I. INTRODUCTION

With the ever increasing power demands of modern systems, as well as the desire for reduced size and lower power consumption, high power density and high efficiency are the key drivers for the advancement of power conversion technologies. However, devices based on Si semiconductor materials are approaching the limits of physical performance with respect to lowering power conversion losses, particularly in terms of reducing conduction resistance and switching loss. Wide band-gap semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) have many advantages including a wide band gap, high saturation drift velocity, high critical breakdown field, etc. As a result, wide band-gap semiconductor devices are more suitable for high-frequency, high-temperature, high power density and high efficiency applications [1]-[4]. Currently, a series of breakthroughs on SiC devices have been made. However, the research and application of GaN devices is still limited [5]-[8]. EPC, Transphorm, GaN systems and Panasonic Inc. have GaN devices. These GaN devices can be categorized into two types defined by their physical structure: single enhancement mode and cascade mode. In these devices, a low voltage Si MOSFET is in series to drive a depletion mode GaN HEMT. Compared to cascade mode devices, these enhanced mode devices usually have a lower on-resistance and smaller size. As a result, they are more attractive for high efficiency and high power density applications.

The authors of [9]-[13] studied the application of EPC enhanced mode GaN devices to MHz Buck converters and LLC resonant converters. They also discussed the impact of GaN device layout, magnetic components and distribution parameters on circuits. [13] shows the 3-D Integrated Gallium-Nitride-Based Point of Load Module Design in detail.

References [14], [15] presented a 90 W AC/DC adapter, which is composed of a buck-PFC stage and a Quasi-Switched-Capacitor (QSC) resonant DC/DC converter. The Buck-PFC evaluation module from TI Inc. achieves 97.1% peak efficiency with a GaN HEMT (TPH3006PD) and a SiC Schottky diode (C4D20120A) at 100 kHz. The 85 V/19 V. 1 MHz QSC resonant converter uses 100 V EPC eGaN FETs. In this case a 10.5 W/cm3 power density and 92.8% peak efficiency at 900 kHz are obtained.

This paper compares both the electrical properties of GaN FETs based on EPC and Si MOSFET, and the electrical characteristics of GaN Schottky diodes and Si fast recovery epitaxial diodes with the same voltage level. An evaluation of a GaN FET based on a flyback-forward high gain DC/DC converter at the soft-switching condition is presented in detail. A power loss analysis of the GaN FET based flyback-forward high gain DC/DC converter is discussed in detail. Finally, a 200W GaN FET based flyback-forward high gain DC/DC converter is established.

II. STRUCTURE AND CHARACTERISTICS OF THE GAN FET

A. Structure and Characteristics of the GaN FET

Fig. 1 shows the structure of a GaN FET. Si material is used as the substrate in the GaN FET, and a GaN crystal layer with a high resistance is grown on the basis of the Si substrate. An aluminum nitride (AlN) insulating layer is added between the GaN layer and the Si substrate layer isolating the device and the substrate. An AlGaN layer exists between the GaN layer and the gate (G), the source (S) and the drain (D) electrodes, and two-dimensional electron gas (2DEG) with high electron mobility and low resistance can be generated between the AlGaN layer and the GaN layer.

Fig. 1.Structure of GaN FET.

The device is voltage-controlled. When the positive gate-source voltage is greater than the threshold voltage, the gate is enabled, and with the 2DEG formed the transistor is turned on. When this is not the case, the transistor is turned off.

A GaN FET is a lateral structure device, as shown in Fig. 1. Unlike a Si MOSFET, a GaN FET has no parasitic body diode. There is no P-type parasitic bipolar region connected to the source electrode under the gate electrode of a GaN FET. This structure makes the GaN FET have a symmetrical transfer characteristic. As a result, the GaN FET can be driven either by a positive gate-to-source voltage (Vgs) or a positive gate-to-drain voltage (Vgd).

The GaN FET from EPC Inc. is an enhancement mode transistor, whose output characteristics with different gate-source voltages are shown in Fig. 2. For power conversion applications, this characteristic increases safety because the device is off when the driving voltage is below its threshold voltage. This decreases the designing difficulties in power conversion systems.

Fig. 2.Electrical characteristics of GaN FET.

B. Characteristics of the GaN Schottky Diode

Compared with Si diodes, the GaN Schottky diode has a lower forward voltage, as shown in Fig. 3. The GaN Schottky diode exhibits a positive temperature coefficient because the forward voltage increases with an increase in temperature, while the Si diode shows a negative temperature coefficient.

Fig. 3.Forward characteristics of GaN Schottky diode (TPS3410PK 600V/6A).

The GaN Schottky diode also has lower on-state losses. In addition, the GaN Schottky diode has no minority carriers, which will greatly reduce transient voltage spikes.

Furthermore, the GaN Schottky diode has a zero recovery charge. Si diodes, taking a Si fast recovery epitaxial diode DSEI12-06A as an example, typically have a 35ns recovery time and their recovery charge is typically 0.5uC under the condition of VR=50V, -diF/dt=200A/us, and IF=14A, T=100°C.

III. TOPOLOGY ANALYSIS AND SIMULATION RESULTS OF FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTERS

A. Operation Principe and Soft-switching Behavior of Flyback-forward DC/DC Converters

A flyback-forward high gain DC/DC converter is shown in Fig. 4 [16]-[18]. The main switches S1 and S2 work in the interleaved mode, and their control signals have a 180 degree phase shift. The active-clamp circuits are mainly composed of auxiliary switches Sc1 and Sc2 and clamp capacitors Cc1 and Cc2. The clamp switches Sc1 and Sc2 are driven complementarily by the main switches S1 and S2, which can recycle the leakage energy, suppress the turn-off voltage spikes on the main switches, and realize ZVS for all of the primary devices. In addition, there are two coupled inductors in the converter L1 and L2, where the primary inductors L1a and L2a are coupled with the secondary inductors L1b and L2b respectively. Llk is the total leakage inductance, which is equivalent to the secondary side. The key waveforms and equivalent circuits in different operational stages are shown in Fig. 5 and 6, respectively. The operation process description is given in detail as follows.

Fig. 4.Topology of flyback-forward high gain DC/DC converter.

Fig. 5.Waveforms of flyback-forward high gain DC/DC converter.

Fig. 6.Equivalent circuits of flyback-forward DC/DC circuit operational time intervals: (a) [t0-t1], (b) [t1-t2], (c) [t2-t3], (d) [t3-t4], (e) [t4-t5], (f) [t5-t6], (g) [t6-t7], and (h) [t7-t8].

[t0-t1]: During this time, the main switches S1 and S2 are on and the two coupled inductors are charged by the input voltage in the flyback mode for energy storage. The auxiliary switches Sc1 and Sc2 are off. The output diodes Do1 and Do1 are both reverse-biased, and the output capacitors Co1 and Co2 provide energy to the load.

[t1-t2]: At t1, the main switch S2 is turned off. Its parasitic capacitor Cs2 is charged so that the drain–source voltage of S2Vds2 increases. Since the GaN FET has a very small Cds, the value of Vds2 increases quickly.

[t2-t3]: At t2, Vds2 increases and the voltage on the primary inductor L2a decreases resulting in a corresponding decrease in the voltage on the secondary inductor L2b which makes the output diode Do1 conduct. During this time, the coupled inductor L1 operates in the forward mode and L2 works in the flyback mode to transfer energy to the load.

[t3-t4]: At t3, the voltage on the parasitic capacitor Cs2 increases to that on the clamp capacitor Cc2 which makes the equivalent antiparallel diode of the clamp switch Sc2 conduct. The coupled inductors L1 and L2 remain in the same mode as [t2-t3].

[t4-t5]: At t4, the clamp switch Sc2 is turned on with ZVS. The current through the equivalent antiparallel diode of the clamp switch Sc2 transfers to Sc2 quickly.

[t5-t6]: At t5, the clamp switch Sc2 is turned on. Due to the parasitic capacitor Cs2, Vds2 decreases linearly and that of the clamp switch Sc2 increases in an approximately linear way. As a result, Sc2 turns off with ZVS. One part of the leakage energy continues to be delivered to the load and another part of the leakage energy is recycled to the input source.

[t6-t7]: At t6, Vds2 decreases to zero. Therefore, its equivalent antiparallel diode starts to conduct. The leakage current falls due to the voltage on the capacitor Co1.

[t7-t8]: At t7, the main switch S2 turns on with ZVS. The secondary diode Do1 still conducts. At t8, the leakage current decreases to zero and the diode Do1 turns off with the zero-current switching operation. The two primary inductors are again charged linearly by the input voltage.

B. Simulation of the flyback-forward DC/DC Converter

PSIM software is utilized to verify the operation principle of the circuit. The simulation parameters are listed as follows: Vin=25V, fs=100kHz, Vo=380V, and the resistive load Ro=722Ω.

Waveforms of the primary side current IL1a, IL2a of the coupled inductor, the secondary diode current IDo1, IDo2 and the driving signal Vgs1, Vgs2 are shown in Fig. 7(a). A waveform of the drain-source voltage Vds of the main switches S1, S2 is shown in Fig. 7(b). The voltage and current of S1 both without an active-clamp circuit and with an active-clamp circuit are shown in Fig. 7(c) and Fig. 7(d) respectively. Compared with the simulation results, without an active-clamp, the circuit with an active-clamp achieves ZVS turn on and ZCS turn off, which eliminates the voltage spikes of S1 and S2.

Fig. 7.Simulation waveforms of flyback-forward high gain DC/DC converter: (a) IL1a , IL2a , IDo1, IDo2, (b) Vds of S1and S2, (c) results without active-clamp, and (d) results with active-clamp.

IV. DEVICE SELECTION AND LOSS ANALYSIS OF THE GAN FET BASED FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTER

The design specifications of the GaN FET based flyback-forward high gain DC/DC converter are shown in Table I.

TABLE IDESIGN SPECIFICATION OF FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTER

A. Device and Component Selection of the Flyback-forward High gain DC/DC Converter

The main switch is selected by the voltage levels and current levels [19]. The voltage gain is:

For a duty ratio of D>0.5, when Vin =40V, N < 2.375 . In this case N=2. The main switch voltage is:

The main switch current amplitude is:

Therefore, two EPC2010 in parallel are employed as the main switches. Both the main and clamp switches have the same voltage stress. Therefore, the parameters of the clamp switches should be the same as the main switches. EPC2010s are selected as the clamp switches.

When the primary-side switch is turned off, the maximum voltage drop of the rectified diodes is about 350V. The peak current of the diodes is:

GaN Schottky diodes TPS3410PK 600V/6A produced by Transphorm Inc. are chosen as rectified diodes. The key parameters are shown in detail as following: VF=1.3V, IR=25uA, Qc=54nC, and C =81pF.

When coupled inductor turns ratio is N=2, the leakage inductance needs to satisfy the following condition:

The magnetizing inductor Lm can be determined by setting an acceptable current ripple, which is given by:

The magnetizing inductor is chosen as Lm = 43.75 μH , fs = 100kHz , and the leakage inductance of the primary side is LLk_P =0.87 μH .

The two coupled inductors are composed of a planar EE core EE32 with 3F3 material and printed circuit boards as windings. The main parameters of the two coupled inductors are the same, and are shown in TABLE II.

TABLE IIDESIGN SPECIFICATION OF A COUPLED INDUCTOR

B. Loss Analysis of the flyback-forward High Gain DC/DC Converter

According to the symmetry of the circuit, switch S1 is taken as an example to make a loss analysis. Po=200W, Vin=25V, the conducting time of the main switch T1 = D*TS, the clamp switches conducting time is T2 = (1 - D)*TS, and the dead time is 1% of the switching period Ts.

The active-clamp circuit leads to ZVS of S1, whose loss is mainly composed of conduction loss and switching loss. According to Fig. 5, during one switching period, the current of S1 in every stage is:

[t0-t3]: The current in one, which follows the primary of the coupled inductor, is defined by:

[t3-t5]: Because of the effect of the active clamp circuit, the leakage current of the secondary ILk increases linearly. Converting this to the primary, the main switch current is:

where: ΔV = NVCc2 - Vo/2 = 2V, LLK = 5μH.

[t5-t8]:

where:

[t8-t9]: During this stage, the main switches S1 and S2 are in the turn-on state. The current flowing through S1 is that of the coupled inductor. According to the above analyses, the RMS current of S1 is given by:

When the RDS of EPC2010 is 18mΩ, the conduction loss of S1 is:

The turn-off loss is:

where tf is the overlap time of IDS and VDS, and Vpeak_S1 is the peak voltage of the drain-source voltage.

Since the clamp switch is turned on under ZVS, the current flowing through the clamp switch can be expressed as:

The RMS current of Sc1 is:

The active clamp switches are the same type as the main switches. Therefore, the conduction loss of Sc1 is:

Turn-off loss of Sc1 is:

The current of the secondary side diode is in the discontinuous current mode. As a result, the loss is mainly conduction loss.

[t3-t5]: The current increases linearly.

[t5-t8]: The secondary-side current decreases linearly.

The average current of Do1 is:

Conduction loss is:

where VF is the forward voltage drop of the GaN Schottky diode, which is 1.3V.

The copper loss of a coupled inductor is estimated as follows.

where Irms_p is the root mean square value of the primary current; Np is the primary turns; lp is the mean path length of the primary coil; ρ is the resistivity of copper at 100℃, which is 2.266*10-6 Ω*cm; and Asp is the sectional area of the primary conductor.

The same method is used to estimate the copper loss of secondary side.

Then, the core loss is PFe, which is given by:

where PV is about 0.4W/cm3 for fs=100kHz, Bpk=0.2T, and Ve=5.38cm3.

The losses of the GaN FET based flyback-forward high gain DC/DC converter are shown in Table III. The total circuit loss is about 4.5W and the theoretical efficiency can reach 97.8%.

TABLE IIIPOWER LOSS OF GAN FET BASED FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTER

The losses of the Si MOSFET based flyback-forward high gain DC/DC converter are shown in Table IV. The total circuit loss is about 8.64W and theoretical efficiency can reach 95.86%.

TABLE IVPOWER LOSS OF SI MOSFET BASED FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTER

V. EXPERIMENTAL RESULTS OF THE GAN FET BASED FLYBACK-FORWARD HIGH GAIN DC/DC CONVERTER

A. Experimental Comparison between the GaN FET and Si MOSFET Based on the flyback-Forward High Gain DC/DC Converter

When an EPC2010 is applied into a flyback-forward high gain DC/DC converter, the prototype is shown in Fig. 8. The unique Land Grid Array (LGA) package of EPC2010 products, the parasitic parameters in the main power circuit, and the driver circuit can be significantly reduced. This imporves the driving stability, while reducing the voltage stress of the main switches.

Fig. 8.PCB board of GaN FET based flyback-forward high gain DC/DC Converter.

Experimental tests are conducted under the condition that the input voltage is 25V, the output power is 200W, the duty cycle of main switches is 0.74, and a GaN Schottky diode TPS3410PK is applied. The obtained experimental waveforms are shown in Fig. 9(a), where channel 1 is the output voltage (100V/div), channel 3 is Vds of the main switches S1 (50V/div), and channel 4 is the primary current of the coupled inductors IL1a (5A/div). The temperatures of the devices are tested at a 30°C room temperature, as shown in the Fig. 9(b), where the hotter part is around main switches EPC2010s.

Fig. 9.Test results of GaN FET based flyback-forward high gain DC/DC converter (a) experimental waveforms, (b) thermal test results.

Fig. 10 shows the experimental results when the main switch is selected as a Si MOSFET IPB107N20N3G 200V/88A under the same operating conditions. Experimental tests are conducted under the condition that the input voltage is 25V and the output voltage is 380V. Only the main switches and the driver are changed.

Fig. 10.Test results of Si MOSFET flyback-forward high gain DC/DC converter: (a) experimental waveforms and (b) thermal test results.

In Fig. 10(a), channels 1 and 2 are Vds of the main switches S1 and S2 (50V/div), channel 4 is the primary current of the coupled inductors IL1a (10A/div), and channel 3 is the output voltage (100V/div). The temperatures of the devices were tested at a 30°C room temperature, as shown in the Fig. 10(b), where the hotter part is around main switches IPB107N20N3G.

From the Fig. 9(a), it can be concluded that there is no voltage spikes when the main switch S1 is turned off, and that switching loss of the GaN FET is small. The output DC voltage of the circuit can be stabilized at 380V. On the other hand, there is a visible voltage spikes of the Si MOSFET based flyback-forward high gain DC/DC Converter, which results in a decreased efficiency. The temperature of the Si MOSFET is higher than that of the GaN FET under the same operating conditions.

The highest efficiency is 95.8% at the 200W power point of the GaN FET based flyback-forward high gain DC/DC converter. Meanwhile, it is only 94% at the 200W power point of the Si MOSFET based converter. From the experimental results, it can be seen that the application of the GaN device can reduce the voltage stress of the switches and increase the circuit efficiency.

B. Experimental Comparison between the GaN Diode and the Si Diode Based on a Flyback-Forward High Gain DC/DC Converter

Based on a flyback-forward high gain DC-DC converter applying a EPC2010, an experimental comparison between the GaN diode and the Si diode with the same 600V voltage level is made to determine the advantages of the GaN diode.

Experimental tests are conducted under the condition that the input voltage is 25V, the output power is 200W, the duty cycle of the main switches is 0.74, and GaN Schottky diodes TPS3410PK are applied. The obtained experimental waveforms are shown in Fig. 11(a), where channel 1 is the current of Do1 IDo1(2A/div), and channel 4 is the forward voltage of Do1VDo1(100V/div). Fig. 11(b) shows the temperature test results of the TPS3410PK at a 30°C room temperature, where highest temperature is 51.1°C.

Fig. 11.Test results of GaN FET based flyback-forward high gain DC/DC converter applying GaN diodes: (a) experimental waveforms and (b) thermal test results of TPS3410PK.

Under the same conditions, Si fast recovery epitaxial diodes DSEI12-06A manufactured by IXYS are applied to the converter for comparative purposes. The results are shown in Fig. 12(a), where channel 2 is the forward voltage of Do1 VDo1(100V/div), and channel 3 is the current of Do1 IDo1(2A/div). Fig. 12(b) shows temperature test results of the DSEI12-06A at a 30°C room temperature, where highest temperature is 64.8°C.

Fig. 12.Test results of GaN FET based flyback-forward high gain DC/DC converter applying Si diodes: (a) experimental waveforms and (b) thermal test results of DSEI12-06A.

From Fig. 11(a), it can be concluded that there is little reverse recovery current when the diode is turned off and it verifies the zero recovery charge. Meanwhile from Fig. 12(a), a larger reverse recovery current appears compared with Fig. 11(a). In addition, the voltage spike is larger than that shown in Fig. 11(a) during reverse recovery time, which reduces efficiency. Comparing Fig. 11(b) and Fig. 12(b), it can be seen that the Si diode has a higher temperature under the same operating conditions as the GaN diode. This indicates a larger loss.

VI. CONCLUSION

This paper describes the development of a GaN FET and presents its structure and electrical properties. A GaN FET manufactured by EPC Inc. is applied in a flyback-forward high gain DC/DC converter. A loss analysis is discussed in detail for the GaN FET based flyback-forward high gain DC-DC converter. The application of the GaN FET can significantly reduce the switch off voltage spike, and reduce switching losses. An experimental comparison between a GaN diode and a Si diode is made to determine the advantages of the GaN diode. Finally, a 200W GaN FET based flyback-forward high gain DC/DC converter is established. Experimental results verify that the GaN FET is superior to the Si MOSFET, and that the GaN Schottky diode is superior to the Si fast recovery epitaxial diode at the same voltage level.