h bridge heating problems

[ian123]

[quote}but note that this is the case only when the mosfet is turned on and stays in that state.

so if i had to switch the mosfet on and then keep it switched on say for two secs then the heat dissipation would be 2.43 watts even though the passive load is drawing 3 amps at 300vdc ?

---------- Post added at 17:28 ---------- Previous post was at 17:17 ----------

The 2.43 W calculation is basically correct. There are also other kinds of transistor losses involved, see my post #10.

The 280 W maximum rating is a more theoretical value, that won't be achievable in most cases, e.g. because you can't provide a respective heatsink size.

So apart from the other losses are you saying that only 2.43 watts is dissipated by the mosfets therefore i should be able to have quite a small heatsink say 60mm wide would give me 12degrees per watt therefore the heatsink would go upto say 25 degrees plus ambient temp?


would my reasoning be correct

would this be enough heatsinking for the mosfets???

---------- Post added at 17:39 ---------- Previous post was at 17:28 ----------

The bridge rectifier power dissipation will be in fact twice the value of a single diode. According to a 8A datasheet that I have (KBU8), it's 2*3A*0.9V = 5.4W, resulting in about 100 K temperature rise with 18 K/W thermal resistance.

please explain 100 "K" does this = kelvin or centigrade

1 Kelvin =273.15 degrees celcius then 100 x 273.15 = 27315 degrees centigrade (about the same as a large furnace).

---------- Post added at 17:42 ---------- Previous post was at 17:39 ----------

would the bridge alone need a 12.5 cm heatsink and the mosfets a 6.0 cm heatsink

 

[alexan_e]

but note that this is the case only when the mosfet is turned on and stays in that state.

so if i had to switch the mosfet on and then keep it switched on say for two secs then the heat dissipation would be 2.43 watts even though the passive load is drawing 3 amps at 300vdc ?

Imagine the behavior or the mosfet like a resistor in series with your load, the power on each resistor will be V*I or I²*R or V²/R where R is the mosfet resistance, V is the voltage across drain-source and I is the common current,
all these represent the same equation and give exactly the same result.

Alex

 

[ian123]

so in this circuit you are saying that each mosfet is 0.27 ohms which is the rdson of the mosfet

so total resistance in circuit is 300 volts /3 amps = 100 ohms

therefore 0.27 X 2 =0.54 ohms therfore the load is 100-0.54 ohms = 99.46 ohms

 

[alexan_e]

The Rds-on depends on the Vgs so it may be slightly different but your calculation is correct if you are trying to figure what load resistor to use in order to have 3A @300v

Alex

 

[ian123]

actually trying to figure out the size of the heatsink i need for the mosfets

i can get a 200 degree celsuis per watt per meter lenght of heaksink.

If the mosfets only dissipate 2.43 watts then in a bridge formation 2.43 X 2 = 4.86 watts

therefore a 60mm lenght of heatsink would do for the mosfets to run at 24 degrees celsius + 25 ambient temp = 50 degrees

would this be correct??

 

[FvM]

1 Kelvin =273.15 degrees celcius

No. You should review this in a physics book or at Wikipedia.

For temperature differences, °C and K is just the same. Traditionally , natural scientists are using K for temperature differences, see e.g. Thermal resistance in electronics - Wikipedia, the free encyclopedia. But I noticed that datasheets are mostly using °C/W these days.

 

[Orson Cart]

how long do you keep the mosfets on for? after a while the boot-strap caps will discharge and eventually the UVLO on the IR2110 will turn the high side off - can you post a picture of your circuit (photo) - is there an inrush current with your load?

 

[ian123]

how long do you keep the mosfets on for? after a while the boot-strap caps will discharge and eventually the UVLO on the IR2110 will turn the high side off - can you post a picture of your circuit (photo) - is there an inrush current with your load?

please check timming chart in my previous replies

 

[Orson Cart]

OK - had a look at the timing - putting an isolated scope on the high side gates will tell you if the bootstrap supplies droop too far. Even with only a few watts in the mosfets and bridge rectifier you will need a reasonable heatsink to keep the sink temp to 60 deg say (this is too hot to touch for more than a second or two) Regards, Orson Cart.

 

[ian123]

OK - had a look at the timing - putting an isolated scope on the high side gates will tell you if the bootstrap supplies droop too far. Even with only a few watts in the mosfets and bridge rectifier you will need a reasonable heatsink to keep the sink temp to 60 deg say (this is too hot to touch for more than a second or two) Regards, Orson Cart.

if each side of bridge is 4.26 watts is that a lot of heat ? do i need a heatsink for this ?

by the way hows the world cup doing are you enjoying it.

 

[Orson Cart]

Yes a proper heatsink is a good idea, to answer your question re "is that a lot of heat?" - try putting 4.26 watts into a 5W resistor (from a bench power supply say) and see how long you can hold onto the resistor - this will give you some idea.

The Rugby is pretty good - especially the clashes between the minor nations - great to watch...

 

[Erika]

Hi how are you? I'm Erika, I'm making a H bridge with the IC IR2184 and 4 mosfets Irf1405, but I have some problems. First of all, two mosfets heat up (the mosfets that are in th bottom in the bridge) when the duty cycle is low and I don't know why. Finally, the motor doesn't work continually, I mean, the motor stop each 2 seconds for example =(. Could anyone help me please? It is urgent

 

[barry]

a schematic would help

 

[Erika]

View attachment Motores del Kid.pdf

This is the schematic, I am using MOSFET IRF1405 not IRF3704 as shown in the schematic.
I am working at 20kHz, sometimes any MOSFET heats up.
In the video the circuit consists in 2 motors (the motors are in PDF attachment) controlled with a potentiometer.
At the begining the motors has max Duty Cycle, in that moment the mosfets dont heat up.
But when I start to decrease DUty Cycle, the motors start to work discontinuously as shown
the video (sometimes, this situation also happens when the duty cycle is hight),
but if I apply a load (in this case stopping the motors with my hands) these start
to work contunously. In both cases with or without load, the mosfets heat up, and this is getting
hotter while the Duty Cycle is smaler. Excuse me for my English, if you dont understand please
ask me again. Thanks a lot. =)

 

[barry]

How hot is "hot"? Any MOSFET will heat up, especially when it's being switched rapidly (it dissipates most of the power while it's being switched). What is your supply voltage? What is the value of the gate resistors? If those gate resistors are fairly large (coupled with the large input capacitance of the MOSFET), you'll have a long transition as the MOSFETs switch from OFF to ON, and lot's of power will be dissipated.

 

[Erika]

Hi, the supply voltage is 12V, the gate resistors are 4.7ohm. The mosfets just heat up when the duty cycle is low, while the duty cycle is maximum the mosfets don't heat up. So, I don't understand why the mosfets heat up just with some specific duty cycles. hot = If I leave the circuit working, the mosfets begin to throw smoke.

 

[barry]

okay, smoke= "pretty hot". Low duty cycle means that the MOSFETs are off most of the time, but the fact that they overheat implies that they're really ON. Can you put a scope on all 4 gate signals and verify that they look like you expect? You shouldn't have top and bottom MOSFETs on one-side of the "H" on simultaneously (The 2184 should take care of that), but you should verify that's not happening. I think you need to snoop around with a scope and see what's happening exactly.