Terry Allinder

Many industrial and automotive applications use the synchronous buck converter power-supply topology; these applications also require low conducted and radiated emissions to ensure that the power supply does not interfere with other equipment sharing the same bus (input voltage [V_IN]). For example, in an automotive infotainment system, electromagnetic interference (EMI) could result in unwanted noise in the car stereo.

Figure 1 shows a synchronous buck-converter schematic, along with its switch-node waveform. The ringing on the peak of the switch-node waveform is a function of both the high-side MOSFET’s switching speed and the high-side and low-side MOSFETs and printed circuit boards (PCB) stray inductance and capacitance. The ringing on the switch-node waveform is unwanted because it can increase the voltage stress on the low-side MOSFET and generate EMI.

To determine the relationship between the switch-node ringing of the buck converter in Figure 1 and the EMI that it generates, I ran conducted emissions testing in accordance with Comité International Spécial des Perturbations Radioélectriques (CISPR) 25 Class 5. Figure 2 shows the results. The measured data shows that the buck converter’s conducted emissions exceed Class 5 limits by 15dBμV in the 30MHz-108MHz frequency range.

The first step to lower the EMI is to reduce the switch-node ringing. There are several methods: the first is to slow down the MOSFET’s turn-on and turn-off time, which controls the rise and fall time of the switch node. You can do this by adding resistors in series (R_HO and R_LO) with the MOSFET’s gate leads; see Figure 3. A second method step is to add a snubber (R_SUB and C_SUB) from the switch node to ground. The snubber circuit dampens the parasitic inductances and capacitances during switching transitions.

An alternative to using the methods describe above to reduce switch-node ringing is to use the LM5140-Q1 automotive-qualified synchronous buck controller. One of the key features of the LM5140-Q1 is slew-rate control. By pinning out the driver’s source and sink leads, you can independently control the turn-on and turn-off time of both the high- and low-side MOSFETs.

During the period when the low-side bottom MOSFET is turning off and the high-side top MOSFET is turning on, the switch-node voltage rises from ground to V_IN. If the high-side top MOSFET turn-on time is too fast, there is a high overshoot in the switch-node voltage during this transition. Increasing the R_HO resistor decreases the drive current to the high-side MOSFET, slowing down this MOSFET’s turn-on time and helping reduce switch-node ringing. Note that slowing down the high-side MOSFET’s turn-off time will increase the switching losses. You will need to make a trade-off when selecting R_HO between low EMI and the high-side MOSFET’s switching losses.

The losses in the low-side MOSFET include R_DS(ON) losses, dead-time losses and losses in the MOSFET’s internal body diode. During the dead time (when the high- and low-side MOSFETs are turned off) the low-side MOSFET’s internal body diode conducts the inductor current. The MOSFET’s internal body diodes typically have a high forward-voltage drop, so there can be a significant reduction in efficiency. Reducing the time that the low-side MOSFET’s internal body diode conducts current improves efficiency.

Using slew-rate control, a resistor (R_OL) can be inserted between the LM5140-Q1 driver output (LO pin) and the low-side MOSFETs gate to increase the time that it takes for the low-side MOSFET to turn-off.  Slowing down the turn-off time decreases the dead time between the low- and high-side MOSFETs’ conduction, increasing buck-converter efficiency. When reducing the dead time for a synchronous buck, make sure that the high- and low-side MOSFETs never conduct at the same time.

I modified the power supply shown in Figure 1 using the LM5140-Q1 controller (see Figure 4). Using slew-rate control optimizes the switch-node rise and fall time, eliminating switch-node ringing.

The next step is to run the CISPR 25 Class 5 conducted emissions. I chose these slew-rate control-resistor values: R_HO = 10?, R_HOL = 0?, R_LO = 10? and R_LOL = 10?. The resistors I selected for this application are a good startup point for any application where the output power is under 50W.

Figure 3 shows the results and summary of the conducted emissions testing.

The buck converter using the LM5140-Q1 with slew-rate control reduced conducted emissions by 21dBμV. It also provided better control of the switch-node rise and fall and eliminated the need for a snubber circuit, which adds circuit complexity and cost.

By picking the correct values of slew-rate control resistors, not only can you reduce EMI; you can also improve system efficiency.

See a live demonstration of the LM5140-Q1 buck converter in TI’s booth (No. 1617) at the Applied Power Electronics Conference (APEC), March 21-23 in Long Beach, California. Follow TI at www.ti.com/apec2016.

Additional Resources

• Watch the video “Engineer it: How to use slew rate control for EMI reduction.”
• See the LM5140-Q1 data sheet.
• Open the “LM5140-Q1 Evaluation Module User’s Guide.”