Starting circuit with low conducting voltage drop

Abstract

The invention provides a starting circuit with low conducting voltage drop. The input voltage can be detected in real time; when the detected input voltage is lower than a set voltage value, an N-MOStube is switched on, so that the difference between the output voltage and the input voltage of the starting circuit is extremely low, and it is ensured that the starting voltage of a post boost circuit follows the input voltage basically; and when the detected input voltage is higher than the set voltage value, a voltage limiting circuit works, and the output voltage of the starting circuit is limited within the range of working input voltage of the post circuit.

Description

technical field [0001] The invention relates to a starting circuit, in particular to a starting circuit with low conduction voltage drop. Background technique [0002] Start-up circuits are widely used in the field of switching power supplies. In switching power supply products, no matter the primary power supply (AC-DC converter) or the secondary power supply (DC-DC converter), it is necessary to obtain energy from the input terminal and provide a relatively stable power supply voltage for the subsequent control circuit or control IC power supply, so as to drive the switch on and off through the control circuit, and control the inductive device to transfer energy periodically. [0003] In general switching power supplies, in order to solve the problem that when the input voltage is lower than the minimum starting voltage of the control circuit or control IC, the control circuit or control IC cannot be in a normal state because it cannot obtain a high enough power supply vo...

AI Technical Summary

Problems solved by technology

However, due to the large voltage drop between the input and output terminals of the voltage clamping circuit itself, the input power supply voltage of the second-stage boost circuit is reduced, resulting in the failure of the second-stage boost circuit to work normally. Finally, when the input voltage is low, no Able to supply power to the control IC normally

[0004] Traditional voltage clamping circuit schemes such as figure 2 As shown (the load is the post-stage step-up circuit), the working principle of the circuit is: the moment the product is powered on, the triode Q1 is turned on, and the emitter current of the triode Q1 rapidly charges the capacitor C2. When the voltage of the capacitor C2 (that is, the startup When the positive output terminal of the circuit provides the voltage of the power supply terminal Vcc of the control IC to the voltage regulation value of the Zener diode D1 minus the conduction voltage drop between the base and the emitter of the transistor Q1, the Vcc voltage will no longer be stable Increase, when the Vcc voltage reaches the working voltage of the booster circuit, the booster circuit starts to work, and the power supply voltage of the subsequent control circuit or control IC is established, but when the circuit is working at low voltage, the triode Q1 works in the amplification area, and the third stage The voltage drop on tube Q1 and resistor R2 is large, which reduces the voltage value of Vcc at low voltage, causing the second-stage boost circuit to fail to work normally, making it difficult to start the switching power supply system when Vin is low

With other types of voltage stabilizing circuits or voltage clamping circuits, the problem of excessive voltage drop under low voltage will also occur.

[0005] To sum up, the limitation of the switching power supply starting circuit with wide voltage range in the industry is: for products using two-stage power supply circuit scheme or single-stage power supply circuit scheme, the first-stage voltage clamping circuit will have excessive input and output voltage difference under low voltage. Large, the problem that the booster circuit or the post-stage control circuit cannot be started, and the starting voltage range from extremely low voltage to high voltage cannot be met

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