Switch Mode Power Supply Blocks

Operation of a Switch Mode Power Supply

How a Switch Mode Power Supply Works? What is the operation of a switch mode power supply? A switch mode power supply works by continuously switching on and off a semiconductor switch. This semiconductor switch is can be a MOSFET or a BJT. The former is more popular than the latter. When the switch turns off, there is no current that can flow so there is also no power dissipation. On the other hand, when the switch turns on, there will be ideally no voltage drop on the semiconductor switch. Thus the power is still zero. In SMPS (short for switch mode power supply), it is very important to drive the semiconductor switch in hard saturation to attain a very small power loss. SMPS has higher efficiency compared to the linear power supply design and this is its primary advantage. In order to elaborate the explanation of the operation of a switch mode power supply, I consider a flyback converter below. A switch mode power supply by the way consists of a switching converter. Flyback is one example of a switching converter.

Ideal Analysis of the Operation of a Switch Mode Power Supply

The core circuit of a switch mode power supply is a switching converter. The laptop adaptor and the smart phone charger you have is a switch mode power supply and its main content is a flyback converter. In order to explain well the operation of a switch mode power supply, let us consider the schematic of a flyback converter below.

figure 1

(Switching power supply is not limited to a flyback topology only. There are lots of topologies that can be used in an SMPS.)

When the Vgate is high, MOSFET S1 will saturate and at saturation there is no voltage drop on the MOSFET. During this time the current is maximum. When the voltage and current is being multiplied the resulting power is zero. On the other hand the D1 will be reversed biased and there is no power consumption. At this time the load is deriving power from the energy stored in the capacitor C1.

When the Vgate level is low, the MOSFET S1 will be completely cutoff and there is no current to flow. Again the power dissipation is zero. During this time, the energy stored in the primary magnetic field of the transformer will be completely transferred to the secondary. The diode D1 will then forward biased. A forward biased diode is ideally shorted so there is no power dissipation as well.

 

Real Analysis of the Operation of a Switch Mode Power Supply

There is no such thing as ideal in a real world. When the MOSFET S1 is operating at saturation, there is a  power dissipation due to the drain to source on state resistance. The power dissipation is dependent to how big the drain to source on state resistance is. High end MOSFETs has lower drain to source on state resistance but with a higher price. Aside from this, there are also switching losses due to the charging of the input capacitance of the device. Larger input capacitance will result to a bigger power loss also.  There is also power dissipation due to the output capacitance of the MOSFET. Basically, a MOSFET with a very small drain to source on state resistance most likely to have a higher output capacitance. In order to make the drain to source resistance very small, the manufacturer will adopt the paralleling techniques. We know that when you parallel something, the capacitance will increase. Drain to source on state resistance and output capacitance may not go in the same direction. The other will increase with the decrease of the other. However the effect of the drain to source on state resistance is more pronounced than the effect of output capacitance, so lowering the drain to source resistance is a good move.

Another contributing loss is the rise and fall times of the MOSFET. This loss in often times negligible compared to the drain to source resistance, output capacitance and input capacitance effects.

When the diode is forward biased, there is power dissipation due to its forward voltage. Moreover a diode with a longer reverse recovery time will also contribute power loss. Another loss contributing factor is the imperfect coupling of the primary and secondary windings.

If all these losses are added however, this is still very much low compare to the losses of a linear power supply. A linear power supply most of the times has only 60-75% efficiency. Switching power supply has more than 90% on the other hand.

Actually the operation of a switch mode power supply is complicated compare to the linear power supply. However, due to the fact that it has high efficiency makes it to be the most preferred method in power conversion nowadays.

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