Designing a Switch Mode Power Supply (SMPS)

Engineers are familiar with the term Switch Mode Power Supply or SMPS. Once you understand what SMPS really is, its countless applications can be easily imagined.

An SMPS is used to convert the electronic power supply efficiently. It is used to supply power to sensitive devices that require stable and high-efficiency power supply. Any SMPS has some storage components that store electrical energy to supply to the charging device and some switching components that turn on and off at high frequencies, charging and discharging the storage components.

Power is supplied to the charging device by discharging the storage component when the switching component is in the non-conducting state. The use of switching regulators differentiates SMPS from linear regulators. The SMPS can be AC ​​to DC, DC to DC, AC to AC or DC to AC source. In this series on how to design SMPS, AC to DC and DC to DC SMPS are covered.

The switching regulators (like transistors) in the SMPS continuously switch between the ON and OFF state. Therefore, they spend much less time in the high-dissipation state, which reduces system power dissipation. In linear regulators, all power is dissipated as heat, which reduces the overall efficiency of the system. Due to the use of switching components that work at high frequency, SMPS can provide high efficiency of up to 95%. SMPS can be used in place of any linear regulator when high efficiency and a small-sized, lightweight power supply are required.

Figure 1: Typical SMPS image

In this series, SMPS are designed using different topologies. To design SMPS (AC to DC type), they can be categorized as follows –

Fig. 2: Image showing different types of SMPS

First, SMPS can be broadly categorized into two categories –

1. Non-isolated SMPS

2. Isolated SMPS

In non-isolated SMPS the input and output share the same ground or it can be said that there is an electrical connection between the input and output. In Isolated SMPS, the input and output are isolated from each other by a transformer that provides safety against any electrical shock from the input power supply (AC).

In non-isolated SMPS, the basic SMPS topologies can be categorized based on input-output voltage as follows –

1.Boost Converter – In this SMPS, the output voltage is always greater than the input voltage.

2. Buck Converter – In this SMPS, the output voltage is always lower than the input voltage

3. Buck-Boost Converter – In this SMPS, the output voltage can be higher or lower than the input voltage

In isolated SMPS, there are two SMPS topologies that are designed in this series. They are as follows –

4. Push-pull converter

5. Flyback Converter

Now the above mentioned topologies can be divided into different types as follows –

In this series of SMPS, a Boost converter and a Buck converter are designed in four ways –

I. Open Loop Boost/Buck Converter – In this Boost Converter, there is no error detection circuit or any feedback circuit. Therefore, the output voltage of this Boost converter will not be regulated.

II. Closed Loop Boost/Buck Converter – In this Boost Converter there will be an error detection circuit or feedback circuit. This feedback circuit helps regulate the output voltage.

III. Open Loop Boost/Buck Converter with Adjustable Output – This Boost Converter will have a variable output voltage, but without any error detection circuitry or feedback circuitry. Therefore, the output voltage of this Boost Converter will be variable but not regulated.

4. Closed-Loop Boost/Buck Converter with Adjustable Output – This boost converter will have a variable output voltage and feedback circuit. Therefore, the output voltage of this Boost Converter will be variable and regulated.

There will be two types of Buck-Boost converters designed in this series –

I. Open Loop Inverting Buck – Boost Converter – In an inverting Buck-Boost converter, the output voltage can be higher or lower than the input voltage. The polarity of the output is opposite to the input voltage, which is why it is called a Buck-Boost inverter converter.

II. Open Loop Inversion Buck – Boost Converter with Adjustable Output

In this Buck-Boost converter, the output voltage can vary from the lowest voltage (voltage lower than the input voltage) to the highest voltage level (voltage higher than the input voltage).

The Flyback converter is the same as the Buck-Boost converter, the only difference is that in the Buck-Boost converter it uses a single inductor winding, in the Flyback converter the inductor is divided into two windings that form a transformer. This provides isolation of the output from the input. In this series, a Flyback converter has been designed which will reduce the input voltage and provide low voltage at the output.

An Open Loop Push-Pull converter will be designed which will be a DC to DC SMPS. This converter uses the transformer to convert a DC voltage to another level. A transformer provides isolation between the input and output. This converter can provide high or low voltage at the output compared to the input. The number of turns of the transformer and the duty cycle will decide whether high or low voltage will be supplied at the output.

In this series, a Push-Pull converter will be designed which will increase the input voltage and provide high voltage at the output

Now let’s understand some basic terminologies that will be used in designing the SMPS –

Based on the output current of the SMPS, it can have two modes of operation –

1. Continuous Mode

2. Discontinuous mode

Normally in SMPS circuit there is an inductor at the input which decides the input current. In continuous mode, the current in the inductor is continuous throughout the entire switching period cycle. Thus, a regulated voltage is obtained at the output in continuous mode. In continuous mode, there is some critical value of output current below which the inductor current is zero during a part of the switching cycle, then the system is considered to be in discontinuous mode. Therefore, for continuous mode operation, the output is always greater than or equivalent to the critical current value.

Below is the waveform showing the inductor current in continuous mode.

Fig. 3: Graph showing current variation in SMPS continuous conduction mode

In this mode, the current in the inductor is pulsating and becomes zero during a portion of the switching time. The current in the inductor starts at zero and reaches peak current and returns to zero again after the completion of a switching cycle. Therefore, in this mode, the output voltage is not regulated. Therefore, discontinuous mode requires an error detection block that regulates the output voltage.

Below is the waveform showing the inductor current in discontinuous mode –

Fig. 4: Graph showing the current variation in the SMPS discontinuous conduction mode

In this series, each circuit will be designed in CCM and DCM modes and there will be standard CCM/DCM equations used to calculate component values.

An SMPS has several advantages over Linear Regulators. Some of the main advantages that leverage SMPS over any linear regulator are as follows –

Any SMPS has switching components that work at high frequency. This reduces the size of components required in the SMPS circuit design.

As small size components are used to make the SMPS circuit, assembling the circuit is less expensive.

Due to the use of switching regulators, the efficiency of SMPS is generally very high. An SMPS can have production efficiency of up to 95%. This means that around 95% of the input power can be delivered to the output load.

By using a transformer in place of the inductor, SMPS can provide good isolation. By providing isolation between the input and output, the output load can be saved from any electric shock or voltage fluctuation of the input source.

The use of SMPS is not just limited to computers. They are used with most of the sensitive devices that essentially require stable and efficient power supply. They are used as low loss current source to drive LEDs and LED circuits. They are used in self-powered devices. They are also used as an interface between the battery and CPU components or notebooks where the voltage demand is lower or higher than the battery voltage.

Therefore, in this series, the following SMPS circuits will be designed –

1. Boost Converters –

a) Open loop boost converter

b) Closed-loop boost converter

c) Open Loop Boost Converter with adjustable output

d) Closed Loop Boost Converter with Adjustable Output

2. Buck Converters –

a) Open Loop Buck Converter

b) Closed-loop Buck Converter

c) Open Loop Buck Converter with Adjustable Output

d) Closed-loop buck converter with adjustable output

3. Buck-Boost Converters

a) Buck Open Loop Inverter – Boost Converter

b) Buck Open Loop Inverter – Boost Converter with Adjustable Output

4. Flyback Converter

5. Push-pull converter