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How a Class E Converter Operates with Dual Frequency

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There are various types of DC-DC converters, each with a unique mode of operation. A DC-DC boost converter that operates in class E with dual frequency signal drive, for example, allows the output power to be adjusted if the load should change—all while ensuring very high efficiency under all operating conditions.

Compared to conventional converters, a class E converter enables the output DC voltage to be higher than the input DC voltage with maximum efficiency.

Fixed and variable load in class E

When a class E converter is designed for a precisely valued load, it has no control over adjusting the output parameters. The output signal characteristics are fixed and always operate under the same conditions. On the other hand, if different loads are used, a series of controls must be arranged to perfectly adjust the output signal.

When the output impedance changes, the circuit’s modes of operation change, as do the waveforms of the signals. Efficiency is not always constant and varies depending on the type of load applied.

With class E converters, it is possible to obtain special waveforms that pass through zero before switching. So, the switching losses are very low. In class E converters, the switching frequency can be greatly increased compared to conventional converters, even by a factor of 50, allowing high efficiency to be achieved. As a result, circuit sizes can be kept small, resulting in a significant reduction in weight, footprint and overall cost.

The electromagnetic interference (EMI) aspect is also dramatically reduced. In normal switching, power electronic switches interrupt the current within very short moments, which causes great stress to the electronic components. Power electronic switches must endure high voltages and high currents simultaneously, causing great power dissipation and stress, which results in power losses and low efficiency.

By adopting resonant circuits in converters, waveforms can be shaped to create zero-voltage switching (ZVS) and zero-current switching (ZCS) conditions. So, by reducing power dissipation (ideally to zero), increasing switching frequency and eliminating transient spikes, overall system efficiency is increased for very low EMI.

The dual control frequency

Class E converters usually work for a single load use case because they are designed to operate only under specific operating conditions. Normally, class E converters are used for a specific type of load, and in this case, the converter operates at maximum efficiency with only one optimum operating point. By changing the load, the waveforms of the circuit change, and therefore optimal operation is no longer guaranteed. A change of load results in a drastic reduction in system efficiency.

To address this challenge, it is possible to modulate the output current using a dual control frequency with which an optimal operating point is always guaranteed, and the efficiency of the entire system is maintained at very high values.

In switching converters, the most critical components are the inductors. In this circuit implementation, these components do not have to be replaced, and the optimal operating point can be equally achieved by adopting two different switching frequencies. Specific precautions must be taken when implementing a dual-frequency class E boost converter, as shown in Figure 1.

Figure 1: Generic schematic shows a dual-frequency class E boost converter.

The same inductors can be used in this circuit without having to replace them. Instead, the optimal operating point at both frequencies is obtained by simply changing the value of the capacitors connected in parallel with the electronic switches. Maximum efficiency is obtained only if the output current and the switching frequency of the transistor remains constant.

The below solution is derived from the single-frequency boost in which there are no electronic switches that alternately connect and disconnect the additional pair of capacitors. It is easy to see that when such switching devices are in conduction, they allow additional capacitors to be connected in parallel to increase the capacitance to those already existing in the circuit.

Circuit operation details

Unlike a circuit with only one operating frequency, the dual-frequency class E boost converter has two switches in series with the two capacitors labeled Cinv1 and Crec1 in Figure 1, allowing the total capacitance to be changed. If the switches are closed, the capacitances of Cinv1 and Crec1 are added to the Cinv2 and Crec2 capacitors, respectively. In this case, the converter works at a lower frequency.

On the other hand, if the additional capacitances are disconnected, the converter works at a higher frequency. Note that the additional switches are two MOS transistors. However, the one in series with Cinv2 is N-channel, while the one in series with Crec2 is P-channel. The converter operates at one of two switching frequencies, depending on the input voltage and load conditions, in the following two ways:

  • If the input voltage is high, or the load is light, the circuit adopts the higher switching frequency so that the ON-OFF transition losses and conduction losses produced by the resonant inductor can be reduced.
  • On the other hand, when the voltage is low, or the load possesses low impedance, the circuit adopts the lowest switching frequency.

At the design stage, the input and output operating voltages where the converter is to work must be chosen. Likewise, the designer must choose the two maximum powers dissipated by the load, at the two frequencies. With these, the currents of the two branches, INV and REC, can be easily calculated. The determination of the inductive components is extremely delicate since it must also consider the two different frequencies involved.

The graphs in Figure 2 show the waveforms of the voltage between the drain and source of the electronic switch V(DS) and the voltage between the cathode and anode of the diode V(KA).

Figure 2: Oscillograms of the voltages V(DS) and V(KA) are shown in the circuit operating at the lower frequency.

In short, the following steps should be taken to obtain two good working conditions at the two chosen frequencies. Usually, it is better to select one frequency twice as high as the other:

  • Choose the input voltage and the output voltage.
  • Determine the maximum relative powers at the two frequencies involved.
  • Calculate the relative currents.
  • Choose the inductors and determine the respective Q-values at the two frequencies.
  • Calculate the switching losses.
  • Optimize the results by slightly adjusting the component values.

The first sizing of the components is done at a given frequency; however, the second sizing is done at twice the frequency. The switching losses are again calculated and, finally, further optimization is performed. This is far from a simple procedure.

A well-designed circuit leads to very high efficiency, and the peak values of the two voltages examined earlier are not critical and are supported by most devices on the market. The value of the average output current is equivalent to the average current of Irec. Parasitic resistive components in series with the reactive components—capacitors and inductors—must be considered when designing the circuit. These parasitic resistances are subject to change in relation to frequency; indeed, they are more relevant at high frequencies.

To make a single circuit work at the two frequencies, it is necessary to combine two equivalent designs to implement a class E boost converter that can work at the two frequencies. As mentioned before, it is worth reiterating that if the two switching devices in series with the capacitors Cinv1 and Crec1 are open, the circuit can operate at a higher frequency.

Conversely, if these electronic switches are closed, the circuit operates at a lower frequency. Therefore, at such frequencies, the presence of parasitic resistances varying according to the frequency itself is inevitable. Such reactance will be minimal as far as capacitors are concerned. For inductors, they may reach important values due to their low Q.

In the graph shown in Figure 3, there is a small step on the oscillogram related to the VDS voltage, just at the instant when the electronic device is turned on and goes into conduction. This step cannot be eliminated altogether and results in a very small increase in the power dissipated in switching, which is completely acceptable and insignificant.

Figure 3: A very small step is formed on the VDS signal during transistor switching, which is completely acceptable.

Efficiency gains

Experimental results show that, with this dual-frequency methodology, the conversion efficiency improves by at least 6-7%. By performing a good analysis and design, the circuits behave as planned, achieving their intended goals.

It is interesting to compare the voltage waveforms at the end of the MOSFET and diode—at the two switching frequencies—to verify the efficiency of the system. It is also useful to analyze the currents transiting the Linv and Lrec inductors.

The oscillograms in Figure 4 show the activation pulse of the electronic switch and its two signals at the end of the two semiconductor components—first two oscillograms above—at the base frequency, respectively. The other two oscillograms below show the activation pulse of the electronic switch and its two signals at the end of the two semiconductor components at double frequency, respectively.

Figure 4: The graphs show the signals at the two operating frequencies.

The second mode of operation uses twice the frequency and half the current on the load. At such high frequencies, these results could not normally be achieved with conventional solutions.

Class E converters are extremely complex and critical, but they provide enormous benefits by allowing loads of varying nature and impedance to be driven. With special arrangements, substantial circuit modifications, and an exponential increase in system complexity, a DC-DC converter can be implemented for different types of loads.

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