Here, Rich Miron, Application Engineer at Digi-Key Electronics, explores the operation of switching power supplies and explores Make Vs. Purchasing Decision Process for Power Supplies. Miron also explores the design of a single output power supply using the fast-return topology and provides a model design using readily available parts and components.
Since all electronic systems require a certain amount of energy, the power supplies are well characterized and understood. The design and choice of power supplies still pose problems for engineers because of the tendency to reduce dimensions. greater efficiency greater reliability; and greater energy integrity in applications ranging from mobile devices to network-powered hardware.
With high-speed data communication systems such as 5G, synchronization and signal-to-noise requirements become extremely high. To solve the problem of efficient and reliable output power in a small form factor, power designers use Switching Power Supplies (SMPS) with a flyback converter topology. Suitable for power levels up to 150 watts, this topology offers low component count designs for small size and low cost. It also provides input / output isolation and good efficiency.
An SMPS or switch is a power source that uses a switching regulator to maintain stable output voltage (s) from an AC or DC power source. The switching regulator uses one or more semiconductor devices, such as. A bipolar junction transistor, MOSFET or IGBT, and switches between the “on” and “off” states to maintain the regulation of the output voltage. These devices can operate with a fixed duration and a variable frequency, or more often with a fixed frequency and a variable work cycle. A high degree of efficiency results from the low power loss of the switching device when it is turned on or off. The device only dissipates power during transitions between states. As the switching frequency is generally of the order of a few tens of kHz, transformers, inductors and capacitors can be much smaller, which results in a high volumetric efficiency.
The advantages of SMPS are the electromagnetic interference potential (EMI). This is due to switching transients and can be mitigated by careful selection of components, layout and shielding. The advantages of the SMPS go well beyond the drawbacks and make it the most used power supply with a linear power supply for the most sensitive electronic applications.
SMPS can be implemented in a wide variety of circuit designs or topologies. There are more than a dozen commonly used topologies (Table 1).
Below, Table 1: The 10 Most Common Switch Mode Network Topologies (Data Source: Digi-Key Electronics)
The flyback converter is the most used SMPS circuit (Figure 1).
Below, Figure 1: Functional diagram of a flyback converter with a single MOSFET switch and a return transformer. (Source: Digi-Key Electronics)
The main advantage of flyback topology is its simplicity. At all power levels, the number of SMPS topology components is the lowest. The power supply can be powered by a DC or AC source. When configured to operate from the AC line, the line is generally rectified to a full wave. The input source (Vi) is DC.
The heart of the circuit is the flyback transformer. Unlike conventional transformer windings, the primary and secondary windings of the fast return transformer do not simultaneously feed the power supply. This is due to the inversion of the winding phase indicated by the dot notation on the windings and the serial diode on the secondary side.
The use of the flyback transformer has several advantages. The first is that the primary and secondary sides of the power supply are electrically isolated. The isolation reduces transient coupling on the primary side, eliminates ground loops, and provides greater flexibility in the output polarity of the power supply.
The transformer allows the generation of several output voltages in the power supply. Additional windings for each voltage are added to the transformer. Regulation is based on a single exit, and secondary exits are usually regulated locally.
The circuit starts by turning on the switch (eg a MOSFET) (Figure 2).
Below, Figure 2: The operation of the return converter power supply displays the main waveforms for each of the two modes of operation. (Source: Digi-Key Electronics)
When the switch is on, VDRAIN is close to zero volts and the current IP flows through the primary winding of the transformer. The energy is stored in the magnetising inductor of the transformer. This current increases linearly with time. On the secondary side, the series diode is polarized in the opposite direction and in the secondary side no current flows. The energy stored in the output capacitor feeds the output.
When the MOSFET switch is off, the energy stored in the transformer is transmitted through the diode to the output capacitor and the output load. The secondary current starts at a high value and decreases linearly. When the secondary current drops to zero before the switch is reactivated, the power supply is called DCM. If the secondary current does not drop to zero, the power supply is called Current Current Mode (CCM). Since the energy stored in the inductor is completely discharged at each switching cycle, the DCM power supply can use a smaller transformer. In addition, the supply is generally more stable and produces less EMI.
The energy stored in the leakage inductance of the transformer flows in the primary when the switch goes out and is absorbed by the input terminal or the “snubber” circuit whose function is to protect the semiconductor switch from high induction voltages. Power is reduced only during transitions between the ON and OFF states of the switch (Figure 3).
Below, Figure 3: Measurement of a feedback flow indicating the voltage and current forms on the MOSFET switch as well as the instantaneous power dissipation. (Source: Digi-Key Electronics)
The upper curve in Fig. 3 is the voltage across the MOSFET switch in a return voltage supply. Colored overlays indicate the state of the MOSFET. The blue overlay indicates that the device is running, while the red areas indicate that the device is off. The middle curve is the current flowing through the device. The lower graph shows the instantaneous power, which is calculated as the product of the applied voltage and the resulting current. Note that power dissipation is most important during switching transitions. Indicators below the graphical display (from left to right): power losses at power on, power on, power off, idle state, and total power loss for all areas.
Controller / controller
The switching device, such as the MOSFET in the diagram (2), is controlled by a switching mode controller or controller. In most cases, the controller applies a pulse width modulated waveform (PWM) to the switch control, which is the gate of the MOSFETs. The output of the power supply is connected to the controller, whereby the duty cycle of the gate control signal is changed to maintain a constant output voltage. Thus, the controller forms a closed-loop control system around the flyback converter.
Controllers can also perform several support functions, such as: As protection of the supply against overloads, overvoltages or low line conditions. It can also manage the power start to ensure a controlled (“soft”) start, minimizing initial current and voltage transients.
Some solid-state component suppliers offer design tools to develop switching power supplies such as Texas Instruments’ WEBENCH Power Designer (Figure 4).
Below is Figure 4: The Texas Instruments WEBENCH Power Design Center homepage lists the basic specifications for an SMPS design of a 25-watt, 5-volt power supply. (Source: Digi-Key Electronics)
The design begins with the user specified power specifications for the supply voltage range, the desired output voltage, and the current. In this case, the desired design was for a 5 volt, 5 amperes AC power supply with an isolated topology. For more complex multi-output devices, there is an advanced Power Architect design tool.
As a result, the software launches a series of designs and prompts the user to select the controller. The user can examine any design to see the BOM, BOM, efficiency and a dozen associated circuit specifications.
When you point to a component in the diagram, you get a detailed description of the part and the ability to select an alternative component. The controller (U1) receives feedback from the output via an optocoupler CELPS2811-1-F3-A. This feedback method maintains electrical isolation between the primary and secondary sections of the circuit. The controller supplies the PWM control signal to the STMicroelectronics M1, STB21N90K5900volt, 18.5 amps, MOSFET switch. The design tool also facilitates the choice or design of the fast return transformer.
The optimizer optimization panel allows the user to optimize the design for the best unit cost, footprint or efficiency. Using this tool allows inexperienced designers to gain experience by examining multiple designs and observing the effects of component changes.
Make or buy?
There is no doubt that if an engineer has no experience with SMPS, there will be a learning curve. If time to market is a big problem, it’s best to buy a standard contract or delivery for a custom network part design. With time and technical staff, especially if several projects are needed, it would be wise to schedule a delivery. That said, repeated exposure to the SMPS design will add the required know-how to the design staff.
Switching power supplies offer high efficiency and small size. For power levels below 150 watts, flyback topology power supplies provide the benefits of multiple outlets, a small number of components, and line isolation.