FEATURE AUTOMOTIVE
HOW TO EFFICIENTLY GENERATE HIGH VOLTAGE power rails in automotive applications
Jerome Johnston, applications engineer at Intersil Corporation examines a pair of easy-to-use 2- phase 55V synchronous boost controllers that efficiently generate power rails of 24V, 36V or 48V in an automotive environment where only a 12V supply is available
W
hile the 12-volt lead acid battery is still the dominant power source in
automobiles, there are new automotive applications that require higher voltages such as trunk audio power amplifiers and in-glass window defrosters. To address these high voltage applications, a new generation of AEC-Q100-qualified synchronous boost controllers has emerged. The controllers are designed to boost the 12-Volt battery Voltage, withstand voltage spikes as high as 60 Volts and deliver the robust reliability required in new car models.
BOOSTING THE 12V BATTERY A constant challenge for system designers is how to achieve higher power efficiency while minimising circuit board space. The ISL78227 and ISL78229 55V synchronous boost controllers from Intersil Corporation address this issue by integrating advanced FET drivers that adaptively adjust switching times to prevent cross conduction while simplifying power-stage design. It’s common for buck converters to use a FET in
place of a diode for the output rectifier function because most buck converters deliver low output voltages. In this configuration, the voltage drop across the rectifying element represents a high proportion of the power lost to produce the output voltage. Replacing the output rectifier diode with a sync FET that switches on and off at the right time greatly enhances efficiency. This is because the FET losses are typically a fraction of the loss in the rectifier diode. In the buck converter, the sync FET is ground referenced and therefore the drive circuitry is relatively simple. The sync FET brings several benefits to the
boost configuration. In a boost converter application, the output voltage is usually several times greater than the input oltage so the losses in the output rectifier element are not as high of a percentage of the total output power. While the boost converter benefits from the sync FET efficiency improvements, the sync FET provides bidirectional current flow, which allows continuous mode operation, even at light loads – a key benefit for applications that require low electromagnetic interference (EMI). Additionally, the use of a sync FET does not
preclude operation in discontinuous mode. The boost controllers can detect negative current flow and can optionally disable the sync FET to emulate the function of the synchronous rectifier diode. It is common for audio signals to change over a
14 JUNE 2016 | ELECTRONICS Figure 1:
Efficiency vs. load, 2-phase boost, three modes operation, fSW=200kHz, VIN=12V, VOUT=36V, TA=+25°C
conduction mode (CCM) operation. But of course, we would then sacrifice the efficiency improvement brought by diode emulation, as shown in Figure 1. In applications like audio amplifiers, an alternative method of attaining light-load efficiency improvements is to have the power supply to the amplifier track the demands of the input with an envelope tracking capability. Many power system applications require the converter's switching frequency to be held constant to minimise the likelihood of interference. Because of this requirement, the ISL78227 and ISL78229 also operate in PWM mode (no pulse skipping). However, in forced PWM mode there are conditions when reverse currents may flow, such as when starting up into a pre-biased output, or anytime the output voltage is boosted to a higher voltage than expected. In a typical system, there is no means to limit the reverse current and this can damage the sync FET. The ISL78227 and ISL78229 overcome this issue
Figure 2:
Two devices wired to support 4-phase operation for higher- power applications
wide magnitude in very short periods of time. One moment the amplifier may need a burst of high power, the next moment it may be very low. The audio may even go silent between audio sessions. When this occurs, the amount of power used in the amplifier drops significantly, and because of this, the power demand on the boost regulator will also drop to a low value. In fact, under light loads the boost inductor current can go to zero. When this occurs, the voltage across the inductor has a higher voltage on its output (the boost voltage) than on its input (the battery voltage). If the sync FET remains on under this condition, current can start flowing backward through the inductor taking charge from the output capacitor. The 55Vboost controllers include circuitry that can optionally avoid this reverse conduction loss by making the sync FET emulate the current- blocking behavior of a real diode. This smart- diode operation is called diode emulation mode (DEM) and functions to turn the sync FET off when the circuitry senses that the inductor current is starting to flow in the wrong direction. If the controller enters into the diode emulation mode and the load is still reducing, the controller will enter into a pulse skipping mode to reduce the number of switching cycles to improve efficiency under extreme light loads on its output. While DEM can improve efficiency at light loads,
it can also present some EMI challenges due to the changing switching characteristics. To avoid EMI issues, it’s often desirable to maintain continuous-
by including a reverse current limiting feature. Limiting the negative current reduces output voltage transients and enhances system reliability. Therefore, the designer can configure the boost controllers in forced PWM mode and not worry about reverse currents getting out of control. The ISL78227/29 sync boost controllers support 2-phase boost operation, enabling two devices to be connected together to achieve four-phase operation (see Figure 2). At heavy loads, the main system losses are due to conduction losses and switching losses, but at lighter loads, the switching losses begin to dominate. To improve efficiency, both controllers can be configured to monitor the magnitude of the system current. If the load drops below a certain threshold, the controller can drop a phase. This reduces switching losses under light load conditions. The phase shedding process is done over 15 switching cycles to prevent a load transient. If the load then increases back above the threshold, the phase is added instantly to manage the increased load. The boost controllers’ output voltage can be scaled
using the 1.6-volt on-chip voltage reference, or they can be scaled to an external tracking voltage, which drives the control loop. With these two devices the external signal used to drive the tracking function is configurable as either an analogue voltage or a PWM signal. These TRACK features support the varying of the output boost voltage dynamically.
Intersil Corporation
www.intersil.com T: +1-408-432-8888
/ ELECTRONICS
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