Power Electronics

Roshan Taneja

Voltage regulators

Must have a higher input voltage
dropout: minimum amount of input voltage must be higher
low noise - pro!
psrr: power supply rejection ration
- measure of how low it gets rid of noise
  - low frequency noise best removed by LDOs
- so like 70db (120hz), is like 99.99% of noise @ 120hz
- 20log(x) = db
  - 10 ^ (db/20) = x
How do they work?
- all it is is a variable resistor, vary that resistor to figure out how to increase/decrease the resistance
- thats why they are low noise
They need input and output capacitors
- input: "well of energy" for the ldo to use
- output: needed for stability of output voltage
  - think: capacitors smooth the output -> reduce oscillations
Downside:
- A in is the same as A out
  - makes sense
- power = Voltage * current right
- example: input is 5v and output is 3.3V and current is 1A
  - 5w power input, 3.3w output
  - conservation of energy, where is the other 1.7w?
  - HEAT!
  - they get hot.

3 types
- Buck Regulator: Hi to low
- Boost regulator: low to hi
- Buck-boost regulator: output voltage can eb higher or lower
can be adjustable/fixed
regulator and converter mean the same thing
How it works
- SW pin generates square wave
- the inductor then has a roughtly constant voltage across it
  - we know that V = di/dt
  - so the increase or decrease in current is linear
- the current through the inductor charges the output capacitors -> output voltage
  - the voltage across the capacitor is at a wave close to a constant voltgage
The output current is actually diferrent from the peak inductor current
- you need to pick
Inductor and capacitor values are based on switching freqency
Output Circuit Design:
- theres a feedback pin, that reads what the incoming voltage is and tries to force it to be 1 volt.
- \(R_{fbb}\)
Unlike LDOs, they have a % efficiency
- the efficiency will vary a bit with most chips
- power dissipation = (1-efficiency) * output power

Short Circuits -> pulls a lot of current, can cause damage
- Use a fuse! PTC Resettable Fuses, will cool down
- can also use ATO fuses
"Reverse Polarity" plugging in the battery backward
- Use a diode, if current is backwards, then current will not flow.
- Power ORing: if you plug in both battery and USB.
  - add a diode from each psu into your circuit
- downside: diodes have a voltage drop, so make sure you can afford a lower voltage
Static Electricity/ESD
- TVS Diodes (Trans Voltage Suppressant)
- if you dial a negative enough voltage against it, it will conduct
- TVS diodes are designed to breakdown when ESD happens
- Placed near connectors

Frequency response, for low frequencies, a capaitor acts as an open circuit
- for hight frequencies, a capaitor acts as a short circuit
- this makes them powerful for filtering noise!
ESR: Equivalent seires resistance
- caps are not perfecct, have a small resistance
Cap types
- Ceramics are the lowest esr
- Aluminum polymer, fairly low if you need more capacitance
- Hybrid have electrolyte solution, still pretty low esr
- Electrolytic have really high esr, do not use them as a filter. Only good cuz theyre cheap
- If I really want a high esr for ringing, then maybe use electrolytic
Decoupling Capacitors
- Capacitors act as a short at high frequency, add them to power inputs near chip, remove high frequency
- why use multiple values?
  - its more bs, watch the video in the slideshow
  - capacitors are capacitors, resistors and inductors
  - At high frequencies, it becames more of a inductor, even though it works, so at really high freqs, you actually dont get any attenuation anymore
    - so you add more, it increases the area of electrical impedence, so we have a wider range of getting rid of noise.
- Must be placed as close as possible to the chip

If you wanna get rid of almost all the noise, use ferrite beads
magnetic beads, so at higher frequencies, they have higher resistance
- Ferrite bead has high resistance -> dissipates high freq as heat
- ferrite bead high resistance and C8 low resistance -> high freq prefer going through c8
- ferrite bead high resistance + C5/c^ -> RC low pass filter
  - A low pass filter keeps low freq but removes high freq
Use them for our analog boards,
- any noise in our power lines show up in our sensor, so it gets rid of as much noise as possible

Components that get hot often have a large "exposed pad"
- often connected to gnd
- solder the pad to parge copper pour to make it a heat sink
your board likely has a gnd layer somewhere
- add vias to ground -> the vias transfer the heat into the internal layer. -> improves dissipation
how to calculate thermals?
- determine the power dissipation in watts
- find r0ja and r0jc thermal resistances
- multipy both by the watts dissipated -> degrees in celsius the chip will heat up
- final tem = ambient + increase calculated