge it is worthwhile considering the Fairchild FAN6961.

Delivering a balanced trade-off between performance, EMI and cost, the DC/DC conversion in the design is implemented by a quasi-resonant circuit. The FAN6300 is suitable for quasi-resonant topologies and could be implemented in this design.

FAN6921 is a solution with a higher level of integration that offers all the functions of both the FAN6961 and the FAN6300, as well as some newly-added functions such as input low-voltage protection, overheat protection, input overpower compensation for both high line and low line.

In addition, the FAN6921 performs well in terms of average operating efficiency and standby power consumption, and its total external component count is reduced by 20 compared with that of above-mentioned design solution that considered using the FAN6961 for PFC and the FAN6300 for DC-DC separately. So, it has the advantages in both performance and component cost, Therefore, for this particular design we finally selected Fairchild's FAN6921.

In this design example, we selected RM for the PFC and PQ type for the DC/DC element of the design, (Figure 2 is the Ae and Aw shape of the PQ type,). As the power supply size of the design is relatively (small), the cores that we have selected both have a low magnetic leakage and a larger Ae and Aw value. This delivers advantages in both power efficiency and EMI suppression to the design.

Now, we calculate the specification parameters of the PFC inductor, and select suitable rectifiers and MOSFETs. Before doing the calculation, we have to assume several parameters of the PFC component, as listed in Table 2.

To calculate the minimum inductance:

(See Eq. 1)

To reduce the power loss that the PFC may suffer at low input voltage, the PFC can be implemented in two lever; in the range of 90~150Vac the PFC output voltage is set to 240~260Vdc and 150~264Vac to 390~400Vdc.

Calculating the peak and RMS current:

(See eq. 3)

Calculate the AP :

(See eq. 4)

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