Integrated LED Driver based on 800V Si L-IGBTs

摘要

The vast majority of commercially available LED drivers contain active solid-state switches. Indeed, to have functionalities such as dimming, a switch mode power supply (SMPS) is required. Vertical devices, MOSFETs, are the most widely used option. However, there are some limitations associated with vertical devices in terms of layout, chip size and packaging. On the other hand, lateral devices provide the possibility to be integrated with the controller and other circuit components due to all terminals being on one side. Lateral IGBTs (L-IGBTs) have the advantage over lateral MOSFETs (L-MOSFETs) of lower on-state resistance and higher current density, better temperature performance and higher nominal voltage rating. These advantages, together with the reduced device footprint, has led to increased attention towards of L-IGBTs in high voltage low power applications [1], which are a preferential choice for size sensitive products such as LED lighting, mobile phones, implantable cardioverter defibrillators (ICDs) and chargers [2, 3]. This work focuses on the integration of 800V Si L-IGBTs, the controller and the driving circuit considering thermal performance and reliability. The integration exercise is carried out with the consideration that the final circuit is to be fitted into a commercial GU10 LED light bulb. LIGBTs offer much higher current densities, up to a magnitude of ten, significantly lower leakage currents, lower parasitic device capacitances and gate charge compared to conventional vertical MOSFETs commonly used in LED drivers. The dimensions of the device (Fig. 1) bring about advantage in terms of size, but bring with it other challenges. An increase in power density means that the thermal performance needs to be taken into account. In [4], an analysis of different packaging configurations have been carried out by including and varying vias, copper foil as heat sink and the thickness of the PCB copper track and comparing it with insulated metal substrate (IMS). The conclusion is that IMS provides the best thermal performance with the least complexity and potentially more reliable (Fig. 2a). As a result, a test sample has been successfully soldered on an IMS (Fig 2b). As well as decreasing the size of the power semiconductor, the size of the passives and the layout need to be taken into account. Reducing the size of the device doesn't have much impact if all the other components and layout stays the same. One of the aims of this work is to reduce the input capacitor size by using the Barracuda rectification technique and also utilise the capability of integrating the L-IGBT, the controller and other circuit components. Barracuda rectification uses a diode bridge rectifier and a small capacitor which generates a rectified sine wave rather than a DC voltage. This is advantageous as near unity power factor can be drawn without the need for a power factor correction stage. This results in a more compact design, and a cheaper product compared to vertical device. In order to evaluate the thermal performance a circuit simulation was carried out to extract a realistic power dissipation. The circuit used was an isolated flyback with the minimum voltage input feed allowable in the UK. The components on the transformer's secondary side had to be referenced back to the primary side via the turns ratio and coupling. The model of the LIGBT was inserted into the circuit in order to obtain the losses in the on-state and during switching at 27°C and 127°C. The total power loss was then obtained at 27°C and 127°C to be 0.663W and 1.0624W respectively. The values were then used in a 3D thermal simulation software to obtain 104°C for 0.663W and 125°C for 1.0624W (Fig.3). The boundary condition used in the simulation is constant temperature of 70°C. This value is derived by connecting a GU10 LED light bulb was connected to the mains supply and left on for a period of 30 minutes to reach thermal equilibrium. The temperature of the casing can b