About Converter (AC)

General purpose

In order to:

Understanding the basic concepts of switching power supplies with transistors
Understand the types and types of switching power supplies with transistors

Switched Mode Power Supply (SMPS)

Special purpose

In order to:

Observing the basic concept of Switched Mode Power Supply circuits
Explains the change in output voltage due to the influence of changes in the ratio of input pulses to switching time (D=t1/T).
Determines the switching time when on (ON) and when off (OFF).
Dimensioning the magnitude of the magnetic energy storage inductor.
Differentiating Switched Mode Power Supply from various simple Converters.
Determine the type or type of Transistor Converter that is appropriate and suitable for the needs of a Switched Mode Power Supply.
Explains the working principles of various basic converter circuits.
Explaining the switching current curve of various simple converter circuits.

Subject

Switched Mode Power Supply (SMPS)
Power Conversion
Buck Converter
BOOST CONVERTER
BUCK BOOST CONVERTER
INDUCTIVE BUCK BOOST
ISOLATED POWER STAGE CIRCUIT
FORWARD CONVERTER
Push-pull Converter Circuit
Half Bridge Converter Circuit
Basic Principles of Full-Bridge Converter
FLYBACK CONVERTER
SMPS EXAMPLE

1. Switched Mode Power Supply (SMPS)

Pwr Supply Linear vs Pwr Supply Switching

The block diagram shown in Figure 1a shows a small difference between a linear power supply and a Switched Mode Power Supply. In a Switched Mode Power Supply, the semiconductor components operate in a non-linear region. In contrast, in a linear power supply, the semiconductor components operate in a linear region, namely the variable region of conduction or inhibition.

A linear power supply consists of a circuit that processes the dc output from the dc input by conditioning the junction or conductance level of the collector emitter by regulating the dc voltage at the base of the transistor.

In Switched Mode Power Supply, the dc input voltage (UIN) is changed to a square voltage through the first chopper circuit (fig. 1b). which is then passed through an LPF (Low Pass Filter).

In Switched Mode Power Supply has a minimum efficiency of 71%, while for linear power supply only has a maximum efficiency of 50%. In linear power supply type, the efficiency level is determined by the change in input voltage and load and the amount of output voltage issued. But that dependence is not owned by Switched Mode Power Supply (SMPS).

Switched Mode Power Supply requires as few filter circuits as possible. Linear Power Pupply operates at 50 Hz mains frequency, while Switched Mode Power Supply operates at around 50 kHz frequency.

2. Power Conversion

Power conversion as in most power supplies, converters, inverters, dc-dc power supplies, regulators etc., requires complex analysis. Converters change input voltage or current into different output voltages or currents, dc-dc converters change dc input voltage levels into different dc output voltage levels while inverters change dc voltage magnitudes into ac voltage magnitudes. In reality in the field, large voltage settings such as in power electronics are applied dc-dc converters.

3. Buck Converter

Basic Principle of Buck Converter

In figure 2a, b, c the formation of a buck converter is shown through switching techniques, while figure d shows the implementation of a buck converter circuit that works as follows:

At the switch position at A (S1A on, S1B off), the voltage will induce coil L, see figure 2b. The current through the inductor increases and will supply current to the resistor load RL and the filter capacitor C. Then after the switch is in position B (S1B on, S1A off), the induced current will flow (see figure 2C), together with the capacitor current to the load RL. The period will repeat itself when the switch returns to position A.

Buck Converter is also called as Choppers, down -converter, step-down converter and down Choppers. The converter formed by Buck Converter is more popularly called Switched Mode Power Supply. If the circuit is developed with an isolation transformer it will be easier to design into forms such as forward, push-pull, half bridge and full-bridge converter.

Figure 2d illustrates the on and off times of switch S1A, followed by the shape of switch current, diode current and inductor current.

Converter Analysis

BUCK OUTPUT VOLTAGE

(UIN - UO) DT = UO (1-D)T
UIN DT - UO DT = UO T - UO DT
UIN DT = UO T
UIN D = UO
UO = D UIN

In SMPS systems usually work at a frequency of 100 kHz, meaning the period T = 1/fs = 10 uS. The ratio of ON time and one pulse period is called Duty Cycle D, which is formulated as:

D= t1/T
Duty cycle is very important in determining/dimensioning the magnitude of the output voltage UO = D UIN
The load resistance RL is the ratio of the output voltage Uo to the output current Io:

RL = Uo / Io

The load must be a Low Pass Filter and have a cut off frequency fc lower than the switching frequency fs. LC load as a Low Pass Filter has better properties to smooth the square waveform and avoid very complex harmonics.

If the cut off frequency fc is greater than approximately 10 times the switching frequency fs, then the LPF will dampen approximately 20 dB, meaning it will reduce the voltage of 1 Vp-p with 100 mVp-p ripple. The greater the ratio of fs to fc, the easier the analysis.

DC voltage can be calculated using a technique called Volt-second balance (fig.3b)

Uon x t1 = Uoff x t2

4. Boost Converter

Basic Principles of Boost Converter

Figures 4a to 4c show the topology of the boost converter and its implementation is shown in Figure 4d.

If the switch position is at A (S1A on, S1B off), the voltage will induce coil L and the induction current IL will increase while the load RL is not connected, so the capacitor current Ic will flow to the load RL.

During this period, energy will be stored in the inductor L. When the switch position is at B (S1B on, S1A off), the energy stored in the inductor will flow to the load RL and Capacitor C. This condition will repeat itself continuously.

BOOST OUTPUT VOLTAGE

UIN DT = (UO - UIN ) (1-D)T
UIN DT = UO T - UO DT - UIN T+ UIN DT
UIN T = UO (1-D)T
UO = UIN /(1-D)

Boost Converter is often called Up-Converter and is rarely used.

5. Buck-Boost Converter

Basic Principle of Buck Boost Converter

Buck-boost converter is a combination of buck and boost converter (figure 5a, b, c, d, e). The basic principle is shown in figure 5a, b, c, while the circuit implementation is in figure 5d.

UO = UIN /(1-D)

Boost Converter is often called Up-Converter and is rarely used.

6. Inductive Buck-Boost

INDUCTIVE BUCK BOOST

When the switch is in position A, the circuit works as a Buck-converter, while when the switch is in position B, the circuit works as a Boost-converter. In the inductive Buck-boost converter, the inductor d-charges by the input voltage and releases the charge to the output circuit. This happens continuously and regularly.

The output voltage can be set larger or smaller than the input voltage, but the polarity of the output voltage is always reversed.

The input current in the form of pulses is smoothed by the filter C (called C"uk) which is located between the converter and the input source voltage.

OUTPUT VOLTAGE

UIN DT = UO (1-D)T
UIN DT = UO (1- D)T
UO = (D/(1-D)) UIN

Figure 6a,b,c,d,e illustrates the working principle of the C"uk Concerter. Capacitor C1 as a charge storage. When the switch position is at A, (S1A on, S1B off), Uin induces L1, so that the induced current increases. C1 is connected to L2 and provides power to capacitor C2 and load RL, through L2.

When the switch is in position B (S1A off, S1B on), then L1 will release current and C1 will discharge through short circuit S1B (direction of forward diode). While one of the legs of L2 is connected to ground because S1B is connected forward bias. Between the input and output voltages there is no longer a pulse, but the result is an inverted output voltage. C"uk is rarely used because the C"uk capacitor requires a large RMS current.

7. Isolated Power-Stage Circuit

Between the input and output voltages, isolation is needed to separate the large input grid voltage with the DC voltage required by the circuit. In addition, to secure the circuit from the grounding system of the grid, so that the circuit ground is separated from the ground / grid voltage period (if not, it usually has fatal consequences when measuring voltage with CRO etc.). This isolation is formed by a power transformer combined with the basic SMPS circuit (Buck converter, Boost converter and Buck-boost converter).

Through Buck converter can be designed with forward, half bridge, full bridge and push-pull configurations. While some circuits based on Boost converter are often used in flyback circuits, while inductive buck-boost converters are often called converters with or without isolation.

8. Forward Converter

FORWARD CONVERTER

Forward converters are divided into two types, namely one-switch and two-switch. Figures 7a and 7b illustrate the One-switch forward converter circuit.

Work principle:

If the position of switches S1 and S2 is at A, the transformer will work to transfer power to the secondary coil and current will flow to L2 and loads C and RL.

When the switch position S1 and S2 are moved to position B, the current will flow through S2B to the coil, while S2A is in reverse bias position. Resistor R1 and Diode D1 function to eliminate the pulse jump due to the inductance effect of the transformer.

Switch Forward Converter

Figure 8 illustrates the basic principles and implementation of a two-switch forward converter circuit.

The two-switch circuit is more efficient by installing two diodes as reset trainsistors. This circuit is formed by two primary switches.

9. Push-pull Converter Circuit

Fig.9a. Push Pull Converter Circuit

Fig.9b. Push Pull Converter Circuit

In principle, the push-pull converter circuit is a combination of two one-switch forward converters with each primary switch S1 and S2 working alternately with a phase difference of 180 degrees.

When Transistor S1 is on, then diode S3A is active and conducts current through inductor L to load C and RL. In the next half period S1 is off, S2 is on, so Diode S3B is on and conducts current to load C and RL through inductor L.

10. Half Bridge Converter Circuit

Fig.10a. Half Bridge

Fig.10b. Half Bridge

Capacitors C1 and C2 function as input voltage dividers and reduce pulse voltage jumps due to inductance effects. D1 and D2 as commutators that secure the transistor's operation. Transistors S1 and S2 work alternately with a phase difference of 180 degrees. And each transistor is controlled by a voltage bias of 1/2 the input voltage.

11. Basic Principles of Full-Bridge Converter

Fig.11a Full-Bridge Converter

Gb.11b Full-Bridge Converter

In the full-bridge converter circuit, the voltage divider capacitor is replaced by two switches S4 and S5. When Transistor S4 is on together with transistor S2, then transistors S1 and S5 are off and vice versa. The phase difference between the two conditions is 180 degrees.

12. Flyback Converter

The flyback converter circuit is formed from an inductive-buck converter plus an isolator transformer.

Flyback converter circuit

When S1 on S2 off, the primary coil Np stores energy in the transformer core. Because the position of S2 is open (diode S2 gets reverse voltage from the secondary transformer whose polarity has a phase difference of 180o to the primary transformer), then capacitor C supplies current to the load RL. When the input transistor is low, then transistor S1 is open, while the secondary transformer provides forward voltage to the diode S2. And the current is passed to the load C and RL.

Flyback converter is very popular for use at low power (<200W). with multiple outputs that can be developed more by adding the number of secondary coils of the transformer.

Characteristics Table of some SMPS

13. Example of SMPS

Converter circuit with direct duty cycle control

The primary switch is used Mos-FET because it has a fast response to changes in on-off conditions (switching) and is very suitable for switching systems. The current source is a source of the formation of a triangular waveform RAMP which is then compared with the dc voltage level Uea in the comparator circuit. Then a pulse width pulse or popularly called PWM (Pulse Width Modulation) will be obtained which will control the Gate of the Mos-FET.

The output voltage from the converter is fed back through the feedback input resistor Za which is compared with the reference voltage Uref which will then produce a voltage level Uea.

Thus, correction and stability of the converter output voltage will be achieved.

Forward Converter Circuit with Current-Mode Control

In both circuits above, the primary and secondary coils of the transformer are isolated. While the output control to the input is carried out by the feedback resistor Za

Continued from this material >> STEP UP DC-DC CONVERTER

Reference

P.6-27/28 The ARRL Handbook for Radio Amateurs.
FUNKSCHAU Magazine 1987.
Elector Magazine.
Udo Leonhard Thiel: Pofesionalle Schaltnetzteil Applikationen-Franziz.
Udo Leonhard Thiel: Schaltnetzteil-erfolgreich planen und dimentionieren-Franziz.
Otmar Kilgenstein, Prof.: Schaltnetzteil in der Praxis-Vogel.
John. D. Lenk: Simplified Design of Switching Power Supplies.

Netizens

Q1: Sir, do you have a full bridge SMPS schematic?

A1: No, bro. The manufacturer usually provides it. Please determine the brand and product series, bro, then browse the official site. Usually the schematic is provided.