When if you want a negative voltage from a positive power supply. You can do to use a Buck/boost converter as shown in figure 1. This is 5V to 12V boost converter circuit or higher using a transistor. Which combines the buck and boost circuit is the same circuit to provide voltage is high than the input.
Figure 1, when the switch (S) closed to causes a current flow through a coil (L). Then, it causes a voltage drop at L is a voltage (UL). And if SW opened, the current on L still flow continuous. But it flows through to diode (D). It is a current collapsed rectifier as the negative voltage appears on output. (Uout)
Both buck / boost combination is the “Buck / boost converter”, but it is not the only function to increase the voltage, we can use it for a lower voltage.
Therefore, it may be also called. Step-up/step-down converter.
Figure 2 The circuit mixed with between the Step-up/step-down converter and the flyback converter circuit.
Consider the circuit in Figure 2. The upper circuit or the section that has elements of the coil L1a is Buck / boost converter, that is when apply the voltage of +5 volt into this circuit causes the current flow through R3 before. The current as described above send to bias a transistor T2 to makes T2 conduct current.
When fully integrated with T2, T1 gives rise to conduction. To provided current out to collector or T1 acts as same a switch connected the voltage source go to drop at L1a. The voltage drop across the coil as above so equivalent power supply.
While the current gradually increased. This higher voltage through C2, R2 to a bias on the T2 and C2 fully charged, has canceled the transfer the current to T2. Makes T2 lower conduct and T1 also lower conduct too.
The electric potential of L1a so changes the direction from poles above the positive, to be negative. and below that is a negative to a positive occurred rectifier or result of the current of the coil through diode D1.
To the output way one, the other one Occurred the reverse bias value with. emitter-base of T2, makes T2 stop and T1 also stop or switch cut the voltage goes out from L1a. The providing current of Buck / boost converter so compete with the basic circuit in figure 1
Then T2 start conduction again and work will continue to provide current to load continuously.
We describe the operation of the transistors these much. In order to accommodate that content. Circuit as shown in Figure 1. The principle of the Switch (S) is a continuous function.
The Circuit needs more than that. This circuit requires a regulation to control the voltage at output constant of -12 volts. So place the Zener diode D3 number ZPD13 have voltage value control 13 volts as the end of the number.
When the output voltage lower than -12 volts makes the Zener diode as above start conduct so can stop working of T2 before timed out work, the time of conduction of T1 and T2 decrease. The providing voltage will lower to stage -12 volts normally.
However, there are problems of design added, is This circuit not only to the negative. But want voltage both positive and negative, Have to apply the principles of the flyback converter circuit principle is shown in Figure 3.
That’s when the switch as figure 3, will cause voltage Uin to drop at the transformer cause is voltage UL. By polarity P of the transformer has positive phase. Now the coil to the Secondary. It also can not apply voltage UL to appear to output.
Because the Electric potential, the reverse diode is shown As black dots. electrical phase in Figure 3.
When switch S cut off, voltage polarity P backward voltage diode so apply a voltage to output. As Uout the simple way is we will turns of coil add to L1b by Number of turns of the coil equivalent L1a.
Or is the Ratio 1: 1. The effect From switching of the circuit. that work with switch T1 to provide the voltage to output by coil L1b.
Which acts as the secondary coil. By has diode D3 act rectifier as the output voltage of +12 volts.
May called the circuit of L1b that the Isolate converter because the voltage as above do not need to connecting common ground with the input circuit.
This circuit has 60% efficiency and should be a transformer El, the same old circuit models used there.
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