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Switching step-up and step-down regulators

2015-10-12 02:13  
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All regular voltage regulators (like 7805) have several disadvantages like output voltage is always lower than input and some power is dissipated in control element. Dissipated power is approximately equal to I(Vin-Vout).

There is another way to generate regulated voltage. Method is different from previous. In switching regulator a transistor operates as a saturated switch which periodically applies the full unregulated voltage across an inductor in short intervals. During each pulse inductor current builds up storing energy on its magnetic field:

switching_reg.PNG

 

Inductor also smooths the output voltage. Feedback circuit with comparator compares output voltage with reference and this way changes oscillators pulse width or frequency.

Switching regulators have few attractive properties that made them popular. Low power dissipation even if there is a large drop from input to output; switching regulators may generate output voltages higher than unregulated input; they can generate opposite polarity than input; switching regulators can be designed to run directly from AC power line with no power transformer. This way modern DC supplies are lightweight. As na example you can take computer power supply block.

But… Switching supplies have side effect- they generate noise and can put hash back to power line. But these problems have been solved.

Step-down regulators

Simple explanation of step down:

step_down.PNG

 

When the MOS switch opens, inductor current continues to flow in the same direction. Output capacitor acts as energy smoother. When switch closes inductor current decreases linearly. Of course complete circuit have feedback circuit that compares output voltage and this way controls pulse width or frequency.

For instance lof current fixed 5V regulator MAX638. This chip includes nearly all necessary elements including resistor divider.

??max638.PNG

 

In MAX638 switching oscillator runs at 65kHz and comparator circuit permits or cuts switching pulses and this way efficiency reaches about 85% independently from input voltage.

Step-Up regulators

When output voltage is needed greater than input or output voltage has to be opposite polarity to input, then step- switching regulators are used:

??step_up.PNG

 

Looking in step-up circuit we can see that inductor current ramps up during switch-on. When switch is off voltage raises at X point as inductor attends to maintain constant current. The diode turns on and the inductor dumps current in to capacitor. Output voltage can be much larger than input.

In the inverting circuit during switch conduction current linearly increases from X point to ground. When switch is off, inductor pulls X point to negative voltage as much as needed to maintain current flow. Output voltage can be larger or smaller depending on magnitude -defined in feedback.

As step up regulator good example is MAX631:

??max633.PNG

 

And as inverting switching regulator MAX637:

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MAX637 can have input of 2V and provide 5V with greatly reduced maximum output current. External components are not critical as in circuit. They are manufacturer recommended, but for instance larger inductance lowers the peak current but increases efficiency.

As wee switching regulators are quite handy elements. Power efficiency and light weight design allows constructing compact design. For instance step-up regulators are ideal where relatively high voltage is required like using fluorescent or plasma technology. Switching regulators have another terminology a€?Dc to DC convertera€?. DC to Dc converters or switching regulators are handy where battery operated design is used. For instance using alkaline battery 9V which voltage actually starts at 9.5V and steadily drops to 6V. So low power 5V regulator will provide steady 5V with high efficiency over mos all battery life what cannot be said about linear regulators like 7805 where input voltage has to be much higher to produce reliable output voltage, and of course voltage difference is dissipated as heat, what makes battery life very short.


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