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Converter types used in SMPS.

With the help of neat circuit diagram explain the working of SMPS.

Marks: 7M,10M

Year: Dec11, Dec12

With the help of neat circuit diagram explain the working of SMPS.

1) In order to obtain almost negligible ripple in the D.C. output voltage, physical size of filters required is quite large.

2) This makes the D.C. power supply as inefficient, bulky and weighty.

3) On the other hand, SMPS works like a D.C. chopper. By operating on/off switches very rapidly, A.C. ripple frequency rises which can be easily filtered by ‘L’ and ‘C’ filter circuits which are small in size and less weighty.

4) It may be therefore inferred that it is the requirement of small physical size and weight that has led to the widespread of SMPS.

5) The four categories of SMPS listed as follows:

a) Flyback Converter

b) Push Pull Converter

c) Half Bridge Converter

d) Full Bridge Converter

6) Flyback Converter

V1 / V2 = N1/N2

Here, V1 = Vs

Operation:

• When Power Mosfet M1 is turned ON, supply voltage Vs is supplied to the transformer primary, i.e. V1=Vs. A corresponding voltage V2, with the polarity as shown in fig 11.2(a) , is induced in the transformer secondary, i.e.V2= (Vs*N2)/N1

• As V2 reverses biases diode D, equivalent circuit of fig 11.2(a) is obtained.

• Filter capacitance ‘C’ is assumed large enough so that capacitor voltage vc(t) = load or output voltage Vo is taken as almost constant.

• When M1 is turned off, a voltage of opposite polarity is induced in primary and secondary windings as shown in fif 11.2(b)

• Voltage across transformer secondary is V2 = -Vo = - (Vs*N2) / N1

• Diode D is forward biased and starts conducting a current iD . As a result, energy stored in the transformer core is delivered partly to load and partly to charge the capacitor ‘C’

Waveforms:

Waveforms for $V_1 , V_2$ , transformer magnetizing current $_im$ and diode current $i_D$ are shown in fig 11.3.

7) Push Pull Converter

SMPS with push pull configuration is shown in fig

• It uses two power MOSFETs M1 and M2 and a transformer with mid-taps on both primary and secondary sides.

• As in flyback converter, an uncontrolled rectifier feeds push-pull SMPS.

• Inductor ‘L’ and capacitor ‘C’ are the filter components.

• When M1 is turned on, Vs is applied to lower half of transformer primary, i.e. $v_1= V_s$.

• As a result, voltage $v_2 = (V_s*N_2)/N_1$ is induced in both the secondary windings.

• Voltage $v_2$ in the upper half secondary forward biases diode D1, therefore load voltage

  $V_0 = ( V_s*N_2)/N_1 = αV_s$

• When $M_2$ is turned on, $v_1 = -V_s$ is applied to upper half of primary winding. Consequently ,

  $v2 = - (V_s*N_2)/N_1$ is induced in both the transformer secondary. As v2 is negative, diode D2 gets forward biased and V0 = αVs

• This shows that voltage on primary swings from $+V_s$ with $M_1$ on to $–V_s$ with $M_2$ on. Power mosfets $M_1$ and $M_2$ operate with duty cycle of 0.5.

• AS both $M_1$ and $M_2$ are subjected to open circuit voltage of 2Vs, this configuration is suitable for low voltage applications only.

8) Half Bridge converter

The circuit for half bridge SMPS configuration is shown in fig 11.5

• It consist of an uncontrolled rectifier, two capacitors $C_1$ and $C_2$, two proper MOSFETs $M_1$ and $M_2$ , one transformer with mid tap on the secondary side, two diodes $D_1$ and $D_2$ and filter components L and C

• Two capacitors $C_1$ and $C_2$ have equal capacitance; therefore voltage across each of the two is (vs/2).

• When M1 is turned on, voltage of C1 appears across transformer primary, i.e. v1 = - Vs / 2 and voltage induced in secondary is $v_2 = - (V_s*N_2)/2N_1$ therefore diode gets forward biased.

• This means that transformer primary voltage swings from – Vs/2 to +Vs/2 .

• Average output voltage, $V_0 = (V_s*N_2) / 2N_1 = 0.5αVs$

• For h.v.dc applications, half bridge converter is, therefore, preferred over push-pull converters.

9) Full bridge converter

• It consists of an uncontrolled rectifier, four power MOSFETs, transformer with mid-tap secondary, two diodes and LC filter circuit.

• The function of control circuit is to sense the output load voltage and to decide the delay ratio of MOSFETs.

• When power MOSFETs $M_1$ and $M_2$ are turned on together, voltage $V_s$ appears across transformer primary, i.e. $v_1 = Vs and v_2 = (Vs*N_2 ) / N_1 = αVs$

• Diode D1 gets forward biased and $V_0 = αVs$

• When $M_3$ and $M_4$ are turned on together, the primary voltage is reversed, i.e. $v_{1v} = -Vs and v_2 = - (Vs*N_2 ) / N_1 = -αVs$ . Therefore , diode D2 now begins to conduct and the output voltage is again $V_0 = αVs$

• It is used for high power applications above 750W.