[hard ten treasures] - 2 [basic knowledge] characteristics of various topological structures of switching power supply

1、 Basic terms

Common basic topologies
  ■ Buck step-down
  ■ Boost boost
  ■ Buck-Boost step-down - boost
  ■ Flyback Flyback
  ■ Forward Forward excitation
  ■ Two-Transistor Forward Dual transistor forward
  ■ Push-Pull push-pull
  ■ Half Bridge Half bridge
  ■ Full Bridge Full bridge
  ■ SEPIC
  ■ C’uk

Basic pulse width modulation waveform
These topologies are related to switching circuits .
The basic pulse width modulation waveform is defined as follows ：

2、Buck step-down

characteristic
  ■ Reduce the input to a lower voltage .
  ■ Probably the simplest circuit .
  ■ inductance / The capacitor filter filters out the square wave after the switch .
  ■ The output is always less than or equal to the input .
  ■ The input current is discontinuous ( Chopping ).
  ■ The output current is smooth

3、Boost boost

** characteristic **
  ■ Raise the input to a higher voltage .
  ■ Same as depressurization , But rearranged the inductance 、 Switches and diodes .
  ■ The output ratio is always greater than or equal to the input ( Ignore the forward voltage drop of the diode ).
  ■ Input current is smooth .
  ■ The output current is discontinuous ( Chopping ).

4、Buck-Boost step-down - boost

characteristic
  ■ inductance 、 Another arrangement of switches and diodes .
  ■ It combines the disadvantages of step-down and step-up circuits .
  ■ The input current is discontinuous ( Chopping ).
  ■ The output current is also discontinuous ( Chopping ).
  ■ The output is always opposite to the input ( Pay attention to the polarity of the capacitor ), However, the amplitude can be less than or greater than the input .
  ■ “ Flyback ” The converter is actually buck - Boost circuit isolation （ Transformer coupling ） form .

5、Flyback Flyback

characteristic
  ■ Such as reducing blood pressure - The boost circuit works the same , But the inductor has two windings , And as both transformer and inductor .
  ■ The output can be positive or negative , Determined by the polarity of the coil and diode .
  ■ The output voltage can be greater than or less than the input voltage , Determined by the turns ratio of the transformer .
  ■ This is the simplest of the isolated topologies
  ■ Multiple outputs can be obtained by adding secondary windings and circuits .

6、Forward Forward excitation

characteristic
  ■ Transformer coupling form of step-down circuit .
  ■ Discontinuous input current , Smooth output current .
  ■ Because of the transformer , The output can be greater or less than the input , It can be any polarity .
  ■ Multiple outputs can be obtained by adding secondary windings and circuits .
  ■ The transformer core must be demagnetized in each switching cycle . The common practice is to add a winding with the same number of turns as the primary winding .
  ■ The energy stored in the primary inductor during the switch on phase , During the switch off phase, it is released through another winding and diode .

7、Two-Transistor Forward Dual transistor forward

characteristic
  ■ Both switches work at the same time .
  ■ When the switch is off , The energy stored in the transformer reverses the polarity of the primary , Turn on the diode .
  The main advantages ：
  ■ The voltage on each switch will never exceed the input voltage .
  ■ There is no need to reset the winding track .

8、Push-Pull push-pull

characteristic
  ■ switch （FET） The driving phases are different , Pulse width modulation （PWM） To adjust the output voltage .
  ■ Good transformer core utilization — Power is transmitted in both half cycles .
  ■ Full wave topology , Therefore, the output ripple frequency is twice the frequency of the transformer .
  ■ Imposed on FET The voltage on the is twice the input voltage .

9、Half-Bridge Half bridge

characteristic
  ■ High power converters are very common topologies .
  ■ switch （FET） The driving phases are different , Pulse width modulation （PWM） To adjust the output voltage .
  ■ Good transformer core utilization — Power is transmitted in both half cycles . Moreover, the utilization rate of primary winding is better than that of push-pull circuit .
  ■ Full wave topology , Therefore, the output ripple frequency is twice the frequency of the transformer .
  ■ Imposed on FET The voltage on the is equal to the input voltage .

10、Full-Bridge Full bridge

characteristic
  ■ Higher power converters are the most commonly used topology .
  ■ switch （FET） Drive in the form of diagonal pairs , Pulse width modulation （PWM） To adjust the output voltage .
  ■ Good transformer core utilization — Power is transmitted in both half cycles .
  ■ Full wave topology , Therefore, the output ripple frequency is twice the frequency of the transformer .
  ■ Imposed on FETs The voltage on the is equal to the input voltage .
  ■ At a given power , The primary current is half that of the half bridge .

11、SEPIC Single ended primary inductive converter

characteristic
  ■ The output voltage can be greater than or less than the input voltage .
  ■ Same as boost circuit , Input current is smooth , But the output current is discontinuous .
  ■ Energy is transferred from the input to the output through a capacitor .
  ■ Need two inductors .

12、C’uk(Slobodan C’uk Patents )

characteristic
  ■ Output inverting
  ■ The amplitude of the output voltage can be greater or less than the input voltage .
  ■ The input current and output current are smooth .
  ■ Energy is transferred from the input to the output through a capacitor .
  ■ Need two inductors .
  ■ Inductors can be coupled to obtain zero ripple inductor current .

13、 Details of circuit operation

The following explains the working details of several topologies
  ■ Step down regulator ：
     Continuous conduction
     Critical conductivity
     Discontinuous conduction
  ■ Boost regulator ( Continuous conduction )
     Transformer operation
     Flyback transformer
     Forward transformer

14、Buck- Step down regulator - Continuous conduction

characteristic
  ■ The inductive current is continuous .
  ■ Vout Is its input voltage (V1) The average of .
  ■ The output voltage is the input voltage multiplied by the load ratio of the switch (D).
  ■ On , Inductive current flows out of the battery .
  ■ When the switch is off, current flows through the diode .
  ■ Ignore losses in switches and inductors , D Independent of load current .
  The characteristics of the step-down regulator and its derived circuit are ：
    ■ The input current is discontinuous ( Chopping ), The output current is continuous ( smooth ).

15、Buck- Step down regulator - Critical conductivity

  ■ The inductive current is still continuous , Only when the switch is turned on again “ achieve ” zero .
     This is known as “ Critical conductivity ”.
     The output voltage is still equal to the input voltage multiplied by D.

16、Buck- Step down regulator - Discontinuous conduction

  ■ under these circumstances , The current in the inductor is zero for a period of time in each cycle .
  ■ The output voltage remains ( Throughout ) yes v1 Average value .
  ■ The output voltage is not the input voltage multiplied by the load ratio of the switch (D).
  ■ When the load current is below the critical value ,D Varies with load current ( and Vout remain unchanged ).

17、Boost Boost regulator

  ■ The output voltage is always greater than （ Or equal to ） Input voltage .
  ■ The input current is continuous , The output current is discontinuous （ As opposed to the step-down regulator ）.
  ■ Output voltage to load ratio （D） The relationship is not as simple as in the step-down regulator . In the case of continuous conduction ：
$$Vo=Vin(\frac{1}{1-D})$$
   In this case ,Vin = 5,Vout = 15,and D = 2/3,Vout = 15,D = 2/3.

18、 Transformer operation （ Including the role of primary inductance ）

  ■ The transformer is regarded as an ideal transformer , Its primary （ magnetization ） The inductor is connected in parallel with the primary .

19、 Flyback transformer

  ■ The primary inductance here is very low , Used to determine peak current and stored energy . When the primary switch is off , Energy is transferred to the secondary .

20、Forward Forward converter transformer

  ■ The primary inductance is very high , Because there is no need to store energy .
  ■ Magnetizing current (i1) inflows “ Magnetizing inductance ”, Demagnetize the magnetic core after the primary switch is disconnected ( The voltage is reversed ).

21、 summary

  ■ The most common circuit topologies in switching mode power supply conversion are reviewed here .
  ■ There are also many topologies , But most of them are combinations or deformations of the topologies described here .
  ■ Each topology contains unique design tradeoffs ：
     The voltage applied to the switch
     Chopping and smoothing input and output current
     Utilization of windings
  ■ Choosing the best topology requires research ：
     Input and output voltage range
     Current range
     Cost and performance 、 The ratio of size to weight

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