Transformers are one of the most common components used in power converters and power supplies, both for AC/DC and DC/DC conversion. Power supplies that accept line voltage inputs with DC output will almost always use a transformer both for safety and for implementing large step-downs with high efficiency. In DC/DC converters, they provide essentially the same functions, but with PWM switching signals instead of AC power.
Some transformers offer a capability to output over multiple rails with high efficiency power conversion. These are multi-winding transformers, where multiple secondary coils are used to step up or down an input voltage to desired values. In power systems where isolation is needed on multiple rails, the easiest implementation is with a multi-winding transformer.
DC/DC Converters That Use Multi-Winding Transformers
The typical circuit diagram for a multi-winding transformer is shown in the image below. These transformers have a single input (primary winding) and more than one output (multiple secondary windings), so a single input AC wave can couple power to multiple outputs at once. The polarity of the coupled power in each output can be controlled through the two winding directions.
Primary and secondary windings in a multi-winding transformer.
These transformers can be found in some of the following power converter topologies:
- Flyback converter
- Bridge converter
- LLC resonant converter
- Forward converter
- Cuk converter
The voltage induced on each winding in a multi-winding transformer depends on the switching frequency, duty cycle, and turns ratio. Because so many power converters control their regulation and ripple value using inductance, the primary and secondary inductances are sometimes used to calculate step-up/step-down voltages. However, inductances in the various current loops in switching regulators also determine ripple/overshoot; there is generally a tradeoff between lower ripple and missing a target regulation voltage.
Functions Implemented With Multi-Winding Transformers
In addition to outputting power over multiple rails, there are two simple but important regulation functions that can be implemented with multi-winding transformers:
- Powering peripherals on the primary side of a converter
- Taking an output voltage measurement on the secondary side
To do this, the appropriate turns ratios would be determined and the multi-winding transformer would look like the following setup:
This arrangement would allow use of a high voltage at the primary side, such as a rectified high DC voltage. The other rail on the primary side would step down power to allow power to primary-side peripherals, such as a gate driver or regulation controllers.
The secondary side rails provide the main output, which could be at high voltage and current. The smaller secondary rail could then be used for a voltage measurement, but without passing the secondary rail voltage output across a current-sense resistor. This could simplify layout for taking a measurement on the output side, and the measured voltage across the output could be passed into an optocoupler to bring the signal back to the primary side for regulation.
Secondary Side Grounding in a PCB
There are two main reasons to use a transformer in a switching regulator: for galvanic isolation and to implement large step-downs as needed. Because transformers are often used in instances where a large unregulated input voltage (DC or AC) is being used, galvanic isolation between the primary and secondary rails is an important safety measure. Without galvanic isolation, there is a danger of exposing the device user to a large DC or AC voltage if there is a fault in the system.
Multi-winding transformers can be used to implement two types of galvanic isolation:
- All rails can be isolated from each other and the primary
- Only the primary rail is isolated, but secondary rails reference the same ground
- Only the primary rail is isolated, and there is a mix of isolations on the secondary rails
In the first case, all rails are connected to different ground nets in the PCB. This is useful if the system needs to galvanically isolate multiple circuits running at various voltages.
Example multi-winding transformer with dual isolated outputs.
In the other two cases, different windings could be connected to the same net. Sometimes these transformers will be center-tapped, so you will have dual-rail output on the secondary side of the transformer with a single common connection to the mutual ground plane for the center-tapped winding.
Example multi-winding transformer with dual-rail output.
Whenever there is galvanic isolation between power rails with different grounds, traces on the PCB must not be routed between these different regions. The reason is that this can create excessive radiated emissions from the system, and/or excessive EMI susceptibility. When there is significant switching noise on the primary side of the system, it’s even possible that excessive noise couples into the secondary side through these highly susceptible control lines.
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