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Introduction to DC-DC Conversion – Cont.

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Presentation on theme: "Introduction to DC-DC Conversion – Cont."— Presentation transcript:

1 Introduction to DC-DC Conversion – Cont.
EE174 – SJSU Tan Nguyen

The switching-mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, inductors, and transformers -- and the use of switches that are in one of two states, on or off. It offers high power conversion efficiency and design flexibility. It can step down or step up output voltage. The term switchmode was widely used for this type of power supply until Motorola, Inc., who used the trademark SWITCHMODE TM for products aimed at the switching-mode power supply market, started to enforce their trademark. Switching-mode power supply or switching power supply are used to avoid infringing on the trademark.

Buck converter: Voltage to voltage converter, step down. Boost Converter: Voltage to voltage converter, step up. Buck-Boost or FlyBack Converter: Voltage-Voltage, step up and down (negative voltages) Cuk Converter: Current-Current converter, step up and down These converters typically have a full wave rectifier front-end to produce a high DC voltages

4 Simple switching-mode power supply Heater:
The heater turns on and off every several minutes to keep the room temperature constant. Examples: Vin = 12 Vdc and the load resistor R2 = 0.25 ohms. The objective is to open and close the switch so that the average voltage across R2 is 5 Vdc. The waveform of the voltage across R2 shown below. Vout = Vin x D where D = Ton / (Ton + Toff) : Duty cycle

5 The Buck Converter The buck converter is known as voltage step-down converter, current step-up converter, chopper, direct converter. The buck converter simplest and most popular switching regulator. It has two operating modes, depending on if the transistor Q1 is turned ON or OFF.

6 Buck Converter Assume large C so that Vout has very low ripple
 Flywheel circuit Assume large C so that Vout has very low ripple Since Vout has very low ripple, then assume Iout has very low ripple Interchange of energy between inductor and capacitor is referred as flywheel effect.

7 Buck Converter What do we learn from inductor voltage and capacitor current in the average sense? the average current through a capacitor operating in periodic steady state is zero the average voltage across an inductor operating in periodic steady state is zero

8 Switch closed for DT seconds
Buck Converter + (Vin – Vout) – Switch closed for DT seconds Where D = Duty Cycle T = Switching period

9 Switch open for (1 − D)T seconds
Buck Converter – Vout + Switch open for (1 − D)T seconds When switch open VL = - Vout, diode is closed (forward biased) so iL continues to flow. This is the assumption of “continuous conduction” in the inductor which is the normal operating condition.

10 Buck Converter If D is duty cycle average output voltage is
Since the average voltage across L is zero The input/output again becomes From power balance,

11 Power Losses in a Buck Converter
There are two types of losses in an SMPS: DC conduction losses. AC switching losses.

12 DC conduction losses in Buck converter
The conduction losses of a buck converter primarily result from voltage drops across transistor Q1, diode D1 and inductor L when they conduct current. A MOSFET is used as the power transistor The conduction loss of the MOSFET = IO2 x RDS(ON) x D, where RDS(ON) is the on-resistance of MOSFET Q1. The conduction power loss of the diode = IO • VD • (1 – D), where VD is the forward voltage drop of the diode D1. The conduction loss of the inductor = IO2 x RDCR, where RDCR is the copper resistance of the inductor winding.

13 Power Losses in a Buck Converter
Therefore, the conduction loss of the buck converter is approximately: PCON_LOSS = (IO2 x RDS(ON) x D) + (IO • VD • [1 – D]) + (IO2 x RDCR) Considering only conduction loss, the converter efficiency is:

14 Power Losses in a Buck Converter
Example: For 12V input buck supply  3.3V/10AMAX output buck supply. Use 27.5% duty cycle provides a 3.3V output voltage. Vout = Vin x D = 12 x = 3.3 V MOSFET RDS(ON) = 10 mΩ Diode forward voltage VD = 0.5V (freewheeling diode) Inductor RDCR = 2 mΩ Conduction loss at full load: PCON_LOSS = (IO2 x RDS(ON) x D) + (IO x VD x [1 – D]) + (IO2 x RDCR) = (102 x 0.01 x 0.275) + (10 x 0.5 x [1 – 0.275]) + (102 x 0.002) = 0.275W W + 0.2W = 4.095W Converter efficiency:

15 AC Switching Losses in Buck Converter
MOSFET switching losses. A real transistor requires time to be turned on or off. So there are voltage and current overlaps during the turn-on and turn-off transients, which generate AC switching losses. Inductor core loss. A real inductor also has AC loss that is a function of switching frequency. Inductor AC loss is primarily from the magnetic core loss. Other AC related losses. Other AC related losses include the gate driver loss and the dead time (when both top FET Q1 and bottom FET Q2 are off) body diode conduction loss.

16 Basic Nonisolated DC/DC SMPS Topologies

17 References:
Linear Technology - Application Note 140

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