开关电源基础知识

Switch Mode Power Supplies.

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Introduction - Some Definitions. Forward and Flyback Circuits.

Step Down Switch Mode Regulators:

The Buck Regulator (or Forward Regulator). The Current-Boosted Buck Regulator. Step Up Switch Mode Regulators: The Boost Regulator.

The Current-Boosted Boost Regulator. Other Types of Switch Mode Regulator: The Flyback Regulator. The Cúk SMPS.

Switch Mode Converters: The Forward Converter. The Flyback Converter. The Push Pull Converter. The Half Bridge Converter. The Full Bridge Converter. SMPS Control Circuits: Control.

Off-Line SMPS Circuits. Switch Mode Converters. SMPS Considerations: SMPS RFI & EMI

SMPS Integrated Circuits. SMPS 'Hiccup' Mode. SMPS Hold Up. References.

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Introduction - Some Definitions.

Switch Mode Power Supplies are the current state of the art in high efficiency power supplies. Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator can, therefore, tend to be very inefficient. The switch mode power supply, however, uses a high frequency switch (in practice a transistor) with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by an

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LC filter.

SMPSs can be used to step-down a supply voltage, just as linear supplies do. Unlike a linear regulator, however, an SMPS can also provide a step-up function and an inverted output function. Typical applications are given below. Typical application for a ste-down switching regulator:

Generation of 5V for TTL-based circuits from a 12V battery (particularly suitable if the 12V battery has limited capacity, as switching regulators are far more efficient than linear regulators). Typical application for a step-up switching regulator:

Generation of 25V from a 5V supply in an EPROM programmer. Typical application for an inverting switching regulator:

Generation of a double-ended supply from a single-ended for OP-AMP. Generation of a negative bias for MOS devices eg Dynamic RAMS.

The term switch mode regulator is used to describe a circuit which takes a DC input and provides a DC output of the same or opposite polarity, and of a lower or higher voltage. Switch Mode regulators use an inductor and there is no input to output regulation.

The term switch mode converter is used to describe a circuit which takes a DC input and provides a single or multiple DC outputs, again of same or opposite polarity and lower or higher voltage. Converters use a transformer and may provide input to output isolation.

The term Switch Mode Power Supply or SMPS is used to describe switch mode regulators and converters.

Forward and Flyback Circuits.

When discussing SMPS circuits, the different topologies are often referred to as 'Forward' or 'Flyback'

A Feed Forward SMPS circuit will supply energy to the output capacitor when the switching element (transistor) is switched on.

A Flyback SMPS circuit transfers energy (from an inductor) to the output capacitor when the switching element (transistor) is switched off.

The Buck Regulator (or Forward Regulator).

The Current-Boosted Buck Regulator.

The current-boosted buck converter uses a transformer to increase output current above the maximum current rating of the switch (which is a transistor in a practical circuit). The current-boosted circuit does so at the expense of increased switch voltage during switch-off time. The increase in maximum output current over a standard buck

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converter is equal to input voltage divided by output voltage, plus turns ratio times the input-output differential. For example, in a 15V to 5V current-boosted buck converter, with a 1:4 turns ratio, the increase in output current is double: 15/(5+1/4x15-5), or 2. This is a 100% increase in output current. However, the maximum switch voltage for a current-boost buck is increased from input voltage to input voltage plus output voltage divided by the turns ratio. Using the 15V to 5V converter, the maximum switch voltage is 15+5/turns ratio, or 15+5/0.25=35.

The Boost Regulator.

In a forward (or buck) regulator power is continuously supplied to the outlet\\filter capacitor. In a boost regulator, however, energy is pumped in a cyclic manner. The filter capacitor therefore has to be of a higher value.

The boost regulator, like the flyback regulator, pumps energy into the outlet\\filter capacitor in a cyclic manner, and it is therefore desirable to operate in the discontinuous mode with a fixed peak current through the inductor.

The diode conduction time in a boost regulator, unlike the flyback regulator, is not fixed, but varies with the input voltage.Ipk = 2 x Iout,max x (Vout / Vin,min) Tdon = (L x Ipk) / (Vout - Vin)

Output voltage is regulated by controlling the duty cycle.Vout = ((Ton / Tdon) + 1) x Vin

Ripple voltage is directly proportional to diode conduction time.Tdon max = (L x Ipk) / (Vout - Vin,max)

The Current-Boosted Boost Regulator.

The Flyback Regulator

The flyback regulator circuit shown below can be used as a step-up or step-down circuit

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In a forward (or buck) regulator power is continuously supplied to the outlet\\filter capacitor. In a flyback regulator, however, energy is pumped in a cyclic manner. The filter capacitor therefore has to be of a higher value.

Flyback regulators can operate in one of two modes: Continuous or Discontinuous. In Continuous mode, a large value of inductor is used such that the current in the inductor never falls to zero. In Discontinous mode, the current in the inductor falls to zero before the switch closes. Usually the circuit is designed such that at worst case conditions (max output current, min input voltage) the current only falls to zero for an instant, ie as soon as the diode stops conducting the switch is closed.

For a flyback regulator, the peak current is given by: Ipk = (Vin x Ton)/Lwhere: Ipk = Peak current - Amps Vin = Input voltage - Volts

Ton = switch conduction time - Seconds L = Inductance - Henries

The conduction time of the diode (which may or may not be the same as the off time of the switch) is given by: Tdon = (Ipk x L) / Voutwhere: Ipk = Peak current - Amps Vout = Output voltage - Volts

Tdon = Diode conduction time - Seconds L = Inductance - Henries

The output power from a flyback regulator is given by: Pout = Vout x Iout = 0.5 x L x Ipk^2 x fwhere: Iout = Average output current - Amps Vout = Average output voltage - Volts Tdon = Diode conduction time - Seconds L = Inductance - Henries Ipk = Peak current - Amps

f = Frequency of operation - Hertz

Iout is also the average current through the diode (since all output current must flow through D: Iout = (Ipk / 2) x (Tdon x f)where: Iout = Average output current - Amps Ipk = Peak current - Amps

Tdon = Diode conduction time - Seconds f = Frequency of operation - Hertz

The reverse polarity output voltage is given by: Vout = (Pout x Rl)^0.5 or:

Vout = Ipk x ((L x f x Rl)/2)^0.5 where: Vout = Average output voltage - Volts Pout = Average Output power - Watts Rl = Load Resistance - Ohms Ipk = Peak current - Amps

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L = Inductance - Henries

f = Frequency of operation - Hertz

The output voltage of the circuit can be regulated by operating the circuit at a fixed frequency and varying the transistor duty cycle. However, because of the pumping action, the output voltage sags while the transistor switch is on and rises when the transistor is off. This makes the circuit difficult to control in a fixed frequency manner. A better approach to controlling the flyback converter when operating in the discontinuous mode is to have a fixed peak current in the inductor and hence fixed anode conduction time. The transistor switch 'on' time can then be varied inversely to any changes in the output voltage. This gives rise to the circuit having variable frequency of operation

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