Eric Lee
The start-stop system in hybrid electric vehicles (HEVs) helps reduce fuel consumption and emissions by stopping the engine during idling, but the battery voltage drops whenever the engine restarts. To provide the minimum required voltage to the loads during the battery voltage drop, pre-boost converters are widely used in automobiles.
Table 1 shows the typical requirements for a pre-boost converter.
Requirements | |
---|---|
Input voltage | Peak 40V, typical 12V, minimum 2.5V |
Output voltage | Maximum 40V, minimum 8.5V |
Output current | From 2A to 4A |
In this blog post, I’ll explain the key parameters which affect the output-voltage undershoot of the pre-boost converter when the battery voltage drops.
According to the traditional small-signal analysis, there are two important parameters which affect the output undershoot: the loop response of the converter and the output impedance of the power stage. Table 2 shows the factors that optimize the loop response and output impedance.
Key parameters | Factors for performance improvement |
---|---|
Fast loop response |
|
Low output impedance |
|
In addition to the two parameters in small-signal analysis, three more parameters in large-signal analysis affect the output undershoot: the error amplifier to pulse-width modulator (PWM) comparator offset, the error amplifier sourcing capability and the wake-up delay. Because all three parameters are device-dependent and none are user-adjustable or user-programmable, device selection is very important when designing a pre-boost converter.
Traditional boost devices have about 1.2V offset from the error-amplifier output to the input of the PWM comparator (see Figure 1). Because the device cannot start switching until the error amplifier output is greater than the offset voltage, minimizing this offset is a key factor to improve undershoot during the battery voltage drop.
Sometimes the undershoot increases due to the limitation of the error amplifier. Ideally, the gain of the error amplifier should be constant over the typical error amplifier operating range, but the gain drops if the error amplifier sourcing capability is insufficient.
Because a pre-boost converter usually falls into a low quiescent current (IQ) standby mode to minimize battery drain when the battery voltage is in normal range, it takes some time to wake up the device from a low-IQ standby mode. Because the device cannot start switching until it wakes up, excessive undershoot can occur during a long wake-up delay.
The LM5150-Q1 is a 2.2MHz automotive boost controller that features ultra-low IQ in standby mode. Specifically designed for use in start-stop systems as a pre-boost converter, the device has only 0.3V offset and a strong transconductance error amplifier. The wake-up delay is less than 6µs, which is among the fastest in the industry.
Figure 2 is a comparison between a traditional boost converter and the LM5150-Q1. While the output undershoot using the traditional boost conversion is big and greatly affected by a long wake-up time, a large offset and an insufficient error amplifier sourcing capability, the output undershoot using the LM5150-Q1 is small and minimized. The traditional boost converter’s parameters are set to 1.2V offset, with a 100µA source current limit, a 2mA/V transconductance error amplifier gain and a 50µs wake-up delay.
A pre-boost converter’s output undershoot is affected by device selection. The output undershoot using the LM5150-Q1 is minimized by its small offset, large error amplifier sourcing capability and a quick wake-up time.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2023, Texas Instruments Incorporated