How to reduce MOSFET losses and improve EMI performance?

This paper mainly expounds the application of MOSFET in module power supply, analyzes the characteristics of MOSFET loss, and proposes an optimization method; and expounds the relationship between the optimization method and EMI.

This paper mainly expounds the application of MOSFET in module power supply, analyzes the characteristics of MOSFET loss, and proposes an optimization method; and expounds the relationship between the optimization method and EMI.

I. Introduction

As one of the main switching power devices, MOSFET is widely used in module power supply. Understanding the loss composition of MOSFET and analyzing it is beneficial to optimize the loss of MOSFET and improve the power of the module power supply; however, blindly reducing the loss of MOSFET and other aspects will cause more serious EMI problems, resulting in the inability of the entire system to work stably . Therefore, it is necessary to take into account the EMI performance of the module power supply while reducing the loss of the MOSFET.

Second, the power consumption analysis of switch MOSFET

How to reduce MOSFET losses and improve EMI performance?

The loss of MOSFET mainly consists of the following parts:

1. On-state loss; 2. Turn-on loss; 3. Turn-off loss; 4. Drive loss; 5. Absorption loss;

As the size of the module power supply decreases, the switching frequency needs to be further increased, which leads to an increase in turn-on loss and turn-off loss. For example, at a driving frequency of 300kHz, the ratio of turn-on loss and turn-off loss is already the main part of the total loss.

Losses will occur during the turn-on and turn-off of MOSFETs. During these two conversion processes, the relationship between drain voltage and drain current, gate-source voltage and charge is shown in Figure 1 and Figure 2. The conversion process is analyzed as an example:

t0-t1 interval: the gate voltage rises from 0 to the threshold voltage Uth, the switch is turned on, and no drain current passes through this interval without causing loss;

t1-t2 interval: the gate voltage reaches Vth, the drain current ID starts to increase, and reaches the maximum value at t2, but the drain-source voltage remains at a high level when it is turned off. It can be seen from Figure 1 that this part has VDS and ID overlaps, MOSFET power consumption increases;

t2-t3 interval: From t2 time, the drain-source voltage VDS begins to drop, causing the Miller capacitance effect, so that the gate voltage cannot rise and a plateau appears. Minimum value; this part has VDS and ID overlap, MOSFET power consumption increases

t3-t4 interval: the gate voltage rises from the platform to the final drive voltage (the module power supply is generally set to 12V), the rising gate voltage further reduces the on-resistance, and the MOSFET enters a fully-on state; at this time, the loss is converted into conduction. pass loss.

The turn-off process is similar to the turn-on process, except that the waveform is opposite; there are many literatures for the analysis process of the turn-on loss and turn-off loss of MOSFET. Here, the summary formula of “MOSFET Analysis by Zhang Xingzhu” is directly quoted as follows:

How to reduce MOSFET losses and improve EMI performance?

Note: is the rise time, is the switching frequency, is the fall time, is the gate charge, is the gate drive voltage and is the MOSFET body diode loss.

Third, the loss optimization method of MOSFET and its advantages and disadvantages

1. Reduce MOSFET losses by lowering the drive frequency of the module power supply[稍微提一下EMI问题及其解决方案]

It can be seen from the loss analysis of MOSFET that the higher the driving frequency of the switching power supply, the corresponding increase in the conduction loss, turn-off loss and driving loss, but the high frequency can make the transformer core of the module power supply smaller and the module volume It becomes smaller, so turn-on loss, turn-off loss and drive loss can be optimized by switching frequency, but high frequency will cause serious EMI problems.

The frequency hopping control method is adopted to reduce the driving loss by reducing the switching frequency of the module power supply under light load conditions, thereby further improving the efficiency under light load conditions, making the system more energy-saving under standby operation, and further improving the battery power supply system. working time, and can also reduce EMI radiation problems;

How to reduce MOSFET losses and improve EMI performance?

2. Reduce the loss of MOSFET by reducing,

Typical low-power module power supplies (less than 50W) mostly use a flyback circuit topology, and the typical control circuit is shown in Figure 3; from the loss analysis of MOSFETs, it can also be known that it is proportional to the turn-on loss, and is proportional to the turn-off loss. Therefore, the loss of MOSFET can be reduced by reducing , and usually, the drive resistance Rg of MOSFET can be reduced to reduce , time, but this optimization method brings serious EMI problems; Jinshengyang URB2405YMD-6WR3 product is Example to illustrate this issue:

1) URB2405YMD-6WR3 adopts 10Ω MOSFET drive resistance, and the bare metal radiation test results are as follows:

How to reduce MOSFET losses and improve EMI performance?

2) URB2405YMD-6WR3 adopts 0Ω drive resistance, and the bare metal radiation test results are as follows:

How to reduce MOSFET losses and improve EMI performance?

From the test results of the two different driving resistances, although they can pass the CLASS A level of the radiation disturbance degree of EN55022, using a driving resistance of 0 ohms, the margin of the test results in the horizontal polarization direction is less than 3dB. The scheme design cannot be approved.

3. Reduce losses by reducing losses in absorption circuits

In the design process of the module power supply, the leakage inductance of the transformer always exists. With the flyback topology, there is often a large voltage spike on the drain of the MOSFET when the MOSFET is turned off. The voltage design margin is enough to bear. In order to improve the overall power supply efficiency, some power supply manufacturers do not increase the absorption circuit

(The absorption circuit is marked with ① RCD absorption circuit and ② RC absorption circuit as shown in Figure 3) to absorb the peak voltage. However, not paying attention to the design of these snubber circuits is often a major cause of substandard EMI designs. Take the absorption circuit of Jinshengyang URF2405P-6WR3 (using ②RC absorption circuit in Figure 3) as an example:

1) The driving resistance Rg is 27Ω, and there is no RC absorption circuit. The test results of radiation disturbance are as follows:

How to reduce MOSFET losses and improve EMI performance?

2) The driving resistance is 27Ω; the absorption circuit is resistance R and C 5.1Ω 470pF, and the test results of radiation disturbance are as follows:

How to reduce MOSFET losses and improve EMI performance?

From the test results of the two different absorption circuit schemes, the scheme without the absorption circuit cannot pass the CLASS A level of the radiation disturbance degree of EN55022, while the absorption circuit can solve the problem of failing the radiation disturbance degree test. Different RC combinations can further reduce radiation disturbance.

4. Summary

The power consumption optimization of MOSFET is actually a system engineering, and some optimization schemes even affect the characteristics of EMI. In the above case, the overall efficiency and EMI characteristics of the power supply are balanced, thereby further optimizing the power supply parameters. The power supply parameters are further optimized to be more compatible with customer systems, and play the role of the real “heart” of the Electronic system to continuously deliver energy.

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