The following topics were discussed in brief in Part1 of this blog,

  • Basic Definition of Power electronics.
  • Different Applications of Power electronics.
  • Various power electronic converters and their applications.
  • Power Semiconductor Switching devices, their classification, and applications in power electronic converters.
  • Basics of Linear Voltage Power Supply and Switched Mode Power Supply (SMPS).
  • Block-level working of Fly-back converter.

 

Key takeaways from the following blog:

  • Comparison between Linear and SMPS.
  • USB power profiles.
  • SMPS working.
    1. Rectification stage: Considering with and without source inductance.
    2. DC converter stage:
      • Their role in SMPS.
      • Isolated and Nonisolated topologies comparison.
      • Steps to convert a non-isolated to its corresponding isolated topology.
      • The main application of a switch in a DC-DC Converters.
      • The basic working of a DC-DC converter using Inductor volt-sec and capacitor charge balance and small ripple approximation.
      • Working of a closed-loop PWM.
      • Buck-boost converter working.
      • Reasons for the occurrence of discontinuity and their application.
      • Working of Flyback converter
      • Different modes of flyback converter their working and its advantages and disadvantages

 

The main motive behind this blog is to eventually build the understanding of Power Electronic Converters in USB applications. It comprises of different topics and concepts under one blog which are discussed in a proper sequential order to make everyone understand the application of PE who are new to this field.

 

Characteristic comparison of Linear and Switched Mode Power Supply (SMPS):

 

 

Linear Voltage Power Supply

SMPS

Definition

It completes the stepping down of AC voltage first then it converts it into DC.

It converts the input signal into DC first then it steps down the voltage up to the desired level.

Efficiency

Low efficiency i.e. about 20-25%

High Efficiency i.e. about 60-65%

Voltage regulation

Voltage regulation is done by the voltage regulator.

Voltage regulation is done by the feedback circuit.

Magnetic material used

Stalloy or CRGO core is used

A ferrite core is used

Weight

It is bulky.

It is less bulky in comparison to the linear power supply.

Reliability

More reliable in comparison to SMPS.

its reliability depends on the transistors used for switching

Complexity

Less complex than SMPS.

More complex than the Linear power supply.

Transient response

It possesses a faster response.

It possesses a slower response.

RF interference

No RF interference

RF shielding is required as switching produces more RF interference.

Noise and Electromagnetic interference

It is immune to noise and electromagnetic interference.

The effect of noise and electromagnetic interference is quite significant; thus, EMI filters are required.

Applications

Used in Audio frequency applications and RF applications.

Used in chargers of mobile phones, DC motors, etc.

 

Inference: From the above comparison, we can conclude that SMPS is the best fit for USB power supplies based on the following characteristic: Efficiency, Voltage regulation, Magnetic material, weight, and copper requirement. 

           

USB standards and their power profiles:

 

Specifications

Maximum Voltage (Volt)

Maximum current

Maximum Power (Watt)

USB 2.0

5

500 mA

2.5

USB 3.0 and USB 3.1

5

900 mA

4.5

USB Battery Charging 1.2

5

1.5 A

7.5

USB Type C

5

3 A

15

USB PD 3.0

20

5 A

Up to 100

 

Standard Voltage profiles associated with USB Power delivery: 5V, 9V, 12V, 15V, 19V and 20V

Standard Power profiles associated with USB Power adapters: 18W, 33W, 45W, 65W, and up to 100W (Power factor correction circuitry is required for power levels above 65W)

 

SMPS (Switched Mode Power Supply)

 

Block diagram:

            The block diagram of SMPS is attached in image6.

 

Explanation:

Rectification stage

Introduction:

  • It is mainly used to convert the AC mains voltage to ripple-free DC voltage by means of diodes. The output is an uncontrolled DC signal.
  • Most of the power electronics applications such as switching dc power supplies, ac motor drives, dc servo drives, etc. use such uncontrolled rectifiers.
  • The dc output voltage of a rectifier should be as ripple free as possible. Therefore, a large capacitor is connected which acts as a filter on the dc side
  • The rectifiers are supplied directly from the utility source without a 50/60Hz transformer. The avoidance of this costly and bulky 50/60Hz transformer is important in most modem power electronic systems for a reduction in size and copper usage.
  • Some of the components connected inline between the AC input and bridge rectifier include fuse, EMI filters, x-cap (for interference suppression), and common mode choke (to decrease common-mode noise which usually arises because of differences in ground references).
  • Since most of the practical loads are nonlinear or inductive in nature, further explanation is carried out considering the same.

Working:

  • The circuit diagram of full-bridge diode rectifier can be seen in the attached image 1.
  • The supply voltage is Vs. The source voltage Vs is in series with its internal impedance, which is primarily inductive. To improve the line current, an inductor may be added in series on the Ac side. The overall inductance is named as Ls.
  • We shall look at the working considering both Ls=0 and Ls=finite value
  • When Ls=0,
      • For simple understanding, the circuit is redrawn in image 2.
      • The circuit consists of two groups of diodes, the top group with diodes I and 3 and the bottom group with diodes 2 and 4.
      • The current id (diode current which is basically an output current) flows continuously through one diode of the top group and one diode of the bottom group.
      • In the top group, the cathodes of the two diodes are at a common potential. Therefore, the diode with its anode at the highest potential will conduct id. That is, when Vs is positive, diode I will conduct id, and Vs will appear as a reverse-bias voltage across diode 3. When Vs goes negative, the current id shifts (commutates) instantaneously to diode 3 since Ls = O. A reverse-bias voltage appears across diode I.
      • In the bottom group, the anodes of the two diodes are at a common potential. Therefore, the diode with its cathode at the lowest potential will conduct id. That is, when Vs is positive, diode 2 will carry id and Vs will appear as a reverse-bias voltage across diode 4. When Vs goes negative, the current id instantaneously commutates to diode 4 and a reverse-bias voltage appears across diode 2.
      • The input and output voltage waveforms can be found in the image3.
      • In this case output voltage Vdo = 0.9*Vs.
  • When Ls≠0,
      • Due to a finite Ls', the transition of the ac-side current is from a value of +Id to -Id (or vice versa) will not be instantaneous. The finite time interval required for such a transition is called the current commutation time or the commutation interval, and this process where the current conduction shifts from one diode (or a set of diodes) to the other is called the current commutation process.
      • The waveforms are like that of the previous case except that there is a slight delay in initial conduction.
      • The output voltage is like the previous case except that an area Au (lost due to current commutation) is lost in every half-cycle from the integral voltage. Therefore Vd = Vdo - (2*w*Ls*Id)/pi

 

Controlled DC Stage

Introduction:

  • The dc-dc converters are widely used in regulated switch-mode dc power supplies. Often the input to these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage, and therefore it will fluctuate due to changes in the line-voltage magnitude.
  • Switch-mode dc-to-dc converters are used to convert the unregulated dc input into a controlled dc output at the desired voltage level.
  • These DC-DC converters are very often used with an electrical isolation transformer in the switch-mode dc power supplies (SMPS) and almost always without an isolation transformer in the case of dc motor drives.
  • Some of the non-isolated converters include Step-down (buck) converter, Step-up (boost) converter, Step-down/step-up (buck-boost) converter, Cuk converter, and Full-bridge converter. (image7)
  • Some of the isolated converters include Flyback converter, Forward converter, Push-pull converter, Half-bridge a Full bridge converter.
  • The maximum power ratings, their typical efficiency, and their applications are discussed in previous blog(part1).
  • The DC-DC converter which is best suited for the USB power application includes an isolated Fly-back converter and a non-isolated buck-boost converter.
  • Nonisolated and their corresponding isolated topology is as given below,

 

Non-isolated topology

Corresponding isolated topology

Buck converter

Forward converter

Buck-Boost converter

Flyback Converter

 

  • Form the above-mentioned block diagram, an isolated DC-DC converter stage includes inverter chopper, output transformer, output rectifier, and filter.
  • The basic application of a switch in a power converter is to control the duration of current flow in the circuit. By achieving the average output voltage and current can be controlled based on the desired value.
  • Consider the circuit as shown in the image4. The average value Vo (output voltage) depends on Ton and Toff as shown in the corresponding image.

Basic Converter working:(explained using buck converter example)

  • In dc-dc converters, the average dc output voltage must be controlled to equal the desired level, though the input voltage and the output load may fluctuate. Switch-mode dc-de converters utilize one or more switches to transform dc from one level to another. In a dc-dc converter with a given input voltage, the average output voltage is controlled by controlling the switch on and off durations (Ton and Toff).
  • Consider the circuit shown in the image9. This is a basic buck converter without the filter circuit.
  • When the switch is toggled between positions 1 and 2 with proper duty cycle, the desired output voltage can be obtained (in this case a stepped-down voltage).
  • When this type of circuit is used without a filter, the output will be comprised of the DC components and AC components with a lot of harmonics.
  • The output voltage waveform of this type of circuit comprising of both AC and DC components is attached in the image10.
  • A filter must be designed with cutoff frequency fo<<<fs, where the average voltage of the waveform i.e. D*Vin will be obtained (which is a straight line with ripples. The ripple content can be minimized by properly designing the filter). (D = ton/Ts)
  • Hence the circuit in image9 along with the inductor and a capacitor connected in series and parallel respectively on the output side acts a buck converter.
  • Since it is a low pass filter, the components less than fo will be passed and reject the components in the Fourier series that are greater than fo. Hence it will reject the components with frequency fs and pass the dc component D*Vin. (which is nothing but the output voltage of a buck converter).
  • One of the methods for controlling the output voltage employs switching at a constant frequency (hence, a constant switching time period Ts = Ton + Toff) and adjusting the turn-on the duration of the switch to control the average output voltage. In this method, called pulse-width modulation (PWM) switching, the switch duty ratio D, which is defined as the ratio of the on duration to the switching time period, is varied. The other control method is more general, where both the switching frequency (and hence the time period) and the on duration of the switch are varied. This method is used only in dc-dc converters utilizing force-commutated thyristors.
  • The basic circuit for implementing PWM controlled converter is shown in the image5.
  • In the PWM switching at a constant switching frequency, the switch control signal, which controls the state of the switch, is generated by comparing a signal-level control voltage Vcontrol with a repetitive waveform as shown in the image5. The control voltage signal generally is obtained by amplifying the error, or the difference between the actual output voltage and its desired value. The frequency of the repetitive waveform with a constant peak, which is shown to be a sawtooth, establishes the switching frequency. This frequency is kept constant in a PWM control and is chosen to be in a few kilohertz to a few hundred-kilohertz range. When the amplified error signal, which varies very slowly with time relative to the switching frequency, is greater than the sawtooth waveform, the switch control signal becomes high, causing the switch to turn on. Otherwise, the switch is off. In terms of Vcontrol and the peak of the sawtooth waveform Vsawtooth, the switch duty ratio can be expressed as

D = ton/Ts =Vcontrol/Vsawtooth

  • The dc-dc converters can have two distinct modes of operation, continuous current conduction, and discontinuous current conduction. In practice, a converter may operate in both modes, which have significantly different characteristics. Therefore, a converter and its control should be designed based on both modes of operation.

Nonisolated Buck-Boost Converter:

  • The main application of a buck-boost converter is in SMPS or regulated output power supplies, where a negative-polarity output may be desired with respect to the common terminal of the input voltage, and the output voltage can be either higher or lower than the input voltage.
  • The buck-boost converter can be obtained by cascading a Buck converter (step down converter, where ideally output is equal to or less than the input) followed by Boost converter (step-up converter, where ideally output is equal to or greater than the input).
  • The circuit diagram of a Buck-boost converter is attached in image8.
  • When the switch is closed, the input provides energy to the inductor and the diode is reverse biased. When the switch is open, the energy stored in the inductor is transferred to the output. No energy is supplied by the input during this interval. In the steady-state analysis presented here, the output capacitor is assumed to be very large, which results in a constant output voltage Vo(t) ~ Vo
  • In steady-state, the ratio of output to input voltages is the product of the output to input voltage ratios of the two converters in cascade (if switches in both converters have the same duty ratio).

Voutput/Vinput = D/1-D

where D = duty ratio (explained in the previous section)

  • From the above-derived output voltage ratio, we can conclude that the output can either be greater than(boost) or less than(buck) the input voltage.
  • There are two modes in which a buck-boost converter operates. Continuous conduction mode (CCM) and discontinuous mode of operation (DCM).
  • Discontinuity occurs due to the following reasons,
  • When the ripples are greater than the inductor current or capacitor voltage. As a result, the direction of current flow changes and discontinuity occurs.
  • It occurs in DC-DC converters where unidirectional switches (diode, BJT, IGBT) are used.
  • It also occurs when the converter is operating in light load conditions.
  • The operating mode is decided based on load conditions and power requirements. This can be achieved by controlling the duty ratio and value of the passive electronic components used in the circuit(inductor).
  • In DCM, the average output voltage decreases. Hence this mode is preferred when Low powered loads are used, or low energy is required. When the diode is used as a rectifier (as shown in the circuit diagram), the converter goes to DCM. When a synchronous rectifier is used, the operation depends on the driving signal.
  • In CCM, the average output voltage increases. This mode is preferred when the load requires high power.
  • When the resistance of the load > resistance of the circuit, the circuit operates in CCM and vice versa.

 

Flyback converter (Isolated Buck-Boost Converter)

  • The overall block and the role of isolated DC-DC converter are attached in image13.
  • The basic working of an isolated converter is as follows,
      • The dc-dc converter block as shown in the image13 converts the input dc voltage from one level to another dc level. This is accomplished by high-frequency switching, which produces high-frequency ac across the isolation transformer.
      • The secondary output of the transformer is rectified and filtered to produce output Vo(shown in image13) The output of the dc supply is regulated by means of feedback control that employs a PWM controller as discussed in the previous section, where the control voltage is compared with a sawtooth waveform at the switching frequency. The electrical isolation in the feedback loop is provided either through an isolation transformer as shown or through an optocoupler.
  • Classification of isolated DC-DC Converter based on their transformer core utilization,
  • Unidirectional core excitation where only the positive part (quadrant 1) of the B-H loop is used (Fly Back and Forward Converter).
  • Bidirectional core excitation where both the positive (quadrant 1) and the negative (quadrant 3) parts of the B-H loop are utilized alternatively (Pushpull, Half bridge and forward converter).
  • Nonisolated converter is converted to an Isolated converter by including Transformer where the average value of inductor voltage becomes zero. Hence in Buck-Boost converter, transformer is placed after the inductor where according to the waveform at that point, the average value of inductor voltage is zero.
  • Working:
      • The basic circuit diagram of open-loop flyback converter is attached in image14.
      • During the switch T1 is in ON position, the current builds up in the transformer primary (and thus storing energy). When the switch is turned OFF, the polarity of primary and secondary coil voltages is reversed. Now the diode D1 is in forward biased and the energy stored in the transformer is transferred to the Capacitor (C1), sequentially to the load.
      • When the switch T1 is in ON position, the voltage across diode D1, which is reverse biased is given as,

Vd = Vo + (Vs * Ns/Np)

When T1 is ON, Diode D1 is reverse biased. Hence voltage across it is the sum of output voltage plus the reflected input voltage.

      • When the switch T1 is in OFF position, the voltage across T1is given as,

Vt1 = Vs + (Vo * Np/Ns)

When T1 is OFF, switch T1 is reverse biased (open). Hence voltage across it is the sum of input voltage plus the reflected output voltage.

      • The waveforms are attached in image15.
      • It has a low parts count.
      • Careful design of the transformer turns ratio between primary and secondary that enable the output to be higher or lower to the input.
      • A Flyback converter can support multiple outputs by adding more windings to the transformer.
      • The Flyback converter uses the single magnetic of a common reference of a transformer behave as the coupled inductor, this transformer combines the functions of energy stored, energy transferred, and isolation. So, the need for a separate LC filter on each output is laminated. This reduces the overall cost of the flyback converter.
      • It is important to note that the diode and switch should not conduct at the same time.
  • Different modes of operation of the Flyback converter:
      • Frequency modulation (FM) and Amplitude Modulation (AM).
      • Discontinuous Conduction Mode (DCM).
      • Valley Switching.
      • Quasi-Resonant
      • Continuous Conduction Mode.
      • Boundary conduction Mode.

 

DCM:

  • When the power stage is designed in such a way to allow the transformer to completely demagnetize during each switching cycle. The simplest form of a DCM flyback is designed with a fixed switching frequency and modulates the peak current to support the load demands.
  • At the start of the switching period, the on-time begins, and the primary side current ramps up from zero.
  • At the end of the on-time, the primary current of collapses back to zero, and the current flows to the secondary windings. It begins at its peak proportional to the turn’s ratio and ramps down to zero, completely demagnetizing the transformer during every switching cycle.
  • After the demagnetizing time, there is a delay before the primary side switch turns on again to start the next switching cycle. This delay is referred to as dead time. During this portion of the switching period, neither the diode nor the MOSFET is conducting. This dead time, where the transformer is completely demagnetized and no current is being conducted, is why this operating mode is known as discontinuous or DCM.
  • During this dead time, a resonant ring is generated by the interaction between the primary inductance of the transformer and the parasitic capacitance at the switch node. A converter in deep discontinuous mode can have a dead time long enough for the resonant ringing to dampen completely, at which point the drain to source voltage will have settled to be equal to the input voltage.
  • A flyback is operating in DCM when the power stage is designed in such a way as to allow the transformer to completely demagnetized during each switching cycle.
  • At the start of the switch period, the in-time begins, and the primary side current ramps up from zero. At the end of the no-time, the primary current collapses back to zero, and the current flows to the secondary windings. It begins at its peak proportional to the turn’s ratio and ramps down to zero, completely demagnetizing the transformer during every switching cycle.
  • After the demagnetizing time, there is a delay before the primary side switch turns on again to start the next switching cycle. This delay is referred to as dead-time or resonant time. During this portion of the switching period, neither the diode nor the MOSFET is conducting.
  • The waveform of the Flyback converter operating in DCM is attached in image16.
  • Advantages of DCM: The advantages of DCM flyback are that there are no reverse recovery losses in the output rectifier because it’s able to ramp down to zero amps during every switching cycle.  The primary inductance is the lowest out of all the flyback, which may result in a smaller transformer. A DCM flyback is inherently more stable because it doesn’t have a right-half-plane to zero in its transfer function.
  • Disadvantages DCM: DCM flyback does have the disadvantage of very large ripple currents, which may require large EMI filters. Fixed frequency DCM flyback has higher losses because they can turn off the switch when the drain to source voltage may be relatively high. It can be ringing higher than the input voltage now of turn-off, this could contribute a considerable hit to efficiency, as the switching losses are proportional to the square of this voltage.

Valley Switching:

  • Valley switching is a specialized form of discontinuous conduction mode and requires a controller that is specifically designed to detect when the resonant ring during the dead time is at a low point.
  • Before turning the MOSFET on to start the next switching cycle, minimizing switching losses.
  • In order to maintain the required average output power, the controller will modulate the switching frequency by skipping one or more valleys from one cycle to the next.
  • Controllers that modulate the frequency to meet the average load demand every cycle are operating in FM (frequency modulation) mode. This is sometimes also called frequency fold back because as the load demand is decreased the switching frequency is also decreased or folded back.
  • Valley switching can occur at any resonant valley during the dead time as long it’s large enough for the control to detect. The switching waveforms may appear to dither as the controller adjusts its dead time in its search for the nearest valley.
  • The waveform of the Flyback converter operating in Valley switching mode is attached in image17.
  • Advantages of Valley Switching: Valley switch flyback has all the advantages of traditional DCM flyback with the bonus of lower switching losses due to consistently turning the MOSFET off when the drain to source voltage is at a low value. This also helps to reduce the turn-on current spike at the current sense resistor. The dithering produced by valley skipping helps to reduce EMI.
  • Disadvantages of Valley Switching, Unfortunately, valley skipping will result in higher output voltage ripple. Also, valley switching is ineffective if the converter is operating in the deep discontinuous mode as the controller will not have enough of a signal to detect a valley

Quasi-Resonant Mode:

  • Quasi-resonant mode or QR may be referred to as critical condition mode or transition mode.
  • Quasi-resonant operation is a specific valley switching the operating mode of DCM where the switching occurs on the very first and deepest resonant valley.
  • QR delivers the maximum amount of power by adjusting both the peak current and the switching frequency to turn the MOSFET on at the first resonant valley for minimal losses.
  • QR controllers operate in AM and FM mode at the same time to meet the demands of energy transfer. QR controllers will decrease the switching frequency as the load increases. This is just the opposite of the frequency fold back mentioned earlier.
  • Most Valley switching controllers can operate in Quasi-resonant mode but only at the specific operating point of maximum load and minimum input voltage when designed accordingly.
  • This limited QR range of operation is due to the control method used in the valley switching controllers, where only the frequency is modulated. Not both the frequency and the peak current like in dedicated QR controllers.
  • The waveform of the Flyback converter operating in QR mode is attached in image18.
  • Advantages of QR mode: QR mode converters switch at the lowest drain to source voltage, they achieve the lowest possible switching losses and have high efficiency over the entire operating range. This is a soft-switching converter only small EMI filters are needed.
  • Disadvantages of QR mode: QR converters are difficult to compensate due to the wide peak current and switching frequency ranges. A considerable phase margin is required to maintain stability over the entire operating range.

CCM:

  • CCM refers to continuous conduction mode. A continuous current is always flowing in the transformer during each switching cycle.
  • When the MOSFET is turned on the primary current ramps up. But it doesn’t start from zero amps as in DCM. In CCM the current ramps from an offset that is due to residual energy that is continuously maintained in the transformer.
  • When the switches turned off energy is transferred across the secondary and the transformer demagnetizes resulting in the secondary side current ramping down. But it does not ramp all the way to zero amps.
  • Residual energy is maintained in the transformer. The next switching cycle begins before the current is completely depleted.
  • As shown, the current waveform on both the primary and the secondary is trapezoidal in shape. This is sometimes referred to as a ramp on a step.
  • Note that there is no dead-time in CCM. Current is always being conducted somewhere in the transformer but also note that despite continuously conducting current the MOSFET and the diode do not conduct at the same time.
  • As the load demand decreases the store of residual energy the step portion of the waveform decreases.
  • CCM flyback transformers are designed based upon the ripple current or ramp portion of the waveform which is considerably less than the ripple seen in DCM flyback. Controllers specifically designed for Valley switching will not operate in CCM as there is no resonant ring available and the transformer is not allowed to fully demagnetize.
  • The waveform of the Flyback converter operating in CCM mode is attached in image19.
  • Advantages of CCM: Lower peak currents mean smaller filter components. The advantages of CCM are the small ripple and RMS current which result in lower capacitor losses. These lower current also help lower conduction and turn-off losses when compared to DCM flyback. Lower peak current means smaller filter components.
  • Disadvantages of CCM: The most noted disadvantage of CCM flyback is the presence of a right-half-plane zero in the power stage transfer function, this limits the bandwidth of the control loop and will impact the converter’s dynamic response. Also, CCM flyback require a larger inductance which may require a large magnetic component.

 

Topics i would be covering in my next blog

  • Different closed loop topologies in flyback converter.
  • 4 switch Buck-Boost Converter.

 

People who are interested to learn more than discussed here and if there are any mistakes or errors in the written blog, please let me know in the comments section.

 

Regards

Abhilash P