原创 Reducing Emissions in the Buck Converter SMPS

Abstract

Switched Mode Power Supply demands are increasing, as the electronics industry requires more DC-DC conversion. In the past, linear regulators have been used to regulate power, but as the difference between supply voltages and desired output voltage increases, they become very inefficient. The BUCK power supply is efficient in converting higher voltages to lower voltages, but unfortunately in the process, both change in current (dI/dt) and change in voltage (dV/dt) are experienced. These changing parameters can cause excessive emissions in the RF spectrum, in conducted and radiated forms. We will examine the modes under which these emissions are allowed to propagate, and investigate techniques used to reduce them.

Introduction

One primary contributor to the low frequency emissions is the switching frequency of the converter, found typically in the 100’s of kHz range. Energy at the fundamental frequency along with several of its harmonics can find its way out onto the wire harness and radiate effectively. These emissions are derived from, among other things, the sudden changes in current flow (dI/dt) as a result of the regulator (SMPS IC in Figure 1) turning on and off during its periodic cycle.

Figure 1. Typical BUCK SMPS Circuit Diagram.

When the SMPS IC turns ON, the current flows through L1, SMPS IC, L2, and is delivered to the load (in parallel with C4). When the SMPS IC turns OFF, the current flowing through the SMPS IC stops. At this same moment, the energy stored in the inductor (L2) is released to the load, as the “free-wheeling” diode (CR1) begins conducting. It is this switching that creates current flow discontinuities at the input to the power supply. These current spikes in turn can drive the wire harness, attached to the product (Position 1 in Figure 1), like a transmitting antenna. Equation 1 can be used to calculate resonant frequencies (Hertz) of the cabling; substitute the length of the attached cable (meters) for l, and the speed of light (3×108 m/s) for C. Using 2, 4, and 20 times the length of the attached cable for l allows other resonant frequencies to be calculated. If any resonant frequencies of the cabling correspond with undesired RF frequencies coming from the Power Supply, the resonance can exaggerate the RF emissions problem.

l = C / f (1)

Equation 1. Wavelength.

Figure 2 shows discontinuities at the input to the power supply (measuring between VIN of the SMPS IC and ground) while the regulator is switching. Notice that the discontinuities at VIN correspond directly to sharp changes in the SMPS IC’s output voltage (measured between Figure 1 position 4, and ground).

Figure 2. Switching waveforms.

At the output of the regulator (position 4 in Figure 1), dur-ing switching states, two separate resonant RLC networks can be defined. These networks produce an under-damped response when “excited” by a step function (ie. switching!) and allow high frequency ringing to oscillate for several cycles. Broadband RF emissions commonly seen anywhere from 40 – 140 MHz are a direct result of this ringing. The R, L, and C components that make up the networks are defined by the path that the current flow takes during each switch state. RLC network #1 is formed when the SMPS IC output turns OFF. In this state the current flows from ground through CR1, L2, C4, and the LOAD. Each of these devices has impedance that is made up of R’s, L’s, and C’s (including the PCB layout traces and parasitics). When combined, these properties form the resonant network that gets “excited” by the step response of the switching (ON – OFF, or OFF-ON). RLC network #2 is formed when the SMPS IC output turns ON. In this state, the current flows from the OUT pin of the SMPS IC (Figure 1), through L2, C4, and the LOAD. Each of these two paths has a unique frequency response, and is tuned differently. An RC snubber circuit (R1 & C3) can be added in parallel to the output of the regulator to create a more “dampened” response in the two unique RLC networks. If values are chosen correctly, the snubber circuit will reduce the amplitude and number of cycles of the unwanted ringing. See waveforms in Figure 3 that illustrate this ringing. Note the frequency of the ringing is directly related to high frequency emissions seen during testing (see Figure 4 @ 60MHz).

Figure 3. Leading edge, ringing waveforms.

Ringing measured on the output of the regulator (position 4 in Figure 1) also affects the current flow through the output inductor, and causes a corresponding change in the magnetic field surrounding the device. This changing magnetic field can couple onto neighboring traces or planes allowing the RF energy to proliferate throughout the board.

Layout Considerations

One of the most important layout considerations for buck converters is to minimize parasitic capacitance and inductance at the output of the regulator. Parasitics contribute significantly to the ringing and other distortion on the output waveform. Do not place a ground plane on any layer of the board stack-up directly below inductor L2; this creates the parasitic capacitance that should be avoided. An option to consider would be to place a power bus (+) beneath inductor L2 if using a multi-layer board.

Loop Areas

Loop areas need to be controlled and minimized (physical area) in the layout to reduce overall emissions. A loop formed between the output pin of the SMPS IC and the ground on the load (connected across C4) contains large amplitude dI/dt waveforms and must be controlled to the fullest extent possible. Another loop is formed between position 1 and position 3 in Figure 1. Finally, a loop area formed between the input to the supply (position 1) (usually at the main connector) and the ground to the SMPC IC, contains small discontinuities (dV/dt and dI/dt). Loop areas are kept small through proper floor planning and routing of traces. Initially our output stage (CR1, L2, C4) was not properly designed and the loop area formed there was approximately 3 in2. Redesigning the output stage allowed us to reduce the area to 1.5 in2. This change brought improvements in the amount of cycles and amplitude of the unwanted ringing on the output (Figure 1 position 4). Although this was not the only change to the design, reducing this loop area had a direct correlation to reducing broadband emissions in the frequency range of 40MHz – 140MHz (Figures 4 & 5).

Feedback Trace

The feedback trace is used by the SMPS IC to sense the status of its output (connected between C4 and the feedback pin of the SMPS IC). Therefore, it is critical to route it away from any noisy circuitry, in particular, the output inductor L2. If using a multi-layer PCB, the feedback trace of the regulator can be embedded in a layer below the top layer (further down the board stack-up) and shielded by ground from the layers above and below. A ferrite may also be placed in series with this trace to reduce RF energy that can enter the SMPS IC and can adversely influence the output waveform.

Component Considerations

Whenever possible, use surface mount devices to minimize lead inductance. L2 should be a closed core inductor. This type of component keeps most of the magnetic field confined within the core during the switching of the regulator. Industry suppliers have various options to choose from. The best approach is to ask vendors for samples, and try each type during testing. Choosing the right device is a key factor in reducing emissions, since the field surrounding the inductor is constantly changing (i.e. ringing, switching current) and can couple to neighboring components and traces, etc.

Snubber Circuit

A snubber circuit can be used in parallel with the output of the SMPS IC (Figure 1 position 4 to ground) to reduce distortion and ringing on the output waveform. Typically this circuit is located between the “free-wheeling diode CR1,” and the output inductor L2. This circuit works like a high frequency shunt to allow RF energy to return to ground (i.e. capacitor to ground). One important factor to consider when choosing to use a snubber circuit is that you must sacrifice some efficiency in the power conversion. Some of the power will be dissipated in the snubber circuit itself. The series resistor is used to limit the amount of RF current taking this path to ground and the capacitor is used to “tune” the frequency. The switching waveform at the location of the snubber circuit is primarily a square wave, and Fourier theory states that it contains high frequency content that is dependent upon the rise time of the waveform. Consequently, without a series resistor in the snubber circuit, there will be significant power dissipation in the shunt capacitor. The snubber circuit can be effective at reducing broadband RF emissions typically seen between 40MHz and 140MHz. Typical values are R = 20 ohms, and C = .01uf. Also, low parasitic inductance components should be used to avoid forming resonant tank circuits. Therefore, avoid using wire-wound resistors, or leaded capacitors.

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