Tools

Performance measurement and analysis of isolated DC-DC push-pull regulator

Theory

The circuit diagram of push-pull converter is given in Fig. 1.

Fig. 1. Circuit Diagram of Push-pull Converter.


The switches (Q1 and Q2: ON/OFF-state) are turned-ON/OFF using 180° phase shifted PWM signals. The converter operation can easily be understood from the following equivalent circuits. Brief mathematical analysis is given below.



Fig. 2(a). Circuit in mode-I (Q1- ON, Q2- OFF).



Fig. 2(b). Circuit in mode-II, IV (Q1- OFF, Q2- OFF).



Fig. 2(c). Circuit in mode-III (Q1- OFF, Q2- ON).




1. Voltage conversion ratio or voltage gain (M)
Voltage across inductor L:



Since the frequency of inductor current/voltage is ‘TS/2’, only Mode-I and Mode-II or Mode-III and Mode-IV is sufficient to formulate the voltage gain:

Mode – I :

..(1)



where n=Ns/Np.

Mode – II :

..(2)



Applying ‘volt-sec’ balance across the inductor (eqn. 1 and 2)

..(3)




Solving eqn. 3 gives,

..(4)


2. Average current through the inductor:
Current through capacitor C:



Since the frequency of both the inductor and capacitor currents is ‘TS/2’, only Mode-I and Mode-II is considered.

Mode – I :

..(5)



 

Mode – II :

..(6)



Applying ‘Charge-sec’ balance to the capacitor (eqn. 5 and 6)

..(7)




Solving eqn. 7 gives,

..(8)



Therefore, average inductor current is equal to load current.



3. Power balance under ideal condition (neglecting losses in the converter):



In ideal conditions, the input power is equal to the output power. Hence,


..(9)



 

..(10)



Substituting eqn. 4 in 10,

..(11)





4. Inductor current ripple:

From eqn. 1,


..(12)



 

..(13)



Therefore, the inductor ripple current is,

..(14)




5. Current through various components:

The current through various components can easily be identified from Fig. 3.


Fig. 3. Circuit Diagram of Push-pull Converter.


  Mode-I
(DTs/2) 		Mode-II
(1-D)Ts 		Mode-III
(DTs/2) 		Average Current
imin imax imin imax imin imax Iavg
iL
iC 0
iQ1 0 0 0 0
iQ2 0 0 0 0
iDi1 0 0
iDi2 0 0


6. Voltage and current stress on various components:


  Voltage stress Current Stress
Inductor (L)
Capacitor (C)
Switches (Q1, Q2)
Diodes (Di1, Di2)



7. Efficiency analysis:


..(15)


..(16)


Power losses occurring in various components are given below:

Power loss in inductor:


..(17)


Power loss in capacitor:

..(18)

Power loss in switches:


..(19)



 

..(20)


Power loss in diodes:


..(21)



 

..(22)




Total power loss:


..(23)



8. Effect of non-idealities on voltage gain expression:



Power losses occurring in various components are given below:

Fig. 4. Circuit Diagram of Push-pull Converter with non-idealities.





Voltage across inductor L


Since the frequency of inductor voltage is TS/2, only Mode-I and Mode-II or Mode-III and Mode-IV will be taken:

Mode – I :



..(24)



Mode – II :



..(25)


Applying Volt-sec balance across the inductor (eqn. 24 and 25),

..(26)

Substituting, and solving the above equation gives,


..(27)


("For more Information on mathematical analysis, see references")