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Classification and Conversion Stages in Switching VDC Output Power Supplies

Key Takeaways

  •  Whenever using VDC power supplies, the important specifications to look for are:

    • Electrical isolation between the input and output

    • Regulation of the DC output voltage

    • High power density for reduced size and weight

    • Control of  power flow direction

    • High efficiency of conversion from AC-DC

    • The need for small filters

    • Power factor control at the AC source side 

  • VDC power supplies can be achieved through two-stage conversions and three-stage conversions. Two-stage conversions include DC-AC and AC-DC conversions, whereas three-stage conversions include AC-DC, DC-AC, and AC -DC conversions. 

  • Based on the techniques used for conversion, switching VDC output power supplies can be classified into switch-mode or PWM DC power supplies, resonant power supplies, and bidirectional power supplies.

Electronic circuits require DC power supplies

VDC power supplies generate regulated DC voltage from unregulated DC, or utility AC, voltage.

Working on FPGAs, voltage sensors, op-amps, and transistors requires a supply of DC power. In the case of FPGAs, sensors, and transistors, only a positive DC supply is needed. However, op-amps require both positive and negative DC voltage. 

In general, most analog and digital electronic circuits require regulated DC power supplies. We often depend on voltage of direct current (VDC) output power supplies—either linear power supplies or switching power supplies. 

Given the bulkiness and poor efficiency of the linear power supply, the switching power supply is considered more advantageous. Semiconductor devices are utilized as switches in switching VDC output power supplies, leading to a significant reduction in power loss. The elimination of transformers is a major size reduction modification in switching power supplies. 

Requirements of the VDC Output Power Supply

In industrial, commercial, and laboratory applications, we often meet the requirements of DC voltage using switching VDC power supplies. Switching VDC power supplies are reasonably priced and extensively used. They should meet these important specifications:

  • Electrical isolation between the input and output

  • Regulation of the DC output voltage

  • High power density for reduced size and weight

  • Control of  power flow direction

  • High efficiency of conversion from AC-DC

  • The need for small filters

  • Power factor control at the AC source side

Conversion Stages of the VDC Output Power Supply

Two-stage and three-stage conversion in switching VDC output power supplies

Switching VDC power supplies—(1) two-stage conversion and (2) three-stage conversion.

With all these considerations, switching VDC output power supplies can be achieved through two-stage conversions and three-stage conversions. In two-stage conversions, the input power supply is converted from DC to AC in the first stage (DC-AC conversion). In the second stage, the AC output of the inverter is rectified to give regulated DC voltage (AC-DC conversion). 

A common practice to obtain DC regulated output voltage is plugging the switching power supply box to utility voltage. In such switching of VDC power supplies, the three-stage conversion is used. The input AC utility voltage is rectified to unregulated DC voltage in the first stage (AC-DC conversion) and further transformed into AC voltage using an inverter in the second stage (DC-high frequency AC conversion). The third stage is the rectification of the AC voltage into the DC voltage of the desired amplitude (high-frequency AC-DC conversion). In switching VDC output power supplies, either pulse-width modulation (PWM) or resonant inverter are used. High-frequency isolation transformers are used to convert DC to high-frequency AC voltage in the second stage. The high-frequency modulation generates fewer voltage ripples in switching VDC output power supplies, thus, the ripple content in the regulated DC voltage is less. The filtering of low ripples can be easily carried out using small filters in three-stage conversion. The use of high-frequency transformers provides isolation in three-stage conversion. 

Classifications of Switching VDC Output Power Supplies

Based on the techniques used for conversion, switching VDC output power supplies can be classified into:

  1. Switch-mode or pulse width modulated (PWM) DC power supplies-These types of VDC output power supplies have high efficiency and can be used for low voltage, high current applications. PWM inverters are utilized in this type of power supply. The common configurations of switch-mode or PWM inverters used in switch-mode power supplies are flyback, forward, push-pull, half-bridge, and full-bridge. 

  2. Resonant power supplies-In resonant VDC power supplies, the inverter AC voltage frequency is the resonant frequency, which is very high. The DC output voltage range is narrow in this type of VDC power supply.

  3. Bidirectional power supplies-In battery applications, the direction of power flow is different for the charging state and discharging state. Bidirectional power supplies are employed when there is a need to change the power flow direction. 

Let's see how a switch-mode VDC output power supply works with a flyback configuration.

Switch-Mode VDC Power Supplies Based on Flyback Converters

Flyback converter

A Flyback converter is a single-switch topology

In switch-mode VDC power supplies where flyback converters are used, the DC-DC conversion is realized by flyback converters, which split the conversion into unregulated DC-high frequency AC and then to regulated DC. Flyback converters are derived from buck-boost converters. They are a single switch topology that utilizes a high-frequency isolation transformer with unidirectional core excitation. The transformer provides isolation as well as energy storage. 

The figure above shows a flyback converter where the input voltage is Vin and the output voltage across the load resistor is Vo. The high-frequency transformer primary and secondary winding turns are denoted by Np and Ns. The primary and secondary windings are arranged so that the polarity is opposite to each other. A diode, D, connected to the secondary side of the transformer rectifies the high-frequency AC voltage. 

When the switch is closed, the diode is reverse biased and cuts-odd the secondary winding from the output resistor side. The capacitor discharges to the load resistor and generates a voltage, Vo. When the switch is turned off, the change of winding polarity makes the following changes: the diode gets forward biased, the capacitor charges, and the current flows to the load resistor from the secondary winding side. The direction of current flow through the resistor remains the same throughout the switch on and off time, and we get regulated DC voltage across the resistor. The output DC voltage of the flyback converter based VDC power supply depends on the secondary winding voltage, the duty-cycle ratio of the switch, the load resistor value, frequency, and primary winding inductance. The equivalent circuit during the switch turn-on and turn-off is illustrated in the figure below.

Two states  of Flyback converter operation

Equivalent circuit of a flyback converter at (1) switch turn-on time and (2) switch turn-off time.

Advantages of Switching VDC Output Power Supplies

Switching VDC output power supplies offer multiple regulated DC outputs of the same or different polarity, with electrical isolation in a smaller footprint than linear power supplies. The other advantages of switching VDC power supplies are lowered costs, high switching frequency, high voltage, and current ratings. To select the appropriate topology of converter for switching VDC power supplies, it is critical to understand the merits and demerits of each configuration. 

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