“There are many reasons why electrically isolated power supplies are widely used in various applications. On some circuits, electrical isolation must be implemented for safety reasons. In other circuits, functional isolation is used to intercept signal interference.

“

There are many reasons why electrically isolated power supplies are widely used in various applications. On some circuits, electrical isolation must be implemented for safety reasons. In other circuits, functional isolation is used to intercept signal interference.

Electrically isolated power supply design generally uses flyback converters. The design of these regulators is very simple. Figure 1 shows a typical design of this type of regulator, which uses an ADP1071 flyback controller. It can be seen that this is a flyback converter because its points do not match the transformer. The primary side power switch (Q1) is used. In addition, a secondary rectifier circuit is also required. This can be achieved with Schottky diodes, but in order to obtain higher efficiency, an active switch (Q2 in Figure 1) is generally used. The corresponding ADP1071 controller is responsible for controlling these switches and providing electrical isolation for the feedback path FB.

Figure 1. A typical flyback regulator (flyback converter) with power up to about 60 W.

Although flyback converters are extremely common, this topology has practical limitations. The transformer T1 in Figure 1 is not used as a typical transformer. When Q1 is in the on state, no current will flow through the secondary winding of T1. Almost all the electrical energy of the primary current is stored in the transformer coil. A buck converter stores electrical energy in a choke (inductance), and a flyback converter stores electrical energy in a transformer in a similar manner. When Q1 is in the closed state, the secondary of T1 will form a current to provide power for the output capacitor COUT and the output. This concept is easy to implement, but the concept itself has limitations at higher powers. Transformer T1 is used as an energy storage element. Therefore, the transformer can also be called a coupled Inductor (choke coil). This requires the transformer to store the required electrical energy. The higher the power level of the power supply, the larger the volume of the transformer required and the higher the cost. In most applications, the upper power limit is about 60 W.

If you need to use an electrically isolated power supply to obtain higher power, then the forward converter is a good choice. The concept is shown in Figure 2. Here, the transformer is really used as a typical transformer. When current flows through the primary Q1, the secondary will also form current. Therefore, the transformer does not need to have an energy storage function. In fact, the reverse is also true. It must be ensured that the transformer is always discharged while Q1 is on to prevent it from accidentally reaching saturation after a few cycles.

Figure 2. Flyback controller (forward converter) with power up to about 200 W.

If it is to achieve the same power, the transformer volume required by the forward converter is smaller than the volume required by the flyback converter. Therefore, even when the power level is less than 60 W, the forward converter is very practical. But there is a shortcoming, that is, it is necessary to prevent the transformer coil from storing electric energy unintentionally in each cycle. This should be realized by the active clamping wiring of switch Q4 and capacitor CC in Figure 2. In addition, forward converters generally require an additional inductor L1 at the output. However, after this, at the same power level, the output voltage ripple will be lower than when using a flyback converter.

Power management ICs (such as the ADP1074 from ADI) provide a very compact forward converter design solution. This structure is usually used when power levels higher than about 60 W are required. Below 60 W, according to the complexity of the circuit and the achievable efficiency, using a forward converter is also a better choice than using a flyback converter. In order to more easily determine which topology to use, it is recommended to use the free circuit simulator LTspice simulation simulation. Figure 3 shows the analog simulation schematic diagram of the ADP1074 forward converter circuit in the LTspice simulation environment.

Figure 3. A circuit example using ADP1074 simulated in LTspice®.

ADP1074

●Current mode controller, realizing active clamp forward topology
●Integrated 5 kV (wide body SOIC package) or 3.0 kV (LGA package) dielectric rated insulation voltage, using ADI’s iCoupler patented technology
●Wide power supply voltage range
●Main side VIN: up to 60 V
●Auxiliary VDD2: up to 36 V
●Integrated 1 A main plane MOSFET driver for power switch and active clamp reset switch
●Integrated 1 A auxiliary MOSFET driver for synchronous rectification
●Integrated error amplifier and ●programmable slope compensation
●Programmable frequency range: 50 kHz to 600 kHz (typical value)
●Frequency synchronization
●Programmable maximum duty cycle limit
●Programmable soft start
●Start smoothly from the pre-charged load
●Programmable dead time
●Power saving light load mode using MODE pin