ISBN 978-80-261-0812-2, © University of West Bohemia, 2019 Tracking Deadtime Algorithm for GaN DC/DC Converter Pavel Skarolek, Jiří Lettl Czech Technical University in Prague Faculty of Electrical Engineering Department of Electrical Drives and Traction, Technicka 2, 166 27 Prague 6, Czech Republic skaropav@fel.cvut.cz, lettl@fel.cvut.cz Abstract – The presented method automatically adjusts the deadtime of gallium nitride (GaN) transistors in half-bridge to increase the efficiency. This removes the need of manual measuring and setting the deadtime of the finished converter. The developed algorithm was tested and compared with the fixed deadtime case. The obtained results show that the developed algorithm is achieving higher and more stable efficiency compared to selected fixed deadtime. Keywords - GaN transistor; deadtime; tracking algo- rithm; DC/DC converter I. INTRODUCTION Gallium nitride (GaN) normally-off transistors are available for power electronics [1]. Typical applications that become useful are DC/DC converters or power factor correctors (PFC) [2]. In such an application, a high frequency hard switched half- bridge is usually employed. The absence of freewheeling diode in GaN transistor brings the benefit of zero reverse recovery current [2]. However, in half-bridge converter, the reverse conduction of the transistor is needed [3]. The reverse conduction characteristic of GaN transistor [4] is such that the gate voltage has to be applied for the transistor to achieve a low voltage drop in the reverse conduction region. The same phenomenon occurs in all metal oxide field effect (MOSFET) transistors. However, the body diode that is present in silicon and also in silicon carbide (SiC) structure has much lower voltage drop. This leads to the decrease in efficiency in particular when increasing deadtime is not significant for MOSFETs except GaN. It is because the body diode conducts the reverse current however, GaN transistors do not have the body diode in its structure [5]. The deadtime must be carefully adjusted for the converter equipped with GaN transistors to keep the high efficiency in a wide range of power [6]. The proposed algorithm is developed for typical buck or boost DC/DC converter employing half-bridge of GaN transistors and working inductor coil as in Fig. 1. One option is to measure the efficiency depending on the converter power and create a look-up map that determines the optimum deadtime for the control circuit [7]. Figure 1. DC/DC converter topology The other option is to add a specific analogue circuit which measures the voltage drop at reverse conduction as a feedback for the control circuit adjusting the deadtime [8]. This technique needs more time to be spent when putting the converter into operation because of the necessity of measuring the deadtime look-up map or adding a new circuit. The proposed tracking algorithm works only with regularly measured values such as output voltage and current to find the optimum deadtime. The tracking algorithm for example [9] keeps adjusting the deadtime online when the converter is running. II. TRACKING ALGORITHM When the added deadtime is longer than needed it forces the GaN transistor to operate in reverse conduction region with no gate voltage applied. It results to higher voltage drop and lowers the converter efficiency. To operate the tracking algorithm, it is necessary to find a value that should be measured in the circuit and that depends on the converter efficiency. For the converter we can write the following equation (1). The output voltage V out depends on the input voltage V in , the converter duty cycle d, and a voltage drop that represents also the voltage loss V loss caused by the deadtime. (1) If we neglect the loss in the inductor coil, as we study only the half-bridge, we can write that the loss voltage depends on the deadtime t dt , switching period T, on-state resistance R DSon , off-state resistance in reverse conduction region R SDoff , and output current I out according to (2).