Energy transfer with a resistor

Transcription

1 nergy transfer with a resistor An emf with a higher voltage can charge an other emf 2 with a lower voltage if you connect them to together with a current limiting resistor. > 2. I R 2 2 I R The delivered energy 2 W It R RI t W2 2It W During charging, there becomes heat losses in R. Stored energy

2 nergy transfer with an inductor With an inductor one can transfer energy from a larger emf to a smaller, > 2, Step Down, but also from a smaller emf to a larger, < 2, Step Up. This in theory is completely without losses.

3 Step Up 2 2 Transistor Diode Pulse width modulation PWM-unit Transistor and diode acts together as a switch controlled by the PWM unit.

4 2 Step Up i 2 2 i DutyCycle D : ton toff D t on ton t off

5 The coil current inertia Pump t 0 i Waterwheel Flywheel di dt i di dt t The inductor has current-inertia. The current can not change immediately. A fluid analogy: The inductor is a water mill with a flywheel.

6 nergy transfer with an inductor i 2 i on et drive an increasing current through during t on. The current then reaches I max. I max I avg I 0 i on t t on

7 nergy transfer with an inductor i I max I avg I 0 2 i off ioff switch the inductor so that now continnues the current through 2. 2 > means that the current will be decreasing. It will reach I 0 again after time t off. 2 The times t on, t off are those that gives us a stationary plot. t t off

8 nergy transfer with an inductor i I max I avg I 0 i on i off 2 Stationary nergy: W on I avg t on ton W off toff ( 2) Iavg toff Won Woff t Stationary 0 2 ton toff t off D 2 D

9 5V 50V? 2 5V 50V Dutycycle equation: 5 2 D D %

10 Resistiv load? 5V 50V C The current to the load will flow intermittently only during t off, so the voltage needs to be smoothed with a capacitor C.

11 Semiconductor diode ( What is a diode? ) ideal diode A K Symbol and characteristic conduction direction blocking direction Fluid Analogy check valve

12 C B ( What is a transistor? ) C Water analogy B h F C B The classic bipolar transistor, is earlier development than the MOS transistor. A small "base current"i B can control up to 00 times (h F ) bigger collector current I C. h h F F I I C B 00 PIC I B ma I 00 ma C

13 Practical dimensioning? It is simple to set up the Step-up converter's output voltage with the DutyCycle D! This we do at the lab. In practice it is much more difficult. An electronics engineer is faced with many questions: At what current "saturate" the coil iron core? How big internal resistance has the inductor? How big are the load variations? What values to and C and f should be choosen? It is common to simulate the circuit with more realistic component models than what we use here. T:s App note has the title Switching Regulators for Poets

14 A problem is that while everyone agrees that working switching regulators are a good thing, everyone also agrees that they are difficult to get working. Unfortunately, switching regulators are one of the most difficult linear circuits to design. Mysterious modes, sudden, seemingly inexplicable failures, peculiar regulation characteristics and just plain explosions are common occurrences. Diodes conduct the wrong way. Things get hot that shouldn t. Capacitors act like resistors, fuses don t blow and transistors do. The output is at ground, and the ground terminal shows volts of noise. Jim Williams

15 Jim Williams at lab

16 Simulation D = 86 % Startup Sequence 0 ms 5 The voltage is stepping upwards Simulation with an ideal switch

17 Stationary at 00 ms = 5V 2 30V D = 86% Simulation

18