Faced with wide supply voltage variations, how do you reduce cost and complexity of your wide input range DC/DC solution, yet maximize its performance and reliability? Take, for example, the automotive powertrain with the addition of start-stop technology: It sees a widely-varying voltage profile (cf. Figure 1), requiring either a pre-boost or buck-boost power stage that must also cater to the battery’s load dump overvoltage transient, 40V or even higher.
Figure 1: Automotive cold-crank waveform example
As more demanding wide VIN applications come to fruition, proper DC/DC converter power stage and control loop design is mandatory to fend off the challenges posed by large input voltage disturbances along with expected load current transients. Fortunately, classic current-mode control is well suited to wide VIN power converter solutions, offering key advantage in terms of ease-of-use, feature integration, high current scalability, and increased performance.
As such, the practicing power electronics engineer should have a good understanding of current-mode control. With that in mind, I recently wrote a two-part article series, "Current-mode Control Stability Analysis for DC/DC Converters," that delves into this topic in more depth.
Figure 2: DC/DC synchronous buck converter schematic with peak/valley current-mode control.
Leveraging the DC/DC Converter’s Control Loop for Wide VIN Performance
Including bandgap reference, error amplifier, and PWM comparator, the structure of the current-mode control loop (cf. Figure 2) is quite similar to that of a voltage-mode control loop with the fundamental difference of the addition of an inner, wide-bandwidth current loop. Peak, valley, and emulated current-mode techniques are now well-proven and established whereby simple operation and dynamics are achieved. Here are the primary advantages:
- Accurate output regulation using relatively straightforward loop compensation;
Better line transient rejection via automatic input-voltage feedforward;
High step-up/down conversion ratio from wide duty cycle operating range;
Easier and reliable design with cycle-by-cycle current limiting of the instantaneous MOSFET current;
True boost converter startup and short-circuit fault protection through input-output disconnection.
Other Benefits
In fact, current-mode control provides an opportunity to meet other performance goals such as multi-phase current sharing/stackability, load current telemetry reporting, and EMC compliance. For the latter, the fixed switching frequency of most types of current-mode control streamlines EMI filter design, enabling easier compliance to the EMC directives dictated by the various issuing bodies. Conformance to regulatory specifications clearly is an increasingly-important power solution benchmark.
So, are you interested in current-mode control applied to wide VIN power solutions? Head over to www.ti.com/widevin to learn more about our purpose-dedicated portfolio of wide VIN controllers, converters and power modules. Feel free to peruse various wide VIN reference designs, tools, and detailed product information.
Resources:
Read “Current-mode Control Stability Analysis for DC-DC Converter (parts 1 and 2)” in how2power.com.
Check out the “Wide VIN power management ICs simplify design, reduce BOM cost, and enhance reliability” whitepaper.
Refer to the “Wide VIN power solutions for industrial automation” app note.
Choose a wide VIN DC/DC power solution here.
Search LM5122 boost reference designs from the TIDesigns™ reference design library.
Order the EVM for the LM5117 buck or LM5122 boost controllers.
Start a design now with WEBENCH® Power Designer.