The role of start-up power supply series transistors in switching power supply design
Switching power supply chip of the two supply voltage, all the way is the starting resistor to provide the starting voltage, all the way is the transformer auxiliary power supply generated by the auxiliary voltage, string in the middle of the starting resistor transistor role, a friend asked me is not an over-voltage protection circuit, the two transistors is not over-voltage protection, but to close the role of the starting resistor, in some occasions, the need for power supply is very low power loss, to improve the efficiency of the power supply, often in the Battery-powered systems, where you can reduce power consumption will definitely be closed, switching power supply design will sometimes take into account a lot of aspects, cost, low power consumption, efficiency, EMC, etc., according to which is sensitive, will correspond to make the programme, may be in order to reduce the power consumption, will increase a few components, thus increasing the cost, other factors will be a compromise choice.
The following is the principle of the starting resistor off circuit, DC bus voltage through resistors R20, R17, to D4 voltage regulator diode regulator, generating 12V, transistor conduction output of 12V voltage, the power supply chip here is uc3843, low start-up voltage, if it is uc3842, the regulator diode should be replaced with 18V regulator, when the power supply chip voltage exceeds 16V at foot 7, 8 foot Reference voltage output, through the resistor R21 into the transistor Q5 base, so that the transistor conduction, collector voltage is pulled down, Q4 transistor cut-off, the start-up resistor is disconnected, is not involved in the work of the power supply chip power supply voltage has an auxiliary winding power supply, since the power supply start-up is complete.
Doing so can reduce the power consumption of the system, increase the life of the start resistor, add a few more components, also met the phenomenon of transistor breakdown, can not cut off the start resistor, diode D4 breakdown, resulting in the power supply does not start, this natural damage is rare, basically is the power supply chip is damaged, along with the peripheral circuitry is damaged, the DC bus voltage is high, the breakdown of the transistor. Auxiliary power supply and shut down the starting resistor circuit is finished, the next time to talk about the role of the 03 transistor. Strive to put every piece of circuit All speak to, if not correct, please advise, the principle of the finished, will take you together to create a switching power supply, easier to understand the role of each component.
Note that the above circuit diagram is a low-voltage 48V input, all circuit parameters are different from 220V.
