On Sun, October 27, 2013 4:09 pm, Chetan Bhargava wrote:
> Thanks for a very informative answer Russell,
>
> I'm driving 12V LED load using Arduino PWM. While looking for transistor
> suggestions and before reading your post, I used MPS2222A (Motorola TO92
> 2222) from by parts bin.
>
> The current draw from the LEDs (with PWM) was 135mA @12v. I determined th=
e
> wattage to be 1.62W. Ta < 25C.
> I used a 1K base resistor and at 100% duty cycle, the base current was
> 3.8mA (hFE ~100)
> At 100% duty cycle the collector current was 136mA
> At 50% duty cycle the collector current was 67mA
> http://i.imgur.com/E8IjHhq.jpg
>
> Although MPS2222A in a TO92 package can only sustain 625mW dissipation, i=
t
> was not running hot. I ran it >24 hours and I saw no heat issues. So you
> are right that small transistors (TO92) like BC337 can be used.
>
> Now my questions:
>
> 1. In a similar case (load, transistor, supply, etc) how much can I expec=
t
> from a TO92 case?
> 2. How to calculate the tipping point to go to TO126 or TO220 package?
> 3. Is it safe to assume that using a MOSFET can even be thermally more
> effective due to low Rds(on)

To properly answer 1 and 2 takes a bit of calculation and knowledge of the
device you're driving and how you're driving it- there isn't an easy
answer. A simple answer is to use the DC values and the thermal
conductivity of the package which will give you the temperature rise per
watt, and make sure that stays within your requirements. This is
definitely a simplification.

A MOSFET can definitely handle more current due to the low Rds(on)-
usually, most of the heat generated in a MOSFET is generated when it is
transitioning from on to off and back again- when it is going through the
linear regions- nothing happens instantaneously. If it's a low frequency
PWM, very often you can just look at the heat dissipated by the Rdson and
multiply that by duty cycle, and you'll be pretty close.

In general, the gate on a MOSFET has a lot of capacitance (higher current
MOSFETS have higher gate capacitance), so it takes some time to charge the
gate up to get the MOSFET to a "fully enhanced" state. For this reason,
there is the FET driver class of device, like Microchip's MCP1415- they
can drive very high currents for a small amount of time.

Since there is capacitance, you have to have the ability to discharge the
gate rapidly (something a FET driver also does) - the linear region exists
on the off-on and on-off transition.

Also, in one of your emails, you mentioned that your load isn't inductive
so you wouldn't need a diode... every load is inductive, it's just a
matter of degree. Adding a diode is a good safety measure. Not perfect,
but cheap insurance.

Matt Bennett
Just outside of Austin, TX
30.51,-97.91

The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with.
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