How to use slew rate to control EMI in automotive and industrial applications

Synchronous buck converter power topologies are used in many industrial and automotive applications; these applications also require low conducted and radiated emissions to ensure that the power supply does not interfere with other devices sharing the same bus (input voltage [V _IN ]). For example, in automotive infotainment systems, electromagnetic interference (EMI) can cause annoying noises in the car stereo.

Figure 1 shows the schematic of a synchronous buck converter and its switch node waveform. The switching speed of the high-side MOSFET and the high-side/low-side MOSFET and printed circuit board (PCB) stray inductance and capacitance all have the function of ringing when the switch node waveform reaches its peak. Switch node waveform ringing is undesirable because it increases the voltage stress of the low-side MOSFET and generates electromagnetic interference.

To determine the relationship between the switch node ringing of the buck converter in Figure 1 and the EMI it generates, I performed conducted emissions testing in accordance with the International Special Committee on Radio Interference (CISPR) 25 Class 5. Figure 2 shows the results of the test. The measured data shows that the buck converter’s conducted emissions are 15dBµV higher than the Class 5 limit over the frequency range of 30MHz-108MHz.

To reduce electromagnetic radiation, the first step is to reduce the ringing noise of the switch node. There are several ways to do this: First, slow down the turn-on and turn-off times of the MOSFET, thereby controlling the rise and fall times of the switch node. This function can be achieved by adding a series resistor (R _HO and R _{LO ) to the gate lead of the MOSFET; see Figure 3. The second step is to add a buffer (R SUB} and C _SUB ) between the switch node and ground . The buffer circuit can suppress parasitic inductance and capacitance during the switching transition.

In addition to using the above method to reduce the switch node ringing noise, there is another method, which is to use the LM5140-Q1 synchronous buck controller that meets the requirements of automotive applications. An important feature of the LM5140-Q1 is the slew rate control. By bringing out the source and sink side leads of the driver, the turn-on and turn-off time of the high/low side MOSFETs can be independently controlled.

During the time when the low-side MOSFET turns off and the high-side top MOSFET turns on, the switch node voltage rises from ground to V _IN . If the high-side top MOSFET turns on too quickly, the switch node voltage will overshoot during the transition. Increasing the R _{HO resistor reduces the drive current of the high-side MOSFET, slowing down the turn-on time of the MOSFET and helping to reduce switch node ringing noise. Note: Slowing down the turn-off time of the high-side MOSFET increases switching losses. There is a trade-off when selecting R HO} between low EMI and switching losses of the high-side MOSFET .

The low-side MOSFET loss includes R _DS(ON) loss, dead time loss, and the loss of the MOSFET's internal body diode. When no-load (both high-side and low-side MOSFETs are off), the low-side MOSFET's internal body diode conducts the inductor current. Generally, the MOSFET's internal body diode has a high forward voltage drop, so its efficiency is greatly reduced. Reducing the time that the low-side MOSFET's internal body diode conducts current can improve efficiency.

Slew rate control allows a resistor (R _OL ) to be inserted between the LM5140-Q1 driver output (LO pin) and the low-side MOSFET gate to extend the time it takes for the low-side MOSFET to turn off. Slowing down the turn-off time reduces the dead time that both the low-side and high-side MOSFETs conduct, improving the efficiency of the buck converter. When reducing the dead time of a synchronous buck, do not conduct the high-side and low-side MOSFETs at the same time.

I modified the power supply shown in Figure 1 using the LM5140-Q1 controller (see Figure 4). I used slew rate control to optimize the rise and fall times of the switch node and eliminate switch node ringing noise.

The next step is to perform CISPR 25 Class 5 conducted emissions. I selected the following slew rate control resistor values: R _HO = 10Ω, R _HOL = 0Ω, R _LO = 10Ω and R _LOL = 10Ω. The resistors chosen for this application are a good starting point for any application with input power less than 50W.

Figure 5 shows the results and summary of the conducted emissions test.

The buck converter reduces conducted emissions by 21dBµV with the help of the LM5140-Q1 and slew rate control. It also provides enhanced control over the rise and fall of the switch node while eliminating the need for a snubber circuit, reducing circuit complexity and cost.

Picking the right value for the slew rate control resistor not only reduces EMI but also increases the efficiency of the system at the same time.