Low EMI automotive power designs: Page 4 of 7

February 05, 2015 //By Jeff Gruetter
Low EMI automotive power designs
Jeff Gruetter, Product Marketing Engineer, Linear Technology Corporation considers low EMI automotive power design issues.
monolithic alternative. Fortunately, new power management ICs that can offer both high voltage and high efficiency via internal synchronous rectification is now found in the marketplace.

Smaller Power Conversion Circuits

There are a few ways to make power conversion circuits smaller. In general, the largest components in the circuit are not the power IC, but the external inductor and capacitors. By increasing the switching frequency of an IC from 400kHz to 2MHz, the size of these externals can be dramatically reduced (about 4x smaller solution footprint). But in order to do this effectively, the power IC must deliver high efficiency at higher frequencies, which historically has not been feasible. However, using new process and design techniques, synchronous power ICs have been developed that deliver efficiencies in excess of 95%, while switching at 2MHz. The high efficiency operation minimizes power loss, eliminating the need for heat sinks. It also has the added benefit of keeping switching noise out of the AM frequency band. 

“Always-On” Systems Need Ultra-Low Supply Current

 Many electronic subsystems are required to operate in “standby” or “keep alive” mode, drawing minimal quiescent current at a regulated voltage while in this state. These circuits can be found in most navigation, safety, security and engine management electronic power systems. Furthermore, each of these subsystems can use several microprocessors and microcontrollers.  Most luxury cars have over 150 of these DSPs onboard and approximately 20% of these require always on operation. In these systems, the power conversion ICs must operate in two different modes. First, when the car is running, the power conversion circuits that power these DSPs will generally operate at full current fed by the battery and charging system.

However, when the car ignition is off, the microprocessors in these systems must be kept “alive,” requiring their power ICs to provide a constant voltage while drawing minimal current from the battery. Since there can be upwards of 30 of these

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