UNDERSTANDING SWITCHING SUPPLIES

The great disadvantage to ordinary linear power supplies is their tremendous waste. At least half of all power provided to a linear supply is thrown away as heat. Most of this waste occurs in the regulator portion of the supply. Ideally, if there was just enough energy supplied to the regulator to achieve and maintain a stable output voltage, regulator waste could be reduced almost entirely, and supply efficiency would be vastly improved. This is the concept behind a switching power supply.

CONCEPTS OF SWITOHING REGULATION

Instead of throwing away extra input energy, a switching power supply creates a feedback loop. A feedback circuit senses the output voltage provided to a load, and then switches the AC primary (or secondary) voltage on or off as needed to maintain steady levels at the output. In effect, a switching power supply is constantly turning on and off in order to keep the output voltage(s) steady. A block diagram of a typical switching power supply is shown in Figure 32-l. There are various possible configurations, but Figure 32-1 illustrates one classical design approach.

Raw AC line voltage entering the supply is immediately converted to pulsating DC, and then filtered to provide a primary DC voltage. Notice that unlike a linear supply. AC is not transformed before rectification, so primary DC can easily reach levels exceeding 170 volts. Remember that AC is 120 volts RMS. Since capacitors charge to the peak voltage (peak = RMSX 1 414), DC levels can be higher than your AC voltmeter readings.

On startup, the switching transistor is turned on and off at a high frequency (usually 20 kHz to 40 kHz) and a long duty cycle. The switching transistor acts as a chopper, which breaks up this primary DC to form.

Chopped DC that can now be used as the primary signal for a step-down transformer. The duty cycle of chopped DC will affect the AC voltage level generated on the transformer’s secondary winding (output). A long duty cycle means a larger output voltage (for heavy loads), and a short duty cycle means lower output voltage (for light loads). Duty cycle itself refers to the amount of time that a signal is “on” compared to its overall cycle. The duty cycle is continuously adjusted by the sensing/switching circuit. You can use an oscilloscope to view switching and chopped DC signals. Figure 32-2 illustrates a more practical representation for a switching supply.

AC voltage produced on the transformer’s secondary winding (typically a step-down transformer) is not a pure sine wave, but it alternates regularly enough to be treated as AC by the remainder of the supply. Secondary voltage is re-rectified and re filtered to form a secondary DC voltage that is actually applied to the load. Output voltage is sensed by the sensing/switching circuit, which constantly adjusts the hopped DC duty cycle. As load increases on the secondary circuit (more current is drawn by the load), output voltage tends to drop. This is perfectly normal, and the same thing happens in every unregulated supply. However, a sensing circuit detects this voltage drop and increases the switching duty cycle. In turn , the duty cycle for chopped DC increases, which increases the voltage produced by the secondary winding. Output voltage climbs back up again to its desired value-output voltage is regulated.

The reverse will happen -as load decreases on the secondary circuit. (Less current is drawn by the load.) A smaller load will tend to make output voltage climb. Again, the same actions happen in an unregulated supply. The sensing/switching circuit detects this increase in voltage and reduces the switching duty cycle. As a result, the duty cycle for chopped DC decreases, and transformer secondary voltage decreases. Output voltage drops back to its desired value. Output voltage remains regulated. Consider the advantages of a switching power circuit. Current is only drawn in the primary circuit when its switching transistor is on, so very little power is wasted in the primary circuit. The secondary circuit will supply just enough power to keep load voltage constant (regulated), but very little power is wasted by the secondary rectifier, Filter, or switching circuit Switching power supplies can reach efficiencies higher than 85 percent (35 percent more efficient than most comparable linear supplies). More efficiency means less heat is generated by the supply, so components can be smaller and packaged more tightly.

 

Simplified diagram of a switching power supply

Unfortunately, there are several disadvantages to switching supplies that you must be aware of. First, switching supplies tend to act as radio transmitters. Their 20-kHz to 40-kHz operating frequencies can wreak havoc on radio and television reception, not to mention the circuitry within the PC or peripheral itself. This is why you will see most switching supplies somehow covered or shielded in a metal casing. It is critically important that you replace any shielding removed during your repair. Strong electromagnetic interference (EMI) can easily disturb the operation of a logic circuit. Second, the output voltage will always contain some amount of high-frequency ripple. In many applications, this is not enough noise to interfere with a load. In fact, most of the noise is filtered out in a carefully designed supply. Finally, a switching supply often contains more components and is more difficult to troubleshoot than a linear supply. This is often outweighed by the smaller, lighter packaging of switching supplies. In virtually all cases today, a defective power supply unit is simply replaced.

In actual practice, sensing and switching functions can be fabricated right onto an integrated circuit. Chip-based switching circuits allow simple, inexpensive circuits to be built as shown in Figure 32-3. AC line voltage is transformed (usually stepped down), and then it is rectified and filtered before reaching a switch-regulating chip. The chip chops DC voltage at a duty cycle that will provide adequate power to the load. Chopped DC from the switching regulator is filtered by the combination of choke and output filter capacitor to reform a steady DC signal at the output. The output voltage is sampled back at the switching chip, which constantly adjusts the chopped DC duty cycle.

CONNECTING A POWER SUPPLY

PC power supplies operate the motherboard directly, as well as a number of internal drives. This part of the chapter presents the typical connection schemes for AT, ATX, and NLX power supplies and highlights the major signals that you should be familiar with.

AT-Style Power Connections

The AT-style power supply is largely considered to be the classic connection scheme for IBM-compatible PCs. An AT-style supply provides four voltages to the motherboard (+5 Vdc, -5 Vdc, +12 Vdc, and -12

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