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MC33365 bảng dữ liệu(PDF) 9 Page - ON Semiconductor |
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MC33365 bảng dữ liệu(HTML) 9 Page - ON Semiconductor |
9 / 11 page MC33365 http://onsemi.com 9 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 17. 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 μA, allowing it to be easily overridden with a pull−up resistor. This is desirable in applications that require secondary side voltage sensing. Bulk Capacitor Voltage Comparator In order to avoid output voltage bouncing during electricity brownout condition, a Bulk Capacitor Voltage Comparator with programmable hysteresis is included in this device. The non−inverting input, pin 11, is connected to the voltage divider comprised of RUpper and RLower as shown in Figure 20 monitoring the bulk capacitor voltage level. The inverting input is connected to a threshold voltage of 1.25 V internally. As bulk capacitor voltage drops below the pre−programmed level, (Pin 11 drops below 1.25 V), a reset signal will be generated via internal protection logic to the PWM Latch so turning off the Power Switch immediately. An internal current source controlled by the state of the comparator provides a means to program the voltage hysteresis. The following equation shows the relationship between VBULK levels and the voltage divider network resistors. RUpper + 20 [VBulk_H * VBulk_L ] in K Ohm RLower + 25 [VBulk_H * VBulk_L ] VBulk_H * 1.25 in K Ohm Figure 20. Bulk OK Functional Operation VBULK RUpper RLower BOK 11 Vref 50 mA 1.25 V Protection Logic 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 MC33365. 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 μF or less. 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 |
Số phần tương tự - MC33365_06 |
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Mô tả tương tự - MC33365_06 |
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