Features - Bodo's Power

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POWER SUPPLY LT3751 has the feedback loop internally compensated in the regulation configuration that simplifies stability compensation and has an on-board DONE pin that goes active when the output capacitor charge voltage is reached. Furthermore, the CHARGE pin initiates a new charge cycle or enables the part in voltage regulation mode. A Figure 3a: Typical Applications Circuit Figure 3b: Efficiency Curve with Secondary-Side Output Sense low 106mV differential current sense threshold accurately limits peak switch current and allows the use of a low power primary side current sense resistor. The LT3751 is packaged in a thermally enhanced 4mm x 5mm QFN-20 package, and is offered in extended and industrial temperature ranges from -40°C to 125°C. Gate Driver & Internal Clamp There are four main concerns when using a gate driver; output current drive capability, peak output voltage, power consumption and propagation delay. The LT3751 is equipped with a 1.5A push-pull main driver, enough to power large 80nC gates. Most discrete MOSFET’s have a gate to source limit of 20V. And so driving a MOSFET higher than 20V can cause a short in the internal gate oxide, causing permanent damage. To alleviate this issue, the LT3751 has an internal selectable 5.6V or 10.5V gate driver clamp. No external components are needed, not even a capacitor. Simply tie the CLAMP pin to ground for 10.5V operation or tie it to VCC for 5.6V operation. Not only does the internal clamp protect the external MOSFET from damage, it also reduces the amount of energy injected into the gate. This increases the overall efficiency and reduces the power consumption in the gate driver circuit. High Input Supply Voltage, Isolated Capacitor Charger As already stated, the differential DCM and VOUT comparators allow the LT3751 to accurately work from high input voltages. A full wave bridge rectified off-line capacitor charger is shown in figure 4. The transformer provides primary to secondary isolation and the output Figure 4 – 100V to 400V Input, 500V Output Capacitor Charger voltage is sensed from the primary side transformer winding. Input voltages of greater than 80V require the use of resistor dividers on the DCM and VOUT comparators. Thus the circuit in figure 4 operates from a 100V to 400VDC input voltage. Note that R14, R15, and Q1 are added to protect the external M1 MOSFET from exceeding the maximum pulse rating. Under normal conditions, this MOSFET must discharge the total equivalent capacitance present on the MOSFET drain node. Since this node can initially be charged to over 400V, significant current spikes occur when the MOSFET is first turned on, which can permanently damage the MOSFET. Inserting R14 sets the maximum current spike to: ISPIKE = 0.7____ = 3.6A R13 + R14 Conclusion The LT3751 allows an easy path for the design of a high voltage power supply and capacitor charger over a wide input voltage. Having the ability of knowing when the output capacitor is fully charged without a physical connection to the high output voltage minimizes the number of parts crossing the isolation barrier. The protection, status and selectable gate drive voltage of the LT3751 minimizes additional external components required for reliable operation. Boundary mode control prevents the converter from going into an over current condition at start-up when powering a capacitive load and allows for a smaller transformer enabling a design that takes up less PCB space. Also, a LT3751-based design can achieve efficiency greater than 85% and can significantly simplify the design. www.linear.com 44 Bodo´s <strong>Power</strong> Systems ® August 2009 www.bodospower.com
EPE 2009 �� Barcelona, Spain 13th European Conference on <strong>Power</strong> Electronics and Applications www.epe2009.com Receipt of synopses: Monday 3 November 2008 Receipt of full papers: Monday 11 May 2009
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