Analysis of Large-Signal Output Capacitance of Transistors Using Sawyer–Tower Circuit

Abstract

A detailed analysis on the Sawyer-Tower method used in the measurement of large-signal output capacitance (Co) of power transistors is presented, followed by important design recommendations to obtain accurate results. Key factors affecting the proper implementation of the technique, such as power amplifier characteristics, load slew-rate, reference capacitor (C ref) and reverse conduction of the device are addressed, with accompanying simulation and experimental results for Si, SiC and GaN devices. A thorough investigation on the selection of C ref is presented, with a new equation to correctly determine its value for a given voltage swing and output capacitance range of the Device Under Test (DUT). We report that the Sawyer-Tower circuit impose the DUT to enter steady-state reverse conduction under certain conditions, leading to charge-voltage (QV) hysteresis patterns unrelated to Co. Our analysis reveals that the origin of this phenomenon is related to DUT's leakage current, and that it could be minimized by proper selection of the excitation frequency. This work intends to provide an effective guide on designing and using the Sawyer-Tower circuit and to induce further scientific insight in characterizing Co.

Stability, Reliability, and Robustness of GaN Power Devices: A Review

“…In its OFF-state, a resonance is produced between the load inductor and the DUT's C OSS ; the C OSS loss is extracted by analyzing the loss in this resonance process based on the DUT's V DS or I DS waveforms. Despite the simpler setup, the accuracy of these electrical approaches may be compromised due to the variability and noise of waveforms and equipment, e.g., limited probe bandwidth, probe delays, and waveform distortion at high frequencies [113], [114]. In addition to the trade-off between thermal and electrical approaches, the Calorimetric and Sawyer-Tower methods only involve the device OFF-state, which disallows for the study of the impact of ON-state current on COSS loss.…”

Active-Device Losses in Resonant Power Converters: A Case Study with Class-E Inverters

“…energy loss, 1 𝐸 diss , and can be calculated using a charge versus voltage (QV) curve as Fig. 1(e) indicates [7], [10]. The OFFstate losses related to the leakage current through the device channel are generally negligible, especially in comparison to 𝐸 diss losses for MHz-range frequencies [10].…”

“…1(e) indicates [7], [10]. The OFFstate losses related to the leakage current through the device channel are generally negligible, especially in comparison to 𝐸 diss losses for MHz-range frequencies [10]. Therefore, the total power loss is in S, at a switching frequency of 𝑓 sw , is given by (4), where 𝑃 diss = 𝑓 sw • 𝐸 diss .…”

“…This is due to the requirement of small circuit-size in high-frequency operation that in turn creates measurement issues: for example, probing difficulties in electrical measurements and loss quantification issues in thermal/calorimetric methods [8] (due to thermal cross-coupling when several devices are involved). Therefore, indirect electrical methods such as the Sawyer-Tower technique [7], [8], [10] or calorimetric methods [11], [12] have been used to predict 𝐶 o -hysteresis losses in a working converter. However, they offer difficulties in certain aspects.…”

On the Practical Evaluation of the Switching Loss in the Secondary Side Rectifiers of LLC Converters

“…However, it has been reported that in the LLC (and other resonant converters), although the high-voltage (HV) primary side devices are soft-switched, their switching loss does not completely disappear [8]. The phenomenon can be interpreted as an equivalent series resistance (R oss ) appearing in series to the output capacitance of the devices (C oss ) during the resonant transitions [9][10][11][12][13][14][15][16]. However, there are no similar reports about the soft-switching loss contribution of the secondary side low-voltage (LV) rectifiers, even though the most mature technology for the LV secondary side rectifiers is the Silicon (Si) Trench MOSFET with shield-plate [17][18][19][20][21], and it is known that in their construction a series resistance appears with the output capacitance [22][23][24].…”