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MC33363BDW bảng dữ liệu(PDF) 10 Page - ON Semiconductor |
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MC33363BDW bảng dữ liệu(HTML) 10 Page - ON Semiconductor |
10 / 11 page MC33363B http://onsemi.com 10 The Power Switch is designed to directly drive the converter transformer and is capable of switching a maximum of 700 V and 1.0 A. Proper device voltage snubbing and heatsinking are required for reliable operation. A Leading Edge Blanking circuit was placed in the current sensing signal path. This circuit prevents a premature reset of the PWM Latch. The premature reset is generated each time the Power Switch is driven into conduction. It appears as a narrow voltage spike across the current sense resistor, and is due to the MOSFET gate to source capacitance, transformer interwinding capacitance, and output rectifier recovery time. The Leading Edge Blanking circuit has a dynamic behavior in that it masks the current signal until the Power Switch turn−on transition is completed. The current limit propagation delay time is typically 262 ns. This time is measured from when an overcurrent appears at the Power Switch drain, to the beginning of turn−off. Error Amplifier An fully compensated Error Amplifier with access to the inverting input and output is provided for primary side voltage sensing, Figure 19. It features a typical dc voltage gain of 82 dB, and a unity gain bandwidth of 1.0 MHz with 78 degrees of phase margin, Figure 6. The noninverting input is internally biased at 2.6 V ±3.1% and is not pinned out. The Error Amplifier output is pinned out for external loop compensation and as a means for directly driving the PWM Comparator. The output was designed with a limited sink current capability of 270 mA, allowing it to be easily overridden with a pullup resistor. This is desirable in applications that require secondary side voltage sensing. Overvoltage Protection An Overvoltage Protection Comparator is included to eliminate the possibility of runaway output voltage. This condition can occur if the control loop feedback signal path is broken due to an external component or connection failure. The comparator is normally used to monitor the primary side VCC voltage. When the 2.6 V threshold is exceeded, it will immediately turn off the Power Switch, and protect the load from a severe overvoltage condition. This input can also be driven from external circuitry to inhibit converter operation. Undervoltage Lockout An Undervoltage Lockout comparator has been incorporated to guarantee that the integrated circuit has sufficient voltage to be fully functional before the output stage is enabled. The UVLO comparator monitors the VCC voltage at Pin 3 and when it exceeds 14.5 V, the reset signal is removed from the PWM Latch allowing operation of the Power Switch. To prevent erratic switching as the threshold is crossed, 5.0 V of hysteresis is provided. Startup Control An internal Startup Control circuit with a high voltage enhancement mode MOSFET is included within the MC33363B. This circuitry allows for increased converter efficiency by eliminating the external startup resistor, and its associated power dissipation, commonly used in most off−line converters that utilize a UC3842 type of controller. Rectified ac line voltage is applied to the Startup Input, Pin 1. This causes the MOSFET to enhance and supply internal bias as well as charge current to the VCC bypass capacitor that connects from Pin 3 to ground. When VCC reaches the UVLO upper threshold of 15.2 V, the IC commences operation and the startup MOSFET is turned off. Operating bias is now derived from the auxiliary transformer winding, and all of the device power is efficiently converted down from the rectified ac line. The startup MOSFET will provide a steady current of 1.7 mA, Figure 11, as VCC increases or shorted to ground. The startup MOSFET is rated at a maximum of 400 V with VCC shorted to ground, and 500 V when charging a VCC capacitor of 1000 mF or less. Output Short−Circuit Condition When the output is short−circuited, the VCC is powered by the internal startup circuit instead of the VCC auxiliary winding. The internal startup circuit turns on and charges up VCC voltage when VCC reaches its UVLO lower threshold of 9.5V. It turns off and VCC voltage drops when VCC reaches its UVLO upper threshold of 15.2V. The device only delivers drain current for the time when VCC goes from 15.2V to 9.5V. No drain current is delivered when VCC goes from 9.5V to 15.2V. As a result, some of the switching cycle is missed as shown in Figure 20. The drain current limit is limited by the cycle−by−cycle current limit. Regulator A low current 6.5 V regulated output is available for biasing the Error Amplifier and any additional control system circuitry. It is capable of up to 10 mA and has short−circuit protection. This output requires an external bypass capacitor of at least 1.0 mF for stability. Thermal Shutdown and Package Internal thermal circuitry is provided to protect the Power Switch in the event that the maximum junction temperature is exceeded. When activated, typically at 135 °C, the Latch is forced into a ‘reset’ state, disabling the Power Switch. The Latch is allowed to ‘set’ when the Power Switch temperature falls below 105 °C. This feature is provided to prevent catastrophic failures from accidental device overheating. It is not intended to be used as a substitute for proper heatsinking. The MC33363B is contained in a heatsinkable plastic dual−in−line package in which the die is mounted on a special heat tab copper alloy lead frame. This tab consists of the four center ground pins that are specifically designed to improve thermal conduction from the die to the circuit board. Figures 16 and 17 show a simple and effective method of utilizing the printed circuit board medium as a heat dissipater by soldering these pins to an adequate area of copper foil. This permits the use of standard layout and mounting practices while having the ability to halve the junction to air thermal resistance. The examples are for a symmetrical layout on a single−sided board with two ounce per square foot of copper. |
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