SWITCHING REGULATORS

INTRODUCTION

A switching regulator switches a series device on or off. The switch's duty cycle sets the amount of charge transferred to the load. Switching regulators have the advantages of higher efficiency, dissipates lesser heat and are small in size, as compared to linear regulators. While linear regulators utilize a variable resistor for controlling the output parameters, switching regulators make use of a switch (BJT, MOSFET). Also, unlike linear regulators, switching regulators are able to generate output voltages higher than the input voltages or voltages of opposite polarity.

There are various types of switching regulators - Buck regulator, Boost regulator, Buck-boost regulator, Flyback regulator, Pushpull converter, Half-bridge converter, Full bridge converter. They are used depending upon the requirement of the output voltage.

There are various factors which affect the performance of a switching regulator. They are - the equivalent series resistance (ESR) and equivalent series inductance (ESL), the input and output capacitors, bypass capacitors, transformer/inductor core and radiated noises, etc. Proper consideration of these factors will help increase the efficiency of a switching regulator.

OPERATION OF A SWITCHING REGULATOR

The switch goes on and off at a fixed rate, usually between 50 kHz and 100 kHz, as set by the circuit. The time that which the switch remains closed during each cycle is varied to maintain a constant output voltage. The diode D1 must be Schottky or any fast switching diode. The capacitor C1 must be of low equivalent resistance type.

Case - 1. When the switch is closed

When the switch is closed, current flows through the inductor L1. L1 tends to oppose the increasing current, and develops a polarity. Initially, the left end of L1 is positive and the right end is negative. Hence, the diode D1 is reverse biased and acts as open switch. Due to the current flowing in the inductor, an electromagnetic field is developed.

Case - 2. When the switch is open

When the switch is open, the electromagnetic field discharges and generates current in the reverse polarity, that is, at this condition, the left end of L1 is negative and the right end is positive. As a result, the diode D1 is conducting and will continue to do so till the field of L1 is diminished.

The minimum load requirement (RL) is determined by the inductor value. Without minimum load, the regulator will generate excessive noise and distortion.

The following types of switching regulators operate on the principle discussed above.

TYPES OF SWITCHING REGULATORS

1. BUCK REGULATOR -

i) It is used to convert a DC voltage to a lower value of the same polarity.

ii)  It uses a transistor as a switch that alternately connects and disconnects the input voltage to an inductor.

iii) The conversion is associated with very little power loss.

During the ON time, the inductor current flows into both the load and the output capacitor, the capacitor charges during this time. The capacitor discharges to the load during the OFF time, contributing to the total current being supplied to the load. The waveform of the current at the inductor is shown below. The dc load current from the regulated output is the average value of the inductor current.

2. BOOST REGULATOR

i) It takes a dc input voltage and produces a dc output voltage of greater magnitude, but of the same polarity.

When the switch is OFF, the forward bias of the diode allows the capacitor to charge upto a voltage that is higher than the input voltage. At this instant, the inductor current flows into the capacitor and the load. During the ON time, the load current is supplied only by the capacitor.

An important design consideration in the boost regulator is that the output load current and switch current are not equal, and the maximum available load current is always less than the current rating of the switch transistor. The maximum total power in any regulator is equal to Vin multiplied by the maximum average input current. Also, since the output voltage in a boost regulator is greater than the input voltage, the output current is less than the input current.

3. BUCK BOOST (INVERTING) REGULATOR

i) It takes dc input and produces dc output voltage that is opposite in polarity to the input.

ii) The negative output voltage can be either larger or smaller in magnitude than the input voltage.

During the ON time, the source of the load current is the discharging of the capacitor. The charge lost from the capacitor in this process is replenished in the OFF time, as the discharging of the inductor causes current supply at both the output capacitor and the load.

4. FLYBACK REGULATOR

i) This type of regulator is used where one or multiple output voltages are required from a single input voltages.

ii) Some of the output voltages may be opposite in polarity to the input voltage.

iii) The output with the highest current is selected to provide PWM feedback to the current loop.

Fig : Single output flyback regulator

When the switch is ON, current flows through the primary winding of the transformer. The dot-negative voltage appearing on the secondary winding turns off the diode. Hence the source of the load current is the discharging of the capacitor. When the switch is OFF, the decreasing current in the primary causes the dot end to become positive and hence the dot end of the secondary also becomes positive. This forward biases the diode. The charge lost from the capacitor is replenished. Also, current flows through the load.

Fig: Flyback regulator for multiple outputs

5. PUSHPULL CONVERTER

i) It uses two transistors to perform dc to dc conversion

ii) It is best suited for lower input voltage application, generally in the range of 12V - 24V.

iii) Use of MOSFET switches in a pushpull converter increases the overall efficiency, due to relatively voltage drop across the switches.

iv) It can generate multiple output voltages, some of which are opposite in polarity.

v) One disadvantage is that they require very precise matching of the switches to prevent unequal ON times, the failure of which may damage the device.

The converter operates by turning ON each transistor at alternate cycles, that is, no two transistors are ON at the same time. When the transistor A is turned ON, the input voltage is forced across the upper primary winding with dot positive polarity. On the secondary side, a dot negative voltage will appear across the upper winding which turns on the bottom diode (the lower part of the sec winding has positive polarity). This allows current to flow inductor to supply both the output capacitor and load. When transistor B is ON, the input voltage is forced across the lower primary winding with positive polarity. The same voltage polarity appears on the upper secondary winding, which forward biases the top diode, causing current to flow in the output capacitor and load.

6. HALF BRIDGE CONVERTER

i) It is a two transistor converter frequently used in high power designs requiring load power in the range of 500W - 1500W and is almost operated directly from the load line.

ii) The transformers used operate at switching frequency typically 50 kHz or higher, which means that they can be very small and efficient.

iii) The regulated dc output is electrically isolated from the ac line.

When transistor A is ON, a dot positive voltage is forced across the primary winding and causes the lower end of the secondary to have a positive polarity, which in turn forward biases the lower diode, supplying current to the output capacitor and load. When transistor B is ON, the polarity of the primary voltage is reversed. The secondary voltage polarity is also reversed, turning ON the upper diode.

7. FULL BRIDGE CONVERTER

i) The full bridge converter uses 4 transistors to perform dc to dc conversion.

ii) It is powered directly from the ac line, providing load power of 1kW to 3kW.

iii) It provides input-output isolation.

The transformer primary is driven by the full voltage Vin when either of the transistor sets ( A set or B set) turns on. The full input voltage utilization means that the full bridge can produce the most load power of all the converter types. Primary and secondary current flows in the transistor during the switch ON times, while the output capacitor discharges into the load when both transistors are OFF.

FACTORS TO BE TAKEN INTO CONSIDERATION -

1. ESR - It causes internal heating due to power dissipation.

2. ESL - It limits the high-frequency effectiveness of the capacitor.

3. Input capacitors - Adequate capacitive bypass should be provided as near as possible to the switching converter input for best results.

4. Output capacitors- The function of output capacitors is filtering. Current should flow in and out of the filter capacitor.

5. Grounding - Single point grounding is preferred. A good high-frequency capacitor (like tantalum) is provided near the input voltage source to provide good grounding.

6. Transformer/inductor core - It should be of the toroidal type, which produces the lowest radiated flux noise.

REFERENCES -

1. www.google.com

2. Electronic Devices and Circuits by G.K. Mithal

3. Fundamentals of Switching Regulators by Anonymous