Negative pressure generation of buck-boost circuit

In some practical applications , We often use occasions that need to provide negative pressure . Design for negative pressure , Xiaobai talked about a way before , using charge pump Methods . However , Due to its insufficient load carrying capacity , It is often not used in some designs . meanwhile , Xiaobai has also seen that you use the reverse connection of transformer isolation winding to generate negative pressure , However, not all circuits need transformers , The design of magnetic components is avoided in some occasions .

therefore , Which other method can generate negative pressure ？

Today Xiaobai will show you DCDC In the circuit BUCK-BOOST circuit , In some OLED In the driver chip , It is often applied to .

about DCDC circuit , Xiaobai wrote in his previous article BUCK circuit ,BOOST circuit . It mainly acts as a circuit for step-down and step-up . However, today we are going to talk about this kind of circuit , It can be said to be a combination of the two , It can realize boosting , It can also reduce blood pressure .

The picture shows the simplest BUCK-BOOST circuit . It mainly uses power V1 MOS tube Schottky diode capacitance Load resistance as well as PWM controller . See these electronic components , Do you feel familiar with , It is similar to what I learned before BUCK circuit BOOST The devices used in the circuit are almost identical .

MOS The tube chosen is PMOS tube , Its grid is made of PWM Control . As long as meet Vgs<Vgs(th) That is to say MOS The conduction of the tube .
MOS The tube acts as a switch , Control the opening and closing of the branch .
When the grid is high ,Vgs>Vgs(th), here MOS The pipe is disconnected , The branch where it is located does not conduct .
When the grid is low ,Vgs<Vgs(th), here MOS Tube closure , Regardless of the pressure drop , It is equivalent to the role of wires .

therefore , This circuit is discussed in two cases .
（1） When MOS When the tube is on , The current flows to ：V1 Positive pole →C1→V1 Negative pole C1 Positive pole →Q1MOS tube →L1 inductance →C1 Negative pole . At this time, the inductance converts electrical energy into magnetic energy for energy storage , The current flowing through both ends gradually increases , Because the inductance has the characteristic of blocking the change of current , So for its polarity , Performance is positive and negative . meanwhile C2 Capacitor discharge , Power the load .

（2） When MOS When the pipe is disconnected , The magnetic energy of inductance is converted into electric energy , Because the current flowing to both ends gradually decreases , And it has the characteristic of blocking the change of current , So for its polarity , Performance is positive and negative . The current flows to ：V1 Positive pole →C1→V1 Negative pole L1 lower end →C2& Load resistance → Schottky diode →L1 Top . capacitance C2 Being charged .

According to the second volt law of inductance ：
VinTon=-VoutToff
VinDT=-Vout(1-D)T
Vout=-Vin*D/(1-D)

When the duty cycle is less than 50% when , The output is reduced voltage .
When the duty cycle is greater than 50% when , Output is boost .
The conventional BUCK-BOOST voltage , The polarity of output is opposite to that of input , The output shows negative pressure .

Circuit characteristics
The output voltage can be greater than / be equal to / Less than the input voltage
Input / The output current is intermittent
The output voltage is negative pressure
MOS tube 、 The diode terminal voltage is (VIN+VOUT), High voltage withstand requirements for devices .
although ,BUCK-BOOST The circuit can realize boost , It can also reduce blood pressure , However, in terms of efficiency , It is still lower than simple BUCK Circuit or BOOST circuit , So in use , Still need to pay attention to the margin .