Exploiting current-mode control for wide Vin DC/DC conversion


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.