This second part of the series focuses on isolated SMPS, covering the general structures that ensure galvanic separation and energy transfer between the input and output stages. A list of possible structures is presented. First, discover how to transfer the input power to the output despite the galvanic isolation, and second, how to regulate Vout by providing feedback from Vout to the input without a direct link.
On the direct path, the challenges include handling high power (e.g., high voltage and high current), efficiency (losses must be minimized), noise emission (e.g., electromagnetic interference (EMI)), integrability, and cost. On the reverse path (feedback), the techniques (in addition to maintaining galvanic isolation) address sensitivity, stability, speed, and, of course, size and cost.
Figure 1: Energy must be transferred from one Side to the other
Transformer
The best element for ensuring energy transfer without a galvanic connection is the transformer. Its operation is based on the energy exchange between electric and magnetic waves. If one neglects the effects of losses and imperfections, a transformer can pass the full electrical power from the primary coil to the secondary coil. As a result, some fundamental and simple equations are established between voltages, currents, and the number of turns:
PP = PS where PP = VP*IP and PS = VS*IS
VP*IP = VS*IS or VP / VS = IS / IP
Since the magnetic field intensity in the core is proportional to the number of windings around it, VP and VS are proportional to the number of turns in the primary and secondary sides, NP and NS, respectively. Therefore VP / VS = NP / NS.
This relationship linking the number of turns has a direct impact:
- If NP = NS, it allows the secondary voltage to be equal to the primary voltage
- If NP > NS, it allows the secondary voltage to be lower than the primary voltage (step-down)
- If NP < NS, it allows the secondary voltage to be higher than the primary voltage (step-up).
Figure 2: A transformer is formed by 2 Coils
Figure 3: Transformer electrical wound around an iron magnetic core relationships
The transformer is the key element used to transfer energy between the input and output without any galvanic link between them. Of course, ideal transformers have imperfections. These non-idealities include copper losses (due to winding resistance), core losses (hysteresis and eddy currents), and leakage flux, which create series impedance and require a small magnetizing current. These factors result in reduced efficiency, voltage drops, and deviations from the ideal voltage transformation. As a result, the perfect transformer model appears more complex.
Figure 4: Real transformer model
In terms of isolation, the key parameter is the level of safety it can provide. It is specified as the maximum isolation voltage in kV.
Figure 5: Isolation specifications-typical values
Isolated SMPS: General Structure
Figure 6: Isolated SMPS-general structure
As noted in this series, the isolation between the input and output parts of an SMPS must be ensured for both the energy transfer and feedback paths, and the power lines (supply and ground) must be clearly separated (Vin and Vout cannot share a common ground).
One can argue that the transformer is also a component and therefore can be damaged, for example, by shorts between the primary and secondary windings or by insulator breakdown between coils. This is why the mechanical design of the transformer is also key (spacing between the primary and secondary, wire diameter, heat dissipation, etc.).
Figure 7: Transformer winding shorts can collapse protection and safety
Isolated SMPS: Energy Transfer Architectures
We will analyze five methods to realize the energy transfer from input to output, particularly developed by Analog Devices:
- Flyback: derived from buck-boost
- Iso-Buck©: derived from a classic buck
- Forward: classic forward with isolation
- Forward with Active Clamp: forward with magnetic energy recovery
- Resonant LLC: can be seen as a DC-to-AC-to-DC converter.
Isolated SMPS: Feedback Schemes
- Optocoupler: light is used as an intermediary
- Third Winding on the Primary Side: transformer in the feedback
- Direct Control via Primary Side: the primary coil is also used as a sensor
- Built-In Iso-Buck: intrinsically built-in iso-buck.
All nine schemes (five power paths and four feedback paths) will be analyzed in detail in the following parts of the series.
Conclusion
Part 2 of this series introduced the transformer as the key element for transferring energy from the input to the output without a direct electrical connection between them. We also discussed the general structure of all isolated DC-to-DC power supplies, along with a list of four forward-path and four feedback-path options. Stay tuned as we explore the architectures in depth.
Read all the blogs in the SMPS Galvanic Isolation series.