Jm Jan Schellekens

Dead-time compensation for PWM amplifiers using simple feed-forward techniques

Dead-time effect is one of the dominant sources of output current and voltage distortion for pulse width modulated (PWM) amplifiers. Practical switching devices have finite turn-on and turn-off time. To avoid short circuit a blanking time is added between turn-off and turn-on of the complementary working switches in a switching-leg. The blanking time, also referred to as dead-time, results in a nonlinear voltage error of the PWM output stage. Especially high-precision applications that require accurate input current for positioning systems suffer from the dead-time effect. Extensive studies have been done on the analysis and elimination, minimization and compensation of dead-time in PWM converters. Most of these techniques rely on the detection of the polarity of the output current of the converter. By using only the polarity of the output current, the inductor current ripple is neglected, which is not sufficient for high-precision applications. In this paper nonlinear feedforward of the current setpoint is used to compensate the dead-time effect. This simple feedforward technique leads to significant improvement of the current tracking during zero crossings over a high output current frequency range, as demonstrated by measurement results.

Generalized harmonic elimination method for interleaved power amplifiers

This paper examines interleaved converters in which tolerances of the cell inductors are taken into account. Normally, an equally distributed phase-shift is applied to the PWM of an interleaved converter, which results in optimal ripple-cancellation for the output current. This is true for the ideal case in which cell inductors are identical. However, in practice, cell inductors are not equal, leading to the return of fundamental switching-frequency subharmonic components in the spectrum of the output current. In this paper it is shown that for this situation an optimal phase-shift exists. A generalized method is proposed to calculate the phase-shift in such a way that harmonics, such as the fundamental switching frequency harmonic component, are removed from the spectrum. For three parallel cells an analytic and a geometric method, supported by simulation results, to calculate the phase-shift is presented. Finally, results from an experimental setup are shown to verify the proposed ideas.

Elimination of zero-crossing distortion for high-precision amplifiers

Switch blanking time, also referred to as dead-time, is one of the dominant sources of output current and voltage distortion in pulse width modulated amplifiers. Extensive studies are known on elimination, minimization, and compensation of the effect. Most techniques achieve a reduction but are not capable of completely removing it. This paper demonstrates that it is possible to fully eliminate dead-time effects by applying the socalled opposed current converter topology in combination with advanced feedforward techniques. The zero-crossing behavior of the opposed current converter is analyzed and compared to a conventional full-bridge converter with equivalently filtered output. Simulations and measurements on a full-bridge and an opposed current converter of 1.5 kW are included to demonstrate the effectiveness of the proposed ideas for high-precision applications.

Harmonics in opposed current converters

Practical switching devices have finite turn-on and turn-off times. Normally a blanking time is added between the turn-off and turn-on of switches to avoid a short circuit during switching. This blanking time results in a current dependent voltage error of the pulse width modulated output of a converter. Topologies based on only one active device in series with a freewheeling diode, like the opposed current converter, do not need blanking time, and the output voltage distortion is greatly improved but not absent. This paper focuses on the harmonic distortion due to nonideal components in the opposed current converter switching leg and compares different modulation strategies. It is shown that the opposed current converter switching leg can be linearized by proper selection of its components. Furthermore, by applying the right modulation strategy it is possible to achieve a double effective switching frequency, as with unipolar switching, while maintaining constant common mode voltage at the output, as for bipolar switching.

Fast-Shared Current Transient Response in High-Precision Interleaved Inverters

A robust self-interleaving mechanism for paralleled hysteresis-current-controlled inverters is proposed, featuring sustained switching under all load conditions. A fast interleaving technique that can be applied when no clamping of the output voltage occurs is combined with a self-interleaving mechanism that ensures correct switching during output-voltage-clamping conditions. The self-interleaving mechanism was analyzed using the state-plane method, extended to multiple modules in parallel. A minimum switching frequency and maximum duty cycle are guaranteed under all load conditions, enabling the use of low-cost bootstrap circuits to drive the high-side switches. The interleaving approach results in reduced volume of the passive components and improved dynamic response. Simulations were conducted to verify the combined operation of both methods, and measurements were performed on a 2.8-kW prototype zero-voltage-switching inverter with a discrete hysteresis current controller.

On real-time optimal control of high-precision switching amplifiers

Currently, in the field of precision power amplifiers rather conservative power densities and bandwidths are used. These limitations are mainly due to the use of linear feedback architectures that do not consider constraints on the power amplifier dynamics during synthesis. Hence, to avoid any instability or inaccuracy due to nonlinearities of the power stage, the amplifier is generally operated with a highly conservative power budget. This paper investigates the applicability of reference governors and model predictive control in the field of precision power conversion. It is shown that the large signal behavior can be significantly improved, in terms of safety and accuracy. We also illustrate that dual-mode model predictive control (MPC), which combines a linear state-feedback with the sub-optimal solution of the MPC problem found by a real-time optimization solver, efficiently eliminates the small signal distortions inherently present in the standard real-time MPC implementation. The paper is organized as a case-study on controller synthesis for the opposed current converter, and as such, is well suited for practicing engineers in the field of precision power amplification.

Volume reduction of opposed current converters through coupling of inductors and interleaved switching

Practical switching devices have finite turn-on and turn-off times. To avoid short circuit during switching, a blanking time is added between the turn-off and turn-on of switches. This blanking time results in a nonlinear voltage error of the pulse width modulated output stage of a converter.

Interleaved switching of parallel ZVS hysteresis current controlled inverters

A robust self-interleaving mechanism for parallel hysteresis current controlled inverters is proposed, with sustained switching under all load conditions. A fast interleaving technique that can be applied for normal load conditions is combined with a self-interleaving mechanism, which ensures correct switching during voltage clamping operation. A minimum switching frequency and maximum duty cycle is guaranteed under all load conditions enabling the use of low-cost bootstrap circuits to drive the high-side switches. The interleaving approach results in reduced volume of the passive components and better dynamic response. The self-interleaving mechanism was analyzed using the state-plane method, extended to multiple parallel modules. Simulations were conducted to verify the combined operation of both methods and measurements were performed on a 3 kW prototype zero-voltage-switching inverter with a discrete hysteresis current controller.

Advances in High-Precision Amplifiers—The Extra L Opposed Current Converter

In existing half/full-bridge high-precision amplifiers, output distortion is present due to the required switch blanking time. The OCC topology does not require this blanking time but has a much higher total inductor volume compared to the half bridge. In this paper, a patented new topology is introduced that has the advantages of the OCC but with a much lower total inductor volume. The basic operation and properties of the ELOCC topology are explained including an extended optimization of the total inductor volume and an average model for control design. A prototype ELOCC current amplifier has been developed. The behavior of this prototype is in good agreement with the obtained simulation results. Even though the prototype is not fully optimized, the linearity compared to a full bridge is already impressive.

The extra L opposed current converter

In existing half/full-bridge high precision amplifiers output distortion is present due to the required switch blanking time. The OCC topology does not require this blanking time but has a much higher total inductor volume compared to the half-bridge. In this paper a patented new topology is introduced that has the advantages of the OCC but with a much lower total inductor volume. The basic operation and properties of the ELOCC topology are explained including optimization of the total inductor volume and an average model for control design. A prototype ELOCC current amplifier has been developed. The behavior of this prototype is in good agreement with the obtained simulation results. Even though the prototype is not fully optimized the harmonic performance is already impressive.