Switched Mode Power Supply

DC to DC converters and DC to AC inverters belong to the category of switched mode power supplies (SMPS). Besides, there are SMPS, operating from mains, called off-line switching supplies. An off-line switching supply can be distinguished from a conventional AC–DC supply, as in case of former AC mains rectified and filtered without using an Input transformer, and DC voltage so obtained is then used as an Input to a switching type DC to DC converter.

In switching power supply, the active device that provides regulation is always operated in switched Mode, i.e. it is operated either in cut-off or in saturation. The Input DC is chopped at a high freq. (10 kHz to 100 kHz) using an active device and converter transformer. The transformed, chopped waveform is rectified and filtered. A sample of Output voltage is used as feedback signal for the drive circuit for switching transistor to achieve regulation.

Power electronics is entirely devoted to switch mode power conversion and deals with modem problems in analysis, design and synthesis of electronic circuits as applied to efficient conversion, control and regulation and electrical energy. Design and optimization of DC to DC converters which offer the highest power efficiency, small size and weight and high performance, are also included in power electronics.

Basic Block Diagram of SMPS

Figure shows the block diagram of the PSU. The input from mains is first filtered to suppress any spikes/surges entering the power supply circuit. This is an important part of the circuit and helps in preventing data loss or erroneous working during power line disturbances.

Filtered mains supply is then rectified by a full-wave bridge rectifier to produce +150V DC for the power converter section. The input supply is switched with the help of this power converter and the energy transferred to the output through a high frequency ferrite transformer.

         The power converter consists of two externally driven transistors operating in push-pull configuration. These are driven by converter driver. The power converter transformer produces low voltage switched wave-forms on the secondary side which are rectified and filtered to produce well-regulated ±5 V and ±12 V DC output voltages.

The ±5 V DC output voltage is sensed by control section of the PSU which essentially consists of a pulse width modulator (PWM) controller IC. This PWM produces suitable drive pulses at about 22 kHz for the Converter driver. The width of these pulses is controlled by the modulator depending upon the output sense voltages, thus ensuring correct operating voltages at the output and providing necessary protection to the PSU.

The input mains supply is applied to the circuit via an on-off switch of fuse F1. Power converters operating directly ‘off-line’ like this one draw heavy current when switched on. This inrush of current causes great stress on input components, switches, rectifiers and capacitors.

For low power applications, simple series resistor is used to limit the initial high voltage and high current. Special high current surge rated resistors are best suited for this application. However, adequately rated wire-wound resistor also serves the purpose and is frequently used.

Connectors from SMPS

Molex Connectors

The first and most common type of connection is called the Molex. The Molex connector is primarily used for devices that need both 12 V and 5 V of power. The Molex connector has chambers (notches), which make for easy installation. These chambers can be defeated if one pushes hard enough, so Always inspect the Molex connection to ensure proper orientation before one installs it.

Mini Connectors

Most systems also provide a mini connector. The mini is used primarily on 3.5-inch floppy drives, because floppy drive makers have adopted the mini connector for that use.

Motherboard Connectors

AT Connectors

Industry standard PC, XT, AT, Baby-AT, and LPX motherboards all use the same type of main power supply connectors. These supplies feature two main power connectors (P8 and P9), each with 6 pins that attach the power supply to the motherboard. P8 and P9 are identically sized and shaped; the only difference is the colors of wires and the order in which they appear. You have to get them plugged in right or you will irreparably smoke your motherboard. Just keep this in mind: black together. Both connectors have a black wire; when the connectors are plugged in properly, the black wire on P8 will be next to the black wire on P9.

ATX Connectors

The industry standard ATX power-supply-to motherboard main connector is the Molex 39-29-9202 (or equivalent) 20-pin ATX style connector. It is used in the ATX, Mini-ATX, Micro-ATX & SFX form factors. This is a 20-pin keyed connector with pins configured as shown in Table 16.4. The colors for the wires listed are those recommended by the ATX standard; however, they are not required for compliance to the specification, so they could vary from manufacturer to manufacturer.

ATX 12 V Connector

To augment the supply of +12 V power to the motherboard, Intel created a new ATX 12 V power supply specification. This adds a third power connector, called the ATX 12 V connector, specifically to supply additional +12 V power to the board.

Power Switch

Every PC needs a power switch. Power switch utilization creates one of the major differences between AT and ATX power supplies. AT power switches simply turn the system on or off, whereas ATX power supplies use a feature called soft power.

AT Power Switch

AT power switches come in only two common

Types: rocker and plunger. Each of these switches has four tab connectors that attach to four color-coded wires leading from the power supply.

The ends of the cable are fitted with spade connector lugs, which plug onto the spade connectors on the power switch. The cable from the power supply to the switch in the case contains four color-coded wires. In addition, a fifth wire supplying a ground connection to the case might be included. The switch was usually included with the power supply and heavily shrink-wrapped or insulated where the connector lugs attached to prevent electric shock. The four or five wires are color-coded as follows:

Brown and blue: These wires are the live and neutral feed wires from the 110 V power cord to the power supply. These are always hot when the power supply is plugged in.

  • Black and white: These wires carry the AC feed from the switch back to the power supply. These leads should be hot only when the power supply is plugged in and the switch is turned on.
  • Green or green with a yellow stripe: This is the ground lead. It should be connected to the PC case and should help ground the power supply to the case.

Troubleshooting tips for SMPS

  • Check the wall outlet. The outlet should be providing between 220-250 V AC current. Just set the voltage-ohmmeter (VOM) to read AC voltage and put one lead in each hole of the outlet.
  • Check the power cord. It should be firmly plugged into the power supply. If you have a spare cord, swap cords. Yes, power cords do fail.
  • Is power getting to the power supply? The fan gets it first, so if it isn’t turning on, the power supply isn’t getting power. When some power supplies are first turned on, the speaker emits a low click.
  • Check to make sure the power supply is connected to the motherboard using the right connectors, whether P8 and P9 (AT) or the single-piece, 20-pin ATX connector.
  • To conclude whether the problem is with the motherboard or SMPS, short the PS ON pin (Green pin) with any black pin i.e. the ground, and observe whether the SMPS fan has started. If it has, then it can be concluded that the source of the problem is with the motherboard, else it will be with the SMPS.
  • If all those things check out correctly, try swapping in a different power supply.


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