How to choose the best 190 W slim PFC power stage solution?

Most power factor correction (PFC) power stages operate in critical conduction mode (CrM), which controls the inductor current to ramp up from zero to a desired peak level and then back down to zero. Because this mode relies on the duration of the current cycle, the switching frequency varies as a function of the AC line current demand. Unfortunately, when the power demand is low, the current drawn from the AC line is low and the switching frequency "shoots up." This means that using a large inductor is the only way to reduce switching losses and interference to acceptable levels.

Frequency-clamped critical conduction mode (FCCrM) is a technique embedded in controllers such as the ON Semiconductor NCP1606 or NCP1631. When operating in this mode, the power factor correction stage operates in CrM under high load conditions, but at medium/light load conditions ( ), the switching frequency is limited to improve efficiency. FCCrM allows the use of smaller inductors compared to traditional CrM circuits (see reference [1]). In fact, interleaved FCCrM PFC appears to further reduce the size and cost of magnetic components. These advantages are demonstrated in a 190 W low-profile power supply.

This article further advances the research shown in reference [1] and explores the overall PFC cost issue in the same 190 W wide mains input range and maximum thickness of 13 mm application.

Inductor Considerations Table 1 reiterates the main conclusions of reference [1]. Since FCCrM clamps the switching frequency, no large inductor is required to pull down the CrM switching frequency range. Therefore, FCCrM significantly reduces the size of the PFC stage inductor, especially when using an interleaved FCCrM solution. In fact, as shown in Table 1, the following magnetic components can be selected for a 190 W (input power) TV application with a wide mains supply range and a maximum thickness of 13 mm:

• CrM solution: two EFD30 in series

•FCCrM solution: single EFD30

• Interleaved FCCrM solution: two EFD20s (one per branch)

Horizontal comparison

Next, to compare the different solutions, we experimented with a 300 W 46-inch LCD TV power supply reference board (see reference [2]) to compare the three PFC solutions. This reference board, developed by ON Semiconductor, embeds an FCCrM interleaved PFC driven by the NCP1631 (see reference [3]). We used this board to compare the three solutions for our 190 W application. Since the integrated inductor in this application is different from the inductor defined in Table 1 (the original coil size in this application is for 300 W power), the application was first modified to ensure that two EFD20 components could be used. In the second step, the circuit was dynamically adjusted to test the CrM and FCCrM single-phase solutions. For each test, the PFC stage was designed to keep the energy efficiency of the three solutions at a close to the same level.

In Figure 1, we can see the board with the modified interleaved configuration, as evidenced by the two “flying” inductors; Figure 2 shows how a CrM controller (i.e., NCP1607, see reference [4]) is used instead of the original NCP1631 interleaved driver.

Comparison of parameters of various solutions

The inductor is not necessarily the only component that needs to be modified in different solutions. The PFC stage must be adjusted according to the solution being tested. Table 2 summarizes the main design guidelines used in building the three solutions and verified by actual tests.

Interleaved PFC consists of two branches, each of which transmits 50% of the total power. Therefore, this solution uses more components, but they are smaller in size. For the sake of simplicity, the specific interleaved design criteria are not discussed here. However, as detailed in reference [5], interleaving technology can optimize the following components:

-Power MOSFET: In each branch, the MOSFET root mean square (rms) current is only half of the current of the 11 A MOSFET used in the single-phase CrM or FCCrM PFC stage. Two 5 A MOSFETs replace the 11 A MOSFET.

-Boost diode: Again, the boost diode of each branch carries half the total current. Therefore, it is possible to use a smaller MUR160 for each branch.

- Large capacitors: The interleaving scheme forces the two legs to operate out-of-phase, which is intended to significantly reduce the RMS current of the large capacitors (to 0.8 A instead of 1.3 A). This makes it possible to use two 39 µF/450 V capacitors instead of three.

-Electromagnetic Interference (EMI) Filter: The interleaved scheme also reduces the current ripple. For example, according to reference [5], in a typical wide mains voltage application, the peak-to-peak ripple varies between 0 and 60%. The reduced ripple simplifies differential mode filtering. As shown in Figure 3, the interleaved PFC uses a 10 µH inductor to pass the EN55022 specification, while the single-phase CrM (or FCCrM) PFC requires a 50 µH differential mode coil.

The FCCrM and CrM single-phase solutions use almost the same power components because they operate in the same way under heavy load conditions and the device parameters are selected for heavy load operation. However, as mentioned earlier, the inductor used in the FCCrM solution is smaller. Instead of two 200 µH EFD30 coils in series, a single 200 µH EFD30 coil is used (in the single FCCrM solution section). Obviously, the controller has also changed. The CrM solution uses the NCP1607 driver (see reference [4]). For convenience, no special controller was used to test the FCCrM single-phase solution. Instead, we reused the NCP1631 interleaved FCCrM controller used in the reference board and simply turned off the output driving the second branch to obtain single-phase FCCrM operation.

summary

Table 3 summarizes the design differences between the three solutions, which lists the main components that may be selected according to the selected solution, including the controller (the dedicated FCCrM controller NCP1605 is used instead of NCP1631 in the single-phase solution). Based on the cost advantages and disadvantages of these design differences, it can be seen that the interleaved solution is the most cost-effective solution. The single-phase FCCrM is the second lowest cost solution, while the traditional CrM solution is the most expensive! If the CrM solution is used as a reference, the advantages provided by the other solutions are summarized as follows (see Table 1):

Table 3-FCCrM single-phase solution uses one less EFD30 inductor

The -FCCrM interleaved scheme also reduces magnetics (using two EFD20s instead of two EFD30s), but further saves a 39 µF/450 V capacitor, allowing the use of smaller differential mode chokes and operation with smaller and cheaper MOSFETs and boost diodes.

It is difficult to calculate the exact cost advantage. However, still using the CrM solution as a reference and taking into account the cost structure of the (high-volume) consumer market, it can be roughly estimated that the interleaved PFC solution has a cost advantage of 0.5 US dollars, while the cost advantage of the (single-phase) FCCrM is halved.

FCCrM single-phase and interleaved solutions are cheaper overall, although the controllers used to drive them (NCP1605 and NCP1631, respectively) are more expensive. These two ICs integrate more functions than the NCP1607 CrM controller, such as input undervoltage protection, standby management functions, or a "pfcOK" signal that shuts down the downstream converter when the large voltage is not the rated value. These additional features can help the final application save components, thus further enhancing the cost advantage of FCCrM single-phase and interleaved solutions.

in conclusion

While single-phase CrM solutions are often considered the cheapest PFC solution for applications up to 200 W, this study shows that the FCCrM interleaved solution is actually the most cost-effective solution for the 190 W application we have shown. This conclusion is not surprising at all when we carefully consider its specific advantages. Interleaving requires more components, but they are smaller and less expensive. In addition, the reduced input and output current ripple also allows the use of cheaper EMI filters and large capacitors. Finally, FCCrM operation significantly reduces the size of the inductor, which makes the single-phase FCCrM solution superior to the single-phase CrM solution. Obviously, these findings are particularly applicable to low-profile (<13 mm) devices, but they still apply in other applications where there is more flexibility in component selection.

References

[1] “Reduce the inductor size, design of the thin PFC section”, Electronic Design Technology, September 2010, http://article.ednchina.com/Other/Reduce_the_inductor_size_the_design_of_the_PFC_power_supply_section_of_thin.htm

[2] Reference design, http://www.onsemi.cn/pub_link/Collateral/TND401-D.PDF

[3] NCP1631 data sheet, http://www.onsemi.cn/pub_link/Collateral/NCP1631-D.PDF

[4] NCP1607 data sheet, http://www.onsemi.cn/pub_link/Collateral/NCP1607-D.PDF

[5] “Interleaved PFC Characteristics”, Application Note AND8355, http://www.onsemi.cn/pub/Collateral/AND8355-D.PDF