source:Industry News Popular:adapter release time:2021-06-19 09:41:56 Article author:sznbone
When C1 and C2 are charged, current flows through R1 and R2 into the base of Q1, making Q1 fully conductive. The Zener diode ZD1 did not conduct at the beginning because the voltage at the base of C3 and Q1 is low. As Q1 turns on, a much larger current will flow into the low-resistance R3 and charge C3.
In this circuit, R3 can have a much smaller resistance value than R1 and R in the circuit shown in Figure 1.8.1. This will not cause too much loss or lower efficiency, because the current will only flow through RQ1 during the start-up period, and will be turned off after C is fully charged, and will operate in a saturated conduction during the following entire start-up period. Status, so its power consumption is still very small. R1 should have a high surge rating, wire wound resistors or carbon hybrid resistors can be used.
After power on, the capacitor C3 will charge fairly quickly, and the voltage between the emitter and base of Q1 will track this rising voltage +Vbe until the voltage at the base of Q1 reaches the Zener voltage ZD1, at which point ZD1 starts Turning on makes Q1 enter a linear operating state and reduces the charging current flowing into C3. At this time, voltage and power consumption are formed on Q1. However, once the normal working state of the converter is formed, the positive feedback from the additional winding on the main transformer will provide current to C3 through D6 and resistor R4 so that the voltage on C3 will be Continue to increase until the base-emitter of Q1 is reverse-biased and completely turned off.
At this time, diode D5 is turned on, and the voltage across C3 will be clamped by Zener diode ZD1 and diode D5. The power consumption of ZD1 depends on the voltage of R4 and the value of the maximum auxiliary current. After Q1 is turned off, the voltage flowing through R3 The current is also zero, so the power consumption of R3 and Q1 will be reduced to zero.
Because the start-up action is very fast, R3 and Q1 here can be used with very small components and do not require a radiator. In order to avoid dangerous power consumption on Q1 and R3 when the converter fails, R should be able to maintain continuous power, otherwise it will be unsafe. Fusible resistors or positive temperature coefficient (PTC) thermistors have inherent self-protection Characteristics, it is ideal for this kind of occasion.
This circuit can support a considerable starting current, which brings greater freedom to the design of the drive circuit.
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