power suplies

Power supplies

EE328 Power Electronics

Assoc. Prof. Dr. Mutlu BOZTEPE

Ege University, Dept. of E&E

Outline of lecture

Introduction to power supplies

Modelling a power transformer

Analysis method of converters including a transformer

Steady-state analysis of

Flyback dc-dc converter

Forward dc-dc converter

Push-pull dc-dc converter

Full-bridge dc-dc converter

Half-bridge dc-dc converter

EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014 2

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Electrical isolation requirement

A basic disadvantage of the dc-dc converters (buck, boost etc.) is the electrical connection between the input and the output.

If the input supply is grounded, that same ground will be present on the output.

A way to isolate the output from the input electrically is with a transformer.

If the dc-dc converter has a first stage that rectifies an ac power source to dc, a transformer could be used on the ac side.

However, not all applications require ac to dc conversion as a first stage.

Moreover, a transformer operating at a low frequency (50 or 60 Hz) requires a large magnetic core and is therefore relatively large,

heavy, and expensive.

3 EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014

Electrical isolation requirement

A more efficient method of providing electrical isolation between input and output of a dc-dc converter is to use a transformer in the

switching scheme.

The switching frequency is much greater than the ac power-source frequency, enabling the transformer to be small.

Additionally, the transformer turns ratio provides increased design flexibility in the overall relationship between the input and the output

of the converter.

With the use of multiple transformer windings, switching converters can be designed to provide multiple output voltages.


Ideal transformer model

(a) Transformer;

(b) Ideal model


Real transformer model

(c) Complete model


Most used transformer model

The leakage inductances L1 and L2 are usually not crucial to the general

operation of the power electronics circuits described in this chapter, but they

are important when considering switching transients.

Magnetic core reset is important! The average voltage of Lm must be zero!

Otherwise the transformer saturates!


THE FLYBACK CONVERTER


Note the transformer

winding direction !!!

Assumptions for the analysis

1. The output capacitor is very large, resulting in a constant output

voltage Vo.

2. The circuit is operating in the steady state, implying that all voltages

an currents are periodic, beginning and ending at the same points

over one switching period.

3. The duty ratio of the switch is D, being closed for time DT and open

(1-D)T.

4. The switch and diode are ideal.


Analysis for the Switch ON

On the source side of the transformer


Analysis for the Switch OFF


Since the net change in inductor current must be zero over

one period for steady-state operation

Output voltage


Note that vsw, the voltage across

the open switch, is greater than

the source voltage.

Switch withstand voltage


Average magnetizing current


Substituting

Substituting

Min&max value of ILm


Continues current operation requires that ILm,min >0

At the boundary between CCM and DCM; ILm,min =0

The output configuration for the

flyback converter is the same as for

the buck-boost converter, so the

output ripple voltages for the two

converters are also the same.

Output voltage ripple


Buck-boost

EXAMPLE 7-1 Flyback Converter


EXAMPLE 7-2

Homework!!

Flyback converter design


FORWARD CONVERTER

Note that transformer

winding direction!!!


Analysis for the Switch ON


Analysis for the Switch OFF


Output voltage


Transformer reset


When switch is on

Transformer reset


When switch is off

Slope for

Transformer reset


When switch is on When switch is off

By combining these two equations, time duration of Tx can be found as

Transformer reset

For example, if the ratio

N3/N1=1 (a common practice),

then the duty ratio D must be

less than 0.5


For proper resetting of transformer it should be t0

Some waveforms


The circuit conguration on the output of the forward converter is the same as that for the buck converter, so the output voltage ripple based on an

ideal capacitance is also the same.

The equivalent series resistance of the capacitor often dominates the

output voltage ripple.

The peak-to-peak voltage variation due to the ESR is

EXAMPLE 7-4

AND 7-5 ARE

HOMEWORK



THE PUSH-PULL

CONVERTER


Sw1 is ON


Sw2 is ON


Same with the previous one

Both switches are OFF


Output voltage

the net change in inductor current over one period must be zero for steady state operation,

Solving for Vo


Ripple voltage on the output is derived in a manner similar to the buck converter.

The output ripple for the push-pull converter is

As with the other converters analyzed previously, the equivalent series resistance of the capacitor is usually responsible for most of

the voltage output ripple.

Recognizing that and using

EXAMPLE 7-6

ARE HOMEWORK



THE FULL BRIDGE

CONVERTER

No reset winding!!!


Note that the maximum voltage across an

open switch for the full-bridge converter is

Vs, rather than 2Vs as for the push-pull

and single-ended forward

converters.


THE HALF BRIDGE

CONVERTER


Multiple output flyback


Multiple output forward


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