Push-pull transformers are forward-mode transformers with center-tapped primary and secondary windings that are used in 2-switch, open-loop, fixed-duty cycle circuits and in closed-loop, wide input voltage range, push-pull converter circuits. They provide circuit isolation and voltage transformation, step-up or step-down, if needed. They are used over a wide power range from 1/4 Watt to hundreds of Watts. This article discusses advantages, basic operation, applications, and selection of push-pull transformers.
A major advantage of push-pull converters is that the 2-switch operation shares the load across two power switches such that each switch only handles every other power pulse. This means that the transformer is effectively only operating at half the switching frequency.
As with the single-ended forward topology, the voltage stress on the FETs in the push-pull topology is two times the input voltage. A typical rule of thumb is to limit the primary current ripple to 10-30% of the average primary current to limit stresses on the switches, generally holding EMI to within reasonable limits.
In addition, push-pull transformers have inherently lower EMI than flyback transformers because they are designed to have less interwinding capacitance than flybacks, which are designed for low leakage inductance and therefore have inherently higher capacitance. Lower EMI also means smaller components can be used if an LC filter is used to reduce the output ripple.
Push-pull transformers used in open-loop operation drive the center-tapped primary at 50% duty cycle with two-quadrant (bipolar) excursions of the BH-loop, the B and H excursions reversing polarity and crossing zero each alternate half-cycle (Figure 1).
Two-quadrant operation means the transformer core flux that builds up in one phase is ideally cancelled by that of the other phase, preventing permanent flux build up. Winding assymmetry can create imperfect matching of flux in each quadrant which can lead to flux build up. In such cases, current-mode control may be used to reset the core.
The relatively low ripple current and two-quadrant operation of push-pull (versus, for example, flyback) results in high efficiency and may allow the use of a smaller transformer. The cancelling of common mode flux also creates lower EMI and better transient immunity. Figure 2 shows typical push-pull topology current paths and waveforms.
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The major design considerations of a push-pull transformer are turns ratio and power handling. Turns ratio directly determines the output voltage: Vout = Vin x D x 2 x Ns/Np (eq. 1).
As with forward mode topologies, higher magnetizing inductance helps improve efficiency, but inductance is not a major design consideration and tight tolerance is not required. Figure 3 shows a 1:4 turns ratio push-pull transformer used in an isolated push-pull power supply circuit to convert 5V to isolated +/- 15V.
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The push-pull topology is an excellent choice for:
Selecting the appropriate off-the-shelf Coilcraft push-pull transformer starts with determining the output power and turns ratio requirements of the application. Consult the datasheet for your selected push-pull driver IC to determine the appropriate turns ratio. An approximation of the output power requirement of the transformer is Vout x Iout, adding an extra few percent to account for imperfect efficiency.
Coilcraft is pleased to present our powerful online Power Converter Transformer Parametric Search tool.
To view a list of all Coilcraft push-pull transformers:
1. Select Power Converter Transformers
2. Select the Topology filter from the Narrow results menu
3. Click the Push-pull checkbox in the Topology filter
4. Click Apply.
Applying the appropriate Narrow results selection filters helps you quickly find a push-pull transformer meeting your application requirements.
1. Select the Output Power filter, and either enter a range or select from the pull-down list.
2. Select or enter the appropriate turns ratio(s) and any other factors considered important to your selection (e.g. Vin, Vout, isolation voltage, size).
Results can also be sorted by any parameter to quickly find the most appropriate solution for your application. For example: Are you looking for a push-pull transformer designed for use with the Texas Instruments SN6507 IC? Select the Designed for filter and select Texas Instruments. Coilcraft TX1 series includes several turns ratio and output power options.
Push-pull converter simplifies isolated power supply design in HEV/EV systems, Texas Instruments, Analog Design Journal 1Q 2020, Anant Kamath, slyt790a, http://www.ti.com/lit/an/slyt790a/slyt790a.pdf
LM5030 Application: DC-DC Converter Utilizing the Push-Pull Topology, Literature Number: SNVA553, Texas Instruments (employs Coilcraft A9784-B) http://www.ti.com/lit/pdf/snva553
How to Design an Isolated, High Frequency, Push-Pull DC/DC Converter by Dawson Huang, Analog Devices, https://www.analog.com/en/technical-articles/high-frequency-push-pull-dc-dc-converter.html
LT3999 datasheet for low noise, 1A, 1 MHz, push-pull DC-DC driver https://www.analog.com/en/products/lt3999.html#
Fundamentals of Power Supply Design, Robert A. Mammano, 2017
