Application of Capacitance Multiplier Technology to Automotive EMI Filters | ECE 480, Study Guides, Projects, Research of Principles of Theater Design

Download Application of Capacitance Multiplier Technology to Automotive EMI Filters | ECE 480 and more Study Guides, Projects, Research Principles of Theater Design in PDF only on Docsity! Application of Capacitance Multiplier Technology to Automotive EMI Filters Dylan Constan-Wahl March 29th, 2007 Abstract: In the design of automotive EMI filters, large value, low ESR capacitors are often required to meet a desired insertion loss over a wide range of frequencies. By applying active capacitance technologies, these expensive capacitors may be replaced with small, lightweight, and inexpensive active circuits. At the same time, filter performance may be increased over passive designs, as the active capacitance has a lower ESR and higher resonance point than the equivalent passive component. Design considerations specific to the use of these active capacitors in the automotive environment are discussed. Keywords: Filter, EMI, Capacitor, Automotive, Noise Introduction Passive EMI filter designs have been used in automotive applications to reduce the conducted noise from wiper motors, radiator fans, power steering pumps, and other high-current, PWM (pulse width modulated) motor systems. Often, balanced or single-ended Pi networks are used in this application, because they allow. However, many of these motor controllers operate at low switching frequencies (in the low kilohertz range). The conducted emissions limits commonly used by the automotive industry (CISPR 25) require substantial attenuation at frequencies as low as 150kHz. To meet these requirements, the filter must utilize high capacitance, low ESR (equivalent series resistance) capacitors which are rated to handle the substantial ripple currents near the fundamental frequency of the PWM controller. These capacitors contribute substantially to the overall filter cost. By substituting active capacitance multiplier circuits for these expensive capacitors, the filter’s cost and size may be reduced substantially. In addition, the filter’s performance may be increased, as the SRF (self-resonant frequency) and ESR are lower than is economically achievable with passive components. Ideal Capacitance Multiplier An idealized capacitance multiplier circuit is shown in Figure 1. Assuming that the resistances R1 and R2 are matched, the output behaves as an equivalent capacitance and equivalent series resistance as shown in Figure 2. R2 R1 R3 C1 C X1 Figure 1 – Schematic of Ideal Capacitance Multiplier R3 C1(R1/R3) C Figure 2 – Equivalent Capacitance Model The capacitance multiplier as shown was designed for instrumentation purposes, and not for use in power filtering circuits. The limitations of this circuit soon Page 2 of 9 as series capacitance coupling of the output would defeat the purpose of the capacitance multiplier. Also note that the buffer must have low noise, low output impedance, and low DC offset to prevent output device saturation. For the purposes of this paper, a Class AB, push-pull output buffer was used; more details on buffer design are available in the References section. Figure 5- Output Impedance against Frequency for the Ideal Capacitance, Multiplier, and Buffered Multiplier Frequency 100KHz 300KHz 1.0MHz 3.0MHz 10MHz V(1)/I(Is) 100m 300m 1.0 3.0 10 Spice Deck for Figure 5 ECE480 active cap ths4001 model Is 1 0 AC 1 R1 1 4 330 R2 2 3 330 R3 1 2 .1 X1 4 3 6 7 2 THS4001 Vp 6 0 14 Vn 7 0 -14 C1 4 0 2.2n .LIB THS4001.lib .ac dec 200 100k 10meg .probe .end ECE480 active cap ths4001 plus buffer model Is 1 0 AC 1 R1 1 4 330 R2 2 3 330 R3 1 2 .1 X1 4 3 6 7 22 THS4001 X2 22 6 7 2 buffer Vp 6 0 14 Vn 7 0 -14 C1 4 0 2.2n .SUBCKT buffer 1 2 3 4 * transistors Q1 2 1 6 Q2N3904 Q2 3 1 5 Q2N3906 Q3 3 6 8 Q2N3906 Q4 2 5 7 Q2N3904 * resistors R1 6 3 1k R2 2 5 1k R3 7 4 2 R4 8 4 2 *output is on 4 Page 5 of 9 .ENDS buffer .LIB THS4001.lib .LIB ECE402.LIB .ac dec 200 100k 10meg .probe .end ECE480 active cap equivalent passive model Is 1 0 AC 1 Rc 1 3 1G C1 1 3 6.6u R1 3 0 0.1 .ac dec 200 100k 10meg .probe .end To test the performance of this circuit in an automotive EMI filter, the design in Figure 6 was constructed. The inductor L1 is a 2.8uH solenoid, with low ESR. The capacitances C1 and C2 are active capacitance multiplier circuits as shown. The circuit was tested using a lab power supply; the filter was matched to 50 ohm source and load impedances. Figure 7 is a plot of the filter transmission loss against frequency over the range of interest. In a final design, the load and source impedances may drop significantly lower, on the order of 100 milliohms; hence, other circuit characteristics such as op amp and resistance noise will further limit the filter performance. Also, lower impedance (higher Beta and imax) output transistors will be required for the unity buffer to provide a low enough ESR for proper operation, as well as a low output impedance, high current, low noise switching supply, to provide a positive voltage rail for the op amp and buffer circuits. Figure 8 demonstrates the effect of device output impedance on filter insertion loss; the source and load impedance are varied over two decades; as shown, this restriction is extremely important to filter performance. Figure 6 – Schematic of Test Pi Filter using Capacitance Multipliers C1 L1 C2 RS RL VS Figure 7 – Plot of Test Pi Filter Forward Transmission Loss Page 6 of 9 Forward Transmission Measurement of Active PI Filter -60 -50 -40 -30 -20 -10 0 Frequency (Hz) |S 21 | ( d B ) Spice Deck for Figure 8 ECE480 pi filter simulation - 50 ohm Vs 1 0 AC 1 Rs 1 2 50 L1 2 3 2.812u Rl1 2 3 972.7k Cl1 2 3 840.64f Rl 3 0 50 X1 2 activecap X2 3 activecap .subckt activecap 1 R1 1 4 330 R2 2 3 330 R3 1 2 .1 X1 4 3 2 opamp C1 4 0 1n .ends activecap .subckt opamp 1 2 3 Ri 1 2 100meg Ea 3 0 1 2 100meg .ends opamp .ac dec 200 100k 10meg .probe .end ECE480 pi filter simulation - 5 ohm Vs 1 0 AC 1 Rs 1 2 5 L1 2 3 2.812u Rl1 2 3 972.7k Cl1 2 3 840.64f Rl 3 0 5 X1 2 activecap X2 3 activecap .subckt activecap 1 R1 1 4 330 R2 2 3 330 R3 1 2 .1 X1 4 3 2 opamp C1 4 0 1n .ends activecap .subckt opamp 1 2 3 Ri 1 2 100meg Ea 3 0 1 2 100meg .ends opamp .ac dec 200 100k 10meg .probe .end ECE480 pi filter simulation - .5 ohm Vs 1 0 AC 1 Rs 1 2 .5 Page 7 of 9