Based on the application of non-isolated DC to DC converter

In the design of DC-DC converters, when the input is equal to the output, if the conversion method when the input and output are not equal is still used, the conversion efficiency will not be improved. At this time, several non-isolated DC-DC conversion methods can be used, including SEPIC, buck-boost method, and buck-boost circuit combination method. This article analyzes four of these methods and pays special attention to efficiency issues in typical applications.

Most non-isolated input-output regulation schemes have a fundamental drawback, that is, when the input is equal to the output, the efficiency is not improved compared to the case when the input and output are not equal. This result is obvious from some common methods such as SEPIC, C'uk and buck + boost combination circuits, which still use completely different switching modes to process even when the input voltage is close to or equal to the output voltage.

If controlled correctly, the classic buck and boost cascade circuit should be more efficient when the input is close to or equal to the output voltage than in other cases. This is not a new discovery, it has been documented and applied in practice, but this application seems to have been ignored because it is not the mainstream of DC-DC applications. It is currently mainly used in high-power three-phase power correction systems for large mainframe computers, so it is not surprising that it is not included in the common DC-DC conversion technology.

Below we will analyze four topologies, three buck + boost combinations and a single-ended primary inductor converter (SEPIC), using typical components in each case and including parasitic losses. Traditional buck-boost converters and C'uk converters are not included here because the polarity of the output and input is opposite in non-isolated circuits.

Circuit Structure

Figures 1 to 4 are schematic diagrams of these circuits, which are boost + buck, SEPIC, buck + boost, and another buck + boost (two switches driven simultaneously) circuits, where D1 and D2 are the duty cycles of switches S1 and S2, respectively. The following is a detailed analysis.

1. Boost + Buck Converter

Although the circuit of Figure 1a is the most complex of the four circuits, it has several advantages. Its input and output currents are smoothed by the inductors, reducing the ripple current at the input and output terminals and the current stress on capacitors C1 and C3. However, this solution also has disadvantages. The current of capacitor C2 will be interrupted whether it comes from CR1 when Vin is less than Vout or from S2 when Vin is greater than Vout, and it requires two inductors.

Although the circuit works with two switches driven simultaneously (its conversion equation is the same as that given in Figure 1d), the most effective control method is to drive S1 through pulse width modulation (PWM) when the boost function is required (Vin is less than Vout) while keeping S2 on, and to drive S2 through PWM when the buck function is required (Vin is greater than Vout) while keeping S1 off. This is a good solution because when Vin = Vout, no switch-mode power processing is required, S1 is off and S2 is on, power is transferred from input to output only through the DC circuit, and when the input is approximately equal to the output, only minimal switch-mode power processing is required.

2．SEPIC

Figure 1b shows the classic single-ended primary inductor converter (SEPIC). Clearly this is the simplest of the four circuits in terms of total number of components, requiring only one switch and one diode, but it does require two inductors (or two inductor windings on one core).

As shown in the conversion equation, when duty cycle D1 equals 0.5, the input and output are equal and the total power transferred from input to output is handled in switch mode, with all power transferred through capacitor C2. Therefore, the ripple current handling function of C2 needs to be carefully considered, and C2 can be a low impedance electrolytic type, and there are many such components with excellent performance available in the market today. Its terminal voltage is equal to the input voltage, which is obvious considering that L1 is connected to the input and L2 is connected to ground, and the average voltage across the inductor must be zero. I believe that the industry has not fully utilized the SEPIC, probably because it has a non-classical configuration, which requires designers to spend more time and effort to analyze and consider compared to simple buck or boost circuits.

3. Buck + Boost Converter

Figure 1c is similar in function to the circuit in Figure 1a, where the buck section is in front and the boost section is in the back, hence the name "buck + boost" converter, which is the opposite of "boost + buck". As you will see later, it is most efficient when the input voltage is close to the output voltage. When Vin = Vout, no switch mode processing is required, S1 is on and S2 is off. Another advantage is that it only requires one inductor. The disadvantage is that both the input current and the output current are discontinuous, so the input and output capacitors must be chosen so that they can handle the ripple current. Like the circuit in Figure 1a, when Vin is less than Vout, S1 remains on and S2 acts as a PWM boost converter. When Vin is greater than Vout, S1 acts as a PWM buck converter and S2 is off.

4. Buck + Boost Converter (D1=D2)

This circuit structure is similar to Figure 1c, but the operation is completely different. In this case, switches S1 and S2 are driven by the same controller and are turned on and off at the same time. The advantage is of course that the controller is much simpler than in Figures 1a and 1c, but it is more complex than a SEPIC controller because two switches must be driven, and only one of them is at ground potential.

The simplicity of the driving scheme is an advantage of this circuit, but the disadvantage of poor efficiency often prevents its use. Because both switches are driven simultaneously, and the duty cycle D is 50% when the input voltage equals the output voltage, too much energy is recycled in the converter. For example, when Vin = Vout (and D = 50%), inductor L1 conducts the input (output) current twice. At the input, S1 is on 50% of the time, forcing it to conduct the average input current twice, of course, this current comes from L1; similarly, at the output, CR2 is on 50% of the time, again drawing current from the inductor. Indeed, when they are on, all four switching elements (S1, CR1, S2, and CR2) conduct the input-output current twice, resulting in large power losses, making this circuit the least efficient of the four circuits, but it is undoubtedly simple and can be used in low-current applications.

Circuit Simulation

1. Component selection

Four circuits were simulated using a set of components with loss characteristics suitable for use in converters with an output of 2A 24Vdc and an input range of 18 to 44Vdc. These parameters were entered into a spreadsheet along with existing current and voltage expressions and the resulting curves were plotted for comparison, with an operating frequency of 100kHz. In the case of two switches, S1 and S2, the ground switch is an N-channel FET and the upper switch is a P-channel FET. The diode is a Schottky type and the forward voltage is assumed to be 0.6V. The inductor is 150μH 4A and the internal resistance is 0.1Ω. The capacitor is a high quality, low impedance type and its losses are calculated to be negligible. The switching losses of the FETs are estimated, assuming a switching time of 100ns, ignoring the switching losses of the diodes, and the losses of the control circuit are also assumed to be negligible.

The characteristics of FET are as follows:

P-Channel:

ON Semi MTD5P06V, RDS(on)=0.45Ω N-channel:

ON Semi NTD15N06，RDS(on)=0.09Ω

The inductor value is chosen to be 150μH, so that the ripple current in the inductor and other components is about 20%, and the current waveform can be regarded as a flat-top current pulse without worrying about the swing.

2. Loss calculation

We created a spreadsheet for each of the four circuits, set the output voltage to 24V and the current to 2A, and then calculated their performance with the input voltage increasing in 2V increments. In the SEPIC and Figure 1d (D1=D2), the process is simplified because the transfer function (Vout/Vin) is the same when the voltage is below or above the output voltage. The other two circuits use different functions depending on whether the input is less than or greater than the output.

Because the conduction losses in FETs are resistive, the on-current is calculated and squared and then multiplied by the resistance and then multiplied by the on-duty cycle (D) to find the average loss during the switching cycle. The transfer functions below Figures 1a through 1d are used to determine the operating conditions for each component (to analyze the operating details of each circuit in detail).

Circuit Performance

Figure 2 shows the performance characteristics of the four circuits. Note the excellent performance of the two dual-mode circuits, especially their efficiency when the input voltage is almost equal to the output voltage (24V). The SEPIC efficiency is quite high, and this is also the case when the input is close to the output voltage. It is more efficient as the input voltage increases because the input current is reduced. Note that the buck+boost circuit with the switches driven simultaneously (D1=D2) has poor efficiency. Figure 3 shows the same data, but without the fourth circuit, so that the vertical axis can be enlarged to compare the first three circuits in more detail.

Note that the boost + buck dual-mode converter is more efficient when the input voltage is lower or higher than the output voltage, because the smooth input and output currents reduce component stress. Although the intermediate capacitor is affected by the ripple current, its effect can be ignored nowadays with low impedance electrolytic capacitors.

Conclusion

Modeling the performance of the four circuits yields laboratory test data for a buck + boost dual-mode converter prototype. The data shows that the circuit has excellent performance when the input voltage is close to the output voltage, while the boost + buck dual-mode converter has good performance over a wider input voltage range. In comparison, the SEPIC circuit is simpler, but not very efficient. The buck + boost circuit with two switches driven simultaneously is easy to control (but not as simple as the SEPIC), but is also less efficient.