How does power efficiently travel from input to output? Part 3 of this series examines the transformer-based architectures that make it possible. Five methods are highlighted:
- Flyback
- Iso-buck
- Forward
- Forward with active clamp
- Resonant LLC
Figure 1: Power Transfer Part
Flyback Architecture
The flyback scheme is a well-known isolated SMPS structure thanks to its design simplicity and operation. It can be seen as a direct derivation from the fundamental (and not isolated) buck-boost or inverter circuit.
Figure 2: Flyback Directly Derived from a Basic Inverter
Starting from the basic inverter (or buck-boost), it is easy to recognize the central vertical branch as the inductor (think of the “I” for Inverter and Inductor).
To make the flyback, duplicate the inductor. The two inductors are mounted on the same magnetic core. On the primary side, the behavior is identical. Energy is stored in the coil when switching on. When switched off, the secondary coil must drive the load depleting the energy stored previously. The coil direction and diode are reversed to produce a positive output (while the inverter gives opposite polarity versus the input).
In addition, with different turns ratios, the flyback and the duty cycle can buck or boost.
The transfer function for the flyback is:
Vout/Vin = + D (NS/NP) /(1-D)
Where D is the switching duty cycle, NP and NS are the number of turns on the primary and secondary coils, respectively. Compared to the -D/(1-D) of a simple inverter, another level of variation is added (but the polarity inversion is lost).
Iso-Buck Architecture
Again, the technique of duplicating the original output on a secondary coil of a transformer is used (as done for the flyback derived from a buck-boost).The iso-buck can be seen as the isolated version of the basic buck controller. Vout is repeated on the secondary side of a transformer and rectified.
Predefine the turn ration NS/NP, and easily step-up on the original buck structure!
Figure 3: Iso-Buck Derived from the Basic Buck Converter
Isolated Forward Architecture
The basic isolated forward converter operates where primary and secondary sides of the transformer are conducting at the same time, when the switch is on. The sign is the “dot” placed identically on both coils.
The advantage of this topology is the low ripple generated at the output voltage. The negative aspect is cost since it requires two magnetic elements to operate: a transformer and a coil, in addition to two rectifier diodes. Another disadvantage is the stress caused at the primary side: on the coil and on the switch. When the switch is off: the magnetic energy accumulated in the coil generates voltage and current spikes.
Figure 4: Isolated Forward Architecture
Figure 5: Example of an Isolated Converter with the LT8310
Isolated Forward with Active Clamp
This is an enhanced version of the isolated forward converter. When you abruptly switch off the current in a coil, the coil reacts by generating a voltage that attempts to maintain the initial current when the switch (Q1) was on. This voltage represents the coil’s stored magnetizing energy. In conventional designs, this energy is normally dissipated as heat through passive clamping components (freewheeling diode, Q2).
The active clamp idea is to store that magnetizing energy in a capacitor (CCLAMP) during the switch off time. That energy is sent back to the source when the switch is on again.
The main advantages are improved efficiency, reduced voltage stress on the switch, and lower electromagnetic interference (EMI) produced thanks to that soft switching. The drawback is additional components and added complexity in the controller.
Figure 6: Principle of the Isolated Forward with Active Clamp
Resonant Circuit
The principle of this scheme is based on the resonant circuit such as an structure; also called a tank. Operating at a frequency equal to or near the natural resonant circuit can achieve zero-current switching (ZCS) or zero-voltage switching (ZVS).
Soft switching techniques dramatically reduce component stress, power losses, and EMI noise in switching transistors and transformer coils. An isolated resonant converter can therefore be seen as a DC-AC-Isolation-DC structure.
Figure 7: Block Diagram of a Resonant Converter
Below are the blocks or functions.
- A Resonant Tank:Made by passive elements with inductor(s) and capacitor linked in series or in parallel. The resonant frequency is (1/√LC) or a variant of it.
- Oscillating Energy Exchange: When the tank circuit is driven at its resonant frequency, energy is exchanged from the inductor and the capacitor in a sine wave form.
- Soft Switching: This energy exchange creates a waveform where the current or voltage reaches zero regularly.
- Switching at Zero Crossing: The power switches in the converter can then transition (turn on or off) precisely at these zero-crossing points, eliminating the hard switching losses associated with conventional converters. The challenge is to have a circuit able to detect the zero-crossing (stay tuned for this discussion in part 4).
Figure 8: Example of a Resonant LLC Converter
Conclusion
We have described five possible structures to transfer energy from the input to the output without any galvanic connection between them. These are the flyback, the iso-buck, the forward, the forward with active clamp, and the resonant LC.
The final part of the series will focus on ways to ensure the regulation feedback without a direct link.
Read all the blogs in the SMPS Galvanic Isolation series.