Power Tips #123: Using a double-boost converter to extends the power range of high-conversion-ratio designs

Any boost-converter design will have a practical limit to how much it can step up a voltage from input to output. Pulse-width modulation (PWM) controllers have timing limits that restrict the minimum allowable on- and off-time of the field-effect transistor (FET). The timing limit will effectively limit the achievable voltage boost ratio, although this drawback is more apparent in topologies that use an inductor as its magnetic instead of a transformer or coupled inductor. In this Power Tip, I will compare a variety of non-isolated, single-ended boost topologies that extend the voltage boost ratio and introduce the double-boost converter as an option for achieving both large conversion ratios and high-current output loading.

Figure 1 shows a standard boost converter with a single inductor, FET and diode. Any boost controller that generates the PWM signal will have a timing limit for its duty cycle.

Figure 1 A standard boost converter uses a single inductor, FET and diode. Source: Texas Instruments

The maximum duty cycle varies between controllers, but in general the limit is lower at higher switching frequencies. While modern boost controllers can exceed 90% duty cycle, this characteristic may still limit the capabilities if a very large output voltage is required. Assuming ideal diodes, Equation 1 expresses the duty cycle of a continuous conduction mode (CCM) boost converter as:

You can also rearrange Equation 1 into Equation 2 to show the boost ratio for the input to output voltage:

Assuming a maximum duty cycle of 80%, a CCM boost converter can step up a voltage by a factor of five from input to output. Operating in discontinuous conduction mode (DCM) is an option for achieving higher conversion ratios; however, the lower inductances required to run discontinuously will lead to higher peak currents throughout the power stage. Then, as output power requirements increase, the higher peak currents will limit what components you can use in the power stage.

Another option for increasing the boost range of a controller is to add a charge pump to the output. Figure 2 adds a diode and a pair of capacitors to a standard boost converter. Adding subsequent capacitor-diode stages can further increase the boost capabilities.

Figure 2 A charge pump uses additional capacitors and diodes to multiply the voltage step-up. Source: Texas Instruments

For m number of charge pump stages, Equation 3 calculates the boost ratio for this topology variation as:

Using the same 80% duty cycle limit as before, a two-stage charge-pump boost converter can step up a voltage 10 times from input to output. If you require a greater boost ratio, adding stages can further increase the range of the charge pump. The drawback to this approach is that each charge-pump stage adds a pair of diodes between the source and load. Each diode pair will increase the total diode conduction losses for the converter and thus make this topology less applicable for high-output-current applications.

A double-boost converter becomes a solid option for designs that require both a large step-up in voltage and heavier sustained loading currents. As the name implies, this topology demonstrates a shared PWM signal controlling two cascading boost converters. Figure 3 shows a double-boost converter running off a single PWM input. While a charge pump proportionally increases the boost ratio for the number of stages used, a benefit of a double-boost converter is how the boost ratio exponentially increases for a given number of stages.

Figure 3 A double-boost converter cascades a pair of stages to optimize the step-up conversion. Source: Texas Instruments

Equation 4 expresses the ratio of output to input voltage for n number of boost stages as:

Keeping with the same 80% duty cycle example, a double-boost converter can step up a voltage 25 times from input to output. This greatly increases the range compared to the five times step-up using a standard boost converter and the 10 times step-up using a two-stage charge-pump boost.

Table 1 lists the differences between the three boost topology variations. In general, the double-boost converter is the highest performing of these topologies. However, the improved performance comes with an increased number of components and design complexity.

Characteristic

Regular boost

Charge-pump

Double-boost

BOM size

Small

Medium

Large

VDS stresses

VOUT

VOUT/m

√VOUTxVIN, VOUT

(Two FETs needed)

Magnetics

Single inductor

Two inductors

Practical application

Smaller boost ratios

(VOUT < 10 x VIN)

Larger boost ratios

(VOUT > 10 x VIN)

Very large boost ratios, high power

(VOUT >> 10 x VIN)

Limitation

Duty-cycle limits

High BOM count

High cost

Table 1 Summary of comparisons between non-isolated boost variations.

When designing the power stage of a double-boost, consider each step-up separately. In other words, when selecting components for a double-boost converter, treat the input to middle voltage differently than the middle to output voltage.

Assuming that both stages of the double-boost converter are operating in CCM, Equation 5 expresses the middle voltage (Vmid) as:

The middle voltage is where much of the performance increase comes from. By splitting the conversion into two steps, the first stage handles higher current stresses with lower voltages across the FET and diode, while the second stage converts higher voltages at a lower current. For each case, each component of the individual power stages can help optimize the power loss. There is also more flexibility in rectifier selection. For example, the low-voltage, high-current section can use a synchronous rectifier to increase efficiency while still using a diode for the high-voltage, low-current section.

The main drawback to the double-boost converter is its complexity and high parts count. Because Vmid changes across the input voltage range, you will have to consider both extremes of the input. High-conversion ratio designs tend to require more components. However, the double-boost converter will need more unique parts compared to a charge-pump boost stage. The higher voltage FET on the second stage will likely require a higher gate drive voltage compared to the first stage. In cases with a very low input voltage, the design may have to include a second gate driver in order to drive the higher voltage FET.

All boost PWM controllers have limitations on their duty cycles and thus limit the voltage conversion capabilities. Topology variations such as the double-boost converter add many options to extend this usable range. The flexibility of the double-boost converter enables you to achieve high-conversion-ratio requirements in your design while still having enough efficiency to handle heavier loading conditions.

John Dorosa is a Systems Engineer in Texas Instrument’s Power Design Services team focused on industrial applications. Since joining the team in 2017 John has developed over 100 unique SMPS reference design boards to meet custom power requirements. His work covers a broad range of non-isolated and multiphase designs to isolated topologies that deliver a few milliwatts to 500 watts. He received a Bachelor of Science in electrical engineering from Michigan State University in East Lansing, MI.

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Power Tips #123: Using a double-boost converter to extends the power range of high-conversion-ratio designs

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