Linear Regulator / switching Thermal - Page 1
Forum section: electronics / beginners

author: NMP
author_rank: Contributor
timestamp: August 04, 2016, 11:07:26 pm
content: Hi,I thought i did understand the way to check how much a regulator whould heat.Check the Thermal resistance to Air, ex 65 some math with the consumption*StepDown Voltage, and calculate the Temperature.Having a Vin of 24V wanted 5V on the output i thought using for instance LM2592. The Junction-to-ambient thermal resistance its not that much different from a linear!Its common knowledge the one advantage of a Switching Buck Reg its the thermal performance...Im confuse now.. I can i calculate how much a regulator whould heat?

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2016, 04:11:23 am
content: The switching regulator produces less heat to dissipate. It does not necessarily have a lower thermal resistance.

author: leeatljs
author_rank: Contributor
timestamp: August 05, 2016, 07:55:18 am
content: Its all about power dissiption.In your linear regulator 24V in 5V out, lets assume an amp load current. The regulator is 'wasting' 24 - 5 = 19VoltsWould dissipate, 19V * 1A = 19Watts. Hot.A switching regulator would probably dissipate about 1 watt, assuming its efficiency is above 90%So the switching regulator would probably not need a heatsink, but the linear one would require quite a large one.

author: MosherIV
author_rank: Super Contributor
timestamp: August 05, 2016, 12:33:21 pm
content: QuoteThis would depend on the process to make the silicone device and the thermal transfer characteristics of the plastic (or other) package used.ie each device data sheet will quote the thermal resistance between the silicone to the package.QuoteAs others have tried to say in a round about way, switching regulators are more efficient because they convert one voltage to another, hence will create less heat in the process.The heat loss from a switcher will be down to inefficiencies of the switching device (MOSFET probably - therefore should be less than 1ohm) and the resistive part of the inductor.Linear regulators actually dissipate (waste) the difference in power ( I x V ) to convert one voltage to another, that dissipated/wasted power is converted to heat. Hence Linear Voltage regulators get much hotter than their switching counter parts.To work out the heat a linear will produce, just work out the voltage drop, multiply by current to find power, then apply the thermal resistance figure from the data sheet.

author: mariush
author_rank: Super Contributor
timestamp: August 05, 2016, 12:51:13 pm
content: Also worth mentioning, most switching regulators require additional components like an inductor and a diode besides basic components that are recommended for linear regulators as well (input and output decoupling capacitors, two resistors to configure the output voltage).There's some power dissipated inside the inductor and inside the diode so unlike linear regulators where all the heat is concentrated in one point, with switching regulators the heat is spread in a larger surface area.It's not always the case but most of the time it is - some switching regulators have the diode built inside them (some don't require diodes at all) , some (esp those running at high frequencies) even have the inductors built inside them (the chip is slightly taller usually to accommodate the inductor) but for most cheaper switching regulators.

author: NMP
author_rank: Contributor
timestamp: August 05, 2016, 06:28:43 pm
content: For linear its pretty strait forwardWhat was not clear to me its how could i, having a switching regulator, calculate how much it heats up.Reading your answers i see that the efficiency is the key.For the example of LM2592 at (1A)Vin= 24V Vo = 5V => 19Whaving a worst cenario of 90% efficient 19/.9 = 21.1WThe difference is 2.1WThe thermal resistance Junction to ambient is 20ºC/W (in datasheet having a double sided PCB with 3inch copper at least)2.1W x 20ºC/W = 42ºCThat means for 1A it will heat to 42ºCIs that it?

author: dmills
author_rank: Super Contributor
timestamp: August 06, 2016, 12:44:14 pm
content: Ah no, try it this way,5V @ 1A = 5W output power.5W / 0.9 = ~5.5W input power, difference is 0.5W which is lost as heat in the switcher.The difference between 19W and 0.5W is why linear regs are seldom used when you have that large a voltage difference, the heat load is just so much less painful with a switcher.Conceptually linear regulators have equal input and output current, switching regulators have equal input and output power (Both assuming ideal devices).Regards, Dan.

author: NMP
author_rank: Contributor
timestamp: August 06, 2016, 01:47:28 pm
content: Perfect!! Even simpler