| 摘要 |
PROBLEM TO BE SOLVED: To provide a power supply unit having a negative power supply circuit, capable of attaining high safety without outputting a positive voltage at least before the negative power supply circuit starts up. SOLUTION: In this power supply unit having a negative power supply circuit, a boost converter 10 and the negative power supply circuit 20 share a voltage reference 6 for outputting a reference voltage Vr, and a series circuit having a resistor 27, a resistor 28 and a first switch 31 is connected with between an output terminal 2o and the voltage reference 6. The first switch 31 turns on when a control signal Vc is at an "H" level and turns off when the control signal Vc is at an "L" level. An error amplifier 29 applies a connecting point potential Vs between the resistor 27 and the resistor 28 to an inverting input terminal, thereby grounding a non-inverting input terminal. Further, an output from the error amplifier is input into a PWM controller 30 to drive a main switch 21 by a pulse signal at an on/off time ratio corresponding to the output. Before the negative power supply circuit 20 starts up, a positive voltage is not output from the voltage reference 6 through the resistor 27 and the resistor 28 by the first switch 31 even if the boost converter 10 starts up earlier than the negative power supply circuit 20 and the reference voltage Vr is output from the reference voltage 6. COPYRIGHT: (C)2008,JPO&INPIT
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