Switch Mode Power Supply Explained with Common Topologies

For small power applications like on board power sources, linear power supply is often used due to its simplicity and low cost. However, when the need for high power density designs arises, linear power supply simply disappears in the picture. It is because linear power supply is very less efficient. Switch mode power supply comes in place. Switch mode power supply corrected the drawback of a linear power supply in terms of efficiency and high power density. However, it is more complicated and can be expensive. I am not totally saying that a switch mode power supply is by default expensive than the linear power supply, it depends. At low power applications like on board supply, and then yes switching solution is expensive. However, for high power, say 500W application, the cost of a 50/60 Hz transformer is may be expensive than the switch mode power supply.

Linear ACDC Power Supply Block Diagram

A typical ACDC linear power supply has a step down transformer, rectifier, filter and regulator. The step down transformer is a 50/60 Hz which is bulky and expensive for high power applications. It needs the step down transformer next to the AC line because linear regulators voltage range is usually below 50V. The rectifier section converts AC to pulsating DC. The filter section conditioned the pulsating DC output of the rectifier to a low ripple waveform. The regulator will do the final job to fine tune the waveform such that it will become a straight line.

The linear regulator maintains the output voltage level by absorbing the difference between the input and output voltage. For instance in the above diagram the filtered voltage which is input to the regulator is 17V and the output of the regulator is maintained at 12V, this is means a voltage drop of 5V will be measured across the regulator. This voltage drop when multiplied by the load current is the power dissipation of the linear regulator. So, the higher the difference between the input and output means a huge loss on the regulator.

Linear DCDC Power Supply Block Diagram

For DCDC linear power supply solution, the circuit is straight forward and very simple. It will consist only of input and output capacitors and the regulator itself.

How Switch Mode Power Supply Works?

The heart and soul of a switch mode power supply are switching converters. There are several types of switching converter that can be used according to applications. We will discuss few of them later.

A switching converter operates in either saturation or cut-off of a semiconductor switch. At saturation, there is ideally zero resistance resulting to a zero power loss. When the switch is cut-off, there is ideally infinite resistance resulting to a zero current and then again, zero power loss. The switch in switching converter is modulated by a PWM signal and controlled by an application specific IC. The operation of this IC is complicated that linear regulator.

 Switch Mode ACDC Power Supply Block Diagram

Above is a basic circuit for switch mode ACDC power supply. The EMI filter is mandatory for international standard compliance (for personal or non-commercial use, this can be neglected). It has also a bridge rectifier to convert AC to pulsating DC. It has also a filter to condition the waveform so that the input to the DCDC converter is almost pure DC. The DCDC converter converts high voltage DC to a low voltage DC. As you noticed, the transformer is put in this section in contrast to the linear mode which is located next with the AC line. By this arrangement, the transformer is can be operated in very high frequency making the physical size very small and cheaper. Q1 is a switch that is pulse width modulated (PWM) so that it will operate in saturation and cut-off only. In saturation, there is ideally zero loss as the resistance is ideally zero. On the other hand, in cut-off, there is also zero loss as the current is ideally zero. The DCDC converter outputs a rectangular or square wave voltage and the output rectifier process it to become DC. The specific diagram above is actually a flyback topology ACDC switch mode power supply.

Basic DCDC Switching Converter Topology

There are several topologies to use on how to build a switch mode power supply. Topology means what type of switching converter is being used. For instance in the above block diagram; it is a flyback employed on the DCDC converter section. There are several topologies to select and we will understand each application.

1. Boost Converter

Boost converter consists of inductor, switch (MOSFET, BJT or IGBT), and diode and output storage capacitor. The switch is modulated by a PWM signal to generate the desired output voltage in way that the switch has ideally zero loss.

A boost converter is a step up DCDC converter. In other words, its output is higher than its input. The output and input is linked by the duty cycle ratio. The ideal duty cycle for a boost converter is

Duty cycle, boost = 1 – (Vin / Vout)

For instance the output is 20V while the input is 5V, the duty cycle is 75%.

Boost Converter Operation when switch is ON

When the switch is ON, the inductor will charge. The diode will be reversed biased. The output capacitor will be the one to supply the power demand of the load.

Boost Converter Operation when switch is OFF

When the switch is OFF, the inductor will reverses its polarity resulting to the forward bias of the diode. This will let the capacitor to recharge. The load power demand will be supplied by the input at this time.

2. Buck Converter

Buck converter is a step down switching converter. In other words, the output is lower than the input. Buck converter is the commonly used DCDC especially those installed on board. It is composed of switch (MOSFET, BJT or IGBT), diode, inductor and output storage capacitor. The switch is modulated by a PWM signal so that to attain the target output voltage with ideally zero loss on the part of the switch.

The same with the boost converter, the input and output voltage of the buck converter is link by the duty cycle ratio. The ideal duty cycle ratio is:

Duty cycle, buck = Vout / Vin

Supposing the input voltage is 20V while the output is 5V, the duty cycle is 25%.

Buck Converter Operation when switch is ON

When the switch is ON, the diode will reverse bias. The inductor will charge. The capacitor will charge as well. This time the power demand of the load is provided by the input source.

 Buck Converter Operation when switch is OFF

When the switch is OFF, the inductor will reverse its polarity and making the diode to forward bias. This time the load power demand will be provided by the energy stored in the inductor and capacitor.

3. Buck – boost

Buck-boost is a combination of buck and boost switching converters. It can operate either the output is higher or lower than the input. There two ways to obtain a buck-boost function; first is inverting and second is non-inverting. Inverting buck-boost has fewer parts counts and cheaper. Non-inverting buck-boost on the other hand has more parts counts and costly. The analysis of the non-inverting buck-boost is can be the same to buck and boost topologies when you break the operation into a buck or a boost. On the other hand, the inverting buck-boost will be analyzed differently.

4. Flyback Converter

Flyback converter is very common solution to use for an off line switch mode power supply. It is very common to laptop adapters and chargers. It is commonly used topology for phone and tablet chargers. It can handle high input voltage range as the available flyback controllers are rated for very high voltage. Flyback converter is efficient at power range less than 150-200W. Higher than this, flyback is may not a suitable solution at all. Flyback topology provides isolation between input (AC side) and output.

Flyback Transformer

Fly Back Transformer is not a conventional transformer. A conventional transformer transfer power or energy from the primary to secondary at ideally real time and perfectly. Fly back transformer stores energy on the primary magnetic field and after a certain period delivers to the secondary side.

Switch

The switch provides ON and OFF times which able to magnetized and demagnetized the transformer

Rectifier and Filter

Rectifier and Filter smoothens the signal from the secondary winding. The capacitor serves as energy storage element.

The rectifier and filter condition the output to be a pure DC.

Flyback Converter Basic Operation

Switch is ON

When the switch is ON (saturation state), the primary of the flyback transformer will simply acts like an inductor and it will charge. There is a current flow from Vin to the ground via the switch S that acts as a short circuit path this time. The diode on the secondary side is reversed biased making the secondary winding open. The load is supplied by the energy stored in the output capacitor, Cout.

Switch is OFF

When the switch turns off, the energy stored in the primary of the flyback transformer will be delivered to the load. A complete delivery will be attained if the flyback operates in DCM or discontinuous conduction mode. A partial energy transfer will be observed if the flyback operates in CCM or simply continuous conduction mode. Practically, flyback is operated in DCM. At this time frame, there will be flyback action observed in the primary especially on the switch drain as indicated by Vds. The diode on the secondary side will conduct as the secondary winding changes its polarity. This time Cout will recharge and the load will be supplied by the secondary winding.

5. Forward Converter

Forward converter is also commonly used topology for off line ACDC switch mode power supply. There are few approaches for a forward converter like one-switch forward, two-switch forward or interleave one-switch or two-switch forward. We will not go into very details to each as the main principle are the common. Below diagram is a simple one-switch forward converter. Take a look on the dot on the transformer winding; they are in phase unlike flyback.

RCD Clamp

In a simple and low power forward converter, a RCD clamp is enough to discharge the transformer in each switching period. It is a must in forward converter to have a mechanism to discharge of empty the transformer core in every switching period to avoid the phenomenon called “flux walking” that will saturate the transformer and introduce catastrophic failure.

In higher power application, a transformer reset winding is needed resulting to a bulky and expensive transformer. Another approach is to use a two-switch method to get rid of the reset winding.

Flyback also needs an RCD clamp to discharge the leakage energy that will cause high voltage spikes on the switching MOSFET. However, it is not bad as forward needed a discharge path.

Transformer

This is a conventional transformer that transfers all the energy in the primary to the secondary in real time, unlike flyback that is storing energy before it delivers to the secondary. Forward converter transformer is may be bulky (but small enough compared to a 50/60 Hz transformer in linear power supply since the switching frequency is high) if a reset winding is being used as a discharge mechanism.

Switch

This can be a MOSFET, BJT or IGBT. It is driven by a PWM signal to generate a square wave voltage on the transformer secondary.

Forward Diode

This is called a forward diode as this diode will follow the action of the primary side. If the primary switch is ON, the transformer will energized and this diode will conduct.

Freewheel Diode

Freewheel diode conducts only when the switch is OFF so that the current will continue to flow on the inductor towards the load.

Forward converter is also can be used in high input voltage like flyback, as there available controllers are rated to high voltage. Forward converter is more efficient than flyback converter since its transformer is not storing energy intentionally as the flyback do. For more than 150W of power, flyback is good topology to use. It can still be used for power less than 150W, but flyback is still efficient below this power level and flyback is simpler and cheaper, so for me personally I will go for flyback for power rating until 150W and forward beyond this power. Forward converter is also providing isolation between input (AC side) and output.

Forward Converter Operation when Switch is ON

When the switch is ON, the current will flow from the input voltage VIN towards the transformer primary and to the switch. The forward diode D5 will be forward biased this time. The freewheeling diode on the other hand is reversed biased. The inductor L1 and output capacitor C2 will charge and the current will be delivered to the output from the secondary winding.

Forward Converter Operation when Switch is OFF

During switch OFF, there will be no current drawn from the input but the current on the primary winding will still continue to flow on the RCD clamp until the energy on the core is gone. When the core is empty with the leakage energy, then the diode on the RCD (D2) will reversed bias. Since there is no current drawn from the input source VIN, the forward diode will reversed bias. The freewheeling diode will forward bias on the other hand. Both the inductor L1 and capacitor C2 will supply the power demand of the load using there charge energy.

There are more topologies to use for DCDC switching converter like half bridge, full bridge, Resonant (like LLC) or push pull. All these topologies are high efficiency due to switching operation. A switch mode power supply is can be made of several switching converters combining the discussed converters above.