Explain the topology of switch mode power supply in one article
Circuit topology refers to the connection between power devices and electromagnetic components in a circuit, while the design of magnetic components, closed-loop compensation circuits, and all other circuit components depends on the topology. The most basic topologies are Buck, Boost, and Buck/Boost, single ended flyback (isolated flyback), forward, push-pull, half bridge, and full bridge converters. There are approximately 14 common topologies for switch mode power supplies, each with its own characteristics and applicable scenarios. The selection principle depends on whether it is high-power or low-power, high-voltage output or low-voltage output, and whether it requires as few components as possible. It is very important to choose a topology appropriately and be familiar with the advantages, disadvantages, and applicability of various topologies. Wrong choices will inevitably lead to the failure of power supply design from the beginning.
In this article, we will delve into step-down, step-up, and step-down step-up topologies from different perspectives.
Buck Converter
Figure 1 is a schematic diagram of a asynchronous buck converter. The buck converter reduces its input voltage to a lower output voltage. When switch Q1 is turned on, energy is transferred to the output terminal.
Figure 1: Schematic diagram of asynchronous buck converter
Formula 1 calculates the duty cycle:
Formula 2 calculates the maximum stress of a metal oxide semiconductor field-effect transistor (MOSFET):
Formula 3 provides the maximum diode stress:
Vin is the input voltage, Vout is the output voltage, and Vf is the forward voltage of the diode.
Compared to linear regulators or low dropout regulators (LDOs), the greater the difference between the input voltage and output voltage, the higher the efficiency of the buck converter.
Although the buck converter has a pulse current at the input, the output current is continuous due to the presence of an inductor capacitor (LC) filter at the output of the converter. As a result, the voltage ripple reflected to the input terminal will be larger compared to the ripple at the output terminal.
For buck converters with small duty cycles and output currents greater than 3A, it is recommended to use synchronous rectifiers. If your power supply requires an output current greater than 30A, it is recommended to use multi-phase or interleaved power stages, as this can minimize component stress, distribute heat generated between multiple power stages, and reduce reflection ripple at the input of the converter.
When using N-FET, the duty cycle is limited because the bootstrap capacitor needs to be recharged in each switching cycle. In this case, the maximum duty cycle is within the range of 95-99%.
Buck converters typically have good dynamic characteristics because they have a forward topology structure. The achievable bandwidth depends on the quality of the error amplifier and the selected switching frequency.
Figures 2 to 7 show the voltage and current waveforms of FET, diode, and inductor in continuous conduction mode (CCM) in asynchronous buck converters.
Boost converter
The boost converter increases its input voltage to a larger output voltage. When switch Q1 is non-conductive, energy is transferred to the output terminal. Figure 8 is a schematic diagram of a asynchronous boost converter.
Figure 8: Schematic diagram of asynchronous boost converter
Formula 4 calculates the duty cycle:
Formula 5 calculates the maximum MOSFET stress:
Formula 6 provides the maximum diode stress:
Vin is the input voltage, Vout is the output voltage, and Vf is the forward voltage of the diode.
By using a boost converter, the pulse output current can be seen, as the LC filter is located at the input end. Therefore, the input current is continuous, and the output voltage ripple is greater than the input voltage ripple.
When designing a boost converter, it is important to know that there will be a permanent connection from input to output even if the converter is not switching. Preventive measures must be taken to prevent potential short circuit events at the output end.
For output currents greater than 4A, synchronous rectifiers should be used to replace diodes. If the power supply needs to provide an output current greater than 10A, it is strongly recommended to use multi-phase or interleaved power stage methods.
When operating in CCM mode, the dynamic characteristics of the boost converter are limited due to the right half plane zero point (RHPZ) of its transfer function. Due to the inability of RHPZ to compensate, the achievable bandwidth will typically be less than one-fifth to one tenth of the RHPZ frequency.
Please refer to formula 7:
Among them, Vout is the output voltage, D is the duty cycle, Iout is the output current, and L1 is the inductance of the boost converter.
Figures 9 to 14 show the voltage and current waveforms of FET, diode, and inductor in CCM mode in asynchronous boost converters.
Buck boost converter
A buck boost converter is a combination of buck and boost power stages that share the same inductor.
Refer to Figure 15.
Figure 15: Schematic diagram of dual switch buck boost converter
The buck boost topology is very practical because the input voltage can be smaller, larger, or the same as the output voltage, and requires an output power greater than 50W.
For output power less than 50W, a single ended primary inductor converter (SEPIC) is a more cost-effective choice as it uses fewer components.
When the input voltage is greater than the output voltage, the buck boost converter operates in buck mode; When the input voltage is lower than the output voltage, it operates in boost mode. When a converter operates in the transmission region where the input voltage is within the output voltage range, there are two concepts for handling these situations: either the buck and boost stages are simultaneously active, or the switching cycle alternates between the buck and boost stages, each typically operating at half the normal switching frequency. The second concept can cause subharmonic noise at the output, and compared to conventional buck or boost operations, the output voltage accuracy may not be as precise, but compared to the first concept, the converter will be more efficient.
The buck boost topology has pulse currents at both the input and output ends because there is no LC filter in either direction.
For buck boost converters, buck and boost power stages can be used separately for calculation.
The buck boost converter with two switches is suitable for a power range between 50W and 100W (such as LM5118), and the synchronous rectification power can reach 400W (the same as LM5175). It is recommended to use a synchronous rectifier with the same current limitation as the unconjugated buck and boost power stages.
You need to design a compensation network for the buck boost converter for the boost stage, as RHPZ will limit the bandwidth of the regulator.
Common basic topological structures
■Buck voltage reduction
■Boost Boost
■Buck Boost voltage reduction boost
■Flyback Flyback
■Forward Forward
■Two Transformer Forward dual transistor forward
■Push Pull
■Half Bridge Half Bridge
■Full Bridge
■ SEPIC
■ C’uk
1、 Basic pulse width modulation waveform
These topological structures are all related to switch mode circuits, and the basic pulse width modulation waveform is defined as follows:
2、Buck
Characteristic:
■Reduce the input to a lower voltage.
■It may be the simplest circuit.
■The inductor/capacitor filter flattens the square wave after switching.
■The output is always less than or equal to the input.
■ Input current is discontinuous (chopping).
Smooth output current.
3、Boost
Characteristic:
■Raise the input to a higher voltage.
■Similar to voltage reduction, but with rearranged inductors, switches, and diodes.
■The output is always greater than or equal to the input (ignoring the forward voltage drop of the diode).
■Smooth input current.
■ Discontinuous output current (chopping).
4、Buck-Boost
Characteristic:
■Another arrangement method for inductors, switches, and diodes.
■Combining the drawbacks of both step-down and step-up circuits.
■ Input current is discontinuous (chopping).
■The output current is also discontinuous (chopping).
■The output is always opposite to the input (note the polarity of the capacitor), but the amplitude can be smaller or larger than the input.
■The "flyback" converter is actually in the form of a step-down step-up circuit isolation (transformer coupling).
5、Flyback
Characteristic:
■It works like a buck boost circuit, but the inductor has two windings that act as both a transformer and an inductor.
■The output can be positive or negative, determined by the polarity of the coil and diode.
■The output voltage can be greater or less than the input voltage, determined by the turns ratio of the transformer.
■This is the simplest isolated topology structure.
■Adding secondary windings and circuits can result in multiple outputs.
6、Forward
Characteristic:
■The transformer coupling form of the step-down circuit.
■Discontinuous input current, smooth output current.
■Due to the use of transformers, the output can be greater or less than the input, and can be of any polarity.
■Adding secondary windings and circuits can obtain multiple outputs.
■The transformer core must be demagnetized during each switching cycle. The common practice is to add a winding with the same number of turns as the primary winding.
■The energy stored in the primary inductor during the switch on phase is released through additional windings and diodes during the switch off phase.
7、Two-Transistor Forward
Characteristic:
■Two switches work simultaneously.
■When the switch is disconnected, the energy stored in the transformer reverses the polarity of the primary, causing the diode to conduct.
Main advantages:
■The voltage on each switch will never exceed the input voltage.
■No need to reset the winding track.
8、Push-Pull
Characteristic:
■The switch (FET) drives different phases and performs pulse width modulation (PWM) to regulate the output voltage.
■Good utilization rate of transformer magnetic cores - transmitting power in both half cycles.
■Full wave topology structure, so the output ripple frequency is twice the frequency of the transformer.
■The voltage applied to the FET is twice the input voltage.
9、Half-Bridge
Characteristic:
■A topology structure commonly used in high-power converters.
■The switch (FET) drives different phases and performs pulse width modulation (PWM) to regulate the output voltage.
■Good utilization rate of transformer magnetic cores - transmitting power in both half cycles. ■Moreover, the utilization rate of the primary winding is better than that of the push-pull circuit.
■Full wave topology structure, so the output ripple frequency is twice the frequency of the transformer.
■The voltage applied to the FET is equal to the input voltage.
10、Full-Bridge
Characteristic:
■The most commonly used topology structure for high-power converters.
■Switches (FETs) are driven in diagonal pairs and pulse width modulation (PWM) is performed to regulate the output voltage.
■Good utilization rate of transformer magnetic cores - transmitting power in both half cycles.
■Full wave topology structure, so the output ripple frequency is twice the frequency of the transformer.
■The voltage applied to the FETs is equal to the input voltage.
■At a given power, the primary current is half that of the half bridge.
11、SEPIC Single-ended primary inductor converter (SEPIC)
Characteristic:
■The output voltage can be greater or less than the input voltage.
■Like a boost circuit, the input current is smooth, but the output current is discontinuous.
■Energy is transmitted from input to output through capacitors.
■Two inductors are required.
12、C'uk(Slobodan C'uk patent)
Characteristic:
■Output in reverse phase.
■The amplitude of the output voltage can be greater or less than the input.
■The input current and output current are both smooth.
■Energy is transmitted from input to output through capacitors.
■Two inductors are required.
■Inductance can couple to obtain zero ripple inductor current.
13、Details of Circuit Operation
■The following explains the working details of several topology structures:
■Voltage regulator: continuous conduction, critical conduction, discontinuous conduction.
■Boost regulator (continuous conduction).
■Transformer operation.
■ flyback transformer.
■ Forward transformer.
14、 Buck voltage regulator continuous conduction
Characteristic:
■The inductor current is continuous.
■Vout is the average of its input voltage (V1).
■The output voltage is the product of the input voltage and the load ratio of the switch (D).
■When connected, the inductor current flows out from the battery.
■When the switch is turned off, current flows through the diode.
■Neglecting losses in switches and inductors, D is independent of load current.
■The characteristics of the voltage regulator and its derivative circuits are:
■Input current discontinuous (chopping), output current continuous (smoothing).
15、 Buck voltage regulator critical conductivity
■ The inductor current is still continuous, but reaches zero when the switch is turned on again, which is called "critical conduction". The output voltage is still equal to the input voltage multiplied by D.
16、 Buck voltage regulator discontinuous conduction
■ In this case, the current in the inductor is zero for a period of time in each cycle.
■The output voltage remains (always) the average value of v1.
■The output voltage is not the product of the input voltage and the load ratio of the switch (D).
■When the load current is below the critical value, D changes with the load current (while Vout remains constant).
17、 Boost regulator
■ The output voltage is always greater than (or equal to) the input voltage.
■Continuous input current, discontinuous output current (opposite to voltage regulator).
■The relationship between output voltage and load ratio (D) is not as simple as in a voltage regulator. In the case of continuous conductivity:
In this example, Vin = 5,Vout = 15, and D = 2/3. Vout = 15，D = 2/3.
18、 Transformer operation (including the role of primary inductance)
■ A transformer is considered an ideal transformer, with its primary (magnetized) inductance connected in parallel with the primary.
19、 Flyback transformer
■ The primary inductance here is very low, used to determine peak current and stored energy. When the primary switch is turned off, energy is transferred to the secondary.
20、 Forward converter transformer
■ The primary inductance is high because there is no need to store energy.
The magnetizing current (i1) flows into the "magnetizing inductor", causing the magnetic core to demagnetize (reverse voltage) after the primary switch is turned off.
Summary
■ This article reviews the most common circuit topologies in current switch mode power conversion.
■ There are many other topological structures, but most of them are combinations or variations of the topology described here.
■ Each topology structure contains unique design trade-offs:
1) Voltage applied to the switch
2) Chopping and smoothing input and output currents
3) Utilization rate of winding
■ Choosing the optimal topology requires research on:
1) Input and output voltage range
2) Current range
3) The ratio of cost to performance, size to weight