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Jun 8 2018

Switch-Mode Power Supply Fundamentals (2)

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 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.

Article Core
SMPS fundamentals
Maintain a stable output voltage
Switch-mode power supply
Electronic power supply
Converting & supplying power
Efficiency higher, smaller, lighter


Chapter 2 The relationship between efficiency and Vout3.3 Merits and faults of asynchronous and synchronous topologies4.2 Isolated topologies4.2.6 Important waveforms of flyback
Chapter 3 Synchronous vs. Asynchronous(1) Merits and faults of asynchronous topology4.2.1 Forward converters4.2.7 Steady-State Analysis of Flyback
3.1 What is asynchronous and synchronous circuits(2) Merits and faults of synchronous topology4.2.2 Flyback converters4.2.8 Design of flyback Converter
(1) Asynchronous3.4 Choosing between synchronous and asynchronous types4.2.3 Comparison of forward and flyback converter topologies4.2.9 Examples & Design Specifications
(2) SynchronousChapter 4 Isolation and non-isolation4.2.4 Characteristics of flyback converter
3.2 Differences between asynchronous and synchronous circuit4.1 Non-isolated topologies4.2.5 Advantages and Applications of Flyback

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 inductor current, and the input Vin=5V, output Io=1A, the losses at output of 3.3V and 1V is shown in the following table:


Table 1 Losses at 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

3.1 What is asynchronous and synchronous circuits

(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 refers to 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.

(2) Synchronous

Synchronization is a new technology which 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 Schottky barrier. Power MOSFET  is a voltage controlled device, having linear voltage-current characteristics in the conducting state. When power MOSFET is used as rectifier, the gate voltage must be synchronized with the phase of rectified voltage to achieve the rectification function, so it is called synchronous rectifier.

3.2 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 a synchronous field effect transistor (FET), as shown in figure 3. 

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


FIG.2  An asynchronous circuit


FIG.3  A synchronous 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  A synchronous Buck circuit

3.3 Merits and faults of asynchronous and synchronous topologies

(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 forward conduction condition and the output current is 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 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.

  • 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 forward 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 additional control circuit, the production process will be relatively simple.

(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.

3.4 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 additional control circuit, so it is much cheaper in terms of component material cost and production cost. Therefore, a asynchronous type is also a nice choice for you if the 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 a 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 are 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 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 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

4.1 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, 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.

4.2 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.

4.2.1 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

4.2.2 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

4.2.3 Comparison of forward and flyback converter topologies


FIG.10 Comparison of forward 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 flyback converter plays dual roles of inductor and transformer. As far as the characteristics of inductor concerned, there is no need for a reset winding.

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 reach the level of saturation.

As for forward 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

4.2.4 Characteristics of 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 flyback converter

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

4.2.6 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 inductor.

4.2.7 Steady-State Analysis of Flyback


FIG.13 Values are transformer related (rutns 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.

4.2.8 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.

4.2.9 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 be 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

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