Understanding Flyback Power Supply Design and Simulation

Key Takeaways

• Learn what a flyback power supply is

• Understand the  unique design specifications for flyback power supplies

• Learn the value of simulation in power supply design

Simplified circuit diagram of a flyback power supply

Many technologies are outpaced by their modern successors. My first computer lesson in the 90s involved a now-historic IBM computer, running on DOS and equipped with a floppy disk drive. Back then, a mouse usually referred to the whiskered creature that is the perpetual foe of a cat.

While most technologies are eventually replaced with their modern counterparts, a few do survive the evolutionary cycles. In electronics design, the flyback power supply is one that has a history stretching back more than 7 decades. Today, the flyback power supply remains a popular power converter in various applications.

What Is a Flyback Power Supply

A flyback power supply involves using a transformer to store energy from the primary winding and relay the stored energy to the secondary winding. This is a unique development of the application of transformers in power supply design, as they are usually used for stepping up or down the voltage.

Generally, a flyback power supply design involves a flyback transformer, a MOSFET converter, which controls the flow of current with PWM, an RCD snubber, and rectifying diodes at the secondary windings. Note that the secondary winding is of the opposite polarity and produces a sudden reversal in the terminal voltage. The operation of a flyback power supply can be broken down into a FET ‘on and ‘off’ cycle. 

In an ‘on’ cycle, the current that flows through the primary winding increases to the maximum. During this cycle, the magnetic energy is stored in the core of the flyback transformer. At this point, the diode connected to the secondary winding is in reverse-biased, thus preventing current flow. 

As the FET turns ‘off’ in the next cycle, the current on the primary winding stops flowing. The sudden cut off of the current flow can result in a huge voltage spike due to inductance leakage at the winding. To prevent the FET from bearing the brunt, an RCD snubber circuit is used to absorb the excessive energy. Meanwhile, the secondary diode is now forward biased and current flows to charge up the output capacitor.

Continuous Mode and Discontinuous Mode Of Flyback Power Supply

There are two modes to operate the flyback power supply and each bears very different results. The first mode is called continuous mode, in which the energy stored in the transformer isn’t completely drained between the cycles. Meanwhile, the discontinuous mode ensures that all magnetic energy is transferred over to the secondary winding and is characterized by a ‘silent gap’ where no current flows before the start of the next positive cycle. 

Simulation is both available and worthwhile for each mode of flyback power supply operation. Using a SPICE tool, you’d be able to ensure proper voltage and current needs are met while also ensuring the device and component tolerances are adequately prepared to handle the power of the design. Furthermore, through frequency domain testing, you can rest assured about the steady-state of the power supply. 

The flyback model is used in switch-mode power supply.

The continuous mode has a comparatively lower peak current compared to the discontinuous mode. This results in lower inductance loss and equally lower ripples on the output voltage. However, the operation of a flyback power supply is susceptible to right half plane zero (RHPZ), which effectively limits the bandwidth of operation. 

The effect of the RHPZ is pronounced when the load current increases. It often results in higher peak current but with shorter conduction time of the diode. This causes a lag that makes the feedback control circuit more difficult to implement.

Comparatively, the discontinuous mode does not suffer from the same problem. It’s also more efficient and has lower switching loss. 

Optimizing Flyback Power Supply Design

The circuit diagram of the flyback power supply is deceptively simple—except that it’s not. Priority should be given to the flyback transformer as a misstep in design could result in issues with efficiency and EMI. 

It’s crucial to get the design of the flyback transformer right.

One of the first steps for a designer is to get the windings ratio right. This ensures that the transformer will deliver the required secondary voltage with the estimated efficiency. It is also important to establish the minimum number of turns on the primary winding to prevent saturation. 

An issue that bogged transformer design is inductance leakage. Despite attempts to reduce the leakage by interleaved windings, there is still a minimal amount that results in a sudden high voltage when the FET is turned off. This is remedied by adding a snubber circuit on the primary winding. 

Beyond layout configurations, of course, the most you can do for your power supply is proper simulation beforehand. Determining adequate voltage and current needs, ensuring power integrity, and having accurate, and having available bode plot simulation are all among the vast capabilities of PSpice. 

Having intuitive bode plot simulation among a horde of other capabilities helps PSpice stand out. 

Using the right PCB design and analysis software can prevent design flaws. With the multiple variables in designing a flyback power supply, it makes sense to simulate the design with PSpice Simulator and identify problematic areas before committing to prototyping. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.