1. Basic principle of push-pull transformer

The push-pull transformer is a high-frequency transformer that is widely used in power electronic devices such as switching power supplies and inverters. Its basic working principle involves utilizing two alternately conducting switching devices (such as MOSFETs or BJTs) to convert the input voltage into a high-frequency AC voltage through the primary winding of the transformer. The secondary winding then rectifies and filters the voltage to output the desired DC voltage.

Basic circuit structure:

• Primary winding: connected to two alternately conducting switching devices.

• Secondary winding: Outputs high-frequency AC voltage, which is rectified and filtered to obtain DC voltage.

• Center tap: The center tap of the primary winding is usually connected to the positive terminal of the input power supply.

Working principle:

• When a switching device is turned on, current flows through half of the primary winding to ground, generating magnetic flux in one direction.

• When another switching device is turned on, current flows through the other half of the primary winding to ground, generating a magnetic flux in the opposite direction.

• The two switching devices are alternately turned on, causing the magnetic flux in the transformer core to alternate in two directions, thereby inducing an AC voltage in the secondary winding.

2. Design characteristics of push-pull transformers

advantage:

• High efficiency: Due to the alternating conduction of two switching devices, the conduction time of each device is relatively short, resulting in low conduction loss.

• High power density: Capable of handling high power, suitable for applications in the medium power range.

• Isolation function: Electrical isolation between input and output is achieved through a transformer, enhancing safety.

Disadvantages:

• Magnetic bias issue: If the conduction times of two switching devices are not completely symmetrical, it may lead to an imbalance in the magnetic flux within the transformer core, resulting in a magnetic bias phenomenon.

• High requirements for switching devices: Since the center tap of the primary winding is connected to the positive pole of the input power supply, the switching devices need to withstand a relatively high voltage.

3. Application scenarios of push-pull transformers

Push-pull transformers are widely used in the following fields:

• Switching power supply: It is used to convert the input DC voltage into a high-frequency AC voltage, which is then rectified and filtered to output the required DC voltage.

• Inverter: Converts DC voltage into AC voltage for driving motors or other AC loads.

• Communication equipment: used in power modules to provide stable power supply.

• Industrial automation equipment: such as PLCs, frequency converters, etc., providing efficient power conversion.

4. Design steps of push-pull transformer

Step 1: Determine input and output parameters

• Input voltage range: Determine the minimum and maximum values of the input voltage.

• Output voltage and current: Determine the output voltage and maximum output current.

Step 2: Select transformer parameters

• Transformer turns ratio: Select an appropriate turns ratio based on the input-output voltage ratio.

• Transformer core: Select the appropriate core material and size to ensure that the core does not saturate.

• Primary and secondary windings: Design the number of turns and wire diameter of the primary and secondary windings according to the current and voltage requirements.

Step 3: Design the drive circuit

• Drive signal: Design two drive signals that are alternately turned on, ensuring that the two switching devices do not turn on simultaneously.

• Dead time: Set an appropriate dead time to prevent short circuits caused by the simultaneous conduction of two switching devices.

Step 4: Design the protection circuit

• Overcurrent protection: Design an overcurrent protection circuit to prevent the transformer and switching devices from being overloaded.

• Overvoltage protection: Design an overvoltage protection circuit to prevent damage to the load caused by excessively high output voltage.

5. Actual design case

Below is a design example of a push-pull DC/DC converter based on LT3999:

Circuit parameters:

• Input voltage range: 10V to 15V

• Output voltage: 5V

• Output current: 400mA

Design steps:

• Selecting a transformer: Choose a suitable high-frequency transformer, ensuring that its turn ratio and core parameters meet the design requirements.

• Designing the drive circuit: Using the LT3999 monolithic DC/DC push-pull driver, design two drive signals that alternate in conduction.

• Design protection circuits: Add over-current protection and over-voltage protection circuits to ensure the stability and safety of the system.

• Testing and optimization: Conduct testing in actual circuits and adjust circuit parameters to optimize performance.

Test results:

• Output voltage: Within the input voltage range of 10V to 15V, the output voltage remains at 5V.

• Power consumption: Over the entire load current range, power consumption remains low and efficiency is high.

• Temperature: By controlling the duty cycle, the voltage difference across the LDO is reduced, thus suppressing the temperature rise.