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Switch-Mode Power Supply Fundamentals (2)

Author: Apogeeweb
Date: 8 Jun 2018

Warm hints: The word in this article is about 3000 words and reading time is about 12 minutes.


The switch-mode power supply fundamentals tutorials consist of five chapters: the type of topology, the relationship between the efficiency with input & output and duty cycle, the definitions of synchronization and non-synchronization, characteristics of isolation and non-isolation, pulse width modulation and frequency conversion and other various control modes. It explains the basic concept in simple language and tends to establish a platform to exchange ideas and info for power designers. This is the second part of the tutorial series on switch-mode power supply fundamentals.



Chapter 2 The Relationship between Efficiency and Vout

Chapter 3 Synchronous vs. Asynchronous

Ⅰ What are asynchronous and synchronous circuits?

  1.1 Asynchronous

  1.2 Synchronous

Ⅱ Differences between Asynchronous and Synchronous Circuit

Ⅲ Merits and Faults of Asynchronous and Synchronous Topologies

  3.1 Merits and Faults of Asynchronous Topology

  3.2 Merits and Faults of Synchronous Topology

Ⅳ Choosing between Synchronous and Asynchronous Types

Chapter 4 Isolation and Non-isolation

Ⅴ Non-isolated Topologies

Ⅵ Isolated Topologies

  6.1 Forward Converters

  6.2 Flyback Converters

  6.3 Comparison of Forwarding and Flyback Converter Topologies

  6.4 Characteristics of the Flyback Converter

  6.5 Advantages and Applications of Flyback

  6.6 Important Waveforms of Flyback

  6.7 Steady-State Analysis of Flyback

  6.8 Design of Flyback Converter

  6.9 Examples & Design Specifications




Chapter 2 The Relationship between Efficiency and Vout

In a switching power supply, the range of input power is known, and so is the setting of output voltage, but what is the relationship between output Vout and efficiency?


We often say that the greater the duty cycle, the higher the efficiency, and the less the loss, so the next question is why? An experienced engineer will derive it from the formula. Why is it most efficient when the duty cycle is highest? Here's an example to explain it.


The efficiency of power supply η:


Pout is the output power and Pd is the dissipation power in the above equation.


Let's use a simplified formula to calculate the power dissipation. Why is it simplified like said? Switching losses include turn-on loss, turn-off loss, conduction loss, drive loss, and for the sake of more obvious demonstration, the following calculation is simply about turn-on loss. Assuming that there is no ripple of the inductor current, and the input Vin=5V, output Io=1A, the losses at the output of 3.3V and 1V is shown in the following table:


Table 1 Losses at the output of 3.3V and 1V

In fact, the calculation of the current effective value of the mos transistor above is wrong. The correct formula is FIG.2_20180605.png

A simplified formula is just for an easier calculation.

From the above calculation, we can see that at 3.3 V output, the efficiency is:


When the output is 1 V, the efficiency is:


Therefore, the efficiency of 3.3V input is relatively high. According to this characteristic, we can also draw the relation between output and power under the same conditions. With the figure below, we can see that the larger the output, that is, the greater the duty cycle, the higher the efficiency.


FIG.1 TPS62400 efficiency vs. Vout (Vin=15V, Iout=300mA)


Chapter 3 Synchronous vs. Asynchronous

Ⅰ What are asynchronous and synchronous circuits?

1.1 Asynchronous

If the high mosfet synchronizes with the low mosfet, we can find in some applications that they are called directly switch tubes, that is, the high-side and low-side MOSFETs.Well, this case is definitely referred to as an asynchronous circuit, so that we are talking about only one mosfet (or switch), there is no need to emphasize whether it is synchronous or asynchronous.


1.2 Synchronous

Synchronization is a new technology that uses MOSFETs, a special power component with very low on-state resistance, to replace rectifier diodes and then to reduce the rectifier loss. It can greatly improve the efficiency of DC/DC converters and there is no threshold voltage caused by the Schottky barrier. Power MOSFET  is a voltage-controlled device, having linear voltage-current characteristics in the conducting state. When power MOSFET is used as a rectifier, the gate voltage must be synchronized with the phase of rectified voltage to achieve the rectification function, so it is called a synchronous rectifier.


Ⅱ Differences between Asynchronous and Synchronous Circuit

The circuits in which both upper and lower transistors used field-effect transistors are synchronous, and in which there is only one upper transistor (switch tube) are asynchronous. For example, see these two buck circuits, as shown in the following two figures: power switches at the primary power level are common to us as shown in figure 2, the lower flyback diode becomes a switch, which is called asynchronous field effect transistor (FET), as shown in figure 3. 


That is, figure 2 shows an asynchronous circuit and figure 3 shows a synchronous one.


FIG.2  An asynchronous circuit


FIG.3  Asynchronous circuit

Another example:

There is a controller with two upper and lower MOS transistors on the periphery, the upper transistor can be used as power transistors and the lower one as a synchronous FET, so we can say that it is a synchronous Buck circuit.


FIG.4  Asynchronous Buck circuit


Ⅲ Merits and Faults of Asynchronous and Synchronous Topologies

3.1 Merits and Faults of Asynchronous Topology

The voltage drop of the diode is quite constant when the output current changes.

The forward voltage drop of the diode is constant when the flyback diode is under forwarding conduction condition and the output current changes. The voltage drop of the germanium tube is 0.2-0.3 V and the voltage drop of the silicon tube is 0.7 V.

  • Low efficiency

Considering the voltage drop of the diode is constant, when the current flowing through the diode is very large, although a very low output voltage can get an impressively high value multiplied by the current. At this point, a small voltage drop of the diode can cause a considerable power dissipation, which makes the efficiency of the circuit decrease at a high current.

  • It's cheaper

Everyone knows that diodes must be cheaper than mosfets under the same conditions (on the same kind of substrate). If one is an ordinary mosfet, the other is a silicon carbide substrate diode, or one is a low-voltage mosfet, and the other is a high-voltage diode, then the diode is not necessarily cheaper than the mosfet.

  • The high output voltage can be used

It is more suitable for high input voltage, because if the output voltage is relatively high, then the proportion of the voltage drop of the diode will be very small in forwarding conduction mode, and the impact on efficiency will also be reduced. In addition, due to its relatively simple circuit structure and there is no need for an additional control circuit, the production process will be relatively simple.


3.2 Merits and Faults of Synchronous Topology

  • The mosfet has a relatively low voltage drop

The mosfet has a very important parameter, the on-resistance Rds on, mostly very small, at the milliohm level, so the voltage drop of the mosfets is very low after conducting.

  • High efficiency

Under the same conditions, the voltage drop of mosfet is much smaller than that of ordinary schottky diode in forward conduction mode, so the power losses of the mosfet is much smaller than that of the diode under the condition of constant current. Therefore using the mosfet would be more efficient than using the diode.

  • Additional control circuits are required

The mosfet needs to add an extra drive circuit to synchronize the upper and lower mosfets, while the non-synchronous diodes are naturally rectified, and no additional control circuits are needed. Therefore, compared with the asynchronous circuit, a synchronous circuit will be more complex.

  • Higher cost

Generally speaking, the price of a mosfet is higher than that of a diode, and the mosfet needs drive circuit or drive IC, so the cost of manufacture of synchronous circuit is more expensive than that of the non-synchronous circuit, and the process is also more complicated.


Ⅳ Choosing between Synchronous and Asynchronous Types

  • Efficiency

After understanding the merits and demerits of both of them, how do we make a choice? Well, if you want to increase the efficiency, and don’t mind a little higher cost, then the synchronous type is your best choice. Like I already said, mosfet can give you efficiency because of its low losses, but it is indeed more expensive and costly.

  • Cost

The asynchronous circuit uses flywheel diodes, which are cheaper than the mosfets and has no requirement for an additional control circuit, so it is much cheaper in terms of component material cost and production cost. Therefore, an asynchronous type is also a nice choice for you if efficiency is not your priority.

  • Reliability

Another thing to consider is reliability. The asynchronous type is definitely more reliable than the synchronous type because mosfet can't be an ideal switch, no matter what, it has a turn-on/off time. Therefore, if dead times of the upper and lower transistors are not controlled well, it will cause an overlap between the turn-off time of the upper transistor and the turn-on time of the lower transistor, resulting in a shot-through current, and will burn the mosfet.


Therefore, timing control (controller ICs) in synchronous circuits is also an important issue. As shown in figure 5, the controller IC is just an integrated power FET, and if its rectifier diode is replaced with a mosfet for synchronization, then its dead-time must be strictly controlled; if the IC in figure 6 is fully integrated with the upper and lower transistors, then you don't have to worry about this issue so much.


FIG.5 Integrated power FET with a rectifier diode



FIG.6  Integrated power FET with upper and lower transistors


To highlight the importance of efficiency issues between the synchronization and non-synchronization, here's an example:

  • Input voltage: Vin=5V  Synchronous mosfet internal resistance: Rds on _sync=0.12ohm

  • Output voltage: Vout=1V  Power mosfet internal resistance: Rds on_PWR=0.2 ohm

  • Output current: Iout=1A  Forward conduction voltage drop of asynchronous flywheel diodes: VF_DIONDE=0.5V



From the comparison above we can see that the main factor affecting the efficiency is the loss of flywheel diodes. The effect of using a synchronous circuit for low output voltage is very obvious, but for higher Vout, synchronization or non-synchronization will not be a problem in high duty cycle mode, the power loss of synchronous FET or diode clamp is relatively low.


Chapter 4 Isolation and Non-isolation

Ⅴ Non-isolated Topologies

The power supplies have different topologies, and figure 7 shows three basic topologies for DC-DC power conversion, which are Buck, Boost and Buck-Boost.


FIG.7 Three basic topologies for DC-DC power conversion


Simple structure, small size, low cost, a wide range of output voltage regulation.



Because it is non-isolated, a person may be in danger of electric shock when in contact with the output or ground end of the power supply when Using municipal electricity as the power supply; 


When it rains and thunders, it is very likely that the whole circuit will be burnt out without isolation; 

For these three non-isolated topologies, the output voltage is not equal to the input voltage.


Ⅵ Isolated Topologies

Since input-to-output isolation is required in many applications, other common topologies are derived based on these three non-isolated topologies of Buck, Boost and Buck-Boost: flyback, forward, push-pull, half-bridge, full-bridge.


Safe isolation; 

Protecting the devices from electrical transient damage; 

Removing ground loops between isolation circuits to improved noise-reduction performance in active noise control systems; 

The output wiring is easily completed in the system without conflict with the main grounding.



Large size or smaller power for the same size.


6.1 Forward Converters

The single-terminal forward converter is derived from the Buck converter. The following figure is the schematic of the Buck converter, which can be obtained by inserting an isolation transformer to the right side of the switch.


FIG.8 Schematic of the Buck converter


6.2 Flyback Converters

The flyback converter is derived from the Buck-Boost converter. The following figure is the schematic of the Buck-Boost converter, which can be obtained by replacing the inductor with an isolation transformer.


FIG.9 Schematic of the Buck-Boost converter

6.3 Comparison of Forwarding and Flyback Converter Topologies


FIG.10 Comparison of forwarding and flyback converter topologies

We can observe the difference between the two schematics, which has been shown in the figure above. See the yellow part, the forward converter adds an isolation transformer as well as an auxiliary inductor Nr; but the flyback converter modified by buck-boost does not have this reset winding. Why? That is because the isolation transformer of the flyback converter plays dual roles of inductor and transformer. As far as the characteristics of the inductor concerned, there is no need for a reset winding.


The inductor is a device for storing energy that stores energy when the mosfet is turned on and releases energy when the mosfet is turned off, so it is always in a state of equilibrium and never reaches the level of saturation.


As for forwarding converter, the Vin has always added to it. when the mosfet is repeatedly turned on, energy is continuously added to the primary side and its core will be easily saturated, at this point if we add an inductor and reset it, we can then release the energy of the primary side inductor.

Let's compare the other features of forward and flyback:

Input filtering
Medium, impulse
Medium, impulse
Output filtering
Providing low continuous output current from inductor
Large output capacitors are required for high pulse output current
Low to medium
Multiple output capacity
Yes, but it is difficult to design the coupled output inductor
Yes, good cross-regulation is achieved by a careful transformer design
Low, no output inductor
Typical power range
Medium, need to reset the transformer

Table 2 Comparison of forward and flyback


6.4 Characteristics of the Flyback Converter


Using a coupled inductor to act as an isolation transformer and to store energy; 

Input-to-output isolation; 

Using the duty cycle and turns ratio to step up or step down voltage; 

Easy to implement multiple outputs;

No need for a separate output inductor;

Best suited for lower power levels.



High output ripple current;

High input ripple current;

The loop bandwidth may be limited by the right-half-plane (RHP) zero.


FIG.11 Characteristics of the flyback converter


6.5 Advantages and Applications of Flyback

  • With the simplest and cheapest isolated topology

  • Minimum number of power components used: 4

  • One of the most widely understood, implemented and widely supported topologies

For these reasons, a flyback converter is a good choice for applications with a power range <150 W.


6.6 Important Waveforms of Flyback


FIG.12 Continuous conduction mode input/output relationship

When the switch is turned on, and the current of the inductor rises, you can see that the pattern of its current is very similar to that of buck-boost, and the only difference is the turns ratio of the primary and secondary side, from which we can realize that the transformer also plays a role of the inductor.


6.7 Steady-State Analysis of Flyback


FIG.13 Values are transformer related (turns ratio, inductance)

The above figure shows the waveforms during turn-on transition and the freewheeling diode waveforms, from which we can also see that the waveform of the switch plus the current waveform of the diode is the current waveform of the inductor.


6.8 Design of Flyback Converter

Appropriate flyback converter components must be selected to handle the necessary current and voltage stresses which are determined by the formulas given in the previous chapter. All these stresses are related to transformer: turns ratio, inductance.


FIG.14 Design of flyback Converter

The duty ratio and the turns ratio are designed by the engineer. It has a corresponding duty cycle for the minimum or maximum input voltage, with a steady working voltage. What you have to do is to optimize the efficiency at this point. Of course, few engineers think so much, because the general use of flyback is in low-power situations where the efficiency issue is not so strictly required.


FIG.15 Basic requirements for flyback design

Please note that multiple thin wires are required in high-switching-frequency transformers due to skin effect. High inductance is required to keep the operations proceed in continuous conduction mode over a wide load range, making the ripple current in primary and secondary circuits become lower.


6.9 Examples & Design Specifications

A design always starts with design specifications, including input voltage range, power level, output voltage, etc. Duty cycle and switching frequency are generally predetermined. In general, a switching frequency between 200kHz and 300kHz can well balance the requirements of switching loss and filter. Actually, it's also used in 65kHz-300kHz, because the frequency is in direct proportion to the switching loss and in inverse proportion to the volume. When the volume in your design is not so strictly required, you can do it at 65 kHz, so that it becomes more efficient. If the switching frequency is between 200kHz and 300kHz, then the core and power will all be made smaller and the ripple is better reduced.


FIG.16 Application & Specifications


Here is a video tutorial series on the design of power converter circuits.

SMPS Tutorial (2): Linear Regulators, Voltage References, Switched Mode Power Supplies

SMPS Tutorial (3): Charge Pumps, Buck Converters, Switched Mode Power Supplies



1. What is a switching power supply 12v?

Switching regulated 12VDC power supplies, sometimes referred to as SMPS power supplies, switchers, or switched-mode power supplies, regulate the 12VDC output voltage using a complex high-frequency switching technique that employs pulse width modulation and feedback.


2. How can I check my PC SMPS power supply?

• Remove the connections that are connected to the motherboard from SMPS.

• Use a paper clip and bend it in U shape. Locate the green and select any one of the black wires of the bigger connector.

• Connect the Power cable and Power on the SMPS.

• The SMPS Fan Will Spin, if it is working.


3. Can I use a switching power supply to drive a DC motor?

A simple unregulated analog power supply may be easier and be able to supply the large starting under load current more than the switching one. DC motors are not too fussy about the supply, and will usually run quite well on unfiltered DC.


4. Does a switching power supply have a transformer?

Switch mode transformers (also known as switch mode power supply transformers and SMPS transformers) are using in a regulated power supply and function to step up or step down voltage or current, and/or provide isolation between the input and output side of a switch-mode power supply.


5. What happens if SMPS not working?

One of the major symptoms of PSU failure is the constant noise from the SMPS. This indicates that the fan bearings of your SMPS have been worn out and it can stop working anytime. After a while, the noisy fan will stop spinning at its original RPM or speed, resulting in the temperature rise in the components of SMPS.



Book suggestions

Principles of Asynchronous Circuit Design: A Systems Perspective (European Low-Power Initiative for Electronic System Design (Series).) 2002nd Edition

This book addresses the need for an introductory text on asynchronous circuit design. The objective in writing this book has been to enable industrial designers with a background in conventional (clocked) design to be able to understand asynchronous design sufficiently to assess what it has to offer and whether it might be advantageous in their next design task.

by Jens Sparsø and Steve Furber


MOSFET Theory and Design 1st Edition

Developed for a one-semester course at the junior, senior, or graduate level, this book presents a clear, in-depth treatment of the physical analysis and design principles for the MOSFET. By focusing solely on the MOSFET, this slim volume recognizes the dominance of this device in today's microelectronics technology while also providing students with an efficient text free of the extra subject matter. 

by R. M. Warner and B. L. Grung


Grounds for Grounding: A Circuit to System Handbook

Grounding design and installation are critical for the safety and performance of any electrical or electronic system. Blending theory and practice, this is the first book to provide a thorough approach to grounding from circuit to system. It covers: grounding for safety aspects in facilities, lightning, and NEMP; grounding in a printed circuit board, cable shields, and enclosure grounding; and applications in fixed and mobile facilities on land, at sea, and in air. It’s an indispensable resource for electrical and electronic engineers concerned with the design of electronic circuits and systems.

by Elya B. Joffe and Kai-Sang Lock


Relevant information about "Switch-Mode Power Supply Fundamentals (2)"

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