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voltages (VIN – VOUT). When the switch is off, the catch diode rectifies the current and the inductor voltage is the same as the output voltage.
MasterGaN system in package (SiP) for LED drivers
Figure 3
Consequently, the CCM PFC is generally more expensive than a TM PFC. Ideally, a zero reverse recovery switch can be used in place of the rectifying diode in CCM PFCs. This makes GaN transistors very good candidates for this application.
Isolation is optional and can be introduced between the input stage and the second stage of power conversion. In this example, isolation is not used, and the input PFC stage is followed by a non-isolated inverse buck stage with CC/CV control. In the cases where isolation is needed, a resonant power converter (LLC, LCC) or a flyback converter can be used depending on the output power requirements of the application. The PFC boost converter generates a regulated DC bus voltage on its output (higher than the peak of the input AC voltage) and passes this higher DC bus voltage to the inverted buck converter stage. The stepdown operation is quite simple. When the switch in the buck is on, the inductor voltage is the difference between the input and output
Figure 4
Along with power density and efficiency, a key challenge for high-voltage lighting applications is the complexity of the design. With the use of wide bandgap semiconductors like GaN, the power density and efficiency of the circuit can be increased. ST’s MasterGaN family addresses that challenge by combining the high-voltage smart-power BCD-process gate drivers with high-voltage GaN transistors in a single package. MasterGaN allows for an easy implementation of the topology shown in Figure 1. It embeds two 650 V GaN HEMT transistors in Half-Bridge configuration as well as the gate drivers. In this example, the entire buck power stage is integrated into a single QFN 9x9 mm package requiring minimal external component count. Even the bootstrap diode, typically needed to supply the isolated high-voltage section of a dual, high-side/low- side, Half-Bridge gate driver, is embedded into the SiP. Consequently, the power density of an application that uses a MasterGAN device can be increased dramatically compared to a standard silicon solution while increasing the switching frequency or the power output. More specifically, in this LED driver application, a 30 per cent decrease in PCB area was achieved and no heat sinks were used. For high-power LED lighting applications, CCM is the best operating mode to use. When implementing CCM with GaN devices, there will be the high-level benefits previously discussed as well as a reduced cost. There
Table 1
would be no need for very low RDSON to serve high power applications due to the reduced switching loss contribution to overall power losses. GaN also mitigates a major drawback of using CCM by eliminating recovery losses and reduced EMI, as GaN experiences no reverse recovery. CCM operation with Fixed Off Time control also makes the compensation of output current ripple dependency on VOUT very easy. It is clear that GaN switch implementation using CCM is a great combination for high-voltage LED lighting applications, as well as many others. The basic scheme of an Inverse Buck topology is shown in Figure 2 along with an implementation that uses the MASTERGAN4. MASTERGAN4 embeds two 225 mΩ (typical at 25°C) 650 V GaN transistors in Half-Bridge configuration, a dedicated Half- Bridge gate driver and the bootstrap diode. This high level of integration simplifies the design and minimizes PCB area in a small 9x9 mm QFN package. The evaluation board that is shown in Figure 3 was designed with the MASTERGAN4 in an inverse buck topology and has the following specifications: it accepts up to 450 V input, the output voltage of the LED string can be set between 100 V and 370 V; it operates in Fixed Off Time (FOT) CCM with a switching frequency of 70 kHz; the max output current is 1 A. The controller in this solution, the HVLED002, is used to generate a single PWM control signal. An external circuit based on simple Schmitt Triggers is then used to generate two complementary signals to drive the low side and high side GaN transistors with a suitable dead time. Two linear regulators are also included to generate the supply voltages needed by
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the MASTERGAN4. The inverse buck topology implemented with MASTERGAN4 creates a solution for increased power density and efficiency, but let the results discussed below speak for themselves.
Experimental results
The efficiency plots in Figure 4 show the advantages of the proposed solution vs. a traditional silicon solution as a function of the LED string voltage for output currents of 0.5 A and 1 A.
The efficiency of MASTERGAN4 stays at or above 96.8 per cent across the entire LED string voltage range. It is possible to observe that across all power levels the gain in efficiency is maximized thanks to the low conduction losses as well as the minimal driving and switching losses of the GaN solution.
Table 1 compares the silicon solution with the MASTERGAN4 based solution. As can be seen, more than 30 per cent overall PCB area reduction is shown with the GaN design implementation. The results show one path that can be taken with GaN in this inverse buck topology. Increasing the switching frequency above 70 kHz can decrease the output inductor and capacitor size at the expense of higher driving and switching losses. At a higher frequency and reduced filter size, electrolytic capacitors can be replaced with more reliable and larger ceramic capacitors. The trade-off between filter capacitor and buck inductor size can be optimized based on the switching frequency required by the target application.
Conclusions
This article discussed the implementation of an inverse buck topology for LED lighting applications based on MASTERGAN4. The system in package configuration has 650 V, 225 mΩ GaN transistors in half-bridge configuration and dedicated gate drivers. The GaN solution vs. silicon shows higher efficiency and reduced PCB area. MasterGaN is the ideal solution for a compact, high efficiency and high-power inverse buck implementation for lighting applications.
https://www.digikey.co.uk/ Components in Electronics November 2022 33
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