MOBILE ZONE

Switch Mode Power Supply For Automotive Applications

Introduction:-

Switch mode power supply (SMPS) is a type of an electronic power converter which switches current at a fast rate to increase, decrease, regulate or condition the output voltage or current.

In other words like every other power supply a SMPS transfers the power from the input to the output like every other power supply but the advantages are discussed later on.

That being said the most important aspect of a switch mode power supply is the high switching frequency. All SMPS operate at a very high switching frequency typically from 25 Khz up to 1Mhz.

Types of SMPS

They are of two basic types:-

Isolated

Non-isolated

Non-isolated switch mode power supplies have no electrical isolation between input and output. These types of SMPS do not use a transformer at the output but instead uses an inductor and capacitor as an energy storing components and by using these and variable duty cycle the output voltage and current can be controlled or converted. These are used in low power applications, typically lower than 100 watts. Some types of non-isolated SMPS are:-

Buck converter

Boost converter

Buck boost converter

Charge pump/switched capacitor converter

Isolated

Isolated SMPS contains a transformer at the output so there is electrical isolation between input and output. This power supply has the advantage over the non-isolated types that it is much safer when used for mains powered power supply. These types of SMPS are used for higher power applications 100 watts and above. These power supplies can easily produce a wide range of voltages within the same power supply by simply using different secondaries .Types of non-isolated SMPS are :-

Push pull

Flyback

Half-bridge

Full-bridge

Forward converter

Advantages And Disadvantages of SMPS

Advantages

Much more efficient as compared to linear power supplies because it uses components as switches rather than resistive elements.

Smaller and lighter.

Cheaper at higher power levels (sometimes in lower power as well).

Regulation is easy and efficient to implement .

Disadvantages

The circuit is much more complex as compared to linear PSUs.

EMI/RFI which is inherent in SMPS is difficult to suppress.

Electronic noise at the output and input terminals.

Implementation

To make a switch mode DC to DC converter for automotive application the topology used is push pull converter. This topology is selected because it uses less amount of switching devices as compared to others.

Circuit Diagram

control board

power stage

SG3525

The control board uses sg3525 PWM controller IC. This IC generate the high frequency pulses required to drive the MOSFETs. It has the following pinout:-

The switching frequency is given by the formula shown. Capacitor Ct is connected between pin 5 and ground and Rt is connected between pin 6 and ground .Rd is known dead time resistor which is explained later.

Pin 14 and 11 are inverted outputs which would be connected to each bank of MOSFETs. The frequency at these pins are half the switching frequency set by the above formula. It can source and sink up to 400 ma peak.

A capacitor is connected at pin 8 is known as soft start capacitor which when used causes the output pulse width to increase slowly when the circuit is switched on. Without it there would a large inrush current during start up which may damage the MOSFETs.

Pin 1(inverting) and 2 (non-inverting) are the inputs to the onboard error amplifier. These pins are used to control the output pulse width. Often they are used with feedback to make regulated power supplies. When the voltage on the inverting input is greater than the voltage on the non-inverting input the duty cycle decrease. Similarly when the voltage in the inverting input is lower than the non-inverting input the duty cycle increase

Pin 16 is the output for the built in voltage reference module whose voltage is equal to 5.1v .This reference voltage is often used for one of the input to the error amplifier to set up feedback with regulation or for a constant duty cycle mode as used in our application.

Working Principle

There are two banks of MOSFETS which turn on alternatively but both the banks never turn on simultaneously.

As seen from this image the gate waveform show that at a time only one MOSFET will be on. Also the on time of one bank is a bit different from the off time of the other bank. This difference is known as dead time. It ensure that one bank of MOSFET do not turn on until the other bank is fully off. Without dead time there would be large ringing and spikes in the primary and secondary.

When one MOSFET turns on it causes the production of magnetic flux in one direction and when the other MOSFET turns on the production of magnetic flux is in the opposite direction. Doing so is very important because otherwise the transformer would saturate due to flux in one direction only causing large losses and rendering the transformer useless.

Transformer Design

For our particular application we need to wind the core ourselves because the transformer we need are not available with the required windings. For high frequency applications the iron core transformer cannot be used because the eddy current losses would be too high. We need a transformer core which have high permeability and low electrical conductivity .It should have initial permeability of 2500-3000 and maximum flux density of 4500-5000 .This is only possible by sing ferrite transformer.

The transformer used is ETD49 .It is a ferrite core transformer. To calculate the number of turns required at the primary we use the following formula:-