EMI Effect of Synchronous Rectifying MOSFET in Quick Charger

小融一号

1.Introduction

The battery volume of smart phone and portable devices increases gradually from below 1000mAh to 3000mAh, even to about 5000mAh to meet the requirement for long time of usage. It will take much more time to charge the battery fully by the traditional charger of 5V/1A or 5V/2A with mini-USB interface ports. As a result, the quick charger with high output current is used in cell phone and portable devices to shorten to great degree the time of charging the high volume battery fully.

So far there are two types of fast charging the battery: traditional 5V with high current and high voltage with 2A or 2.5A current. The former, known as direct charging, flash charging or super charging, is safe at critical conditions and does not change the power structure inside smart phone due to common low voltage of 5V. But the charging current is limited by the USB cable resistance around 300mohm, which causes too much voltage drop and power loss at high current. The current of this solution is from 4A to 5A, 6A, even to 8A so it needs dedicated output wires with higher diameter and ports with wide contact area for the charger and smart phone. The latter increases output voltage of the charger, that is, the input voltage of the phone, at the fixed current based on the current micro USB cables and ports, for higher input power to the phone. Then, inside the phone, high input voltage is converted to 5V with much higher current to realize the fast charging to the battery. It is clear that this solution changes the power structure of smart phone inside and requires special communication protocols to make sure safe voltage is delivered to devices.

All parts of the quick charger with high output power are sealed inside the close plastic case. It is so demanding that the charger should have lower power loss, higher efficiency and lower temperature rise compared with the traditional charger. The traditional chargers of the phone use the diode as secondary rectifier with fly-back topology, which has high power loss due to high forward voltage dropout of the diode. In order to improve the efficiency and reliability, synchronous rectifier with MOSFET as the switching device is taking the place of passive rectifier with the diode.

The characteristics of the intrinsic diode inside MOSFET and the output capacitance Coss of power MOSFET is totally different from the fast recovery diode, which will cause many application issues. The EMI effect of radiation emission will be discussed and researched in this paper when MOSFET is used as the secondary rectifier. The rest, such as primary MOSFET and related circuits, the filter, etc. are not the subjects for this paper.

The fly-back converter can operate at discontinuous current mode DCM and continuous current mode CCM. Usually, thereis not the reverse recovery of the diode at DCM because secondary synchronous rectifying MOSFET, that is SSR MOSFET, is turned off when the output current decays near to zero and turned on at very small current. Accordingly, it has little effect on the radiation emission of EMI when fixing the rest of the fly-back converter. However, SSR MOSFET will undergo the reverse recovery of the diode at CCM so it will lead to very great effect on the radiation emission of EMI.

2.The schematics and SSR waveforms

The basic schematic of CCM fly-back charger is shown in Figure 1. The SSR MOSFET is put on the low side for easy driving due to the source of MOSFET connected to the output ground, which does not need high side boot strap floating driving. The external schottky diode and RC snubber network are placed in parallel with the SSR MOSFET.

The diode is used for improving the efficiency at the medium load. The time of SSR MOS turning off is based on the comparison between the reference of SSR controller and RDS(ON)*Is. If SSR MOSFET is turned off a little early, the paralleled schottky diode is on to increase the efficiency due to its low forward voltage.

From the waveforms, it is found that at thetime of SSR turn off, its current is not zero but has a certain value, that is, at CCM mode.

(a) The schematic

b) The waveforms

Figure 1: The schematic and waveforms

3.EMI design and consideration

The voltage spike at the time of SSR MOSFET turning off will be caused due to the diode reverse recovery and the parasitic inductance of the board and the device package. The dv/dt and voltage spike during the SSR MOSFET turning off will cause not only EMI issue but also reliability issue. The voltage spike will increase the possibility of overstress or avalanche at the worst condition. The common method for damping the voltage spike is to place RC snubber in parallel with SSR MOSFET, of course, which will decrease the efficiency of the system. The values of RC network should be optimized by the requirement of voltage de-rating and the efficiency. The values of RC network s in Figure 1 are selected be 10Ohm and 1nF.

The value of dv/dt will also greatly affect the EMI performance of the system. The higher the value of the capacitance is, the smaller the value of dv/dt. The lower dv/dt will be helpful to EMI but this is at the price of sacrificing the efficiency.

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The dv/dt and spike voltage are also related with the capacitance characteristics of Coss and the value of Coss of MOSFET, which also obviously affect the performance of EMI. The test of several MOSFETs from different vendors with different capacitance characteristics of Coss and the value of Coss are done and the worst results are shown in figure 2, even though the rating voltage is similar and the gap of their R DS(on) is not so big.

(a) Part 1 peak: 3.79dB  

(b) Part 2 peak: 5.54dB

(c) AON7262E peak: 7.04dB

Figure 2: The test result of EMI

The waveforms of VDS and the current of SSR MOSFET are shown in Figure 3. The test condition is 230V AC input and 5V, 3A output. It could be found that the value of dv/dt is greatly relevant with EMI results. From the test result, there are two peak points near to the limit: one is about 50MHz,the other is 80-100MHz.

The capacitance characteristic of Coss of AON7262E is tuned inside to give a smoother switching performance, and help suppress the the EMI noise, besides the value of Coss. It has suitable value of dv/dt and optimum capacitance characteristics of Coss, as a result the circuit was able to pass all limits at whole frequency band width with enough margins. The voltage spike is also limited to acceptable level.

Figure 3: The waveforms of VDS and ID

4.Conclusion

AON7262E,AO4262E and AON6262E are dedicated for SSR MOSFET in quick charger with thepackage of DFN3*3, SO8 and DFN5*6 and AO4262E respectively. The different packages give the customer more options that they can easily select based on their requirement of the system. They have low RDS(on) and excellent Qgd*RDS(on) product (FOM) to ensures on state and switching losses are minimized with AOS proprietary advanced AlphaSGT TM technology. They have optimized capacitance curve for better dv/dt, voltage spike and EMI results with softer body diode recovery and higher ESD capability, comparing with the two competitors.

AOS developed a series of 60V, 80V and 100V new MOSFET include AON6260, AON6262E, AON6284A, AON6220, and so on, They are specially applied in SSR MOSFET of fly-back topology quick charger with logic level (4.5V) drive. It is very easy for the customer to select from the portfolio of products with low RDS(on) ranging from 1.0m ohm to 13.5 mOhm and different packages options of SO8, DFN5*6 and DFN3*3.