Switching power supply buck circuit CCM and DCM working mode

This article can be downloaded ：《 Switching Mode Power Supply Buck circuit CCM And DCM Working mode 》

One 、Buck Switch type regulator :

Two 、CCM And DCM Definition ：

1、CCM (ContinuousConduction Mode), Continuous conduction mode ： In one switching cycle , Inductive current never reaches 0. Or inductance never “ Reset ”, It means that the inductive flux never returns during the switching cycle 0, When the power tube is closed , There is still current flowing in the coil .

2、DCM,(Discontinuous Conduction Mode) Discontinuous conduction mode ： During the switching cycle , The inductive current will always reach 0, It means that the inductance is properly “ Reset ”, That is, when the power switch is closed , The inductive current is zero .

3、BCM（Boundary Conduction Mode）, Boundary or boundary line conduction mode ： The controller monitors the inductive current , Once the current is detected, it is equal to 0, The power switch closes immediately . The controller always waits for inductive current “ Reset ” To activate the switch . If the inductance value is high , And up to the slope is quite flat , Then the switching cycle is extended , therefore ,BCM The variator is a variable frequency system .BCM The converter can be called critical conduction mode or CRM（Critical Conduction Mode）.

chart 1 Three different working modes are represented by the flower inductance current curve .

chart 2 Three modes of inductance operation ：CCM、DCM、BCM

The midpoint amplitude of the current slope is equal to the average value of the DC output current , The difference between peak current and valley current is ripple current .

3、 ... and 、CCM Working mode and characteristics

according to CCM Definition , Test the waveform of the buck converter working in continuous mode , Here's the picture 3 Shown .

wave form 1 Express PWM graphics , Trigger the switch to on and off . When the switch SW Conduction time , Common point SW/D The voltage on is Vin. contrary , When the switch is off , Common point SW/D The voltage will swing to negative , At this time, the inductive current is opposite to the diode D Provide bias current , Negative depressurization occurs —— Afterflow .

wave form 3 The change of voltage at both ends of the inductor is described . At the equilibrium point , inductance L The average voltage at both ends is 0, And S1+S2=0.S1 The area corresponds to the product of voltage and time when the switch is on ,S2 The area corresponds to the product of voltage and time when the switch is turned off .S1 Simply multiply the height of the rectangle by , and S2 Also rectangular height -t multiply （1-D）.

From the above formula, we can see that it is with D( Duty cycle ) Changing . Ideally , The transfer characteristic is independent of the output load . But the book says this description , Not very accurate , I'll tell you the details after I read them carefully .

In fact, let's look at the last waveform above , When the switch is closed ,SW/D The point current waveform has a big spike , I have measured the voltage waveform , Use voltage chip ACT4065 And ACT4065A, Pictured 4、 chart 5 Shown , There are two specific reasons .

chart 4 chart 5

First of all 、 Because when the switch is closed , Will act on the cathode of the diode , The conduction cycle of the diode is suddenly interrupted . about PN diode , First, when it is necessary to turn on the positive PN When the junction transformer returns to electric neutral PN junction , Remove all minority carriers . It takes a certain time for the diode to recover to its disconnected state after removing all the injected charges , Before full recovery , It presents short circuit behavior . For Schottky diodes , There are metal semiconductor silicon junctions , It has no restorative effect , However , There is a large parasitic capacitance , There are also junction capacitors . When the diode is on , Once discharged ,SW Soon, the voltage is applied through the discharge capacitor , Produce current spikes . So slow down the closing switch SW Time will help reduce the peak current .

second 、 It is related to the current shape . The output ripple can be seen from the image （ Capacitive current waveform ） Very small . The output ripple is very smooth ,“ No pulse ”. It means that the output current signal can be well accepted by the subsequent circuit , That is, the pollution in the power supply is small . in addition , The input current has not only spikes , And it looks like a square wave . If inductance L The value of tends to infinity , The waveform of the input current is a real square wave . therefore , The current is “ pulsation ” electric current , Contains a large amount of pollution , It is more difficult to filter than the general sinusoidal current .

square wave : It is composed of odd harmonics of sine wave , That is, by sine 1,3,5,7…n Equal frequency composition .

There are also spikes at the moment when the switch is turned off , I think it should also be related to diodes and SW The parasitic capacitance of the foot is related to the junction capacitance .

From the above, we can conclude CCM Characteristics of step-down converter ：

1、D Limited to less than 1, The output voltage of the step-down converter is always less than the input voltage ;

2、 If various ohmic losses are ignored , Transformation coefficient M Independent of load current ;

3、 By changing the duty cycle D, The output voltage can be controlled ;

4、 The step-down converter works in CCM, It will bring additional losses . Because it takes time for freewheeling diode to recover charge in reverse , This is for power switches , Is the additional loss burden ;

5、 The output has no pulse ripple , But there is pulse input current .

Four 、DCM Working mode and related characteristics

Switching devices work when the load current is large CCM Pattern , But when the load current decreases , Ripple current will decrease as a whole , Pictured 2 Shown , When the load current is reduced to half of the peak value of harmonic wave , namely

, The lowest point of the slope just drops to zero , At this low point , The inductive current is zero , Inductive energy storage is zero . If the inductive load current is further reduced , Inductance will enter DCM Working mode , The voltage and current waveforms will change greatly, as shown in the figure below 6 Shown , And the transfer function will change a lot .

chart 6

From waveform 4, You can see that the inductive current drops to 0, Cause the freewheeling diode to cut off . If this happens , The left end of the inductance is open . Theoretically , The voltage at the left end of the inductor should return to , Because of inductance L No more current , No oscillation . But there are many parasitic capacitors around , Such as diode and SW The parasitic capacitance of , An oscillation circuit is formed . Like a curve 2 Sum curve 3, Sine signal appears , And disappear after a few cycles , This is related to resistance damping . But there may still be differences in actual tests , For example, I am in ACT4065A In the test , test SW/D Waveform of , The oscillation is in the middle , Here's the picture 7 Shown , The supplier engineer said it was in DCM Pattern , But I didn't find the relevant information to verify .

chart 7

Buck The transformer controls the output voltage at a fixed value in the whole load range , Even if the inductor enters discontinuous operation mode . Therefore, it is easy for us to misunderstand , It is considered that the inductor entering the discontinuous working mode has no effect on the circuit operation . actually , The transfer function of the whole circuit has changed , The control loop must adapt to this change .

about Buck an adjuster , There is no problem for the inductor to enter the discontinuous working mode . Before entering discontinuous mode , DC output voltage

Note that this formula is independent of the load current parameter , So when the load changes , There is no need to adjust the duty cycle D, The output voltage remains constant . actually , When the output current changes , The conduction time will also change slightly , because Q1 The conduction voltage drop and inductance resistance of vary slightly with the change of current , This needs to be Ton Make appropriate adjustments .

Get into DCM After the work , The transfer function will change ,CCM The transfer function of will no longer be applicable , The on time of the switch will decrease with the decrease of the DC output current . Here is DCM Transfer function in working mode , The duty cycle is related to the load current , namely

Because the control loop should control the output voltage to be constant , Load resistance R It is inversely proportional to the load current . hypothesis Vout,Vin、L、T、 Constant , In order to keep the voltage constant , The duty cycle must change with the load current .

At the critical switching current , Transfer function from CCM Turn into DCM. Work CCM when , The duty cycle remains constant , It does not change with the load current ; Work in DCM when , The duty cycle changes as the load current decreases .

From the above, we can conclude DCM Characteristics of step-down converter ：

1、M Dependent on load current ;

2、 For the duty cycle of imagination ,DCM Transfer coefficient under M Than CCM Large working depth at low load current DCM,M Easy to reach 1.

5、 ... and 、Buck Regulator inductance selection ：

In order to reduce the critical output load current when entering the intermittent mode , We can increase the inductance L, To reduce the critical output load current . Make the circuit work in continuous mode within the expected load current range .

Theoretically, we should use 303uH, But in practice, we only use 68uH, Part of it is related to cost , It is also related to the characteristics of my product , The space should be small , If the large inductance cannot be put down at all , Actually, I think , Enough is enough. .

Here is the test ACT4065A when , Some waveforms of the output load current critical value varying with the inductance ：

1）、L1=27uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=27uH when , When the load current gradually increases, the oscillation waveform width decreases , achieve 100mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 100mA

2）、L1=33uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=33uH when , When the load current gradually increases, the oscillation waveform width decreases , achieve 55mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 55mA

3）、L1=47uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=47uH when , When the load current gradually increases, the oscillation waveform width decreases , achieve 45mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 47mA

4）、L1=68uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=68uH when , When the load current gradually increases, the oscillation waveform decreases , achieve 30mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 30mA

5）、L1=136uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=136uH when , When the load current gradually increases, the oscillation waveform width decreases , achieve 20mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 20mA

6）、L1=204uH,Uo=12.51V

By changing the load current , Observe the output waveform , stay L1=204uH when , When the load current gradually increases, the oscillation waveform width decreases , achieve 12mA when , There is no oscillation waveform when the waveform is turned off , Reach the normal switching state .

0mA 12mA

Comprehensively speaking, based on the described and test waveforms , For chips ACT4065A, The inductance increases gradually ,SW When off , The width of oscillation waveform decreases ; The greater the inductance , The smaller the load current, the less the oscillation waveform can be eliminated , But in 10mA This situation exists in .

5、 ... and 、CCM And DCM Compare ：

1、DCM It is the characteristic of technical leadership , Can reduce power consumption ,DCM Mode conversion efficiency is higher , It belongs to complete energy conversion ;

2、 Work in DCM Pattern , Ripple ratio of output current CCM Big ;

3、 Work in DCM Pattern , When the inductive current is 0 When , There will be oscillation ;

4、 Work in CCM Pattern , The output voltage is independent of the load current , When working in DCM Pattern , The output voltage is affected by the load , In order to keep the voltage constant , The duty cycle must change with the load current .

Reference to the original ：《 Switching Mode Power Supply Buck circuit CCM And DCM Working mode 》