Working principle of push-pull converter

A push-pull DC/DC converter, or simply push-pull converter, is a power electronic device that uses two power switching transistors to alternately switch on and off to achieve DC/DC conversion. It can be viewed as a combination of two single-transistor forward DC/DC converters, and its output rectification and filtering circuits are basically the same as those of a forward DC/DC converter.

VOOHU Electronics Technology, with nine years of experience in the communications electronics field, has launched a push-pull converter solution, featuring a compact push-pull transformer that reconfigures power isolation solutions with millimeter-level dimensions. It is available in various package types, such as WHST06E, WHST060, WHST06D, and WHST06L.

Topology of Push-Pull DC/DC Converter
The topology of a push-pull DC/DC converter is shown in Figure 1. The core components and their parameters are defined as follows:
High-frequency push-pull transformer (T):
Contains primary windings (N_P1、N_P2) and secondary windings (N_S1、N_S2). Both primary and secondary windings have center taps.N_{P1 and} N_P2  have the same number of turns, and N_S1 and N_S2 have the same number of turns. The polarities of the primary and secondary windings are consistent, and the positions of the corresponding terminals are marked in the figure.
Power switching transistors (VT₁、VT₂):
Generally, transistors or MOSFETs are used to control the circuit's on/off state.
Output rectifier diodes (VD₁、VD₂):
Rectify the secondary induced voltage and unify the current direction.
Output filter components:
Filter inductor (L), filter capacitor (C), used to stabilize the output voltage and current and prevent sudden changes.
Load and output:
External load resistor (R), DC output voltage (U_O);
Control core:
Pulse width modulator (PWM), generates two control signals (U_A，U_B) with a 180° phase difference to drive...  VT₁ and VT₂  work alternately, and the PWM duty cycle is theoretically less than 50%, which generates a dead zone.


Working Principle of Push-Pull DC/DC Converter
The control signals  U_A and U_B output by the pulse width modulator (PWM) are alternately complementary: when U_A is high, U_B is low, and vice versa. Power switches VT₁、VT₂  alternately turn on and off under the control of the PWM signal (equivalent to a high-speed mechanical switch); for ease of analysis, the on/off states of switches  S₁、S₂ correspond to the on and off states of VT₁、VT₂, respectively.


VT₁ On (VT₂ Off) State
When U_A is high, VT₁ is on and VT₂ is off. The current path is shown in Figure 2:
The input voltage (U_I) is applied across the primary winding N_P1, and the primary current (IP1) increases linearly. The polarity of the induced electromotive force (EMF) in NP1 is "-" at the top and "+" at the bottom.
According to the principle of electromagnetic induction, the polarity of the induced voltages in the primary winding N_P2 and the secondary windings N_S1 and N_S2 is "-" at the top and "+" at the bottom. At this time, VD₁ is off and VD₂ is on.
The induced voltage (U_S2) in the secondary winding N_S2 acts on the left end of the filter inductor L, forming a linearly increasing secondary current (I_S2, i.e., the forward current of VD₂). The energy stored in the inductor L increases, and its induced EMF polarity is "+" on the left and "-" on the right.
I_S2 charges the filter capacitor C (charging current I1) and provides the output current (IO) to the load R_L, satisfying I_S2 = ... I₁ + I<sub>O.

VT2 On (VT1 Off) State</sub>
When U_B is high, VT₂ is on and VT₁ is off. The current path is shown in Figure 3:
The input voltage (U_I) is applied across the primary winding N_P2, and the primary current (I_P2) increases linearly. The polarity of the induced electromotive force (EMF) of N_P2 is "+" at the top and "-" at the bottom.
According to the principle of electromagnetic induction, the polarity of the induced voltages of the primary winding N_P1 and the secondary windings N_S1 and N_S2 is "+" at the top and "-" at the bottom. At this time, VD₁ is on and VD₂ is off.
The induced voltage (U_S1) of the secondary winding N_S1 acts on the left end of the filter inductor L, forming a linearly increasing secondary current (I_S1, i.e., the forward current of VD₁). The energy stored in the inductor L increases, and its induced EMF polarity is "+" on the left and "-" on the right.
I_S1 charges the filter capacitor C (charging current I₂) on one hand, and provides the output current (I_O) to the load R_L on the other hand, satisfying I_S1 = ... I₂ + I_O.


Dead Time Design
When the PWM control signal duty cycle (D) is < 50%, there will be a period of time when both U_A and U_B are low. To avoid short-circuit damage caused by VT₁ and VT₂ conducting simultaneously, a dead time (DT) needs to be reserved, typically set to 1~3μs; and the smaller the duty cycle D, the longer the period of time when both U_A and U_B are low.
Tip：
To eliminate the risk of both power switches conducting simultaneously, the duty cycle (D) of the two control signals output by the pulse width modulator (PWM) must be strictly less than 50%.

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