Push–pull converter is the most widely used SMPS configuration.
Push-pull converter is not recommended for off-line operation as in a push–pull converter, switching transistors operate at collector stress voltages of at least twice the DC input voltage.
Depending on the mode in which the transformer primary is driven, push-pull converters can be categorized as follows.
• Two-transistor, one-transformer push–pull converter (both self-oscillating and extremely driven type)
• Two-transistor, two-transformer push–pull converter
• Half-bridge converter
• Full-bridge converter
Figure below shows the conventional self-oscillating type of push–pull converter.
Basic self-oscillating type push-pull converter
Base resistors RB1 and RB2 are equal in magnitude. The converter can be considered as equivalent to two alternately operating self-oscillating flyback converters. When transistor Q1 is in saturation, energy is stored in the upper half of the primary winding. When the linearly rising current reaches a value where the transformer core begins to saturate, the current tends to rise sharply. This is not supported by a more or less fixed base bias and the transistor starts to come out of saturation. This is a regenerative process and ends up in switching off transistor Q1 and switching on transistor Q2. Transistors Q1 and Q2 switch on and off alternately. When Q1 is ON, energy is being stored in the upper half of the primary and the energy stored in the immediately preceding half cycle in the lower half of the primary winding (when transistor Q2 was ON) is getting transferred. Therefore, the energy is stored and transferred at the same time. The voltage across secondary is a symmetrical square waveform, which is then rectified and filtered to get the DC output
Self-oscillating push–pull converters are frequently used along with a voltage multiplier chain to design a high-voltage, low-current power supply. Refer to figure below. The basic push–pull converter converts the low DC input voltage to a stepped-up square waveform, which is then multiplied using a chain of diodes and capacitors. This configuration is used for designing helium–neon laser power supplies.
Push–pull converter with voltage multiplier chain
This circuit has a center-tapped primary and only half of the primary winding is active at a time. Therefore, the main transformer is not utilized as well as it is in the case of other forms of push–pull converters, like half-bridge and full-bridge converters.
The transformer provides both power transformation as well as power switching. As the power switching is done at output power levels, the converter efficiency lowers quite a bit for a high-power converter. Also, the transformer core must be the expensive square loop material with a large maximum flux density rating. The peak collector current depends upon the available base voltage, transistor gain and input characteristics and is dependent on load. As there is a wide variation in the characteristics from device to device, the circuit performance depends upon the particular device chosen. These problems are overcome in the two-transformer, two-transistor push–pull converter.
Figure below shows the circuit diagram of an externally driven push-pull converter.
Externally driven push–pull converter
Figure below shows the circuit diagram of a two-transformer, two-transistor push–pull converter.
Two-transformer, two-transistor push–pull converter
Figure below shows the circuit of a half-bridge converter. Transistors Q1 and Q2 operate alternately.
Half-bridge converter
The half-bridge converter has the advantage that it allows the use of transistors with lower breakdown voltages.
Half-bridge converters are used for high power applications.
Figure below shows the circuit diagram of a full-bridge converter.
Full-bridge converter
The full-bridge converter has the advantage that the highest voltage any transistor is subjected to is only VIN against 2VIN as in case of push–pull converter. Owing to reduced voltage and stress on the transistors, full-bridge converter offers a great reliability.
Figure below shows the configuration of the converter along with relevant diagrams. Step-by-step design procedure is outlined as follows.
• First step is to determine the size of the core in terms of the minimum area product required to deliver the desired amount of power to the load for the chosen values of operating frequency and maximum allowable temperature rise of the core.
• The selection of the core size is done either with the help of nomograms provided by the manufacturer or by using the generalized expression given by
• From the first principles,
Where,
NP is the number of primary turns in the collector circuit of either of the two switching transistors = half of the primary turns.
• The magnetic flux varies from -Φ max to + Φ max or + Φmax to -Φ max when either of the two transistors is conducting. In both cases, change in magnetic flux equals 2 Φ max. That is
• Also, this change in flux occurs in a time period equal to half of the total time period. That is
Where,
T is the time period of switching waveform
T = 1/f, f being the switching frequency. Therefore,
Therefore,
(a)
(b)
Self-oscillating push–pull converter: (a) circuit diagram; (b) waveforms Now,
Where,
Bmax is the maximum flux density in the core and Ae the effective area of core cross-section.
• This gives
Therefore,
• Number of secondary turns NS can be computed from
• From the known values of primary turns and core parameters, primary inductance can be determined from
• The primary inductance then determines the peak value of primary current, which can be computed as follows.
Therefore,
• Maximum value of transistor’s collector current is then given by
The chosen transistors should be capable of handling this amount of collector current.
• Maximum value of collector-to-emitter voltage appearing across each of the transistors equals [2VIN + VBE (winding)]. VCEO(max) of the chosen transistor should be about 25–30% higher than this value.
• NB can be determined in the same way as in the case of self-oscillating flyback converter. That is
• RB should be such that it produces a voltage of 0.6 V at the center tap of the feedback winding. That is
• D1–D4 are rectifier diodes. These diodes should have the requisite PIV and forward current ratings.
• Capacitor C is chosen to meet the specified output ripple requirement. Capacitor C is usually chosen to make CRL time constant much larger than (usually 100 times) the period T. That is
• Substituting
in the expression for C, we get
