How to Make an Isolated Mains Power Supply Without a Transformer - Multiple Output
Isolated or non-isolated power supply without being mistaken
As a PCB designer, at some point in your career you may need to meet certain regulatory requirements. Whether intended for the medical, automotive, military or any other such field, your design will certainly be scrutinized and must meet very strict standards.
Often, when these regulations are in effect, power isolation (or lack thereof) is a topic that suddenly takes on major importance.
What is electrical isolation and what is an isolated power supply? As the name suggests, electrical isolation involves isolating the power supply from other circuits in a system.
This is a common measurement in power systems, and for good reason. For example, if a non-isolated power supply powers your medical PCB, there is a greater risk that the power supply will cause shocks or surges and affect your device, which could injure the user (or even the patient!).
Understanding the concepts of isolated and non-isolated power is essential to ensure the safety of designers and users.
This is not just about the AC or DC power supply units that you can find in the lab. Many digital and embedded systems integrate the power supply on the board without it appearing as a single integrated circuit.
Power isolation, even when the power supply is on board or in a multi-board system, helps protect the end user as well as other equipment.
So err on the side of caution and think carefully about the issue of isolated or non-isolated power before you start your design.
What is an isolated power supply?
An isolated power supply is a power supply that is electrically isolated from the rest of the circuit it supplies, usually through an isolation transformer. This means that power and voltage are transferred from the input to the output without a direct electrical connection between the two sections.
These power supplies can take a large mains input voltage and then convert it to a lower voltage.
Later PFC and regulation stages can be used to restrict the output current to a stable value, which helps protect downstream components from surges and current spikes at the input of the power supply.
If we take the case of a professional quality AC or DC power supply, it should be noted that the user will have to interact with the output stage of the isolated power supply. In other words, it is likely to connect or disconnect wires, adjust certain settings on the front panel or otherwise manipulate the power supply unit.
Isolating the input from the output minimizes the risk of shock to the end user when interacting with the power supply.
A common topology for converting AC to DC with an isolated power supply is shown below.
In the topology above, a step-down transformer appears at the input (between any input EMI filtering and the rectifier circuit) for the purpose of AC-DC conversion. Note however that the transformer can also be positioned after the rectifier and the PFC, especially in the presence of an isolated DC-DC switching converter.
In general, for high current DC-DC switching converters that require isolation, the strategy adopted will require driving a full-bridge or half-bridge network of MOSFETs with a pulse train provided by a circuit of grid command. This is essentially what is done for LLC resonant converters.
The pulse output of this circuit is then reduced to a lower voltage by a transformer and smoothed with a capacitor bank. This approach is also used for flyback converters, which are a common type of isolated switching DC-DC converter, although the topology may differ.
However, there is something that the topology above does not show you explicitly: how the isolation is actually implemented and the overall grounding strategy.
Primary Ground (PGND): This is the ground area on the primary side of the transformer, i.e. the input side of the power supply. If the device is connected to single-phase or three-phase alternating current, a ground connection may also be present (see below) on the input side. She will act asas connection in an input EMI filter circuit. This ground zone should extend to the primary side of the transformer, its edge defining where the isolation occurs in the system.
Secondary Ground (SGND): This ground area begins on the secondary side of the transformer and provides the reference ground for the rest of your system. This zone can be floating. However, in the case of high power systems, then significant noise is likely to appear if the secondary ground oscillates around the level of the reference ground, the secondary side acting as a floating conductor. This phenomenon disappears in alternating current with the use of a capacitor Y on the two GND zones.
Chassis ground (PE or GND): If present in your isolated power system, it is usually a safety ground. This should not be connected to the output side of your isolated power supply or to the power return of any downstream equipment or cards in your system. The chassis is not intended to be a current conductor unless it fails.
Note that the designations "PGND" and "SGND" are not required. You can name your signals whatever you like.
How we connect these zones in a way that eliminates noise and provides safety while maintaining DC isolation will depend on the application your system is intended for.
How to Make an Isolated Mains Power Supply Without a Transformer - Multiple Output
Isolated or non-isolated power supply without being mistaken
As a PCB designer, at some point in your career you may need to meet certain regulatory requirements. Whether intended for the medical, automotive, military or any other such field, your design will certainly be scrutinized and must meet very strict standards.
Often, when these regulations are in effect, power isolation (or lack thereof) is a topic that suddenly takes on major importance.
What is electrical isolation and what is an isolated power supply? As the name suggests, electrical isolation involves isolating the power supply from other circuits in a system.
This is a common measurement in power systems, and for good reason. For example, if a non-isolated power supply powers your medical PCB, there is a greater risk that the power supply will cause shocks or surges and affect your device, which could injure the user (or even the patient!).
Understanding the concepts of isolated and non-isolated power is essential to ensure the safety of designers and users.
This is not just about the AC or DC power supply units that you can find in the lab. Many digital and embedded systems integrate the power supply on the board without it appearing as a single integrated circuit.
Power isolation, even when the power supply is on board or in a multi-board system, helps protect the end user as well as other equipment.
So err on the side of caution and think carefully about the issue of isolated or non-isolated power before you start your design.
What is an isolated power supply?
An isolated power supply is a power supply that is electrically isolated from the rest of the circuit it supplies, usually through an isolation transformer. This means that power and voltage are transferred from the input to the output without a direct electrical connection between the two sections.
These power supplies can take a large mains input voltage and then convert it to a lower voltage.
Later PFC and regulation stages can be used to restrict the output current to a stable value, which helps protect downstream components from surges and current spikes at the input of the power supply.
If we take the case of a professional quality AC or DC power supply, it should be noted that the user will have to interact with the output stage of the isolated power supply. In other words, it is likely to connect or disconnect wires, adjust certain settings on the front panel or otherwise manipulate the power supply unit.
Isolating the input from the output minimizes the risk of shock to the end user when interacting with the power supply.
A common topology for converting AC to DC with an isolated power supply is shown below.
In the topology above, a step-down transformer appears at the input (between any input EMI filtering and the rectifier circuit) for the purpose of AC-DC conversion. Note however that the transformer can also be positioned after the rectifier and the PFC, especially in the presence of an isolated DC-DC switching converter.
In general, for high current DC-DC switching converters that require isolation, the strategy adopted will require driving a full-bridge or half-bridge network of MOSFETs with a pulse train provided by a circuit of grid command. This is essentially what is done for LLC resonant converters.
The pulse output of this circuit is then reduced to a lower voltage by a transformer and smoothed with a capacitor bank. This approach is also used for flyback converters, which are a common type of isolated switching DC-DC converter, although the topology may differ.
However, there is something that the topology above does not show you explicitly: how the isolation is actually implemented and the overall grounding strategy.
Primary Ground (PGND): This is the ground area on the primary side of the transformer, i.e. the input side of the power supply. If the device is connected to single-phase or three-phase alternating current, a ground connection may also be present (see below) on the input side. She will act asas connection in an input EMI filter circuit. This ground zone should extend to the primary side of the transformer, its edge defining where the isolation occurs in the system.
Secondary Ground (SGND): This ground area begins on the secondary side of the transformer and provides the reference ground for the rest of your system. This zone can be floating. However, in the case of high power systems, then significant noise is likely to appear if the secondary ground oscillates around the level of the reference ground, the secondary side acting as a floating conductor. This phenomenon disappears in alternating current with the use of a capacitor Y on the two GND zones.
Chassis ground (PE or GND): If present in your isolated power system, it is usually a safety ground. This should not be connected to the output side of your isolated power supply or to the power return of any downstream equipment or cards in your system. The chassis is not intended to be a current conductor unless it fails.
Note that the designations "PGND" and "SGND" are not required. You can name your signals whatever you like.
How we connect these zones in a way that eliminates noise and provides safety while maintaining DC isolation will depend on the application your system is intended for.
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