APEC: Y-capacitance multiplier IC shrinks EMC inductors
Author: EIS Release Date: Mar 27, 2023
At APEC in Florida, Texas Instruments introduced a family of ICs designed to shrink the size of EMC anti-emission common-mode filters in single and three-phase mains and automotive power converters.
TI TPSF12C1-Q1 active EMC filter app
In principal, the ICs implement a capacitance multiplier circuit to emulate the Y-capacitors in a conventional passive filter design.
They do while sitting amongst smaller filter inductors and capacitors (top diagram) while summing the high-frequency components of the voltages from the two or four ac power conductors, and then injecting an anti-phase ac current derived from these signals back into the neutral.
TI conventional two-stage input emc filter for switching psuCompare the active circuit with this conventional passive EMC filter circuit
“The effective active capacitance is set by the circuit gain and the injection capacitance,” according to TI. “The active EMC filter sensing and injection impedances use relatively low capacitance values with small component
footprints.”
[Scroll down to the bottom of this article for a discussion on where exactly currents flow in this scheme]
TI active EMC filter response graphWhat ever the mechanism, the result, said TI, is a low impedance path for common-mode noise enabling “15 to 25dB of CM noise reduction over, for example, 150kHz to 3MHz, helping to reduce the size of common-mode chokes”. It also said: “as much as 30dB at between 100kHz and 3MHz” and that they are capable of “helping to meet CISPR 25 Class 5 EMI limits for conducted and radiated emissions”.
TI TPSF12C1-Q1 active EMC filter blockThe difference between single and three phase versions is only the number of inputs provided to sense voltages – two or four respectively (diagram left, the three-phase version has four input load resistors). All of these signals are summed internally into a single ac signal, which is filtered appropriately before being re-injected.
The sense and injection capacitors (see diagrams) have to be Y-rated components.
The other passive components on the output are for damping – to manage resonance between the still-needed common-mode choke inductance and the injection capacitance – which appear in the active loop gain as a pair of complex zeros.
There are four devices: TPSF12C1 and TPSF12C3 for single- and three-phase commercial applications, then TPSF12C1-Q1 and TPSF12C3-Q1 for automotive use. Volume production is scheduled for the second quarter of 2023, with additional active EMI filter ICs appearing later this year.
Operation is over 8 to 16V (18V withstood) and up 105°C ambient (150°C junction).
Protections include under-voltage lock-out, and thermal shut-down, and there is an enable pad.
They “meet IEC 61000-4-5 surge immunity requirements, minimising the need for external protection components such as transient voltage suppression diodes”, said TI.
Packaging is 4.2 x 3.3mm 14pad SOT-23
Applications are foreseen in on-board chargers, servers and uninterruptable power supplies.
This application note has the clearest information about these ICs and includes an example of inductor size reduction.
Electronics Weekly is wondering exactly how this scheme works – for example, where the return loop for the injected current is. We will be talking to TI about it next week, so watch this space – and if you have any questions for TI regarding this IC before 27mar2023, please post them below in the comments.